CONTENT and DESCRIPTIONS OF ORE DEPOSITS
Image: Section of the Kalgoorlie Super-pit.
Porter GeoConsultancy assisted the Agência para o Desenvolvimento Tecnológico da Indústria Mineral Brasileira - ADIMB of Brazil, - to organise a study tour for 13 senior Brazilian geologists to 10 significant gold and base metal ore deposits across Australia during September 2008. The tour was divided into three modules, as follows.
South-west Module in Western Australia
- Archaean greenstone belt gold,
St Ives - Archaean greenstone belt gold,
Cawse - lateritic nickel,
Kambalda - Archaean massive sulphide nickel,
Forrestania - Archaean massive & disseminated sulphide nickel,
North-east Module in NW Queensland - Mesoproterozoic IOCG style copper-gold,
George Fisher - Mesoproterozoic sediment hosted zinc-lead-silver,
Mt Isa Copper - Mesoproterozoic transgressive sediment hosted copper.
Central Module in western New South Wales and northern South Australia - Mesoproterozoic metamorphic hosted zinc-lead-silver,
Olympic Dam - Mesoproterozoic IOCG style copper-gold-uranium.
The tour commenced when the group assembled independently in Perth, Western Australia on the evening of Sunday 7 September, 2008 and ended in Adelaide, South Australia, on the morning of Tuesday 23 September, 2008, after which the group dispersed and returned to Brazil. A few participants only took part of the tour, as suited their interests or availability.
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Kalgoorlie Super-pit - Gold ...................... Monday 8 September, 2008.
The KCGM Super Pit lies within the Kalgoorlie Gold Field, some 600 km east of Perth. It exploits the Golden Mile string of deposits (#Location: 30° 46' 39"S, 121° 30' 10"E).
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The Kalgoorlie Gold Field is developed within the Archaean Norseman-Wiluna greenstone belt of the Eastern Goldfields province in the eastern Yilgarn craton. It is hosted by greenschist facies, marine volcano-sedimentary rocks, divided into narrow blocks by NNW-trending regional wrench faults. The volcanic succession comprises, from oldest to youngest: (1) the Lunnon Basalt (~2720 Ma) - pillowed to massive tholeiitic lavas; (2) Kambalda Komatiite (2709±4 Ma) - ultramafic flows; and (3) Devon Consols and Paringa Basalts - two pillowed to massive magnesian lavas, separated by the 10 m thick, 2692±4 Ma, Kapai Slate marker horizon of sulphidic and tuffaceous shale.
This volcanic succession is overlain by black shales, greywackes, and volcaniclastic rocks of the Black Flag Group. To the south of Kalgoorlie, near Kambalda, this latter unit is intruded by the 2680±8 Ma Condenser Dolerite sill and 2678±8 Ma rhyolite porphyry dykes. In the Kalgoorlie district it is intruded by the 750 m thick Golden Mile Dolerite, which is the principal host rock to lode structures, and comprises a folded and metamorphosed, differentiated sill, emplaced at the same stratigraphic position as the Condensor Dolerite. The youngest mineralised rocks in all gold mines located between Kalgoorlie and Kambalda are 2675 to 2660 Ma stocks and dykes of hornblende-plagioclase porphyry.
Deformation of the volcano-sedimentary succession in the Kalgoorlie-Kambalda area was subdivided by Swager (1989) into four events: D1, characterised by recumbent folding and nappe-style thrusting; D2, which comprises upright folding that produced regional northwest-trending folds; D3, a post ~2660 Ma episode of sinistral wrench faulting in a transpressional regime, with major NNW-trending faults; and D4 which involved dextral wrench faulting, mostly NNE-trending.
The Kalgoorlie Gold Field comprises more than 1000 discrete lodes, controlled by brittle-ductile shear zones, clustered in a geometric array on both sides of the steeply dipping, NW to NNW-trending, sinsitral Golden Mile fault, which transects folded Golden Mile Dolerite in the centre of the mining district. The most significant of the lodes make up the Golden Mile, Mt Charlotte, Mt Percy and Hannan's South ore zones. An envelope of chlorite-calcite alteration, surrounds the entire mining area, replacing metamorphic actinolite and albite in all mafic rocks. The main lode systems are contained within a volume of these altered rocks, which is up to 5 km in length, by 1 to 2 km in width, and to a depth of 1000 m. They are controlled by a complex series of steeply dipping shears, and are largely hosted by ultra-mafic and mafic rocks and sills, the most important of which is the composite mafic sill, the Golden Mile Dolerite, and to a lesser extent the Paringa Basalt.
Two groups of structures, the Fimiston and Oroya lodes have been recognised. The Fimiston Lodes occur in steeply dipping (70 to 90°) shear zones, commonly parallel to the main Golden Mile fault, although others have different strike orientations. Individual lodes are up to 2 km long by 1.3 km in vertical extent, with high-grade shoots located at the intersections of shear zones. They are characterised by breccia bodies and cavity-fill veins surrounded by (1) an inner sericite-ankerite-siderite-quartz-hematite-pyrite±telluride alteration zone, containing most of the gold, and (2) an outer ankerite-sericite-quartz-pyrite zone where chlorite and calcite are progressively replaced. The ores are mineralogically refractory and complex, containing free native gold (often intimately associated with gold-bearing arsenical pyrite) and a significant proportion of Au-Ag-Hg-Pb telluride minerals. These Fimiston Lodes are subdivided into the Eastern and Western Lode System on the flanks of the Kalgoorlie Syncline. The Eastern Lodes System comprises a swarm of lodes (areas of pyritic and hydrothermal alteration) with mineralisation confined to shoots at lode-lode and lode-fault intersections. The Western Lode System is less complex, although the lodes are more persistent and well defined. Individual lodes occupy 20 to 50% of a lode channel and may be 30 to 1800 m long, 0.1 to 10 m thick and extend 30 to 1160 metres down dip. Mueller et al. (1988) suggest that the Fimiston Lode shear zones formed during D3 sinistral wrench faulting, whereas Bateman et al. (2001a) and Bateman and Hagemann (2004) conclude that the mineralised shear zones formed as flat, late D1 thrusting structures, subsequently rotated into their present subvertical position during D2 folding.
The Oroya Lodes represent the high-grade "green leader" ores, characterised by green vanadian muscovite, ankerite, quartz, pyrite, native gold, and gold-silver tellurides, the principal example of which is the 1500 m long Oroya shoot on the Paringa mine leases of the Golden Mile. This lode is controlled in large part by the 50°W dipping, reverse
Oroya shear zone system. This style of ore also occurs in the brecciated cores of steeply dipping Fimiston lodes, and may represent a late-stage of the D3 transpressional regime that generated the Fimiston lodes (Mueller et al., 1988), or a separate mineralisation event (Bateman et al., 2001).
Mount Charlotte occurs as a quartz vein gold deposit, present as a series of steeply plunging, pipe-like vein stockwork orebodies in massive 2692±2 and 2678 to 2670 Ma metagabbro of the Golden Mile Dolerite. These, and other related quartz-vein stockworks cross-cut the Fimiston and Oroya lodes, and are apparently controlled by district-scale NNE-trending strike-slip D4 faults. The Mt Charlotte mine exploits the main Charlotte and Reward orebodies and the satellite Maritana and Northern bodies. The Charlotte orebody extends more than 800 m vertically, from the surface to -1000 m RL, 250 m north-south along strike and over a width of 50 to100 m east-west. Reward extends from the surface to -800 m RL, 250 m north-south along strike and 50 m east-west. The orebodies are restricted to the most differentiated (and competent) unit of the host sill and are usually found adjacent to major steeply dipping faults where these cut the sill. The stockworks have two sets of veins that were developed as hydraulic fractures and were filled simultaneously and are of equal significance. Gold is in pyrite or pyrrhotite bearing metagabbro around the stockwork veins and to a lesser degree as free gold in the veins and along vein margins.
The final 'Super Pit' open pit is designed (as of 2008) to have dimensions of 3.8 x 1.35 km and a depth of 500 m below the surface by 2018. Production in 2006 was 85 Mt of mined rock, 12 Mt of which was milled. The remaining tonnage was stockpiled low grade mineralisation and waste. In 2007, the underground Mt Charlotte mine produced 1 Mt of ore @ 3 g/t Au.
Total production from 1893 to 2005 was ~1475 t Au (47.5 Moz), with a further ~110 t from 2005 to 2010.
The total open pit and underground reserves plus resources at December 31, 2010 were (Barrick, Newmont, 2011):
Proved + probable reserves - 153 Mt @ 1.71 g/t Au, for 260 t Au; (reserves in addition to resources)
Measured + indicated resources - 95.6 Mt @ 0.76 g/t Au, for 72 t Au;
Inferred resource - 2.24 Mt @ 4.47 g/t Au, for 10 t Au.
The Kalgoorlie Super Pit open pit and Mt Charlotte underground operations are owned by KCGM, a 50:50 JV between Newmont Mining and Barrick Australia. Production in 2010 totalled approximately 24.5 t of recovered Au (Barrick and Newmont, 2011).
St Ives - Gold ...................... Tuesday 9 September, 2008.
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The St Ives Complex of gold orebodies lie within the Archaean Norseman-Wiluna greenstone belt of the Yilgarn Craton in Western Australia, and comprises a string of separate orebodies distributed over a 30 km long, NNW-SSE orieneted trend parallel to the Boulder Lefroy Shear Zone, to the south of Kambalda, approximately 75 km SSE of Kalgoorlie.
The main mines and complexes include Redoubtable, Intrepide, Delta, Revenge, Orchin, Brittania-Sirius, Victory, Defiance, Junction and Argo with lesser past, current, future and subsidiary mines at Formidable, Bahama, Santa Ana, Hunt, Apollo, Orion, Reward, Nelsons Fleet, Repulse, Thunderer, Mars, Belleisle, Cave Rock, Agamemnon, Conquerer, Leviathon and others.
Like the New Celebration gold complex some 30 km to the NNW, the St Ives complex is located within the core of a regional-scale anticlinal dome, the NNW trending Kambalda Anticline which also envelopes the Kambalda Dome to the NNW. The Kambalda komatiite hosted nickel-sulphide deposits are distributed around the flanks of the Kambalda Dome which lies between the two gold complexes.
Within the Kambalda Anticline, the Norseman-Wiluna greenstone belt sequence comprises the +2000 m thick Kalgoorlie Group volcanics (mainly basalts/komatiites with lesser sediments) and the 1000 m thick Black Flag Group felsic volcanics and sediments, dated at 2700 to 2660 Ma, overlain by the post tectonic conglomerates and terriginous arenites of the Merougil Beds. Doleritic to gabbroic dykes and sills intrude the Kalgoorlie and Black Flag groups. Gold mineralisation, although found in nearly all stratigraphic units, is best developed in the mafic dominated part of the Archaean sequence, above the Kambalda Komatiite (host to the nearby major nickel orebodies). Hosts range from basalts to dolerites to slates.
Gold mineralisation is disseminated within shear zones or hosted by quartz veins or breccias, localised within low-displacement reverse shear zones up to 1 km long with maximum displacement of up to a few hundred metres. Many of the mineralised shears are local structures, adjacent to larger NNW-trending regional shear zones, such as the Playa Shear Zone, to which they are interpretted to be related. The Playa Shear Zone, which is the largest structure in the immediate vicinity of most of the St Ives deposits and extends for an interval of 10 to 15 km south of the Kambalda Dome, is considered to be a second order structure, related to the major regional Boulder-Lefroy Shear Zone 5 km to the north-east.
Ore is contained within 2 to 10 m wide thrust/shear zones trending north to NNW. Gold deposits are also associated with stockworks and vein arrays that have been developed in brittle host rocks, such as the Kapai Slates and the felsic to intermediate lithologies, in breccia zones and central, quartz rich and mylonitic parts of shear zones which are invariably only minor structures. They are associated with post-peak metamorphic faults and thrusts which post date felsic dykes and are splays from the nearby regionally extensive Boulder-Lefroy Fault. The alteration zonation has a core development of albite-ankerite/dolomite-quartz-pyrite, with gold being associated with the pyrite; enclosed by a biotite zone; passing in turn into a chloritic halo and then the regional greenschist metamorphic assemblage.
To June 1997, the total production at St Ives had totalled:
15.6 Mt @ 4.2 g/t Au underground, and 14.3 Mt @ 2.8 g/t Au open pit.
The total resource in December 1999 was:
23.5 Mt @ 6.4 g/t Au underground and 33.9 Mt @ 2.5 g/t Au open-cut.
At December 31 2006 the reserves and resources were quoted at (Gold Fields, 2008):
proved + probable reserve - 27.74 Mt @ 2.7 g/t Au,
measured + indicated + inferred resource - 56.6 Mt @ 2.9 g/t Au.
In 2007, underground production was 0.1336 Mt @ 5.28 g/t Au, while open pit mining produced 3.928 Mt @ 2.23 g/t Au, with a waste to ore ratio of 6.38.
The complex of more than 22 deposits has been historically operated by WMC Resources Gold Division, before being purchased by the Gold Fields Group.
By 2007, the majority of poduction was mined from the Thunderer and Delta North pits with operations and development continuing in the Argo, North Revenge and Bahama pits, the Mars pit and the Belleisle underground mine below that pit, the Cave Rock underground mine, the Agamemnon pit, the Conquerer underground mine and the open pits of the Leviathon group of mines.
Cawse - Lateritic Nickel ...................... Wednesday 10 September, 2008.
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The Cawse project area is located some 55 km to the north of Kalgoorlie in Western Australia (#Location: 30° 23' 11"S, 121° 9' 13"E).
It lies within the Archaean Norseman-Wiluna greenstone belt of the Archaean Yilgarn Craton. The Cawse deposit is on the SW side of the Coongarries-Mount Pleasant anticline, a regional fold with a width of about 20 km, with a core of granitoid, overlain on the limbs by a greenstone sequence. The mafic-ultramafic sequence of the greenstone succession on the SW side of the anticline is 10 km thick and can be broadly correlated with the sequence in the Kalgoorlie-Kambalda region to the south, and comprises from the base:
Pole Group - not present near Cawse, mainly basalts;
Linger and Die Group, divided into,
Walter Williams Formation - coarse-grained olivine adcumulate dunite composed of fresh olivine and minor chromite; with a thin orthocumulate present at the base and top of the formation, with the upper orthocumulate separated from the underlying adcumulate by a thin harrisitic, olivine layer. The contact with the overlying Siberia Komatiite is a complex zone of peridotite, pyroxenite, gabbro and high Mg basalts.
Siberia Komatiite - a 2600 m thick sequence of thin spinifex-textured kamatiite flow units, and includes minor high Mg basalt and gabbro.
These are overlain by a sequence of epiclastic sedimentary rocks.
The lateritised protoliths of the Cawse deposit are serpentinised olivine mesocumulates and adcumulates of the Walter Williams Formation. In this area the unit 300 m thick and strikes NNW. Orthocumulates are present at the upper and lower margins of the unit and locally intertongued within the dunitic host. The protolith is cokposed of forsterite, antigorite, lizardite, minor magnetite, brucite and hydroxy-carbonates with minor chromite, magnesite, dolomite, talc, chlorite and silica. The un-enriched protolith contains 2500 ppm Ni and is enriched in olivine-compatible elements (mean wt%, 42.35% MgO, 36.6% SiO2, 8.33% Fe2O3, 100 ppm Co, 820 ppm Mn, 9 ppb Ir, 12 ppb Os, 7 ppm Ru); and is depleted in olivine compatible elements (Al2O3, TiO2, Cu, Zr, Pt, Pd, and Rh).
The deposit comprises an up to 500 m wide corridor that extends for around 25 to 30 km containing lateritic nickel and cobalt mineralisation. This corridor reflects a fault structure related surface "channel" cutting the Siberia Komatiite within the Archaean Norseman-Wiluna greenstone belt.
Around 80% of the mineralisation in the main Cawse Central tenements consists of a shallow, flat lying zone associated with limonitic clays in the upper 40 m of the weathering profile in this structure, overlying a barren saprolite.
The lateritic profile comprises, from the base:
i). Immediately above the saprolite a 10 to 30 m thick smectite layer is developed, composed of lime green. mottled green and brown nontronitic clays with variable overprinting silica, while talcose clays occur in subvertical shear zones;
ii). Limonite zone, which is 10 to 30 m thick, 1.7% Ni, 0.09% Co, composed of residual clay with finely divided irnoxides particles adding a orange to yellow tinge. A distinctive 1 to 54 m thick interval of indurated silica and manganese oxide with pisolitic, crustiform or botryoidal texture is consistently developed at the top of the limonite zone containing high grade silica-cobalt with 1.6% Ni, 0.73% Co and 5% Mn. A narrow zone of manganiferous cobalt ore occurs at the transition from the limonite to the underlying siliceous ores and carries similar grades of Ni and Co, but higher Mn;
iii). Lateritic duricrust - 0 to 10 m thick, comprising a pisolitic ironstone, cemented in part near the surface to form a duricrust;;
iv). Transported soil, alluvial silt and clay from 0 to 3 m thick.
There are four zones of Ni-Co laterite enrichment:
i). Nontronitic ore, comprising green to chocolate brown smectitic clays; ii). shear controlled high grade Ni in talcose zones; iii). a 1 to 10 m thick siliceous Mn-Ni-Co horizon at the top of the limonite zone; and iv). a sub-horizontal blanket of limonitic clay extending over the full length of the deposit containing Ni and Co.
Around 90% of the resource at Cawse has <0.5% Mn, although the Co rich zones may contain up to 20% Mn. The deeper nontronitic clay and talc zone is found on the margins of bedrock structures.
The oxide (limonite) ore at Cawse is the result of a second phase of weathering that oxidises the first stage nontronite formed from the protolith, which at Cawse was a dunite, not a peridotite.
In 1997 the total resource amounted to 213 Mt @ 0.7% Ni, 0.04% Co, over the 25 km interval.
Proven + probable reserves were of 30.3 Mt @ 1% Ni, 0.11% Co. The ore deposit is an oxide laterite.
The operation was originally developed by Centaur Mining & Exploration Ltd, but has since (2001) been sold and is now owned by OMG Cawse Pty Ltd, a member of the US based OM Group, and has subsequently been purchased by Norilsk Nickel.
Kambalda (Juan-Otter & Miitel) - massive sulphide nickel-copper ...................... Thursday 11 September, 2008.
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The Kambalda nickel mines lie within the south-central section of the Archaean Norseman-Wiluna greenstone belt, 60 to 100 km SSE of Kalgoorlie in Western Australia. More than 22 deposits worked as part of the Kambalda operation, includes those on the flanks of the Kambalda and Widgiemooltha domes, and within the St Ives, Tramway and Golden Ridge - Carnilya Hill belts, and other equivalent exposures of komatiitic flows, as listed below.
The lowest member of the host succession within the Kambaldo Dome sequence is the Lunnon Basalt which is overlain by the host 2710 Ma Kambalda Komatiite, followed by the Devon Consols Basalt and a package of basalts and sediments (slates and greywackes) deposited from 2710 to 2670 Ma. These are intruded by a 2662 Ma granitoid stock which forms the core of the oval shaped Kambalda Dome around which the individual orebodies are distributed in an annular zone of approximately 8 x 3 km, elongated in a NNW direction. Peripheral porphyry dykes associated with the granitoid stock cut both the hosts and ore.
The sill like Kambalda Komatiite, which lies more or less conformably between the two basalts, is composed of the upper Tripod Hill Member and the lower Silver Lake Member, with Fe-Ni mineralisation being generally restricted to the lowermost sections of the latter. In each, flow, lateral and vertical variations in composition, degree of differentiation and distribution of interflow sediments define channel flow and sheet flow facies. Channel flows may be up to 100 m thick, 500 m wide and 15 km long, occupying channel structures in the underlying Lunnon Basalt.
The Fe-Ni sulphides are usually restricted to the base of the lowermost channel flows - contact orebodies - but are occasionally also in higher flows - hangingwall orebodies. The contact orebodies, which have historically accounted for 80% of the reserves, occur as elongate, lensoid and tabular ribbon like bodies up to 3 km long and 300 m wide and usually <5 m thick, containing <0.5 to 10 Mt ore lenses. The individual orebodies grade upwards from the around 2 m thick basal massive (>80%) sulphides to around 2 m of matrix (40 to 80%) sulphide, to disseminated and blebby sulphides.
Deposits around the Kambalda Dome include Lunnon, Hunt, Victor, Long, Gibb, Durkin, Otter-Juan, Gellatly, McMahon, Coronet, Fisher, Ken, Hunt, Alpha and Beta.
Mining commenced in the Kambalda district in 1967, and from 1972 to 1988 exploration maintained resources at an almost constant level of around 25 Mt @ 3.2% Ni. Total production from 1976 to 1996 was approximately 34 Mt @ 3.1% Ni. The bulk of this production was from the main Kambalda Dome deposits.
Similar bodies have been outlined and mined in the surrounding district as part of the Kambalda Nickel Operations. These include: i). the Foster and Jan deposits in the St Ives belt, approximately 10 to 15 km to the south; ii). the Helmut, Schmitz, Edwin and Lanfranchy deposits in the Tramways belt, 45 km to the south, iii). the Redross, Mariners, Miitel, Widgie, Mount Edwards and Wannaway deposits on the Widgiemooltha Dome, 30 to 40 km to the SSW, iv). the Cameron and Stockwell deposits in the Bluebush Line between the Tramway belt and Widgiemooltha Dome, some 40 km to the south, and 10 to 15 km east of the Widgiemooltha Dome, v). the Blair and Carnilya Hill mines at the Golden Ridge-Carnilya Hill Belt 40 km to the NE of Kambalda. All of the distances are with respect to the Kambalda Dome.
Subsequent to the development of original deposits of the Kambalda Dome and the exhaustion of many, operations have commenced at the Miitel, Mariners, Redross, Wannaway and Carnilya Hill Mines, while Kambalda Dome deposits such as the Coronet/McCloy, Otter-Juan and the McMahon have continued production.
The Mariners, Miitel, Redross and Wannaway deposits, which produced at a rate of around 440 000 tonnes of ore per annum in 2005-06, at grades of 2.2 to 2.9% Ni, occur on the flanks of the Widgiemooltha Dome and have a very similar geology to the deposits of the Kambalda Dome. At Miitel, which is representative of the Widgiemooltha Dome deposits, faulting has repeated the lower contact zone of mineralisation which has a lateral extent of 15 km. The main Miitel deposit is a sheet-like body of sulphides lying on the basalt contact. It consists of three parallel, elongated ore zones which dip at about 80° to the east, and plunge at 35° to the south. The largest and highest grade of these, the central ore zone, has dimensions of approximately 1000 metres along plunge, 50-120 metres in dip-direction, and 1-3 metres thick.
The orebody at Miitel comprises a lower 0.2 to 1.5 m thickness of massive (essentially 100%) sulphides, the most abundant of which are pyrrhotite (~50%), pentlandite (~35%), and minor amounts of pyrite, chalcopyrite, chromite and magnetite, with a grade of 10 to 14% Ni. Lesser millerite zones are also present. The massive sulphides lie directly on basalts and are overlain in turn by up to 1.5 m of matrix sulphides, which consist of a net-textured rock composed of intermixed silicate (mainly olivine) of the host and interstitial sulphides. Grade range from 3 to 8% Ni. The matrix sulphides pass upwards into a zone of disseminated sulphides comprising a fine-grained (0.5 to 2 mm) sprinkling of sulphides scattered throughout the ultramafic host rock carrying 0.5 to 2.0% Ni. This zone usually has a gradational upper boundary with the unmineralised overlying host rock.
The Mariners deposit is longitudinally arcuate and is characterised by a high As content of up to 30 000 ppm and Ni-As sulphides such as gersdorffite and niccolite, as well as PGE group elements, particularly Pd, and has a medium Ni tenor of 8 to 12%. It has an erratic thickness variation and continuity, with a series of pods and the presence of a 25 m thicj pyrrhotitic sedimentary unit in the hangingwall.
At Helmut, in the Tramway Belt, the deposit occurs within talc-magnesite-magnetite altered olivine cumulate rocks located 3 to 7 m above the footwall basalt contact. The host flow is up to 110 m thick and 400 m wide, one of the largest channelised olivine cumulate flows of the Kambalda district. It is overlain by interflow sulphidic to carbonaceous to cherty sediments and passes laterally into strongly brecciated flow units of the flanking facies. The ore deposit is predominantly disseminated to matrix mineralisation, with a low Ni tenor, which increases from <5 to 8-12% as the total sulphide content increases. The ore profile is composed of a small core of massive sulphide which passes outwards into matrix and then disseminated sulphide mineralisation.
The Blair deposit in the Golden Ridge-Carnilya Hill Belt, is immediately underlain by up to 15 m of carbonaceous pelites, epidosites and cherts, rather than tholeiitic basalts which are stratigraphically below these sediments. The Carnilya Hill, in the same belt, has a high tenor of 10 to 16% Ni. It comprises massive mineralisation within an up to 20 m thick low Mg amphibole-chlorite altered picrite and pyroxenite unit which underlies across a sharp contact, un-mineralised, high-Mg talc-chlorite altered komatiitic olivine cumulate.
Historical mining and reserve figures for the Kambalda Dome deposits to 1997 are listed above. In 1997 proven and probable reserves totalled 10.5 Mt while resources amounted to an additional >20 Mt of comparable grades.
To 1999, reserves plus production amounted to 70 Mt @ 2.9% Ni. The 8 biggest orebodies varied in size from 0.9 to 10 Mt at grades of 2.3 to 3.9% Ni. The operation was originally owned by WMC Limited - 100% until acquired by Mincor Resources in 2001.
In June 2004 reserves + resources totalled 3.85 Mt @ 3.2% Ni.
At June 2007, total measured + indicated + inferred resources totalled: 3.72 Mt @ 3.9% Ni.
This included total proved + probable reserves of 2.243 Mt @ 2.8% Ni. (Source Mincor, 2008)
The resource figures comprised: Mariners - 0.784 Mt @ 4.0% Ni; Redross - 0.276 Mt @ 3.7% Ni; North Doordie - 0.151 Mt @ 1.5% Ni; Miitel - 1.096 Mt @ 3.6% Ni; Wannaway - 0.073 Mt @ 2.6% Ni; Carnilya Hill - 0.230 Mt @ 4.9% Ni (Mincors 70% of total resource); Otter Juan - 0.404 Mt @ 4.9% Ni; McMahon/Ken - 0.392 Mt @ 4.0% Ni; Durkin - 0.285 Mt @ 4.6% Ni; Gellatly - 0.029 Mt @ 3.4% Ni.
As at 30 June 2012, total measured + indicated + inferred resources totalled: 3.557 Mt @ 3.7% Ni.
This included total proved + probable reserves of 0.747 Mt @ 3.5% Ni. (Source Mincor, 2013)
The measured + indicated + inferred resource figures comprised: Mariners - 0.521 Mt @ 4.5% Ni; Redross - 0.244 Mt @ 3.2% Ni; Burnett - 0.219 Mt @ 3.6% Ni; Miitel - 0.771 Mt @ 3.6% Ni; Wannaway - 0.126 Mt @ 3.1% Ni; Carnilya Hill - 0.080 Mt @ 3.0% Ni (Mincors 70% of total resource); Otter Juan - 0.211 Mt @ 3.8% Ni; McMahon/Ken/Coronet - 0.340 Mt @ 3.6% Ni; Durkin - 0.366 Mt @ 5.1% Ni; Gellatly - 0.029 Mt @ 3.4% Ni; Stockwell - 0.554 Mt @ 3.0% Ni; Cameron - 0.096 Mt @ 3.3% Ni.
Forrestania - massive and disseminated sulphide nickel ................ Friday 12 September, 2008.
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Travelling from Southern Cross to Kalgoorlie to Perth to Brisbane to Mt Isa ...................... Saturday 13 and Sunday 14 September, 2008.
The Forrestania group of Ni-Cu deposits are located approximately 400 km east of Perth, 210 km south-west of Kalgoorlie and 150 km SSE of Southern Cross in the Forrestania Greenstone Belt which is some 150 to 200 km west of the main Norseman Wiluna Greenstone Belt that hosts many of the other significant Ni deposits of the Yilgarn Craton (#Location: 32° 25' 16"S, 119° 41' 28"E).
The 2.9 Ga, 300 x 40 km Forrestania Greenstone Belt is a SSE trending continuation of the Southern Cross Greenstone Belt which lies to the north. It is bounded by Archaean granitoid and gneisses, intruded by less deformed granite and pegmatite and cut buy east-west trending Proterozoic dolerite dykes.
The greenstone pile comprises a lowermost sequence of of tholeiitic basalt with up to six ultramafic members after komatiites, and numerous thin banded iron formations and chert units. This sequence is overlain by psammitic to pelitic schists that are found in the core of a regionally north-plunging syncline. Dips are moderate to steep and locally overturned, with only the western ultramafic belt dipping east. The sequence has been subjected to upper amphibolite facies metamorphism and multiple (at least three) deformation events. Thin komatiites have been recrystallised to assemblages of tremolite, chlorite, serpentine, anthophyllite, enstatite and metamorphic olivine. The thicker adcumulate to mesocumulate komatiites have preserved cores of original olivine cumulate that comprise bladed to granular metamorphic olivine, serpentine, talc, anthophyllite and enstatite. Basalt is recrystallised to amphibolite, while the banded iron formations are well preserved. The psammitic and pelitic sequence has been metamorphosed to quartz-muscovite-sillimanite schists.
There are four main areas of mineralisation along the trend of the greenstone belt, namely: Cosmic Boy, Digger Rocks, New Morning and Flying Fox - Lounge Lizard, and numerous areas of disseminated mineralisation. Other occurrences in close proximity to and along strike from the main deposits/zones include Spotted Quoll, Liquid Acrobat, New Morning, Mt Hope, Beautiful Sunday, Carr Boyd, Seagull and Rat Rat. All of the deposits occur within the Eastern and Western ultramafic belts, mostly in the lowermost of the two main ultramafic units. Cosmic Boy and Digger Rocks occur in the Eastern ultramafic belts, while Flying Fox, Spotted Quoll, New Morning and Beautiful Sunday are in the Western ultramafic belt.
Flying Fox Zone
The Flying Fox deposits are found around 30 km NW of Cosmic Boy, within the centre of the Western ultramafic belt, where it dips at around 40°E and comprises a well layered, strongly foliated succession of intercalated ultramafic flow units, sulphidic cherts, basalts and psammitic to pelitic meta-sediments, all overlying a thick sequence of psammitic schists. The lowermost ultramafic unit, which is up to 50 m thick, is the thickest and most olivine rich of the entire sequence. It is capped by pyroxenite and thin komatiite flows. The succession changes upwards from relatively thin, olivine-rich flows to multiple thin-flow sequences of less olivine-rich flows. The succession is cut by flat lying granitoid and pegmatite dykes. Faults located on the margins of the dykes show a consistent displacement of the lower block to the east.
The Flying Fox mineralisation occurs as a series of high grade massive sulphide lenses of around 0.2 Mt of ore with 5 to 9% Ni. These lenses are within a dipping zone that is around 400 m in strike length, plunging at 25 to 80°E to depths in excess of 1300 m below the surface. Individual lenses are 400 m long and extend around 200 m down dip and are 0.1 to 10 m in thickness. The uppermost lens contained around 0.2 Mt @ 3.1% Ni. The next down-dip is T1, with 0.342 Mt @ 5.1% Ni, followed by T4 with around 0.315 Mt @ 4% Ni, then T5 with approximately 0.55 Mt @ ~1% Ni. Disseminated sulphides with up to 1.5% Ni are found in the basal ultramafic host unit above the massive sulphide and as a low grade halo to the north, south and up-dip.
The upper-most massive sulphides are within the oxidation zone and have been replaced by a supergene assemblage of pyrite-violarite and large pyrite porphyroblasts. The lower massive sulphide lenses are below the supergene zone. The primary sulphide assemblage comprises pyrrhotite-pentlandite±pyrite and chalcopyrite.
Digger Rocks Zone
The Digger Rocks and Diggers South nickel sulphide occurrence are continuous and may be regarded as the same zone. Both are located in the basal portion of a large, complexly zoned ultramafic which is around 5 km long and up to 425 m thick, which comprises a central, 100 to 150 m thick section with some preserved cumulate textures that grades out into bladed-granular forsterite-talc±anthophyllite±enstatite rock. Below the cumulate zone, there is a zonation from forsterite-talc to forsterite-talc-anthophyllite and forsterite-talc-enstatite near the mineralisation. Another 10 to 75 m thick, separate ultramafic known as the 'footwall ultrabasic unit' occurs below the mineralisation, composed of alternating forsterite-tremolite-anthophyllite-enstatite rocks.
Nickel sulphide mineralisation appears to be structurally controlled, with footwall rocks varying from banded iron formation in the lower northern part of Digger rocks, to the footwall ultrabasic unit below the main Digger Rocks, while an 8 to 10 m thick tectonic sliver of quartz-biotite-garnet schist is found in the southern part of Digger Rocks and Diggers South. Hanging wall rocks comprise tholeiitic amphibolites.
Mineralisation occurs over strike length of 1200 m and up to a width of 125 m, comprising both massive and disseminated matrix sulphides, which is best developed towards the northern end of the zone, the Main Digger Rocks zone. The Main Digger Rocks zone is around 240 m in strike length and up to 125 m thick, dipping at 50 to 65°W and pitching at 50°N. A typical cross section of this mineralisation comprises a lower massive nickel sulphide accumulation up to 10 m thick containing 6 to 7% Ni, overlain by 10 to 40 m of heavily disseminated to matrix mineralisation with 0.8 to 2.5% Ni, which is in turn overlain by 30 to 80 m of disseminated mineralisation containing 0.4 to 1.0% Ni. The southern two thirds of the overall mineralised interval, Diggers South, comprises disseminated pyrrhotite-pentlandite-pyrite-chalcopyrite that forms a homogeneous mass ranging from 9 to 25 m in thickness, dipping steeply west. This latter interval is similar to Cosmic Boy, but is richer in pyrrhotite.
The bulk of the higher grade portions of the Digger Rocks lens lies between 30 and 80 m below the surface, directly below a siliceous weathering cap and is strongly influenced by supergene alteration characterised by an Fe-rich assemblage of pyrite-violarite-pentlandite-pyrrhotite-magnetite with a variable Ni:Cu ratio. Most of the massive sulphide mineralisation occurs at the contact with the 'footwall ultrabasic unit', although an appreciable portion occurs as cross-cutting stringers of pyrrhotite-pentlandite-pyrite that cut matrix mineralisation and extend for up to 20 m into the footwall banded iron formation. The disseminated sulphides in the Main Digger Rocks zone consists of pyrrhotite-pentlandite-pyrite with variable supergene alteration to pyrite-violarite. An additional 200 m long by up to 20 m thick Ni-sulphide zone is found in the hanging wall, within the central cumulate ultramafic, comprising disseminated pentlandite-millerite with up to 1% Ni, which is intergranular to the cumulus olivine grains.
Cosmic Boy Zone
This deposit occurs within the Eastern ultramafic belt in an interval where the generally west-facing ultramafic belt is characterised by relatively thin (<60 m thick), laterally extensive ultramafic units with intercalated banded iron formation and tholeiitic basalts which dip at ~50°W. The orebody is found on the basal, eastern contact of the lowermost ultramafic unit. The lowermost ultramafic unit is composed of several, up to 900 m long, discrete lenses of olivine mesocumulate which are linked by differentiated flow units that are up to 50 m thick. Other smaller, relatively minor olivine mesocumulate lenses occur in the lowermost ultramafic unit and within most other ultramafic units. The ultramafic sequence grades upwards to the west to more evolved, less olivine rich, thin spinifex-textured komatiite flows.
The deposit occurs as two parallel ore zones. The basal orebody is ~800 m in strike length and extends down dip for more than 500 m at the base of a 40 to 60 m thick olivine mesocumulate, overlying a prominent BIF unit. The up- and down-dip peripheries are the result of thinning of the sulphide zone, while the lateral margins to the north and south are faulted. The sharp footwall is defined by a fault with the underlying BIF, while the upper margin is marked by a reduction in Ni grade to the background 0.3% Ni of the 'barren' ultramafic.
The primary sulphide assemblage comprises a strong dissemination of pyrrhotite-pentlandite±pyrite and chalcopyrite to between 20 and 40% sulphides by volume, and Ni grades of 0.7 to 5.0%. Violarite is a significant component in the supergene zone. Sulphides form either triangular domains within bladed metamorphic olivine-talc grains and as partially connected networks around silicate minerals. No massive sulphides are found in the basal orebody.
The upper orebody occurs as a complex series of thin, discontinuous lenses of disseminated sulphides hosted by a 10 to 20 m thick olivine mesocumulate unit, close to the 'upper' contact of the second ultramafic unit, which may be an isoclinal repetition of the lower ultramafic unit.
The total reserves/resources at Forrestania in 1981 were 10.8 Mt @ 2.46% Ni, or 14 Mt @ 1.3% Ni.
Reserve and resource figures in mid 2008, as quoted by Western Areas NL, 2008, were:
Flying Fox + Diggers - 3.307 Mt @ 2.8% Ni
Flying Fox Zone:
Flying Fox - 2.307 Mt @ 4.7% Ni,
New Morning/Daybreak - 2.144 Mt @ 1.4% Ni,
Spotted Quoll - 0.545 Mt @ 6.3% Ni,
Beautiful Sunday - 0.480 Mt 1.4% Ni
Total Flying Fox Zone - 5.477 Mt @ 3.3% Ni
Cosmic Boy Zone - 0.376 Mt @ 2.4% Ni
Diggers South Core - 3.000 Mt @ 1.48% Ni,
Diggers South Halo - 4.8 Mt @ 0.74% Ni,
Digger Rocks Core (indicated) - 0.055 Mt @ 1.1% Ni,
Digger Rocks Core (inferred) - 0.172 Mt @ 1.1% Ni,
Digger Rocks Halo - 1.441 Mt @ 0.7% Ni,
Total Diggers Zone - 10.028 Mt @ 1.0% Ni.
Total resource - 15.880 Mt @ 1.8% Ni.
Reserve and resource figures in at 31 March, 2013, as quoted by Western Areas NL, website in 2013, were:
Flying Fox Area - 1.7288 Mt @ 4.0% Ni - Probable
Spotted Quoll - 2.9420 Mt @ 4.2% Ni - Probable
Digger South - 2.0160 Mt @ 1.4% Ni - Probable
Digger Rocks - 0.0930 Mt @ 2.0% Ni - Probable
TOTAL reserves - 6.7798 Mt @ 3.3% Ni - Probable
Flying Fox and Lounge Lizard:
High grade massive ore - 1.7321 Mt @ 5.7% Ni - indicated + inferred
Disseminated ore - 4.9830 Mt @ 0.8% Ni - indicated + inferred
TOTAL Flying Fox and Lounge Lizard - 6.7151 Mt @ 2.1% Ni - indicated + inferred
Massive ore - 0.3218 Mt @ 3.7% Ni - indicated
Massive ore - 0.0931 Mt @ 3.5% Ni - inferred
Disseminated ore - 1.0698 Mt @ 0.9% Ni - indicated
Disseminated ore - 0.6592 Mt @ 0.9% Ni - inferred
TOTAL New Morning/Daybreak - 2.1439 Mt @ 1.4% Ni - indicated + inferred
Massive ore - 2.3795 Mt @ 6.0% Ni - indicated
Massive ore - 0.5397 Mt @ 5.1% Ni - inferred
TOTAL Spotted Quoll - 2.9192 Mt @ 5.1% Ni - indicated + inferred
TOTAL Beautiful Sunday - 0.4800 Mt @ 1.4% Ni - indicated
TOTAL Western Belt - 12.2582 Mt @ 2.8% Ni - indicated
Cosmic Boy Area
Cosmic Boy - 0.1809 Mt @ 2.8% Ni - indicated
Seagull - 0.1950 Mt @ 2.0% Ni - indicated
TOTAL Cosmic Boy Area - 0.3759 Mt @ 2.4% Ni - indicated
Diggers South Core - 3.0000 Mt @ 1.48% Ni - indicated
Diggers South Halo - 4.8000 Mt @ 0.74% Ni - indicated
Digger Rocks Core - 0.0549 Mt @ 1.1% Ni - indicated
Digger Rocks Core - 0.1723 Mt @ 1.1% Ni - inferred
Digger Rocks Halo - 1.4410 Mt @ 0.7% Ni - inferred
Purple Haze - 0.560 Mt @ 0.9% Ni - inferred
TOTAL Diggers Area - 10.0282 Mt @ 1.0% Ni - indicated + inferred
TOTAL Western Areas resource - 22.6623 Mt @ 2.0% Ni.
Ernest Henry - IOCG style copper-gold .................. Monday 15 September, 2008.
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The Ernest Henry IOCG style Cu-Au deposit is located 35 km NE of Cloncurry, 150 km east of Mt Isa and 750 km west of Townsville in north-west Queensland (#Location: 20° 26' 40"S, 140° 42' 21"E).
The deposit lies to the east of the Cloncurry Overthrust, within the Cloncurry-Selwyn zone of the Cloncurry Terrane, which comprises the eastern exposed margin of the Mount Isa Inlier of North-west Queensland. It contains IOCG deposits that are hosted by Palaeoproterozoic (1760-1660 Ma) silici-clastic metasedimentary and metavolcanic rocks that were deposited during periods of ensialic rifting.
The Ernest Henry deposit is hosted within the Eastern Succession of the Mount Isa Inlier, that consists of a poly-deformed Palaeo- and Mesoproterozoic volcano-sedimentary succession which is largely composed of evaporite-rich Cover Sequence 2 and silici-clastic-rich Cover Sequence 3 rocks (CS2 and 3). CS2 and 3 were deposited between 1790 and 1690 Ma and from 1680 to 1610 Ma respectively. To the west, these sequences overlie an older crystalline basement and a core of predominantly Cover Sequence 1 felsic volcanic and related intrusive rocks that correspond to the 1870 to 1850 Ma Barramundi Orogeny of northern Australia. Basement is not exposed in the Cloncurry district. Both CS2 and 3 were deposited in intracontinental rift settings, although the relationship between some parts of the sequence is obscured by the deformation history. Both sequences were also accompanied by the emplacement of various intrusive and volcanic rocks.
The first significant deformation to affect CS2 (but not CS3) was the 1750 to 1735 Ma Wonga extensional event. CS2 was extensively intruded by the 1750 to 1730 Ma Wonga Granite to the west, while the coeval Mount Fort Constantine volcanics are found to the NE. Minor tonalites, granitoids and diorite emplaced between CS2 and 3 have been dated at 1686 to 1660 Ma (including the Ernest Henry Diorite).
Thin skinned deformation of the ~1600 to 1520 Ma Isan Orogeny terminated deposition of Cover Sequence 3, and resulted in gross eastward tectonic transport, interleaving of major lithostratigraphic units, and a dominant north-south tectonic grain. This deformation has been divided into: a D1 event, which involved overall north-south compression, and is characterised by large-scale thrusts and isoclinal folds, thrust reactivation of large, km-scale, basin bounding extensional faults with CS3 rocks thrust over CS2, resulting in overturned limbs and a penetrative rock mass foliation; a D2 event, involving horizontal east-west compression producing major north-south upright to isoclinal folding of CS2 and 3 rocks, and a penetrative cleavage, which peaked at 1595 to 1580 Ma with a regional greenschist to upper amphibolite facies metamorphism and the development of anatectic pegmatites; and a D3 event which includes NW- and NE-trending brittle-ductile corridors of faulting, kinking and folding with steep plunges to the NW and NE, and dominantly north-south trending shear and fault zones and associated breccia formation.
Both CS2 and 3 were intruded by the voluminous Williams and Naraku granite batholiths at 1540 to 1500 Ma (including the 1530 Ma Mt Margaret Granite immediately to the east of the E1 deposits; Marshall and Oliver, 2007; Page and Sun, 1998). These represent the youngest felsic intrusions in the inlier, and have an outcrop exposure of >1500 km2. They were emplaced in an intracratonic environment, and have a pre-, syn- and post-D3 timing, and are largely composed of alkaline to sub-alkaline, K-rich, A-type, magnetite-bearing granitoids. They range from diorite to syenogranite in composition and are typically more oxidised than similar older (~1670 Ma) granitoids in the Western Fold Belt
of the Mount Isa Inlier. Sodic intrusions of similar age are rare.
A regionally extensive Na-Ca hydrothermal system in the Cloncurry district (>1000 km2) affected all rock types, especially the resultant calc-silicate-rich lithologies of cover sequence 2. This alteration appears to have been formed by multiple periods of hydrothermal activity that locally overlapped and is most intense in breccia zones along large structural conduits and within calc-silicate-rich units. The bulk of the sodic-calcic alteration, dominantly regional albite and scapolite, was associated with fluids that were initially mostly sedimentary formation waters with lesser magmatic components, prior to and during peak metamorphism (Kendrick et al., 2008; Oliver et al., 2008; Baker et al., 2008). Subsequent more structurally controlled albite-actinolite-magnetite-titanite±clinopyroxene assemblages, were synchronous with major granite (e.g., Williams-Naraku batholiths) emplacement (Baker et al., 2008), involving a larger magmatic fluid component, and coincided with formation of the majority, but not all of the significant oxide Cu-Au deposits. These deposits may have some stratigraphic control, but are usually associated with brittle and brittle-ductile shear and fault structures which acted as conduits for the transport of high temperature (300 to 500°C) saline fluids into the host rocks (Williams, 1998).
The Ernest Henry deposit is concealed by 35 to 60 m of extensive Phanerozoic cover and does not outcrop. While the exact stratigraphic position of the host rocks is not known, they have been tentatively correlated with the 1730 ±10 Ma Mount Fort Constantine Meta-volcanics towards the top of Cover Sequence 2. The Mount Fort Constantine metavolcanics comprise dacite and andesite with subordinate metabasalts and calc-silicate metasedimentary rocks. The only other outcrop in the district is the 1480 Ma Mount Margaret granite some 12 km to the east. Within Cover Sequence 2, volcanism is common between 1790 and 1780 Ma, and 1760 to 1720 Ma, with later 1540 to 1450 Ma granitoids.
Within the immediate orebody area the principle lithologies encountered are: i). altered plagioclase phyric andesitic volcanic/hypabyssal rocks (ca 1740 Ma) which host the orebody where they are brecciated; ii). various siliciclastic, calc-silicate-rich and graphitic metasedimentary rocks that occur as <10 m thick intercalations within the metavolcanic rocks; and, iii). medium-grained metadiorite (ca 1660 Ma).
Structural analysis suggests that ore deposition accompanied reverse-fault movement between two northeast trending bounding shear zones and formed a pipe-like zone of dilation in the K-feldspathised metavolcanic rocks. The breccia pipe, plunges at ~45° to the SSESSE, nested between the ductile shear zones (Rusk et al., 2010). The orientation of this dilational zone is consistent with the shape and dip of the Ernest Henry ore breccia.
Four stages of alteration are recognised at Ernest Henry:
i). Regional pre-ore Na-Ca alteration, occurring mainly as albitic plagioclase-, magnetite-, clinopyroxene- and amphibole-rich veining and fault-related breccia-fill.
ii). Pre-mineralisation potassic-(manganese-barium) alteration which only contains minor sulphides, and is typified by multiple stages of K feldspar-, biotite-, amphibole-, magnetite-, garnet- and carbonate-bearing veins, and by fault-related breccia and alteration.
iii). Mineralisation associated alteration, characterised by K feldspar veining and alteration. K feldspar alteration is most intense in the vicinity of copper-gold mineralisation, but forms a halo extending from several hundred meters up to 2 km beyond the ore body (Mark et al., 2006), although this outer halo may represent part of pre-ore regional alteration zone. Mineralisation is divided into two main stages, characterised by similar mineral assemblages. The first stage of economic Cu-Au mineralisation was the main ore-forming event, associated with a matrix-supported hydrothermal breccia that is enveloped by crackle veined K feldspar altered meta-volcanic rocks. The second stage of mineralisation occurs as a network of veins cutting earlier infill-supported ore-breccias, and contains a largely identical mineralogy to earlier stage. The ore-bearing assemblage dominantly comprises magnetite, pyrite, chalcopyrite, carbonate and quartz, with lesser apatite, barite, titanite, actinolite, biotite and fluorite. In the upper levels of the deposit, the bulk of the ore is present as hypogene chalcopyrite infilling between K feldspar-altered breccia clasts, while at greater depths, it both infills between, and replaces clasts. Electrum and native gold are closely associated with pyrite and chalcopyrite (Foster et al., 2007).
iv). Post-ore, volumetrically minor, multiple stage calcite-dolomite- and/or quartz-rich veining and alteration which lacks magnetite, and only carries a little gold. Deeper in the deposit, breccias include rounded clasts of previously mineralised breccias containing magnetite, pyrite and chalcopyrite, indicating multiple superimposed brecciation events (Rusk et al., 2010).
Rusk et al. (2010) interpret the data from Ernest Henry to be consistent with the following genetic trend:
i). Rapid devolatilisation (of possibly both chloride-rich brines and CO2-rich fluids) within the source magma chamber;
ii). Fluid over-pressuring in the roof of the magma chamber as a result of volatile exsolution and vapour expansion, assisted by a seal created by magma solidification, sodic-calcic alteration and/or contact metamorphism in the carapace of the igneous complex;
iii). Possible leakage of over-pressured magmatic fluid along structures controlling the location of the later breccia pipe, producing a pre-ore potassic alteration halo;
iv). The eventual failure of the seal and sudden release of fluid pressure, resulting in a high-energy fluid flow event driving brecciation and upward transported and milled clasts. The resultant breccia mass permitted the mixing and/or subsequent ingress of basinal brines circulating within fractured rocks several kilometres above the magma chamber. Fluid mixing, rapid depressurisation and resultant cooling led to ore precipitation within the matrix porosity between breccia clasts at the top of the orebody, where, as the fluid flow, temperature and pressure declined the breccia was sealed;
v). At depth, closer to the heat source, the temperature and pressure gradient degraded more slowly, allowing for fluid-rock reaction to be more protracted, such that prolonged chemical interaction between K feldspar-rich host rocks and ore fluids led to replacement style mineralisation within clasts, with the same mineral assemblage as observed in the shallower parts of the deposit.
vi). At the deepest levels, repetition of the cycle may have resulted in the release of a new pulse of fluids which brecciated and tapped earlier formed magnetite-chalcopyrite rich rocks, telescoping mineralised clasts upwards into the orebody along narrow channels, thereby upgrading ore.
The brecciated volcanic mass that hosts the ore forms a plunging elongate body, some 250 m thick, 300 m average length and extending at least 1000 m down plunge to the SSE. The breccia ranges from the unbrecciated volcanics, to crackle fracture veining to clast supported and matrix supported breccia to total clast digestion (massive matrix). The breccias typically contain 5-20 mm subrounded to rounded meta-volcanic and rare biotite altered meta-sedimentary clasts. The matrix is largely composed of magnetite, calcite, pyrite, biotite, chalcopyrite, K feldspar titanite and quartz. Accessory minerals include garnet, barite, molybdenite, fluorite, amphibole, apatite, monazite, arsenopyrite, a LREE fluorcarbonate, galena, cobaltite, sphalerite, scheelite, uraninite and tourmaline. The bulk of the economic mineralisation is restricted to breccia zones with more than 10% matrix.
The total reserve + resource prior to the commencement of mining in 1998 was 166 Mt @ 1.1% Cu, 0.54 g/t Au.
As of June 2003 the remaining resource totalled 117.9 Mt @ 1.13% Cu, 0.52 g/t Au.
At 30 June 2006, the reserves and resources were (Xstrata, 2007):
Open cut proved reserves - 41 Mt @ 0.9% Cu, 0.5 g/t Au + probable reserves of 20 Mt @ 0.8% Cu, 0.4 g/t Au,
Open cut measured + indicated resources were the same as, and included the proved and probable reserves,
Open cut inferred resources - 1 Mt @ 0.4% Cu, 0.2 g/t Au,
Underground indicated resources - 21 Mt @ 1.5% Cu, 0.7 g/t Au + inferred resources of 23 Mt @ 1.4% Cu, 0.7 g/t Au,
Open pit as at December, 2011 (Xstrata, 2012):
Total resource and reserve - depleted during 2011 from 17 Mt @ 1.0% Cu, 0.5 g/t Au, 23% magnetite at December 31, 2010
Underground as at December, 2011 (Xstrata, 2012):
Measured resource - 4 Mt @ 1.3% Cu, 0.7 g/t Au, 32% magnetite
Indicated resource - 71 Mt @ 1.3% Cu, 0.7 g/t Au, 28% magnetite
Inferred resource - 13 Mt @ 1.2% Cu, 0.6 g/t Au, 26% magnetite
Total resource - 88 Mt @ 1.3% Cu, 0.7 g/t Au, 28% magnetite
Total ore reserve (all probable) - 74 Mt @ 0.95% Cu, 0.5 g/t Au, 23% magnetite.
The operation is controlled by Ernest Henry Mining Pty Ltd, a subsidiary of Glencore plc.
George Fisher - sediment hosted zinc-lead-silver ...................... Half Group on each of Tuesday 16 & Wednesday 17 September, 2008.
The George Fisher South (previously Hilton or P49) and George Fisher North (L72) are located approximately 20 km north of the city of Mt Isa, in north-western Queensland, Australia and 975 km west of Townsville (#Location: George Fisher North - 20° 32' 56"S, 139° 28' 6"E; George Fisher South - 20° 34' 4"S, 139° 28' 35"E).
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Whilst lead-zinc mineralisation had been discovered at what is now Mount Isa in March 1923 by prospector John Campbell Miles, and mining commenced in 1931, the Hilton deposit was not discovered until 1947, when S R Carter, a Mount Isa Mines (MIM) geologist, recognised outcropping cerussite at surface. Diamond drilling to test this discovery was commenced in August 1948, with the first drill hole intersecting a narrow interval of zinc mineralisation. From then until 1957, a significant follow-up drilling program was undertaken, and by 1950, the Hilton ore reserves stood at 26 Mt. The drilling program was curtailed in 1957 due to a fall in metal prices and heavy capital expenditure at the main Mount Isa operation. In 1966, MIM consolidated its mining lease holdings by taking up all the ground between Hilton and the Mount Isa operations within a single Special Mining Lease and diamond drilling recommenced at Hilton. The Hilton reserve was subsequently increased to 37 Mt. MIM decided to develop Hilton in 1969 but market factors delayed start-up of the 1 Mt/y mine and concentrator until 1990. In 1981, similar mineralisation was located 2 km further north at Hilton North, later to be renamed George Fisher North. Mining commenced at George Fisher in 2000.
Continental to Regional Setting
The George Fisher and Mount Isa orebodies lie within the Leichhardt River Fault Trough of the Western Fold Belt, part of the Mount Isa Inlier (or Domain) of Northwest Queensland. The Mount Isa Inlier comprises three major elements from west to east: the Western Fold Belt, the Kalkadoon-Leichhardt Belt and the Eastern Fold Belt, which are predominantly north-south trending sedimentological and structural domains (Blake and Stewart, 1992; O'Dea et al., 1997). The Western Fold belt is bounded to the east by 1870 to 1850 Ma felsic volcanic and coeval granitoid rocks of the Kalkadoon-Leichhardt belt, interpreted to represent a remnant of a magmatic arc related to the Palaeoproterozoic Barramundi Orogeny. To the NW, the Western Fold belt is separated from the McArthur Basin in the Northern Territory by the 1853±4 Ma Murphy Metamorphics basement exposed as the narrow ENE-WSW trending Murphy Inlier.
The Western Fold belt is further divided into the narrow Leichhardt River Fault Trough immediately to the west of the Kalkadoon-Leichhardt Belt, and the broader Lawn Hill Platform further to the west, each separated from its neighbour by a major north-south trending terrane boundary fault zone (Blenkinsop et al., 2008; Foster and Austin, 2008).
The 1300 x 60 to 200 km Mount Isa Inlier-McArthur Basin succession comprise Late Palaeo- to Mesoproterozoic sequences that are, in turn, part of the more extensive 1800 to 1580 Ma Northern Australian Platform cover, a 5 to 15 km thick volcano-sedimentary succession draped across the northern third of the continent. Deposition within the Mt Isa-McArthur basin system took place in three super-basins which represent three nested cycles of deposition and exhumation, specifically the Leichhardt (1798 to 1738 Ma), Calvert (1728 to 1680 Ma) and Isa (1667 to 1575 Ma) super-basins, terminated by the 1590 to 1500 Ma Isan Orogeny, which was followed by the younger Roper super-basin.
All of the major stratabound Zn-Pb-Ag deposits of northern Australia, including the George Fisher and Mount Isa deposits, are hosted by the Isa Super-basin in the McArthur Basin (e.g., McArthur River), Lawn Hill Platform (e.g., Century and Lady Loretta), Leichhardt River Fault Trough (e.g., George Fisher, Mount Isa Zinc) and Eastern Fold Belt (e.g., Dugald River).
A ~25 to 30 m.y. hiatus marked the end of deposition and inversion of the Calvert superbasin within the Mount Isa inlier. During this period the Calvert Super-basin sedimentary rocks were uplifted and incised. Towards the end of this hiatus, the linear, north-south trending, voluminous, lopolithic 1671±8 Ma Sybella Granite, a foliated coarse porphyritic biotite granite, was intruded along the boundary between the Leichhardt River Trough and the Lawn Hill platform immediately to the west of Mount Isa. The exposed and sub-outcropping batholith occupies an area of ~220 x 15 to 30 km and defines the Sybella Domain of Withnall and Cranfield (2013). The northern extremity of this batholith passes into the NNE trending Mount Gordon Fault Zone which separates the Leichhardt River Trough from the Lawn Hill platform in the northern part of the Mt Isa Domain.
The depositional hiatus during Calvert super-basin inversion ended with renewed extension, development of the Isa Superbasin and recommencement of sedimentation from 1670 to 1590 Ma (Jackson et al., 2000). This superbasin is interpreted to have resulted from thermal subsidence of the lithosphere, the sag phase of Etheridge and Wall, 1994, and of Betts and Lister, 2001. This subsidence was most intense to the west and NW of the Leichhardt River Domain, where an extensive, thick blanket of carbonaceous shale, stromatolitic dolostone, and turbiditic sandstone and siltstone were deposited on the Lawn Hill platform the ~10 km thick McNamara Group (Krassay et al., 2000; Southgate et al., 2000). A similar sequence was deposited in the McArthur basin to the north, the 5.5 km thick McArthur Group, which hosts the McArthur River deposit in the Northern Territory (Rawlings, 1999). Within the narrower (40 to 90 km wide) Leichhardt River Trough, the Isa Superbasin sequence is thinner and and spans a narrower temporal range, represented by the up to 7.5 km thick, ~1670 to ~1647 Ma Mt Isa Group, which hosts the Mount Isa, George Fisher South (previously Hilton) and George Fisher North deposits close to its western margin, immediately adjacent to the regional-scale Mount Isa Fault. The Lady Loretta deposits are located within a similar sequence in the lower sections of the overlying McNamara Group on the eastern margin of the Lawn Hill platform. Century is located further to the north, also within the Lawn Hill Platform, adjacent to the major Termite Range Fault, which may merge with the Mount Isa Fault to the SE.
The regional structure of the Western Fold Belt of the Mount Isa Domain is dominated by faults and shear zones striking north-south (e.g. Mt Isa Fault), NW to NNW (e.g., Riversleigh and Termite Range faults) and NNE (e.g., Mount Gordon Fault), with subsidiary east-west, NW–SE or NE–SW structures.
Subsequent to the 1870 to 1850 Barramundi Orogeny, the rocks of the Mount Isa Domain were subjected to at least three phases of extensional faulting and rifting, and at least as many episodes of intervening inter- and post-rift compressional deformation and basin inversion spanning the three superbasin cycles. These events culminated in the most intense episode, the 1590 to 1500 Ma Isan Orogeny that terminated deposition of the Mount Isa Group (e.g. Blake, 1987; O'Dea et al., 1997; Betts et al., 2006; Gibson et al., 2012). Thrusting during inversion of the superbasins is interpreted, in many cases, to have reactivated listric faults active during the preceding extensional events.
The intensity of deformation and metamorphism varies markedly across the Mount Isa Inlier/Domain, although, in contrast to the more strongly deformed Eastern Fold Belt, it rarely exceeds greenschist facies in the rocks of the Lawn Hill Platform and Leichhardt River Fault Trough (Gibson et al., 2016). Metamorphism was largely low-pressure sub-greenschist facies during the Isan Orogeny (Jacques et al., 1982; Rubenach 1992; Oliver 1995; Rubenach and Barker 1998), although higher metamorphic grades to upper amphibolite facies are exposed in the hanging wall of the Mt Isa Fault, adjacent to the Sybella Batholith (Betts et al., 2006).
During deposition in the Leichhardt Superbasin, the Eastern Creek Volcanic pile and the thicker syn-rift sedimentary rocks, accumulated in an elongate, fault-bounded basin which, following deformation, is preserved as a 50 to 80 km wide belt (Bain et al., 1992; Blake, 1987; Derrick, 1982; Eriksson et al., 1983; Gibson et al., 2012; Jackson et al., 2000; Scott et al., 2000), largely restricted to the area of the Leichhardt River Fault Trough. This basin was bounded by faults typically striking NNW and dipping steeply to both the east and west, with displacements varying from a few hundred metres to several kilometres. Variations in this pattern indicate this is not a single basin but a series of highly asymmetric, overlapping, half-grabens, each oriented north-south and up to 70 to 130 km long, controlled by pre-existing fabric in the underlying basement (Gibson et al., 2016). Rifting ceased by no later than ~1740 Ma, with intrusion of post-kinematic granites and gabbro, whilst the regional north-south trending Leichhardt Anticline developed between 1740 and 1710 Ma within the Leichhardt River Fault Trough during east-west shortening inversion. Compression was followed by an episode of thermally induced subsidence (Gibson et al., 2016).
The western margin of the northern half of the Leichhardt River Fault Trough, separating it from the Lawn Hill Platform, is defined by the major, up to 7 km wide Mount Gordon Fault Zone, along which, the most recent activity has been dextral displacement of up to 10 km cutting all other fault sets. Towards its southern extremity, the Mount Gordon Fault curves north-south and then SSE to merge with and become part of the north-south Mt Isa fault corridor. Further south, the latter swings to the SSW again to become the Rufus and Mount Annabele fault zones.
The intensity of deformation within the Leichhardt River Fault Trough increases and is more complex in the vicinity of the Mt Isa Fault Zone where at least five deformation events have been interpreted based on overprinting relationships (Betts et al., 2006), including faulting and folding and overprinting of different strands within the zone (Connors and Lister 1995), three of which are indicated by post crystallisation dating of metamorphic minerals from the Sybella Granite at 1610±13, 1544±12 and 1510±13 Ma (Bell 1986). These deformational events are detailed in the 'Structure' and 'District Scale Structure and Metamorphism' sections below.
The Lawn Hill Platform differs from the Leichhardt River Fault Trough in that much of the Calvert and Isa superbasin successions are preserved in contrast to the latter, which is more deeply eroded, with only remnants of these sequences remaining. In the northern Lawn Hill Platform, immediately west of the Century deposit, a series of major NW-SE to NNW-SSE trending, east dipping faults occur over a width of ±20 km, including, from SW to NE, the Riversleigh, Little Range and Termite Range fault zones. These structures appear to have controlled deposition as listric faults bounding asymmetric, SW thickening half grabens. They also accommodated subsequent basin inversion as SW vergent thrusts, mainly during the Isan Orogeny. Depositional thickening and thrust displacement is most intense across the Riversleigh Fault zone, influencing the Leichhardt, Calvert and Isa Superbasin sequences. These structures approach, but do not appear to intersect the Mount Gordon-Mount Isa fault zones. Other NNW-SSE structures on the Lawn Hill Platform include the possibly sinistral May Downs Fault further to the south, which curves south to follow the western margin of the Sybella Granite just to the west of Mount Isa. To the east of these structures, over the platform, there appears to be little variation in thickness of the Calvert Superbasin sequence across other faults, suggesting little structural activity within the interior of the superbasin during deposition (Gibson et al., 2016), although ENE-WSW, mainly Calvert-age structures have been variously attributed to NW-SE crustal extension on the Lawn Hill Platform (Betts et al., 1998, Southgate et al, 2006).
At least two composite, regional Isan Orogeny basin inversion events are recorded in the Western Fold Belt, including:
i). ~north-south shortening and inversion of pre-existing east-west rift related structures between 1595 and 1570 Ma to produce east-west oriented axial-planar foliations in the immediate hangingwall of inverted extensional faults in the Leichhardt River Fault Trough and ENE-WSW structures in the Lawn Hill Platform (O'Dea and Lister 1995; O'Dea et al., 1997; Lister et al. 1999);
ii). a major phase of ~east-west shortening that developed crustal-scale north-south trending, shallowly plunging folds, and north-south trending reverse faults and thrusts across the Leichhardt River Fault Trough, and NE-trending folds throughout the Lawn Hill Platform between 1570 and 1550 Ma (Betts and Lister, 2002; Betts et al., 2004; Blaikie et al., 2017). Between ~1550 and 1540 Ma, the structural regime changed to strike-slip and oblique faulting and regional wrenching (O'Dea et al., 1997; Lister et al., 1999). This late east-west shortening was accommodated along NW trending sinistral, and NE trending dextral strike-slip faults. During inversion, zones of anomalous strain and buttressing occurred near normal faults and granitic plutons (Betts et al., 2006).
The Mount Isa terrane is considered to have been cratonised after the Isan Orogeny and the architecture of the region has not changed since the Late Mesoproterozoic.
It has been suggested that the Mount Isa Fault Zone is a Palaeoproterozoic Barramundi Orogen terrane-bounding suture (e.g., Hobbs et al., 2000). However, geophysical and geochemical data studied by Bierlein and Betts (2004) showed that basement rocks on either side of the Mount Isa Fault have similar densities, which is consistent with geochemical observations and Sm-Nd data that suggest basement lithologies on either side of the structure are geochemically and isotopically indistinguishable from each other, and that the Mount Isa Fault is unlikely to represent a suture zone that separates different Palaeoproterozoic terranes. Their data also indicates the crustal blocks on both sides must have been in close proximity of each other since the Palaeoproterozoic, and that the Western Fold Belt was part of the ancestral North Australian Craton well before the Barramundi Orogeny.
Regional to Deposit Scale Geology
The host sequence in the Mount Isa-George Fisher area is the Mount Isa Group, which unconformably overlies the Surprise Creek Formation of the Calvert Superbasin, and older rocks of the Leichhardt Superbasin, as follows:
• May Downs Gneiss - which is found ~5 km to the west of Mount Isa, between the overlying Mount Guide Quartzite and an intrusive contact with the 1671±8 Ma Sybella Granite to the west. It comprises plagioclase (microcline)-quartz gneiss and schist containing minor biotite, sillimanite and muscovite. It is reported to be younger than 1789±4 Ma and may be a more intensely metamorphosed equivalent of the Bottletree Formation (Australian Stratigraphic Units Database, viewed April, 2019).
• Bottletree Formation - up to 3000 m of porphyritic, rhyolitic to dacitic lava flows and ash-flow deposits, with interlayered greywacke and greywacke conglomerate, meta-arenite, quartzite, epidotic quartzite and grit. Sheared, schistose, amygdaloidal to massive metabasalt is common at or near the base and top of the formation. It unconformably overlies granites and volcanics rocks of the Kalkadoon-Leichhardt Belt, and is conformably followed by the Mount Guide Quartzite, the basal unit of the Haslingden Group;
• Mount Guide (and Leander) Quartzite - of the Guide Supersequence which comprises a >4000 m package of alluvial sheet, braided river and marginal lacustrine and marine deposits composed of a lower sequence of mainly greywacke type metasediments, including conglomerate and micaceous quartzite, and an upper suite of mostly ridge-forming quartz-rich sandstone metamorphosed to quartzite (Blake, 1987).
Deposition of the Bottletree Formation and Guide Supersequence was in an east-west extensional regime. The overlying sequences were deposited in a north-south directed extensional environment (Blaikie et al., 2017).
• Eastern Creek Volcanics - a 1790 to 1740 Ma, 6 to 15 km thick tholeiitic continental volcanic suite, principally composed of amygdaloidal to massive metabasalt, with interbedded clastic sedimentary rocks, deposited in an extensional setting. It has been subdivided into the Cromwell Metabasalt, Lena Quartzite and overlying Pickwick Metabasalt. These volcanic rocks were extruded under subaerial or shallow water conditions and are part of a larger bimodal igneous province represented in the Eastern Fold Belt by mafic and felsic volcanic rocks, including 1780 Ma rhyolites and ignimbrites of the Argylla Volcanics and the 1760 Ma Bulonga Volcanics (Neumann et al., 2009; Withnall and Hutton, 2013; Gibson et al., 2016);
• Myally Subgroup - formerly known as the Judenan Beds. This sequence constitutes the Myally Supersequence and is an up to 4000 m thick succession, from base to top, principally of quartzite, feldspathic quartzite to siltstone with associated pebbly to conglomeratic sandstone, mudstone, arkose, shale, quartzite, schist, dolomitic sandstone, oolitic and stromatolitic dolostone. Some felsic tuff occurs at the top, and metabasalt near the base of the sequence (Blake, 1987). This unit is the uppermost member of the Haslingden Group;
• Quilalar Formation - which is largely only exposed on the eastern margin of the Leichhardt River Fault Trough, distal to the Mount Isa area. It constitutes the Quilalar Supersequence and represents a transition to ~1750 to 1740 Ma post-rift sedimentation characterised by storm-, tide- and wave-dominated marine shelf and continental facies quartzite composed of feldspathic quartzite, orthoquartzite, conglomerate, arkosic grit, shale, siltstone, minor limestone and dolostone.
• Bigie Formation, a dominantly fluvial succession in a NW-SE directed extensional basin setting that formed as a SE thickening half graben between ~1710 and 1690 Ma. The sequence comprises up to 800 m of purple-brown hematitic sandstone, pebbly sandstone, tuff, conglomerate and red-brown siltstone.
• Fiery Creek Volcanics - a suite of bimodal volcanic rocks dated at 1708±2 Ma, comprising up to 750 m of rhyolite, agglomerate and amygdaloidal/vesicular altered basalt, interbedded with sandstone, conglomerate and siltstone.
• Surprise Creek Formation - a 2500 m thick, generally upward fining sequence of alternating sandstone and quartzite sheets and wedge shaped siltstone horizons.
Gun Supersequence - which is separated from the top of the Prize Supersequence by an unconformity and emplacement of the 1671±8 Ma Sybella Granite and 1678±2 Ma Carters Bore Rhyolite. It comprises, from the base to top:
Mount Isa Group which constitutes the Gun Supersequence in the Leichhardt River Fault Trough, and the lower sections of the Loretta Supersequence on the Lawn Hill Platform.
• Warrina Park Quartzite - a thin, discontinuous basal unit resting unconformably on rocks of the Calvert and Leichhardt superbasins. It comprises an up to 300 m thick, white conglomeratic orthoquartzite to blocky feldspathic sandstone with abundant ripple marks and cross-bedding;
• Mondarra Siltstone - which is generally 300 to 1000 m thick with a maximum of 1700 m. It is a thinly laminated, weakly micaceous, dolomitic siltstone with fine sandstone and black dolostone, overlain by dolomitic sandstone and micaceous slate.
• Breakaway Shale - up to 300 m of thinly laminated, siliceous siltstone/shale, locally chert, typically bleached at surface, but carbonaceous at depth. It is principally composed of quartz, albite, chlorite and muscovite with accessory carbon, microcline pyrite and tourmaline.
• Native Bee Siltstone - a carbonaceous, dolomitic shale and siltstone sequence up to 800 m thick, with rare, thin, vitrophyric tuff bands. Two main fancies have been recognised: i). a featureless carbonaceous shale; and ii). a rock type consisting of alternating white calcite and dark carbonaceous quartz-dolostone layers about 2 to 20 mm thick known locally as the 'zebra-shale'. It contains stromatolites, halite pseudomorphs, flat pebble conglomerates and cross-bedded channel deposits.
• Urquhart Shale - which has been dated at ~1655 Ma and has a maximum thickness of 1050 m. Where unmineralised, it comprises a carbonaceous, dolomitic shale and siltstone, closely resembling the Native Bee Siltstone. It contains numerous potassium rich marker beds, some of which have been positively identified as tuffs. Five facies have been recognised within the unit at George Fisher, each intercalated as bands ranging from a few mm to 25 m in thickness (after Chapman, 2004):
- Medium-bedded stylolitic mudstone - occurring as units with sharp boundaries that range from 5 to 15 m in thickness, composed of individual planar, laminated to massive beds that are 5 to 10 cm thick. This facies occurs as sheet-like bodies with consistent widths across the deposit. The mineralogy comprises quartz, calcite, and/or ferroan dolomite with accessory white mica, K feldspar and carbonaceous matter;
- Banded mudstone - occurring as units that range from 2 to 4 m in thickness with gradational boundaries. It is composed of discontinuous 10 to 100 m long lenses to sheets that extend across the deposit. Beds are more commonly massive than laminated or graded, and are 2 to 5 cm thick. The mineralogy is the same as for the medium-bedded stylolitic mudstone. White, layer parallel carbonate bands are common.
- Carbonaceous Siltstone - occurring as units with gradational boundaries that range from 2 to 50 cm in thickness, composed of mm to sub-millimetre scale alternating light grey and dark wavy laminations with intermittent millimetre thick planar laminations. The light grey laminations are predominantly composed of ferroan dolomite and quartz. The dark laminations have a similar composition, but in addition, contain abundant carbonaceous material. These bands are interleaved with banded mudstones.
- Pyritic Siltstone - occurring as units with gradational boundaries that range from 1 to 5 m in thickness, composed of mm to sub-millimetre scale alternating light grey and dark wavy laminations with intermittent millimetre thick planar laminations. The thickest intervals occur as sheets that are continuous across the deposit. However, at gradational boundaries with banded mudstone, individual laminations may be discontinuous. The light laminations have the same composition as those of the carbonaceous siltstone, while the darker laminations are composed of fine grained spheroidal pyrite and minor carbonaceous material. White, layer parallel carbonate bands and nodular carbonate are common to abundant. Pyritic Siltstone beds tend to be interbedded with banded mudstone to form units from a few to >100 m in thickness and constitute >80% of the central and upper sections of the Urquhart Shale.
- Tuffaceous Marker Beds - occurring as 0.5 to 10 cm thick beds that are continuous across the deposit. They have sharp contacts and are composed of quartz and K feldspar with accessory chlorite and white mica.
A different suite of constituent facies of the Urquhart Shale have been mapped at Mount Isa, although the variations are largely nomenclatural.
• Spear Siltstone - composed of rhythmically laminated siliceous dolomitic siltstone and siliceous dolostone up to 160 m thick. It contains stromatolites, halite pseudomorphs, flat pebble conglomerates and cross-bedded channel deposits.
• Kennedy Siltstone - which is very similar to the underlying Spear siltstone, differing only in its course bedded to massive character over much of the sequence which has a maximum thickness on 320 m. Sedimentary brecciation is common to both units.
• Magazine Shale - a carbonaceous siliceous shale unit that is the uppermost formation of the Mount Isa Group. It's upper section, along its entire strike length, is truncated by the Paroo Fault, with a maximum preserved thickness of 220 m. Outcrops have a characteristic reddish colour due to contained iron oxides.
A succession of deformation events has been recognised within the Leichhardt River Fault Trough, best reflected in the vicinity of the Mount Isa Fault, although not all are evident in the broader Western Fold Belt. These are as follows:
• D1, as defined by Bell (1983) is a pre- to early Isan Orogeny subhorizontal, north-south shortening that occurred between ~1610 and 1590 Ma. It produced local thrusting, east-west trending folds, variably developed bedding-parallel carbonaceous seams (S1), and a north-south trending mineral lineation at the Mount Isa mine (Bell, 1991), but is not recognised at the George Fisher deposit (Chapman, 2004). It is generally characterised by east-west trending, near vertical foliation parallel to bedding, with a north-south trending mineral elongation (Bell and Hickey, 1998).
• D2 which produced the bulk of the regional folds with steeply dipping north-south trending axial planes. It was caused by a east-west shortening event during the Isan Orogeny at ~1544 Ma (Page and Bell, 1986), which at George Fisher was responsible for upright, tight folds with steeply dipping, subvertical, north-south striking axial planes and an associated S2 foliation. It is also defined by concentrations of carbonaceous material. It accompanied folding of and reverse displacement on the Paroo and Hanging Wall faults and dyke intrusion (Chapman, 2004). S2 is the dominant regional schistosity, commonly with a steeply south pitching L2/2 stretching lineation (Bell and Hickey, 1998). The Mount Isa and George Fisher deposits are located on the western limb of a regional D2 anticline (Davis, 2004).
• D3, previously known as D2.5 of Bell and Hickey 1996; Perkins 1996; Bell and Hickey 1998; Chapman 2004 and others. This deformation was first recognised after D2 and D3 had been established in the literature, and hence was designated as D2.5 by Bell and Hickey (1996). However, subsequent authors (e.g., Davis, 2004) referred to it as D3 and then referred to the the subsequent D3 event as D4, etc. (c.f., Wilde et al., 2006; Long, 2010). This deformation, which is evident at both Mount Isa and George Fisher, was the result of NE-SW to ENE-WSW compression (McLellan et al., 2014). It rotated D2 structures (specifically S2) into gently dipping microfolds (or crenulations) to macroscopic folds with subhorizontal axial planes and east vergence, but no recorded foliation. These folds may either die out along their axial planes into mineralised layers, or curve to merge into S3 (Perkins, 1996).
• D4 of Davis (2004) and subsequent authors, (previously D3 of earlier authors as listed above), which is another shortening event, but reflects SE-NW shortening and took place at ~1510 Ma. It resulted in further development of folds with steeply dipping, subvertical axial planes and gently plunging axes that trend NW-SE to NNW-SSE and have an axial planar schistosity, S4 (Page and Bell, 1986; O'Dea et al., 1997; Bell and Hickey, 1998). These folds are more localised than those of D2. The largest D4 structure in the Mount Isa deposit area is the Mount Isa Fold, a syncline-anticline pair with a wavelength of 200 to 400 m, amplitude of 130 m, and a near vertical common short limb. Other D4 folds have wavelengths no wider than 10 to 20 m.
• Post D4, described by Chapman (2004) as Post D3, which is interpreted to have produced local NW-SE trending folds; brittle NNW-SSE splays from the Mount Isa-Paroo faults, including the Transmitter and Gidyea Creek faults between George Fisher and Mount Isa; and reactivation of the Mount Isa and Paroo fault zone. This deformation is assumed to encompasses the D5 and/or D6 listed below, but may well represent the late stages of D4.
• D5, which was previously described by Bell and Hickey (1998) as D4, is only manifested in the Mount Isa District as flat lying kinks, mainly to the west of the Mount Isa Fault in more schistose rock types. These kinks may be distinguished from D3 crenulations in that the latter have not been refolded by D4. The sense of shearing is generally east-vergent (Bell and Hickey, 1998).
• D6, is only very weakly and sparsely developed, forming crenulations or poorly developed crenulation cleavage. Where observed to the west of the Mount Isa Fault, it is associated with local retrogression. While not significant in the Western Fold Belt, it is more important in the Eastern Succession, being related to gold mineralisation at Starra (Bell and Hickey, 1998).
District Scale Structure and Metamorphism
In the Mount Isa-George Fisher area, the Mount Isa Group rocks occurs as a west-dipping, north-south striking sequence on the western limb of a regional
D2 anticline. The stratigraphic package is ~4000 m thick, decreasing north of the Transmitter fault (~2 km south of Hilton) where sections of the group have been removed by faulting (Valenta, 1994). The formations of the Upper Mount Isa Group are truncated to the west by the Paroo and Mount Isa faults and are locally fault-bounded against the lower Mount Isa Group to the east across the Barkly shear zone. The latter structure roughly separates the Urquhart Shales and Native Bee Siltstone.
The Paroo fault is interpreted to have originally formed as a listric extensional fault during deposition of the Kennedy Siltstone and Magazine Shale (Long, 2010), but may be a reactivated predecessor structure that bounded a half graben during Leichhardt Superbasin deposition (Lister, 2002; Betts et al., 2003). The related Mount Isa Fault was active by D2. Whilst both the Paroo and main Mount Isa faults were reactivated by brittle deformation during D2 and D3 to D4, Long (2010) and McLellan et al. (2014) propose that the Paroo Fault was folded during these deformations, while also undergoing reverse shearing. Long (2010) suggests this folding was influenced by uplift of the Sybella batholith which acted as a buttress during D2 shortening (Betts and Lister, 2002). The main Mount Isa and Paroo Faults are broadly parallel at surface to the west of the Mount Isa deposit, northward to beyond George Fisher. Over this interval, the Paroo Fault is in the footwall, and immediately east, of the steeply west dipping Mount Isa Fault. Down dip at Mount Isa however, the Paroo Fault diverges eastward from the Mount Isa Fault where it takes the form of gently north plunging, fault dislocated synform and then an antiform, before dipping steeply east to depth. As a result, the 60 to 65°W dipping Mount Isa Group was truncated at depth by the flat lying section of the folded Paroo Fault and juxtaposed with the underlying similarly steeply dipping Eastern Creek Volcanics and Guide Quartzite at 1655±4 Ma (Southgate et al., 2000; Valenta, 1994; Perkins et al., 1999). To the south of the Mount Isa deposit, this fold is evident where it intersects the surface, and the Paroo Fault diverges sharply from the main Mount Isa Fault juxtaposing the same two sequences. Brittle deformation related to reactivation of these two faults is the major manifestation of D4 brittle deformation in deposits of the Mount Isa district (Chapman, 2004). D2 and D3 shearing and reverse displacement along the folded Paroo Fault is regarded as being important in producing zones of dilation exploited by mineralising fluids (Long, 2010).
Peak metamorphism apparently occurred between D2 and D3 to the west of the Mount Isa and Paroo faults, where metamorphic grades are significantly higher, increasing from biotite to sillimanite-K feldspar grade westward toward the Sybella batholith (Connors and Page, 1995). Metamorphic temperatures within the Mount Isa Group declined northward from ~350 to 300°C at Mount Isa (Rubenach, 1992) to ~200°C at George Fisher (Chapman,1999, 2001). The age of the peak of metamorphism has been estimated at 1550 to 1570 Ma (SHRIMP U-Pb of zircons in folded pegmatite - 1554 ± 10 Ma; Connors and Page, 1995; chemical Th-U-total Pb isochron method or CHIME of monazite - 1570 Ma with no errors quoted; Hand and Rubatto, 2002; and Pb-Pb step-leach dating of tourmaline from quartz-tourmaline veins -1528±51 and 1577±48 Ma; Duncan et al., 2006).
Deposit Scale Structure
At George Fisher North, at the deposit and ore lens scale, mudstone and siltstone units generally dip at 30 to 80° west, with small domains extending for ~50 to 100 m down-dip characterised by near-vertical to slightly overturned bedding. These and other observations are interpreted to imply the presence of open folds with both steep and gently dipping axial planes (Chapman, 2004).
Small, centimetre to tens of centimetre scale tight to open folds are evident in some of the pyritic siltstone units, particularly where these units are mineralised with sphalerite and galena. These small scale folds have a wide range of fold profiles and orientations, with refolded folds and several cleavages evident. The distribution and geometry of these folds have no obvious parasitic relationship to the larger scale open folds detailed above and contrast to the more simple structure of the deposit at the larger scale. Also, in contrast to the Zn-Pb mineralised pyritic units, the largely barren mudstone beds lack similar small scale structures (Chapman, 2004).
At the mineralised unit scale, Chapman (2004) recognises four foliations at George Fisher, denoted as GFS1 to 4 inclusive. The earliest observed cleavage is usually the bedding-parallel GFS1, characterised by anastomosing penetrative seams of carbonaceous material in mudstones and a slaty cleavage in pyritic siltstones. The earliest folds are tight to isoclinal, east-vergent and north-south striking with 0.5 to 2 m amplitudes, and an axial plane cleavage, GFS2, developed in their hinges. These structures are refolded by younger GFF3 recumbent, and upright GFF4 folds with tight to isoclinal profiles around north-striking axes. These structures commonly produce complex interference patterns at 0.5 to 2 m scales. Chapman (2004) noted that it was not clear whether these small scale folds and multiple GFS1 to GFS4 cleavages were developed during a single continuous deformation or as successive, discrete events comparable to the regional D1 to D4.
The George Fisher mines are some 20 km to the north along strike from the Mt Isa orebodies, and are located immediately to the east of the major, north-south trending Mt Isa fault system. Both George Fisher South (Hilton) and North are hosted by similar facies in the upper sections of the same host Urquhart Shale which also embrace the Mt Isa Zn-Pb ores, although the orebodies are generally thinner and more disrupted by faulting than the similar ores at Mt Isa. Little copper is known in association with the zinc lead ores at both Hilton and George Fisher.
Economic grade mineralisation at George Fisher occurs within a north-south striking, west dipping stratigraphic package that is 350 m thick, has a strike length of 1200 m, and extends down dip for >1000 m. It is obliquely truncated by the NNW-SSE trending Spring Creek and Gidyea Creek faults to the north and south respectively, and to the west at depth by the Hanging Wall Fault.
The George Fisher North deposit comprises 11 mineralised stratigraphic intervals that occur as stacked stratabound lenses, hosted by the Pyritic Siltstone and interbedded Pyritic Siltstone and Banded Mudstone units of the Urquhart Shale as described above. These units are separated by thicker bands of barren Banded Mudstone.
Mineralised intervals are denoted, from the stratigraphic base upward, as 1, 2 and A to I inclusive, with internal subdivisions denoted by a numerical subscript. The C and D orebodies contain almost half of the known George Fisher resource at grades of around 10.7% Zn, 5.9% Pb and 111g/t Ag. Each of the mineralised interval has distinctive bedding characteristics and tuffaceous marker beds, enabling detailed stratigraphic correlations throughout the ore-bearing
sequence (Johnston et al., 1998, Tolman et al., 2002).
Each of these mineralised intervals is segmented along strike by NE-SW to NNE-SSW striking faults which Chapman (2004) sugget are post-D4, but may be late D4. These faults contain pods of coarse calcite ±dolomite ±quartz ±sphalerite ±fluorite ±pyrite, localised in jogs, linked by graphitic fault gouge.
The George Fisher South (Hilton) deposit has 7 to 10 stacked ore lenses within a 250 m stratigraphic interval, which have been complicated by intense shortening associated with the Isan Orogeny. The two deposits are separated by a 2 km strike length of intensely faulted, barren shale, mudstone and siltstone.
These Pyritic Siltstone and interbedded Pyritic Siltstone and Banded Mudstone units carry 10 to >50% fine-grained diagenetic pyrite, distributed over a thickness of 800 m, enveloping both the George Fisher North and South deposits and extending over a strike length of >10 km.
The George Fisher North deposit has undergone a long and complex history of hydrothermal activity. The the earliest base metal sulphide-bearing assemblages of the deposit were preceded by four distinct gangue-forming events that included:
i). early dolomite, ankerite and ferroan dolomite cement replacing primary sedimentary grains and rock matrix, such that the rock is enriched in Fe and Mn relative to background Urquhart Shale;
ii). nodular and banded calcite that predates stylolite formation, manifested as as pervasive bleaching of barren mudstone while nodular calcite is rhythmically interwoven with the fine-grained diagenetic pyrite in finely laminated siltstone and finely banded mudstone;
iii). widely developed, fine-grained pyrite precipitated within some siltstone units and along carbonaceous stylolite surfaces;
iv). development of celsian, hyalophane, K feldspar, calcite ±ferroan dolomite ±quartz veins and alteration halos.
The subsequently introduced economic sulphide mineralization predominantly comprises sphalerite and galena with traces of native silver, tetrahedrite and chalcopyrite. Some of the individual ore lenses can be distinguished on the basis of their metal and gangue mineral assemblages. Alteration that is spatially associated with the late, weak, syntectonic Cu mineralisation includes pyrrhotite, siderite, ferroan ankerite, biotite, chlorite, muscovite and magnetite.
Four distinct styles of strata-bound Zn-Pb mineralisation are distinguished at George Fisher, based on the dominant sulphide minerals, grain size, structural setting and paragenesis (from Chapman, 2004):
• Layer-parallel disseminated sphalerite, which is conformable to bedding at a mm to cm scale, and stratabound at all other levels. The sphalerite is very fine grained, with µm to mm widths, and is subhedral to anhedral with a honey to light brown and red-brown colour. It is disseminated in mm to cm thick diffuse bands subparallel to host rock layering, and is very variable, comprising between 5 and 45% of a particular band. Individual sphalerite grains and aggregates mimic, or are finer grained than the host rock. However, at a mining scale, mineralisation of this style alone is not economic.
Where developed within siltstone, disseminated sphalerite is dispersed within quartz-calcite-ferroan dolomite laminations, but is rare in pyritic and carbonaceous bands. Where it is found within nodular carbonate layers, it occurs as isolated interstitial grains, infill after rhombohedral calcite, and as mm sized crystal aggregates with irregular grain contacts with carbonates. In mudstone, sphalerite of this type, locally with traces of galena, occurs as grains that are interstitial to carbonate and quartz and are variably intergrown with bitumen.
Sulphides occur as irregular 'flecks', aggregates, fine disseminations and concentrations that follow and mimic the sedimentary beds, including crossbeds. Some tuffaceous marker beds and white carbonate bands also contain disseminated sphalerite with characteristics that are similar to the mudstone hosted sphalerite.
These observations indicate disseminated sphalerite mineralisation was precipitated in open space within older nodular carbonate material (which itself transgresses stratigraphy at a district scale) and occurs as interstitial pore fillings in mudstones with bitumen. As such, zones of disseminated sphalerite are also discordant to bedding, as well as being discontinuous at the deposit scale, and replace layers that define current produced bedding at finer scales.
• Layer-parallel vein-hosted sphalerite, occurring as fine to medium grained, typically red-brown, semi-massive crystalline sphalerite in discrete layer-parallel bands that are interpreted to be veins. It is coarser grained than sphalerite in the disseminated layer-parallel style, and occurs interstitial to, and surrounds, gangue minerals that have preserved euhedral to subhedral outlines typical of material deposited in open fractures. These veins vary from 1 to 30 mm in thickness, averaging <10 mm, and either occur alone or in stacks of 5 to 10 veins over a width of 10 to 30 cm. They are usually found rhythmically intercalated with pyritic siltstones and at contacts between thin-banded pyritic siltstone and mudstone units. They have planar-parallel or folded margins, and variably sharp to diffuse boundaries. The latter occur where they are bounded by sedimentary layers with abundant disseminated sphalerite. These veins, which comprise 30 to 95 vol.% interstitial sphalerite also carry various combinations of paragenetically associated quartz, calcite ±K feldspar ±hyalophane [(K,Ba)Al(Si,Al)Si2O8] ±celsian [Ba(Al2Si2O8)] ±hydrophlogopite ±ferroan dolomite.
Silicate and carbonate vein filling minerals have significantly coarser grain sizes and are readily distinguishable from those of the wall-rocks. Some of the quartz and carbonate have relict euhedral crystal outlines, whilst all other associated minerals, except hydrophlogopite, have brecciated and corroded grain contacts with sphalerite. These relationships imply sphalerite and hydrophlogopite were deposited after carbonate, quartz and feldspar during a discrete but closely related episode of open-space vein fill. These textures are further interpreted to suggest earlier precipitated silicate and carbonate vein infill was brecciated on a small scale, dissolved and/or replaced by sphalerite during later vein opening and mineral precipitation. Layer-parallel disseminated, sphalerite and hydrophlogopite in the adjacent wall rocks are interpreted to be an alteration selvage to the veins (Chapman, 2004). Crenulation of sphalerite vein margins by GFS2 and intensification of the GFS2 cleavage along vein margins are taken to imply formation of sphalerite veins prior to GFD2.
• Breccia-hosted sphalerite, is characterised by a finer grain size and greater internal structural complexity compared to the vein-hosted sphalerite, although both have an overall strata-bound character. The breccia matrix is composed of very fine grained sphalerite with minor pyrrhotite and galena, enclosing 5 to 50 vol.% clasts. The clasts include irregular, mm to cm size subrounded fragments of folded and boudinaged mudstone and pyritic siltstone, sugary ferroan dolomite vein fragments, and aggregates of calcite ±ferroan dolomite ±K feldspar ±hyalophane ±quartz, similar to the infill in the layer-parallel sphalerite veins.
This brecciated mineralisation postdates sugary ferroan dolomite veining, which crosscuts both vein-hosted and layer-parallel disseminated sphalerite, but occur as clasts in the breccia. Textural relationships suggest breccia formation was synchronous with, or postdated GFF2 fold development.
Breccia-hosted sphalerite is interpreted to be the product of mechanical deformation of pre-existing vein-hosted sphalerite, resulting from strain partitioning in sphalerite veins due to rheological differences between sulphides, mudstone layers and ferroan dolomite veins at cm to tens of cm scales (Chapman, 2004).
• Vein and breccia hosted galena, comprises:
i). Discordant vein hosted galena composed of fine to coarse grained galena ±pyrite ±sphalerite ±pyrrhotite ±carbonate ±chlorite ±quartz. Subhedral galena is interstitial to euhedral pyrite, forming irregularly interlocking crystal aggregates with sphalerite and pyrrhotite. Veins generally have planar to irregular margins, are a few mm thick, and are discontinuous with tapering extremities. They typically form orthogonal networks perpendicular to bedding and are structurally continuous with galena breccias.
ii). Breccia hosted galena occurs as the following variants:
- Coarse-grained galena-rich breccias with a matrix principally composed of ~40 to 70% galena and accessory pyrite, enclosing fragments that vary from a few mm to a few cm in size to larger blocky to subrounded clasts that predominantly comprise mudstone and pyritic siltstone as well as layer-parallel, disseminated and vein-hosted sphalerite mineralisation. Additionally there are microscopic clasts of wall rocks and irregular, subrounded monominerallic silicate, carbonate or sphalerite. These breccias vary from 1 to 100 cm, but are mostly 2 to 10 cm thick, occurring as concordant bands with planar, parallel and discordant margins.
- Fine-grained galena-rich breccias, where the contained galena has a dull lustre and occurs as a fine grained 40 to 75% matrix enclosing microscopic to cm sized clasts similar to those of the coarse-grained galena breccias described above. Clasts usually have oval to spheroidal to elongate shapes and are rounded, sometimes with a bedding-parallel preferred orientation. These breccias occur as conformable bands with planar-parallel margins and range from 1 to 15 cm in thickness.
- Mixed sulphide-rich breccias comprise ~70% matrix composed of variable amounts of fine-grained galena, sphalerite, pyrrhotite and pyrite. These sulphides are irregularly distributed, producing mineralogically distinct bands to wavy and flamelike domains within the breccia matrix at a mm to cm scale. Clasts range from sub-mm to a few cm in size, and are typically well rounded to ovoid to irregular in shape, and include fragments of variably folded host rock, layer parallel, disseminated and vein hosted sphalerite mineralisation, as well as monomineralic calcite, ferroan dolomite and quartz. These breccias are typically 3 to 10 cm thick and have sharp to wavy margins, subparallel to bedding.
There is a consistent timing relationship of galena rich mineralisation compared to all zinc mineralisation styles on a deposit scale, indicating it represent the youngest ore-forming event at George Fisher. The discordant galena veins are structurally continuous with the breccias, indicating the two are coeval. The veins commonly occur in NW-SE to NE-SW trending conjugate sets that crosscut GFF2 and GFF3 folds, with an orientation compatible with their formation during GFF4 fold development.
After extensive Ph.D. investigations of the George Fisher North ore deposit and hosts at all scales, Chapman (1999; 2001; 2004) concluded that the earliest sphalerite mineralisation is hosted by bedding parallel veins. Sphalerite in these veins occurs as infill with hydrophlogopite, and as an apparent alteration product of precursor carbonate-quartz-Ba-K feldspar vein fill. Layer-parallel, disseminated sphalerite occurs as infill after the replacement of carbonate in siltstones, mudstones and nodular carbonate layers and is interpreted to represent a halo to the veins. It was further observed that the earliest sphalerite mineralisation postdated the emplacement of nodular carbonate, which transgresses stratigraphy at a district scale, and occurred prior to district-wide folding marked by the onset of GFD2 at George Fisher. The emplacement of galena in late tectonic structural sites post-dated all sphalerite mineralisation and was emplaced during GFD4 deformation. There is an intimate deposit scale spatial association and grade variation between galena and sphalerite, suggesting a common origin for Zn and Pb and remobilisation in the late stages of deformation, supported by the observation of significant
textural reconstitution of ore during regional folding. Mineralisation styles indicative of sea-floor deposition were not observed, although their complete obliteration and remobilisation cannot be entirely excluded.
George Fisher North is concealed and is partially connected to George Fisher South (Hilton) on its northern margin. Both mines are underground operations and are owned and operated by Glencore's Mt Isa Zinc Division.
Sub-economic mineralisation follows the strike of the Urquhart Shale for 4 km to the north and south of the two economic orebodies.
The original resources were estimated as follows (Forrestal 1990, Valenta 1994, Chapman 2004):
George Fisher South (Hilton): 120 Mt @ 10.2% Zn, 5.5% Pb, 100 g/t Ag,
George Fisher North: 108 Mt @ 11.1% Zn, 5.4% Pb, 93 g/t Ag.
JORC compliant Ore Reserves and Mineral Resources (at June 2006) were as follows (X-Strata 2007):
George Fisher South underground
Proved reserves - 12.5 Mt @ 8.3% Zn, 5.7% Pb, 127 g/t Ag
Probable reserves - 5.9 Mt @ 7.8% Zn, 5.8% Pb, 126 g/t Ag
Measured resources - 25.3 Mt @ 9.7% Zn, 6.9% Pb, 150 g/t Ag
Indicated resources - 10.6 Mt @ 9.2% Zn, 6.6% Pb, 139 g/t Ag
Inferred resources - 10 Mt @ 10% Zn, 6% Pb, 100 g/t Ag
George Fisher North underground
Proved reserves - 11.3 Mt @ 8.9% Zn, 4.7% Pb, 91 g/t Ag
Probable reserves - 15.1 Mt @ 8.3% Zn, 3.9% Pb, 75 g/t Ag
Measured resources - 14.5 Mt @ 10.4% Zn, 5.2% Pb, 101 g/t Ag
Indicated resources - 27.9 Mt @ 9.5% Zn, 4.0% Pb, 74 g/t Ag
Inferred resources - 45 Mt @ 9% Zn, 4% Pb, 80 g/t Ag
Mine production in 2005 totalled 2.7 Mt @ 8.3% Zn, 5.0% Pb, 115 g/t Ag.
JORC compliant Ore Reserves and Mineral Resources (at 31 December, 2011) were as follows (X-Strata 2012):
George Fisher South underground
Proved + probable reserves - 19.6 Mt @ 6.5% Zn, 4.4% Pb, 96 g/t Ag
Measured + indicated resources - 49.8 Mt @ 8.6% Zn, 5.9% Pb, 125 g/t Ag
Inferred resources - 23 Mt @ 8% Zn, 5% Pb, 113 g/t Ag
George Fisher North underground
Proved + probable reserves - 61.8 Mt @ 7.5% Zn, 3.6% Pb, 62 g/t Ag
Measured +indicated resources - 106.2 Mt @ 8.6% Zn, 3.7% Pb, 63 g/t Ag
Inferred resources - 65 Mt @ 8% Zn, 4% Pb, 69 g/t Ag
Handlebar Hill open pit primary
Proved + probable reserves - 2.1 Mt @ 7.6% Zn, 3.4% Pb, 54 g/t Ag
Measured +indicated resources - 6.9 Mt @ 6.9% Zn, 2.3% Pb, 43 g/t Ag
Inferred resources - 1 Mt @ 5% Zn, 2% Pb, 30 g/t Ag
Handlebar Hill open pit oxide
Proved + probable reserves - 0.5 Mt @ 0.4% Zn, 8.5% Pb, 89 g/t Ag
Measured +indicated resources - 0.6 Mt @ 0.4% Zn, 7.8% Pb, 85 g/t Ag
Inferred resources - Nil
JORC compliant Ore Reserves and Mineral Resources (at 31 December, 2018) were as follows (Glencore Resources and Reserves report, 2018):
George Fisher South underground
Proved + probable reserves - 16.7 Mt @ 6.2% Zn, 4.4% Pb, 99 g/t Ag
Measured + indicated resources - 56 Mt @ 8.3% Zn, 5.0% Pb, 106 g/t Ag
Inferred resources - 27 Mt @ 8% Zn, 4% Pb, 87 g/t Ag
George Fisher North underground
Proved + probable reserves - 72 Mt @ 7.0% Zn, 3.1% Pb, 52 g/t Ag
Measured +indicated resources - 168 Mt @ 8.9% Zn, 3.5% Pb, 55 g/t Ag
Inferred resources - 53 Mt @ 9% Zn, 4% Pb, 57 g/t Ag
Handlebar Hill open pit primary
Proved + probable reserves - Nil
Measured +indicated resources - 5.2 Mt @ 6.6% Zn, 2.2% Pb, 37 g/t Ag
Inferred resources - 0.8 Mt @ 5% Zn, 2% Pb, 30 g/t Ag
Handlebar Hill open pit oxide
Proved + probable reserves - 0.5 Mt @ 0.4% Zn, 8.5% Pb, 89 g/t Ag
Measured +indicated resources - 0.6 Mt @ 0.4% Zn, 7.8% Pb, 85 g/t Ag
Inferred resources - Nil
NOTE: Mineral Resources are inclusive of Ore Reserves.
Mine production at George Fisher North and South for the period 1 January 2018 to 31 December 2018 totalled:
2.9 Mt @ 7.3% Zn, 3.9% Pb, 68g/t Ag.
Mt Isa Copper - transgressive sediment hosted copper ...................... Half Group on each of Tuesday 16 & Wednesday 17 September, 2008.
The Mt Isa copper orebodies are located with north-west Queensland, Australia, adjacent to and exploited separately from the Mt Isa lead-zinc deposits (#Location: 20° 43' 42"S, 139° 28' 47"E).
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Travelling from Mt Isa to Brisbane to Sydney to Broken Hill ...................... evening of Wednesday 16 & Thursday 17 September, 2008.
This record will be substantially updated in the near future. For details of the setting and host stratigraphy see the separate Mount Isa Zinc and George Fisher records,
The Mount Isa copper ore deposits are hosted within the Mesoproterozoic (1653 Ma) Urquhart Shale, an around 1000 m thick succession of carbonaceous, pyritic, dolomitic siltstone that belong to the Mt Isa Group, which lies within the Leichhardt River Fault Trough, and belongs to Cover Sequence 3 in the Western Fold Belt of the Mt Isa Inlier.
In the Mt Isa mine area, the Mt Isa Group strikes north-south and has a persistent westerly dip of 65°. It is around 4000 m in thickness and comprises a sequence of alternating units of dolomitic shale and dolomitic siltstone, with relatively minor conglomerate and sandstone at the base. These latter sediments thicken to the east.
Cover Sequence 3 unconformably overlies the thick mafic volcanics and quartzites of Cover Sequence 2 which includes the 7000 m thick Eastern Creek Volcanics.
The individual copper orebodies are contained entirely within a single large irregular silica-dolomite alteration mass which lies to the south of and overprints the Zn-Pb-Ag orebodies, but lie within the Urquhart Shale.
There is evidence to suggest that the silica-dolomite and Cu ore are substantially younger than the Zn-Pb-Ag ore, possibly being emplaced during D3 deformation.
The silica-dolomite mass has a strike length of at least 2600 m, maximum width of 530 m and up-dip extent of near 1000 m. Its boundaries cut across bedding. The main gangue minerals are ferroan dolomite and quartz with locally important talc, chlorite and K-feldspar. The silica-dolomite comprises an early progressive growth of exaggerated dolomite grains and porphyroblastic dolomite replacement forming pseudo-breccias via replacement outwards from fractures. This stage destroys earlier textures. The silica replacement stage results in partial to complete pseudomorphic silica replacement of the dolomitic pre-cursors and preserves pre-existing textures.
The main sulphides are pyrite and chalcopyrite with lessor pyrrhotite and cobaltite. They are predominantly present as replacements forming coarse grained aggregates, pseudo-breccias and discontinuous veinlets. Chalcopyrite deposition is largely controlled by coarsely crystalline dolomite precursors.
Past production and reserves indicate a total resource of around 225 Mt @ 3.3% Cu.
JORC compliant ore reserves and mineral resources (at June 2006) are as follows (X-Strata 2007):
X41 Mine, 1000 & 1900 orebodies
Proved reserves - 28 Mt @ 2.1% Cu
Probable reserves - 21 Mt @ 1.8% Cu
Measured resources - 76 Mt @ 2.1% Cu
Indicated resources - 7 Mt @ 1.8% Cu
Inferred resources - 10 Mt @ 2% Cu
Enterprise Mine, 3000 & 3500 orebodies
Proved reserves - 32 Mt @ 3.6% Cu
Probable reserves - 5.7 Mt @ 3.2% Cu
Measured resources - 61 Mt @ 3.3% Cu
Indicated resources - 8.1 Mt @ 2.7% Cu
Inferred resources - 1 Mt @ 2% Cu
Inferred resources - 70 Mt @ 1% Cu
Mt Isa copper open pit
Measured resources, - 98 Mt @ 1.4% Cu
Indicated resources - 69 Mt @ 1.2% Cu
Inferred resources - 110 Mt @ 1% Cu
JORC compliant ore reserves and minerals resources (at 31 December 2011) were as follows (X-Strata 2012):
X41 Mine, 1000 & 1900 orebodies
Proved + probable reserves - 31 Mt @ 1.8% Cu
Measured + indicated resources - 60 Mt @ 1.9% Cu
Inferred resources - 8 Mt @ 2% Cu
Enterprise Mine, 3000 & 3500 orebodies
Proved + probable reserves - 24 Mt @ 3% Cu
Measured + indicated resources - 45 Mt @ 3% Cu
Inferred resources - 1 Mt @ 2% Cu
Inferred resources - depleted
Mt Isa copper open pit
Measured + indicated resources - 150 Mt @ 1.2% Cu
Inferred resources - 133 Mt @ 1% Cu
The X41, Enterprise and 500 orebody mines are underground, block caving operations. Mt Isa copper open pit exploits the upper sections of the silica-dolomite hosted copper orebody. The resource is based on a 0.5% Cu cut-off and comprises approximately 60% promary chalcopyrite, with the remainder is oxidised or partially oxidised with minor supergene chalcocite.
The mine is operated by X-Strata Copper.
Broken Hill - metamorphic hosted zinc-lead-silver ...................... Friday 19 September, 2008.
The Broken Hill zinc - lead - silver orebodies are located 40 km east of the South Australian border in far western New South Wales, approximately 950 km WNW of Sydney and 450 km north-east of Adelaide (#Location: 31° 57' 50"S, 141° 28' 03"E).
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Travelling from Broken Hill to Adelaide & free weekend ...................... Saturday 20 & Sunday 21 September, 2008.
The Broken Hill deposits lie within the Willyama Block or Broken Hill Domain, which extends from far western New South Wales into eastern South Australia. This block is the southeastern segment of the 200 x 400 km Curnamona Province, and is composed of Palaeo- to Mesoproterozoic metamorphic rocks and minor intrusives. The Willyama block is overlain by Neoproterozoic sedimentary cover which separates it from the similar rocks of the Euriowie Block to the east, while the equivalent Olary Block to the west in South Australia is separated from the Willyama Block by younger cover.
The Neoproterozoic Adelaide Rift Basin (related to extension preceding the late Neoproterozoic Rodinia break-up) separates the Curnamona Province from the Gawler Craton to the west, and masks the mid- to late-Palaeoproterozoic suture between the Gawler craton and Curnamona province. The northeastern, eastern and southeastern margins of the preserved Curnamona province are determined by the Tasman Line, which marks the Rodinia break-up and separation of the eastern sections of the pre-breakup cratonic mass. Prior to the break-up of Rodinia, the Gawler Craton, Curnamona Provinces, and cratonic elements in North America and Antarctica were all part of the larger Mawson craton.
The Willyama Block is largely occupied by the metamorphic rocks of the 7 to 9 km thick Willyama Supergroup, which comprises amphibolite to granulite facies schists and gneisses, ranging from andalusite-muscovite, through sillimanite-muscovite, sillimanite-potassium feldspar-garnet to hornblende-granulites, with lesser amphibolite and basic granulite. It has been interpreted to represent metamorphosed protoliths of clayey and sandy sediments with intercalated basic and felsic volcanics and minor sub-volcanic intrusives. Basement rocks to the Willyama Supergroup are not exposed, but inheritance indicates the presence of Archaean and older Palaeoproterozoic crystalline crust..
The Willyama Supergroup is interpreted to have been deposited in the epi-continental Curnamona rift basin between 1720 and 1640 Ma, and was accompanied by bimodal magmatism. The Olary Domain, exposed to the west in South Australia, represents the palaeo-basin flank to this rift basin, further from the rift axis (Conor and Preiss 2008). The Willyama Supergroup has been interpreted to represent three stages of progressive up-sequence deepening in the central part of the rift basin, comprising:
• Early syn-rift deposition, from ~1720 to ~1690 Ma, within a fluviatile to shallow marine shelf sequence in a restricted saline, oxidised basin, grading to a marine succession. The oldest rocks in the Curnamona Province are the ~1720 to 1710 Ma Curnamona Group which is only exposed in South Australia. This succession commenced with the Wiperaminga Subgroup which includes variably magnetite-rich, micaceous, siliceous and albitic metasedimentary rocks, including interbedded migmatitic gneiss and local metaconglomerate. These are followed by the Ethiudna subgroup mostly psammitic rocks with some calc-silicate minerals, including basal basalt and quartzite, further albitic metasedimentary and quartzo-feldspathic to pelitic rocks, and A-type meta-volcanic/granitic units which intrude and are intercalated with the Curnamona Group, along with mafic intrusive rocks (Conor and Preiss, 2008). This sequence was succeeded by metamorphic rocks of the Thorndale Composite Gneiss (and laterally overlapping Rantyga Group) and the Thackaringa Group, interpreted to have been formed from protoliths that were relatively sandy shallow marine sedimentary rocks, with a possible evaporitic, or hypersaline component represented by albitic horizons (Stevens et al., 1988; Fitzherbert and Downes, 2015).
• Rift deepening and magmatic escalation, between ~1690 and ~1670 Ma, which commenced with the Broken Hill Group, that conformably and gradationally overlies the Thackaringa Group and includes the Allendale Metasediments and Purnamoota Subgroup. This group grades from a pelite-dominant base to interbedded pelite and psammite at its top. The succeeding Sundown Group contains identical lithologies to that of the upper part of the Broken Hill Group, comprising interbedded pelitic, psammopelitic and psammitic metasedimentary rocks, except that it lacks any syn-depositional igneous units. The siliciclastic sedimentary protoliths to both groups has been interpreted as a deep water turbidite sequence (e.g., Wall, Etheridge and Hobbs 1976; Plimer 1986; Laing 1980; Willis et al., 1983), although others (Stevens et al., 1988), suggesting a slightly shallower water shelf setting with minor intervening storm beds. The 'Potosi-type' Parnell and Hores gneisses are interpreted to be coherent volcanic and volcaniclastic mass flow deposits (Stevens et al. 1988), deposited during transgressive phases of sedimentation (Dyson 2003) and active normal faulting (Conor and Preiss 2008). The abundant basic gneisses of the Annandale and Silver King Metadolerites of the Lady Louise Suite, probably formed as syndepositional, high-Fe tholeiitic dykes and sills.
Deposition resumed in the lateral Olary Domain after a 20 m.y. hiatus, commencing with the Saltbush Group, which disconformably overlies the Ethiudna Subgroup, commencing with a regional calc-silicate that is correlated with a similar calc-silicate at the base of the Broken Hill Group. This unit is followed by graphitic metasiltstone, a psammite-dominated suite, overlain by the pelitic rocks containing manganiferous banded quartz-magnetite-grunerite-garnet iron formation and quartz-garnetite rocks similar to the main Broken Hill Group section (Conor and Preiss 2008). Tholeiitic amphibolite sills of the Lady Louise Suite intrude this succession to the top of the upper psammites.
• Post-rift sequences, from ~1670 to ~1640 Ma, representing deeper water, sag phase deposition post-dating the climax of rifting, with the stratigraphic progression reflecting an up-sequence change from deeper shelf/plateau sedimentation to shallow water deposits, partly with shoreface characteristics (Dyson 2003). It is represented by the Cartwrights Creek, Bijerkerno and Dalnit Bore metasediments of the Paragon Group, and comprises thinly bedded graphitic, pelitic to psammopelitic phyllite, with lesser fine-grained graphitic, felspathic psammite, with calc-silicate and albitic interbeds.
The Strathearn Group is the stratigraphic equivalent of the Paragon Group in the Olary Domain, where a similar sequence of interlayered graphitic and aluminous pelite is reflected by the Alconie Formation, which grades into albitic, psammitic rocks of the overlying Mooleulooloo Formation and pelitic schist of the uppermost Dayana Formation.
These sequences were subjected to deformation by the early Mesoproterozoic 1600 to 1580 Ma Olarian Orogeny. The only Mesoproterozoic rocks in the Broken Hill Domain belong to the syn- to post orogenic granites (1595 to 1580 Ma) of the Mundi Mundi Suite. However, to the west in South Australia, extensive suites of volcanosedimentary, volcanic and granitic rocks are mapped, e.g., the 1595–1580 Ma Radium Creek Group in the Mount Painter and Mount Babbage Inliers; the 1585 to 1575 Ma Ninnerie Supersuite granites with synchronous silicic volcanism and the extensive rhyolitic to dacitic A-type Benagerie Volcanic Suite, as well as sequences of shallow water to deltaic quartzite, schist, gneiss, phyllite and calc-silicate (Fitzherbert and Downes, 2015).
Subsequent deposition was during the Neoproterozoic, related to rifting that produced the Adelaide Rift Complex, and eventually led to the breakup of Rodinia. It developed over the deformed Palaeoproterozoic basement, representing continental rift to passive continental margin deposition. In the Broken
Domain sedimentation took place in two stages, the Poolamacca Group, between ~850 and 810 Ma, which was related to NE–SW extension, with the deposition of fluviatile to shallow marine, mature sandstone and lesser conglomerate, followed by stromatolitic limestone and eruption of the 827±9 Ma Wilangee Basalt, interpreted to be a continental flood basalt. These were followed by a depositional hiatus occurring between ~810 and ~700 Ma in the Broken Hill Domain, attributed to Sturtian tectonism, uplift and extensive erosion of the Poolamacca Group. However, in South Australia, to the west, sedimentation continued, with the deposition of the fluviatile to shallow marine sequences of the Burra Group. Sedimentation resumed in the broken Hill Domain in the mid Cryogenian withthe deposition of the Torrowangee Group, which persisted until ~630 Ma. This unit represents a syn-rift depositional environment and includes the two globally distributed glacial successions, the Sturtian from ~700 to 660 Ma, and Marinoan from ~645 to 630 Ma, separated by carbonate-rich interglacial deposition (Fitzherbert and Downes, 2015).
The Torrowangee Group was unconformably overlain by the late Neoproterozoic ~630 to 580 Ma Farnell Group which marked the end of a period of basement instability and the initiation of sedimentation on a stable shelf environment. The sequence represents marine transgression, with upward-coarsening sequences reflecting subsequent marine regression, and comprises a basal cap dolomite, through a thick siltstone-rich package, into an upward-coarsening sequence of quartzite and sandstone. This succession was followed by a prolonged depositional hiatus punctuated by the intrusion of a northwest-trending dyke swarm to the west of Broken Hill at ~580 Ma, succeeded by deposition of shallow marine sediments of the ~540 to ~515 Ma Arrowie Basin which were unconformably deposited on the Palaeoproterozoic basement.
Inversion of the Neoproterozoic-Cambrian passive margin is thought to have occurred on the eastern Gondwana margin during the 515 to 500 Ma Delamerian Orogeny (Foster et al., 2005). During this deformational event (D4), the Neoproterozoic sequences at Broken Hill underwent low-grade metamorphism (lower greenschist facies) with large-scale, relatively open folding along a north-south or NW-SE axis.
Regional Deformation and Metamorphism
Inversion of the Curnamona Rift Basin took place during the 1620 to 1580 Ma, Olarian Orogeny, although heating and partial melting of the Curnamona Province is evident prior to any pervasive deformation. Textural evidence indicates widespread aluminosilicate-cordierite crystallisation and partial melting took place across the Broken Hill and Olary domains under low strain and possible low pressure conditions (Stevens 2006). Monazite dating (e.g., Forbes et al., 2008; MacFarlane and Frost, 2009) indicate a thermal event between 1630 and 1620 Ma, which may reflect early heating..
The compressional Olarian orogenesis comprised northward-directed crustal shortening, followed by continued shortening and the development of approximately north-south oriented folds (Wilson and Powell 2001; Betts and Giles 2006).
Two periods of deformation, accompanied the prograde metamorphism in the Broken Hill and Euriowie Blocks, namely D1 which produced large recumbent to shallowly inclined folds and a layer parallel S1 foliation (Laing et al., 1978; Marjoribanks et al., 1980; Hobbs et al., 1984) with a NW vergence, and the development of a high-T, low-P, metamorphic field array, sub-parallel to the regional stratigraphy (Binns 1964; Phillips 1978; Webb and Crooks 2006). This event produced a metamorphic gradient that progressed from granulite facies at the lowest stratigraphic levels in the south of the Broken Hill Domain, to upper greenschist facies in the upper units to the north.
Continued crustal shortening produced upright, broadly north-south trending D2 folds with steeply dipping axial planes that formed under very similar metamorphic conditions to D1 and affected the distribution of the D1 metamorphic field array (Wilson and Powell 2001).
Anatexis, which had commenced during the high-T event prior to D1, continued through D1 and D2 events in the upper amphibolite to granulite facies sections of the terrane (Stevens, 1978; Burton, 1996, 1998, 2001; Wilson and Powell, 2001; Page et al., 2005), at temperatures up to 800°C (Binns, 1964, Phillips and Wall, 1981). Extensive partial melting, as much as >50% in some lithologies (White et al., 2004) and a relatively efficient melt extraction process (White and Powell, 2002; White, Powell and Halpin, 2004) resulted in the intrusion of voluminous sheeted pegmatite and leucogranite layered sill complexes into the overlying lower amphibolite and upper greenschist facies rocks throughout the prograde to peak metamorphic evolution of the terrane at ~1660±10 Ma. Both D1 and D2 resulted in sillimanite grade axial plane schistosity.
The pegmatites in the upper parts of the main sill complex within amphibolite facies rocks at Yanco Glen and possibly in the vicinity of Mount Robe, have associated tungsten mineralisation, whilst tin mineralisation is generally associated with smaller isolated pegmatites hosted by lower amphibolite to upper greenschist facies rocks at the Waukeroo and Euriowie tin fields, and at the Kantappa mine. Feldspar-rich pegmatites with recognised potential for feldspar and beryl are found in the core of the granulite terrane.
A third phase of deformation, D3, produced folds on all scales throughout the Broken Hill Block, generally with near vertical, retrograde axial planar foliation (S3), generally defined by muscovite-chlorite±biotite (Marjoribanks et al., 1980), although higher-grade assemblages have also been described locally (e.g., Stevens 1978; Marjoribanks et al., 1980). Extensive retrograde schist zones transect the Palaeoproterozoic basement and may contain all or some of the retrograde assemblage of staurolite-kyanite-chloritoid-biotite-chlorite-muscovite assemblages. These schist zones have widths of from a few metres to several hundred metres, while in some cases they constitute complex retrograde belts several kms wide, and have been regarded as D3, formed during the late stages of the Olarian Orogeny (Marjoribanks et al., 1980). However, other authors suggest these retrograde schists zones were related to a later thermal event, possibly Delamerian (e.g., Stuwe and Elhers, 1997), or to have been generated early in the D3 phase, before 1570 Ma, and reactivated during the Delamerian event at 500±20 Ma (Stevens 1986). These shear zones are generally aligned in a NE-SW or NW-SE direction, while some are east-west. All of the wide complex belts are characterised by numerous, close spaced, anastomosing, intense, retrograde schist zones, by pervasive retrogression of intervening high grade rocks and by an abundance of granitoids, pegmatite, aplite, migmatite and granite.
Late syn- to post-orogenic granites (~1595 to1580 Ma: Nutman and Ehlers 1998; Page et al., 2005) including the Mundi Mundi Suite, intrude all levels of the terrane, although the larger plutons are found within the upper Paragon Group. Abundant, sill-like intrusions of Mundi Mundi Suite granite, known as the Umberumberka type (Brown et al., 1983), are associated with the Fe-rich breccias in the Silverton to Purnamoota areas. IOCG-type Cu-Au-Fe-U mineralisation at Copper Blow may be of a similar age, consistent with the timing of iron oxide-associated Cu-Au-U mineralisation related to the 1600 to 1570 Ma Ninnerie Suite elsewhere in the Curnamona Province (Skirrow et al., 1999; Conor and Preiss 2008).
The Broken Hill orebodies and their wall rocks have been subjected by two major periods of regional metamorphism. The first of these, M1, coincides with the Olarian Orogeny and the D2 and D3 deformational events. D2 commenced at or immediately prior to the peak of metamorphism and continued through the culmination of M1, whilst D3 represented the waning stages. Most of the deformation affecting the mineralised system occurred during D2 and the first phase of D3 (D3A). The second period of metamorphism, M2, coincides with the Delamerian Orogeny and D4 (Webster, 2004).
All of the economic mineralisation within the immediate Broken Hill district is hosted by the Broken Hill Group (Suite 4) of the Willyama Supergroup. Haydon and McConachy (1987), described the following sequence and nomenclature, which is utilised in much of the literature, prior to 2008:
Clevedale Migmatite (Suite 1) composed of >425 m of leucocratic quartzo-feldspathic migmatites and composite gneisses (quartz-albite-K feldspar-biotite±cordierite rocks), with meta-sedimentary migmatites, leucocratic sodic plagioclase-quartz rocks and concordant to transgressive basic gneiss. The Suite 1 Redan Gneiss has been dated at 1710±4 Ma (Page et al., 2005);
Thorndale Composite Gneiss (Suite 2), comprising 1000 to 2000 m of poorly to well bedded psammitic to psammo-pelitic meta-sedimentary composite gneisses, composed of quartz-feldspar-biotite-sillimanite±cordierite±garnet, interlayered with subordinate sodic, plagioclase-quartz rock and concordant to transgressive basic gneiss;
Thackaringa Group (Suite 3), 1000 to 3000 m, averaging 1500 m thick, dominantly composed of quartzo-feldspathic gneisses with lesser pelitic and mafic gneisses. Rocks in the basal and upper parts of this unit have been dated at ~1704 and 1683±3 Ma respectively (Page et al., 2005);
Broken Hill Group (Suite 4) composed predominantly of pelitic to psammitic gneisses with minor quartzo-feldspathic and mafic gneisses. This unit has been dated at between 1693±4 (Ettlewood Calc-silicates) and 1685±3 Ma (Hores Gneiss, which hosts the Broken Hill orebody; (Page et al., 2005);
Sundown Group (Suite 5), 5 to 1350 m thick, mainly pelitic to psammitic gneisses, all of which occur in tightly folded repetition. This group represents a major change in the Broken Hill Domain, from the underlying oxidised and albitised succession, to deposition of largely pelitic and psammitic units in a reducing environment. Maximum depositional ages of 1688±6 and 1672±7 Ma have been determined from different parts of the Inlier (Page et al., 2005);
Paragon Group (Suites 6 to 8), characterised by graphitic meta-sediments and is best developed in the lower grade northern sections of the Broken Hill Block. Maximum depositional ages of 1655±4 and 1657±4 Ma have been determined from different parts of the Inlier (Page et al., 2005).
The Broken Hill Group (Suite 4) is 1000 to 1500 m thick, and from its base, marks a significant stratigraphic change from the dominantly feldspathic meta-sediments of suites 1, 2 and 3, to the more pelitic meta-sediments, intercalated with basic gneisses, which characterise the upper half of the Willyama Supergroup, in which no further significant feldspathic gneisses/rocks occur. The Broken Hill Group also contains scattered zoned calc-silicates nodules. It has been sub-divided, from the base (Haydon and McConachy, 1987), into:
Allendale Meta-sediments, 0 to 1500 m, generally 500 m thick - thinly bedded pelitic, to psammo-pelitic/psammitic, to locally psammitic meta-sediments/gneisses with minor basic gneiss, quartz-gahnite rock, tourmaline-quartz rock and well the bedded calc-silicates of the 0.5 to 10 m thick Ettlewood Calc-silicate Member near the base of the unit, generally occuring as a well bedded quartz-Ca plagioclase±clinopyroxene±amphibole±epidote calc-silicate rock, containing high background Pb, Zn and W, and is interpreted as an original impure dolomitic limestone,
Purnamoota Subgroup, 600 m thick, which makes up the remainder of the Broken Hill Group, and is characterised by meta-sediments intercalated with basic gneiss (Fe rich, commonly with abundant garnet and pyroxene), felsic gneiss/rock and quartz-gahnite and quartz-garnet rocks 'lode horizons'. In the northern section of the Block the subgroup is well developed and can be readily sub-divided, while in the south these subdivisions are not as obvious. To the NW it is dominated by basic gneisses which apparently have well developed relict igneous textures. Across the thick developments of the Thackaringa Group granite gneisses around the Stephens Creek the subgroup is thin with poorly developed felsic and basic gneisses. It may be further sub-divided into,
Parnell Formation, 150 to 500 m thick - comprising mainly pelitic, to psammo-pelitic /psammitic metasediments, intercalated with massive to thinly layered basic gneiss which is Fe rich with abundant garnet or orthopyroxene. The basic gneisses are associated with lenticular units of quartz-plagioclase-biotite-garnet±K feldspar gneiss. Small occurrences of Broken Hill type Pb- Zn-Ag mineralisation and associated quartz-gahnite, garnet-quartz and banded iron formation 'lode horizon' are widespread, while tourmaline-quartz rocks and garnet-epidote-amphibole calc-silicates commonly host stratabound W mineralisation.
Freyers Meta-sediments, 50 to 190 m thick -composed of well bedded pelitic, to psammo-pelitic /psammitic metasediments, which are thinly and planar bedded, with rare graded bedding and sporadic calc-silicate nodules, and contain rare basic gneiss, quartz-gahnite and quartz-tourmaline rocks. In the mine area they are pelitic to psammitic, containing banded iron formation lenses and directly underlie the orebodies.
Hores Gneiss, 60 to 400 m thick - is similar to the felsic gneisses of the Parnell Formation, and is generally a quartz-plagioclase-biotite-garnet±K feldspar gneiss. It contains variable proportions of intercalated meta-sediments and is typically associated with minor amphibolite, tourmaline-quartz rock (usually anomalous in W) and rare quartz-gahnite rock. At Broken Hill it is unusually rich in meta-sediments, and consists mainly of pelitic meta-sediments/gneisses intercalated with lenticular, elongate bodies of quartz-feldspar-biotite-garnet gneiss, thin (few cm's to a few metres thick) banded iron formations and the main Broken Hill orebodies.
Silver King Formation, <1 to 50 m thick - the Hores Gneiss passes laterally and upwards into the Silver King Formation with an increasing proportion of basic gneiss and meta-sediments. It occurs mainly in the NW of the Broken Hill Block where massive, extensive and conformable basic gneisses predominate, and rarely in the central section of the Block. It comprises mainly pelitic to psammo-pelitic/psammitic meta-sediments and abundant amphibolite, with subordinate lenticular bodies of quartz-feldspar-biotite±garnet gneiss, in places with tourmaline-quartz rock and minor quartz-gahnite rock. The Silver King mineralisation is hosted by this unit. The top of the formation is marked by the abrupt termination of basic gneiss by the meta-sediments of the Sundown Group.
In the vicinity of the Broken Hill mine the Broken Hill Group has been broken into units 4.1 to 4.8, where 4.1 corresponds to the lower sections of the Allendale Meta-sediments; 4.2 the Ettlewood Calc-silicate Member which is largely absent in the mine area; 4.3 averages 80 m in thickness and is a coarsely garnetiferous gneisses; 4.4 a suite of mafic, hornblende rich basal amphibolite which grade upwards into felsic and garnetiferous variants and averages 25 m in thickness; 4.5 is composed of banded pelitic to psammitic meta-sediments, with massive blue-quartz rich psammites, interbanded with thick poorly to well banded pelites, become more common upwards,with minor zones of disseminated sphalerite, pyrrhotite, galena and chalcopyrite within the psammitic horizons towards the top; 4.6 the Freyers Meta-sediments, described above, contains local lenses of banded iron formation that are few cms to 2 m thick; Unit 4.7, the 60 to 400 m thick (averaging 150 m) Hores Gneiss, which contains all of the economic Broken Hill orebodies, is generally a psammitic gneiss, characterised by the presence of lode rocks, comprising garnet-quartzite, garnet sandstone, blue quartz-gahnite lode, lode pegmatite, calc-silicate lenses and sulphide orebodies; 4.8 corresponds to the Silver King Formation as described above.
More recently, the sequence within the broader Curnamona Province has been subjected to further mapping and reinterpretation, as follows (e.g., Stevens et al., 2008; Conor and Preiss 2008; and others, as summarised by Fitzherbert and Downes, 2015):
• Curnamona Group, which outcrops extensively 20 km or more to the NW and west of Broken Hill in the Olary Domain, which is predominantly in South Australia and was deposited between 1720 and 1710 Ma. It includes the Wiperaminga Subgroup interbedded albitic and migmatitic gneiss and local metaconglomerate, and the Ethiudna subgroup that is mostly psammitic, but also contains calc-silicate minerals. The latter contains basal basalt and quartzite, with the remainder dominantly albitic metasedimentary and quartzo-feldspathic to pelitic rocks. The Basso Suite within the group contains A-type meta-volcanic/granitic units that intrude and are intercalated within the Curnamona Group in the Olary Domain in South Australia, as well as mafic intrusive rocks. A depositional hiatus occurred in the Olary Domain after 1715 Ma, whilst sedimentation continued to the east in New South Wales in the Broken Hill Domain (Conor & Preiss 2008; Fitzherbert and Downes, 2015).
• Thorndale Composite Gneiss, including the Cleveland Migmatite low in the succession, as described above and occurs 5 to 10 km SE of Broken Hill. This suite temporally overlaps the upper Curnamona Group and is a lateral equivalent of the Rantyga Group;
• Rantyga Group, which was deposited between 1710 anD1705 Ma occurs along the southern margin of the Broken Hill Domain, and is not exposed in the immediate Broken Hill area, ~20 km to the NW. It includes the Cleveland Migmatite, Ednas Gneiss, Mulcula Formation and Farmcote Gneiss. These units comprise a sequence of quartz-albite-magnetite±hornblende gneisses, that are interpreted to be shallow marine, siliciclastic sedimentary rocks with an evaporitic component (Stevens et al., 2008), while the basic gneiss of the group represents tholeiitic lavas or subvolcanic intrusions (Stroud et al., 1983). Quartz-albite-K feldspar-magnetite gneiss from the basal Redan Gneiss may represent an altered metavolcanic rock with a protolith age of 1710±4 Ma (Stevens et al., 2008) and has been correlated with the uppermost units of the Curnamona Group in South Australia. The upper unit of the Rantyga Group, the Farmcote Gneiss (1705±5 Ma; Stevens et al., 2008), has been correlated with the Lady Brassey Formation at the base of the Thackaringa Group (Stevens and Corbett 1993). According to Fitzherbert and Downes (2015), the Rantyga Group overlaps the Thorndale Gneisses and the upper Curnamona Group, with the Redan Gneiss at the base of the latter being at least in part equivalent to the upper Ethiudna subgroup (Conor and Preiss, 2008).
• Thackaringa Group, which is exposed in the immediate Broken Hill area, where it includes the Lady Brassey, Cues and Himalaya formations. It is interpreted to comprise a sequence of variably albitic, psammite dominated metasedimentary rocks, derived from a succession of relatively sandy shallow water sediments, probably marine shelf for the Cues Formation, with a variable evaporitic or hypersaline component for the Lady Brassey and Himalaya formations (Conor and Preiss 2008).
• Broken Hill Group, which conformably overlies the Thackaringa Group. The transition from the metapsammite-dominated Thackaringa Group to the metapelite-dominated lower Broken Hill Group reflects a deepening during marine transgression (e.g. Parr and Plimer 1993; Conor and Preiss 2008). The Broken Hill Group is composed of siliciclastic, pelite-dominated rocks of the basal Allendale Metasediments, which are overlain by the Purnamoota Subgroup. The Purnamoota Subgroup is composed of two bands of garnet-bearing quartzo-feldspathic 'Potosi-type' gneiss, the lower Parnell Gneiss and upper Hores Gneiss, separated by pelitic rocks of the Freyers Metasediments. This group also hosts minor calc-silicate, garnet-rich banded iron formation, quartz-gahnite and quartz-garnet rock. It also includes basic gneiss sills, which are locally very abundant (e.g., the Annandale and Silver King Metadolerites of the Lady Louise Suite. Overall, the Broken Hill Group grades from a pelite-dominant base, with minor calc-silicate (e.g., the Ettelwood Calc-silicate Member within the Allendale Metasediments) to a sequence of interbedded pelitic and psammitic metasedimentary rocks at its top. Very minor stratabound base metal mineralisation has been found throughout the group with the main Broken Hill Pb-Zn-Ag lode system being hosted by metasedimentary rocks of the Hores Gneiss. High-precision Pb isotope data suggest the Broken Hill orebodies formed over a ~6 Ma period from ~1685 to ~1680 Ma, synchronous with and marginally postdating felsic volcanism (Parr et al., 2004).
• Saltbush Group, which is confined to the Olary Domain, marking the end of a 20 m.y. hiatus, and was disconformably deposited over the Ethiudna Subgroup of the Curnamona Group. It commenced with the Larry Macs Subgroup, which includes the calc-silicate-rich Bimba Formation that correlates with the Ettlewood Calc-silicate Member low in the Broken Hill Group. The upper Larry Macs Subgroup is occupied by graphitic metasiltstone of the Plumbago Formation, which is followed, in turn by the psammite-dominated Black Maria Formation and the pelitic Oonartra Creek Formation, both of which belong to the Raven Hill Subgroup. The Oonartra Creek Formation contains manganiferous banded quartz-magnetite-grunerite-garnet iron formation and quartz-garnetite rocks similar to the correlative Purnamoota Subgroup of the Broken Hill Domain (Conor and Preiss, 2008). The Lady Louise Suite amphibolites are found no higher than the Raven Hill Subgroup.
• Silver City Suite, which is an intrusive suite of granitoids, the oldest of which is the Alma Granite Gneiss (1704±Ma; Page et al., 2005), emplaced during the deposition of the Cues Formation, and taken to be temporally equivalent of the A-type granite gneisses also intruded the Farmcote Gneiss (Ashley et al., 1996; Pageet al., Gibson 2005). Most of the remaining Silver City suite granitic sills, the Georges Bore, Stephens Creek, Wondervale Well and Rasp Ridge granite gneisses were emplaced into the Broken Hill Group between 1695 and 1683 Ma, coincident with the depositional age range of the felsic volcanogenic 'Potosi-type' gneisses and the intrusion of voluminous mafic sills of the Lady Louise Suite (e.g., Page et al., 2005; Stevens et al., 2008). The cessation of syndepositional igneous activity at the top of the Broken Hill Group is coincident with the end of stratabound mineralisation in the region (Fitzherbert and Downes, 2015).
• Sundown Group, which followed the cessation of igneous activity, is as described above in the Broken Hill district.
• Paragon Group, which conformably overlies the Sundown Group, which commences with the Cartwrights Creek Metasediments, comprising a lower sequence of graphic siltstones and aluminous metapelitic rocks that includes the thinly bedded King-Gunnia Calcsilicate Member in its upper sections. These metasediments grade upward into the Bijerkerno Metasediments, composed of albitic, rippled and cross-laminated psammitic and interbedded phyllitic rocks, followed by the thinly bedded graphitic, pelitic to psammopelitic phyllite, with lesser fine-grained graphitic, felspathic psammite of the Dalnit Bore Metasediments.
• Strathearn Group, the stratigraphic equivalent of the Paragon Group in the Olary Domain, comprising a similar sequence of interlayered graphitic and aluminous pelite, divided into the Alconie Formation, which grades into albitic, psammitic rocks of the overlying Mooleulooloo Formation and pelitic schist of the uppermost Dayana Formation.
Three phases of deformation and metamorphism, D1, D2 and D3, have been recognised within the Broken Hill mine leases. The first two, D1 and D2, were accompanied by the growth of high grade metamorphic minerals of sillimanite grade, while D3 was accompanied by a retrograde metamorphic event leading to the growth of biotite, muscovite and chlorite. The following is drawn from Laing et al., (1978) and Marjoribanks et al., (1980).
• First Deformation, D1 - D1 is generally agreed to have taken place during granulite grade metamorphism. Within the mine area, the S0 and S1 surfaces are parallel, with S0 being determined from the main lithological layering, principally between psammitic and pelitic gneisses which exhibit apparent sedimentary structures and are interpreted as representing original shales and sandstones. The layering of these lithologies also parallels the gross lithological changes within the sequence. S1, which is a well developed schistosity, is the dominant mesoscopic fabric observed. It is defined by the orientation of planar aggregates of sillimanite and fibrolite and by the preferred orientation of sillimanite needles and biotite laths. Garnet and orthoclase porphyroblasts and the inclusions within them are also typically elongate within the schistosity. S1 is most strongly developed within the pelitic gneisses. Within granite gneisses, S1 is defined by metamorphic segregation foliation composed of regularly alternating quartzo-feldspathic and mafic laminae up to 1 cm thick. No F1 folds with S1 as an axial plane have been observed in the mine area, although rare F1 folds have been identified in the surrounding region.
In pelitic gneisses the elongation of sillimanite and fibrolite defines a prominent lineation within S1, while in granite gneiss it is marked by a streaking of quartzo-feldspathic and mafic minerals also within S1.
Detailed studies of preserved sedimentary structures within the meta-sediments (psammitic, psammo-pelitic and pelitic gneisses) has provided a large number of facing directions which with symmetry of lithological sequences suggest fold closures. From this it is apparent that the sequence has been overturned, and with the structural relationships observed, that the D1 deformation resulted in large scale recumbent folding producing nappes. White et al. (1995) suggested that sheath folding was at least a component of the earlier phase of deformation in the Broken Hill Domain and had produced the doubly plunging geometry of the Broken Hill orebodies. In the mine leases, the F1 fold axes are at a low angle, while the L1 lineament plunges to the SW at 30 to 50°. The transport direction of these nappes was from the SE. The host rocks within the mine leases was therefore located on the lower overturned plate of a D1 nappe that was thrust to the NW.
Gibson (2000) considered D1 to be an initially low-pressure, high-temperature extensional event and that metamorphism was driven by the high heat flow caused by crustal thinning and magmatic heating.
• Second Deformation, D2 - The second deformation was predominantly a folding event, responsible for the greater part of the present outcrop geometry and intense NE-SW grain of the region, and took place at granulite grade. The resultant structures range from tight to isoclinal folds in the southern Broken Hill Domain, to broad open warps in the north. They are recognised by their superposition on structures of the first and their relationship to the third phase. Second phase folds F2, are present at all scales and refold S0 and S1, generally being represented by tight to isoclinal folds in the mine leases. Where fully developed in pelitic rocks S2 is defined by folded and flattened aggregates of fibrolite and sillimanite and by the preferred orientation of new sillimanite and biotite. It is therefore similar to S1, although it is typically only developed (or recognised) within or adjacent to F2 hinges where it is oblique to the layering of S1. The principal interpreted F2 structures within the mine leases include the Hangingwall Synform to the NW, with a core of Unit 3.10 (granite gneiss) of Suite 3, the Thackaringa Group; the broader Broken Hill Synform to the SE with a core of Suite 3 metamorphics; and the intervening Broken Hill Antiform with a core of Suite 5, Sundown Group meta-sediments. These structures have folded an already overturned sequence. They have been deduced from the cleavage, repetition of lithologies and detailed facing observations, all of which have been obtained from surface mapping, underground openings and extensive drilling. The orebody is everywhere on the western limb of the Broken Hill Anticline.
Whilst the observations above appear to provide reasonable evidence of the fold closures, they do not always indicate the precise location of axial planes, particularly for the Broken Hill Antiform towards the closure of Unit 4.7 (the host to ore). The plunge of the antiform varies from around 35°SW in the southern leases, to 45°NE in the northern leases.
With the exception of a domain between the British and de Bavay Shear zones, mesoscopic F2 folds plunge to the SW and are parallel to the high grade lineation L1-2 which is developed within S1. This lineation is denoted L1-2 because over much of the mine leases the fold axes of F1 and F2 are parallel and consequently the lineations are parallel. These fold axes to the south of the British Shear, and from a little to the north of the de Bavay Shear, generally plunge to the SW at angles from the horizontal to 35°. Between the British and de Bavay shears the F2 folds plunge to the NE at varying angles. Over the same interval, L1 has also been folded with the S1 foliation and parallels the F2 axes, plunging to the NE. However within the axial plane cleavage S2, which dips steeply to the NW, the dominant mineral lineation is oblique to F2 fold axes and plunges to the SW and has been labelled L2. In some exposures, both the SW plunging L2 and the NE plunging L1 are observed. This is the only area where the composite L1-2 is differentiable into its two components, i.e., while L2 has a constant plunge to the SW, L1 is generally parallel, but where folded by F2 may be seen at an angle to L2.
Within the mine leases the three main D2 structures are tight to isoclinal folds. However, in their hinge zones, the Broken Hill and Hanging wall Synforms are relatively open, although their limbs are near parallel. Structural evidence suggests that both of these synforms are D2 structures, and consequently they must be separated by an antiform. There is a lack of symmetry within this interval and no antiformal closure is evident at surface. In drilling down dip from the orebodies, abundant, consistent sediment structure facings and an inverse repetition of the sequence indicates the position of the axis of the Broken Hill Antiform, although no flattening of S0 is obvious. Similarly while evidence suggests a closure immediately to the east of the orebody in the upper sections of the southern leases, lithological banding is everywhere steep to vertical. The asymmetry in the interval is also in part due to the presence of a set of D3 structures, particularly the Western Antiform and the Eastern Synform (described below). In this zone also, there is a major composite shear zone, referred to as the Main Shear, which may be traced from towards the south-western end of the southern leases to the British Shear zone in the NE.
The Main Shear zone is associated with what is known as the 'Belt of Attenuation', a zone of high strain characterised by the development of numerous tight mesoscopic folds and associated high grade axial plane schistosity. This high grade zone of schistosity was described by Gustafson et al., (1950) and according to Laing et al., (1978) is a D2 feature, superimposed on layer parallel S1, the orientation of which is still preserved in the Western Anticline. Overprinting of S3 on S2 of the 'Belt of Attenuation' may be observed in surface outcrop. Within this zone, both F2 and F3 folds are observable. Both generations are basically co-axial but have differently dipping axial planes. In the vicinity of the ore lenses the complementary F3 Synform (the Eastern Synform) is difficult to distinguish from the tight F2 folds and the Main Shear, both in the 'Belt of Attenuation'.
The 'Belt of Attenuation', as a steep generally tabular zone of high strain which affects the orebodies and the sequence for some distance above and below, the Broken Hill Group Unit 4.7, may have a large simple shear component and thus be responsible for the general failure of the rock units to define the hinge of the Broken Hill Antiform (Laing et al., 1978). It may therefore represent the sheared hinge of the tight isoclinal F2 Broken Hill Antiform. Gustafson et al., (1950) traced the 'Belt of Attenuation' from the southern leases to the de Bavay Shear.
Van der Heyden and Edgecombe (1990) state that within the central part of the orebody in the old Broken Hill South/MMM leases the Main Shear is a steeply NW dipping zone of mylonitic sericite schists up to 30 m wide, containing unsheared pods of meta-sediments and lode horizon rocks (including ore grade material). They also quote observations that the Main Shear has a west block up vertical movement of 200 to 300 m and an inferred sinistral displacement. Where the Main Shear diminishes another similar sub-parallel intersecting structure is developed immediately to the east. This is the Thompson Shear, which dips to the east at 65° and continues to the British Shear within the 'Belt of Attenuation'. The Thompson Shear is displaced across the British Shear where it continues to follow the orebody and the 'Belt of Attenuation'. Both the Main and Thompson Shear Zones strongly influence the orebody shape and position as discussed below. As such the 'Main Shear system' within the 'Belt of Attenuation' is not a single shear, but a zone of interconnecting structures. Gustafson et al., (1950) reported that .."the Main Shear occupies at least three planes offset en echelon between the Zinc Corporation and the British Fault". The Main Shear System also appears to represent composite D2 and D3 structures.
To the NE of the British Shear the 'Belt of Attenuation' diverges from the axial plane of the Broken Hill Antiform but is still coincident with the orebodies. Laing, et al., (1978) suggest that in this area the observed 'Belt of Attenuation' may be related to another subsidiary structure observed in between the British and de Bavay Shears which has absorbed the shear component rather than the Broken Hill Antiform. The folding in the 'Belt of Attenuation' between the British and de Bavay Shears is interpreted to be F2 without superposed F3 folds, as appears to be the case to the SW of the British Shear. These F2 folds are described by Carruthers (1965) as being of relatively small amplitude compared to the folds south of the British Shear, with the ore layers to the north exhibiting the form of crenulated west dipping limbs.
Gustafson et al., (1950) observed that .."The ore bodies occur only where the favourable rock is intersected by a steep tabular belt of intense plastic deformation known as the 'Belt of Attenuation'". This belt of attenuation appears to be of the order of 100 to 150 m in width, with the most intense deformation being on its eastern margin. It comprises a number of intersecting zones of more intense shearing, but also embraces an anomalous zone of tight F2 and F3 folds (see accompanying cross section).
O'Driscoll (1968) showed experimentally that a component of simple shear accounted for the geometry of orebody folds in the 'Belt of Attenuation', with the differences in shape between the northern and southern leases being a reflection of the angle between the dips of the folded body and the 'Belt of Attenuation'. In the southern leases the shape is consistent with shearing with a close but opposite dip to the deformed tabular body, while in the northern leases both had the same direction of dip. King and O'Driscoll (1953) suggested that the structures in the 'Belt of Attenuation' within the ZC-NBHC leases "... consist essentially of a main, somewhat overthrust, anticline and deep attenuated syncline with the [lead lode] ore occupying the crest and flanks of the anticline and the limbs of the syncline. This basic structure appears to be common to all mines of the field, the intense lead-zinc mineralisation being largely confined to the more severely deformed beds beneath the sillimanite gneiss which marks the No. 2 lens overwall".
• Third Deformation, D3 - The third deformation is characterised by the onset of widespread retrogression and shearing developed across the domain in equal intensity (Stevens, 1986). Axial planes of the regional F2 folds dip to the northwest at between 10° and 90°. This variation in dip of S2 is taken to reflect open regional F3 generation of post F2 folding. The mine area lies on the steep limb of a large monoclinal flexure, with beds and S2 surfaces decreasing in dip to the northwest into an extensive shallow dipping limb. Further north S2 steepens again into the long limb of the F3 fold, thus forming the Broken Hill Arch. This refolding of the major F2 folds is also evident underground in the mines area. Penetrative retrograde axial plane cleavage, S3 is developed in association with this deformation.
The principal F3 folds in the mines area are the Western Antiform and the adjacent Eastern Synform. King and O'Driscoll (1953) noted that the axes of all of the folds in the ore zone were not parallel and that tight synforms on the lower surface of the 'lead lodes' migrated progressively from the eastern to the western limb of the Western Antiform over a number of sections. Carruthers (1965) described these same structures as an en echelon pattern of minor folds developed on the main fold pattern. This has been explained by the former being F2 drag folds overprinted by the F3 Western Antiform.
It can be shown (Laing et al., 1978) that the Western Antiform and Eastern Synform have a constant sense of symmetry which is not relevant to the F2 Broken Hill Antiform. The F3 folds possess an axial plane schistosity defined by the crenulation of earlier high grade schistosity and by a new retrograde schistosity characterised by a vertical muscovite-chlorite±biotite schistosity (S3) that, at its highest grade, may also contain sillimanite. Drilling data show these structures not only affect the lode horizon but all of the underlying units, including the granite gneisses. These two structures are therefore restricted to the common limb of the F2 Broken Hill Antiform and the Hangingwall Synform, but are incongruent to those latter structures. They are interpreted to be F3 structures, closely associated with the discontinuous retrograde schist zone known as the Main Shear, which is developed largely within the initially F2 'Belt of Attenuation'.
These F3 folds are generally co-axial with the F2 structures, both structures plunging parallel to the high grade lineation and showing a steepening plunge when traced to the SW from the British, Shear ranging from the horizontal to around 35°. However there are local steepenings and plunge reversals over this interval, with angles of more than 45° to near 60° over limited intervals, and localised flattenings to horizontal and up to 25° to the NE in other areas. These reversals commonly correspond to zones of oblique cross shearing cutting the drag folds and are episodic.
In contrast to the generally co-axial F2 structures, the axial planes of the F3 folds are nearly vertical to SE dipping, so that they transect the NW dipping axial plane orientation of the F2 folds. As stated above the Eastern Synform is hard to distinguish in the vicinity of the ore lenses due to the superposition of both the F2 folds and the Main Shear retrograde zone.
The D3 deformation is also apparently responsible for the extensive retrograde ductile shear zones in the mine area, characterised by the generally north-south British and de Bavay which dip at 65° to the east, and the ENE-WSW Globe Vauxhall Shear zone which dips at 60° to 70° to the SE. The displacement on the British and de Bavay shears is apparently sinistral with east block up, while on the Globe Vauxhall it is also sinistral with SE block up.
The D3 retrograde shear zones were reactivated during the Delamerian event at around 500 Ma.
• Fourth Deformation, D4 - Although Stevens (1986) interpreted the Willyama Supergroup to have been cratonised prior to the 458 to 520 Ma Cambro-Ordovician Delamerian Orogeny, Webster (1996) considered that gentle refolding F4 of the basement occurred during this event producing dome and basin structures throughout the Broken Hill Domain and gently refolded the orebodies along a NNW-SSE axis. Stevens (1986) recognised D4 deformation predominantly in the Neoproterozoic Adelaidean cover rocks, as well as in the Palaeproterozoic basement in the form of resetting of isotopic systems, reactivation of Olarian shear zones and possibly the intrusion of dolerite dykes and plugs, and late zoned pegmatites. Webster (1996) also suggested D4 was associated with a second period of retrogression in the Willyama Supergroup. Dolerite dykes were intruded into mineralisation where they were altered and dismembered by plastic flow in the sulphide-silicate rocks, whilst some hydrothermal activity occured along faults and shears. This hydrothermal activity produced laminated fault and vein sets within the orebodies and the Thackaringa style of galena-siderite-quartz veins were formed within re-activated shears. High temperatures were generated within faults, locally recrystallising rhodonite and bustamite (Webster, 2000). On the basis of investigations in the Southern Cross area, to the north of Broken Hill, Wilson and Powell (2001) recognised that deformation became progressively localised into high strain zones in the Willyama Supergroup over time.
See Webster (2004) for a more recent and detailed description of the structural evolution of the Broken Hill deposit.
Mineralisation and Alteration
Ore occurs as both massive and disseminated sulphides comprising almost exclusively course sphalerite (martite) and galena with minor pyrrhotite, chalcopyrite and loellingite. Apart from a local pyrrhotite mass on one margin, there are very few other sulphides, nor is magnetite an important constituent of the ore. The gangue mineralogy includes quartz, calcite, garnet, calcium and manganese pyroxenes and pyroxenoids. There is a gross, but not well defined banding to the ore as defined by gangue and sulphides, although the principal foliation is evident in low grade mineralisation. The sulphides are generally medium to coarse grained, seldom <1 mm in crystal size. Gangue seldom forms distinct bands with the ore, generally being present as irregular zones and blocks within the higher grade ore. Large calcite or quartz veins with variable amounts of sulphide are found cutting the high grade ore.
The nine, folded, linear, elongate Broken Hill ore lodes are distributed over a strike length of 7.3 km, a 'stratigraphic thickness' of less than 150 m and down dip extent of 300 to 1000 m. The lodes include, from the structural footwall, upwards (see cross section above), the No.s 3, 2 and 1, the A, B and C lodes and their splits. The No. 3, 2 and 1 Lodes (the 'Lead Lodes') are characterised by massive, coarsely-crystalline, galena and sphalerite, with average grades of 8 to 16% Pb, 12 to 22% Zn and 50 to 300 g/t Ag in a gangue dominated by similarly coarsely-crystalline quartz, calcite and rhodonite. The structurally higher, or 'Zinc Lodes' contain a higher proportion of sphalerite are less massive, not as coarse, and have a predominantly quarz and garnet gangue, with grades of 3 to 4% Pb, 6 to 12% Zn and 30 to 35 g/t Ag. The 'Lead Lodes' are best developed and predominate in the northern sections of the line of lode, while the 'Zinc Lodes are more significant in the Southern Leases.
Each of the lodes overlaps the lode above and below along the direction of elongation. They are steeply dipping, but exhibit a shallowly doubly plunging 'coat hanger-like' structure extending below the surface to the north and south of the exposed apex. The orebodies are restricted to two adjacent tight 'drag folds' over the full length of the line of lode. Within these structures, the ore occupies the keel of an eastern synform, the crest of a western antiform, their common limb and a shear zone merging into the eastern limb of the synform. The elongation and plunge of the lodes and the alignment of their vertical en echelon development parallels the F2 and F3 fold axes, both locally and regionally, while the elongation of the lodes corresponds closely to the orientation of the L1-2 lineation and L1 where it is distinguishable from L2.
Grades and gangue minerals of the Broken Hill Lodes (after Haydon and McConachy 1987)
||Quartz, garnet, biotite.
||Gahnite, manganhedenbergite, clinozoisite, feldspar.
||Si, Mn, (Ca, Fe)|
||Manganhedenbergite, garnet, apatite, gahnite, damourite, rhodonite, calcite.
||Si, (Mn, Ca, Fe, P)|
||Quartz, rhodonite, manganhedenbergite.
||Garnet, calcite, cummingtonite, dannemorite, wollastonite.
||Si, Mn, Ca, (Fe)|
||Wollastonite, bustamite, manganhedenbergite, epidote.
||Si, Ca, (Mn, Fe)|
||Bustamite, manganhedenbergite, knebelite, quartz, garnet, fluorite, apatite.
||Ca, Mn, (Si, Fe, F, P)|
|3 Lens SW
||Si, F, (Mn, Fe)|
|3 Lens NE
||Mn, F, (Si, Fe)|
Within the Broken Hill orebody, markedly transgressive high grade ore extend upwards and downwards from the orebody. These developments are known locally as 'droppers', and carry up to 42% combined metal, although typically assaying around 15% Zn, 10% Pb, 80 g/t Ag. Droppers range from simple cusp features to major piercement structures up to 50 m or more in length. The droppers are often irregularly developed along strike, varying greatly in thickness between mine sections, generally with strike lengths of up to 120 m. Droppers may comprise up to 10 m thick of high grade ore on one section, and be a barren shear on the next 20 m spaced section. Examples are described in more detail in the Discussion below.
The orebodies origninally outcropped as a >1 km long by 30 to 40 m wide manganiferous gossan on a long narrow ridge formed by the resistant siliceous lode rocks which stood approximately 50 m above the surrounding gneisses. The gossan was characterised by coronadite, limonite, pyrolusite, psilomelane, quartz and garnet with minor cerussite, silver halides, pyromorphite and a little galena, that assayed from 60 to 900 g/t Ag and 10 to 15% Pb, although leaching of Zn was almost complete. Generally oxidation was complete to 100 m and in places below 200 m, while the outcropping zone of Fe and Mn oxides extended to an average depth of 75m.
Away from the main high grade lodes detailed above there are substantial tonnages of low grade mineralisation, examples of which are the Western Mineralisation (300 m down dip to the NNW of the main lodes in the Kintore-Delprat shafts interval) which surrounds a higher grade core of comprises 15 Mt @ 3.2% Zn, 2.5% Pb, 31 g/t Ag; the Centenary Mineralisation, (across the Globe Vauxhall Shear Zone to the NW of the Western Mineralisation) estimated to contain a higher grade core of 9 Mt @ 3.2% Zn, 2.1% Pb, 33 g/t Ag; and the White Leeds Mineralisation 1.5 km to the SW and up dip from the southern leases which contains 2.5 Mt @ 2% Zn, 2% Pb 30 g/t Ag. The Western and Centenary Mineralisation zones have dimensions of the order of 2000 x 250 m and 1400 x 300 m respectively, plunging generally parallel to the main orebodies up dip. Lower grade dispersed to strongly geochemically anomalous base metals are found between the higher grade zones along strike and up and down dip, as reflected by the regional distribution of the 'lode rocks'.
The Western Mineralisation occurs within a sequence of three stratabound sequences: i). a downdip extension of the garnet quartzite, composed of spessartine±rhodonite; ii). a hedenberg-rich unit equivalent to the B Lode and iii). a quartz-gahnite sequence equated with C Lode. The Centenary Mineralisation is located structurally below the Globe Vauxhall Shear which separates it from the Western Mineralisation, and may be stratigraphically equivalent. The Centenary Mineralisation appears mostly in blue quartz lode, with garnet and calc-silicate gangue in addition to the garnet quartzite. These low resources are higher grade cores within a lower grade mass of mineralisation estimated total ~100 Mt of >3% Pb+Zn mineralisation.
Away from the main orebody, 'lode rocks' with small massive sulphide accumulations and anomalous sulphide mineralisation has been traced over a cumulative strike length of 300 km. Within this mineralised trace, there are a number of other smaller zones of higher grade mineralisation, including the Potosi and Pinnacles and Silver Peak prospects and deposits. Potosi and Silver Peak are approximately 2 km north-east of the North Mine.
The Potosi deposit, which is also hosted by the Hores Gneiss, has been exploited as an open pit and occurs on the opposite limb of the 'Hanging Wall Synform' to the Broken Hill deposit which is to the south-west. The 'Hanging Wall Synform'is cut by two major sub-parallel structures, the Globe-Vauxhall and Western shear zones, and occurs between these two structures and are limited by the Potosi shear zone to the south-west. Mineralisation is dominantly coarse black, iron-rich sphalerite (marmatite) with lesser galena, chalcopyrite and pyrrhotite in a gangue of gahnite, garnet and blue-quartz. It occurs in four main ore types, i). stringers to sub-massive marmatite with 3 to 20% Zn; ii). 4 to 5 m thick zones of massive marmatite with a 'pebbly' durchbewegung texture, generally containing 30% Pb+Zn; iii). varying amounts of disseminated galena-chalcopyrite with fine veinlets and stringers of marmatite with lesser galena and chalcopyrite in a blue quartz, garnet and gahnite gangue, typically with 7% Pb+Zn; and iv). stringer marmatite in a blue quartz psammite, averaging 10% Pb+Zn. To the SW the mineralisation occupies a synformal structure, while to the NE it becomes a west dipping flexure as it approaches the Western Shear. In 1996, prior to mining the proved + probable reserve quoted by Pasminco was 1.0 Mt @ 2.3% Pb, 9.1% Zn, 28 g/t Ag in a 400 x 200 by 90 m deep pit outline. The indicated + inferred resource outside of the pit amounted to 1.8 Mt @ 3.7% Pb, 13.7% Zn, 45 g/t Ag, 0.28% Cu.
The Pinnacles Mine, some 17 km to the south-west of Broken Hill comprises 0.15 Mt @ 2.5% Zn, 11% Pb, 500 g/t Ag which has been mined from one of six generally 2 to 4 m (locally up to 15 m) thick concordant lodes distributed over a 150 m stratigraphic thickness. The lodes are siliceous (quartzites) with garnet, gahnite, hedenbergite, galena and sphalerite with minor pyrrhotite, chalcopyrite, arsenopyrite and pyrite within or adjacent to a quartz-magnetite rock. The mineralisation occurs in a strongly folded interval between two retrograde schist zones. The host has prominent quartz-magnetite and fine sodic plagioclase-quartz rock components. This lode zone and the other similar occurrences within the Willyama Inlier, is within the Suite 3 Thackaringa Group, which lies below the Broken Hill Group that hosts the main Broken Hill lodes.
Discussion - Distribution and Controls of Mineralisation
The orebodies at Broken Hill are restricted to Unit 4.7 of the Suite 4 Broken Hill Group within the Willyama Supergroup. The mineable ore deposit occupies eight separate stratigraphic positions within the 7.3 km of continuous strike length over which the orebody is developed. Ore lenses within these eight stratigraphic positions virtually completely overlap vertically, parallel to their elongation, within a stratigraphic interval of approximately 150 m. However while these ore lenses show a close vertical correspondence within the 'Belt of Attenuation', their strike extent from NE to SW results in a progressive offset with the stratigraphically lowest 'C' Lode being restricted to the southern leases, as are the other 'zinc lode' bodies, 'B' and 'A' Lodes and No. 1 Lens. The southern limit of No. 2 Lens is north of the southern termination of the 'zinc lodes, while the corresponding end of the stratigraphically higher No. 3 Lens is further NE again.
While the original pre-erosion 'lead lodes' appear to have been continuously developed, the 'zinc lodes' are present as several lenses separated by lower grade intervals.
The individual ore lenses are markedly elongated. The width of the 'zinc lode' lenses is generally of the order of 250 to 300 m in contrast to a strike length of 2.5 to 3 km, while the 'lead lodes' are from 500 to 600 m wide with a strike length of 6 to 6.5 km. The exception is the No. 2 lens where it is markedly attenuated within the Main Shear in the ZC leases and consequently is up to 1200 m across.
All of the high grade economic mineralisation appears to be located within the 'Belt of Attenuation' as described by Gustafson, et al., (1950) and discussed above in the 'Structure' segment. Carruthers (1965) stated .."That there is a remarkable association of ore and folds is not questioned". He went on to argue however that the folds were a result of the location of the ore rather than the converse. However as can be seen from the plan above and the discussion in the 'Structure' segment above, these structures involve a far broader stratigraphic interval than the immediate orebodies, which tends to counteract the latter argument.
As detailed above, the elongation of the ore lenses and the alignment of their vertical en echelon development lies within the 'Belt of Attenuation' and parallels the F2 and F3 fold axes, both locally and regionally. In addition the plunge of the orebodies and their elongation corresponds closely to the orientation of the L1-2 lineation and L1 where it is distinguishable from L2.
Empirically ore is restricted to 'Belt of Attenuation' which to date has only been found on the common limb between the tight Broken Hill Antiform and the adjacent Hangingwall Synform. This zone of attenuation may largely be related to the thrust core of the Broken Hill Antiform. Within the 'Belt of Attenuation' ore is localised within the following structural positions,
• The attenuated and sheared out core of a synformal dragfold structure and the adjacent limbs, but generally progressively thinning and lensing out away from the nose.
• The crest and common limb of an adjacent antiform, again thinning and fading out progressively away from the crest within less than 100 m. An exception is the 'Western A Lode' which re-develops on the western limb of the Western Anticline drag fold anticlinal limb adjacent to, and terminated by, the Termination Shear.
• As thin, but high grade vein like massive developments within the 'Main Shear System' shear zone adjacent to where it cuts the ore bearing band. This style of mineralisation is most obvious in the old Broken Hill South and MMM leases where thin planar bodies of ore follow the Main and Thompson Shears. This Main Shear ore passes into the old ZC (Southern) leases where it merges into the ore of the attenuated eastern limb of the Eastern Synform.
The plunge reversals along the Western Antiform-Eastern Synform described in the 'Structure' segment above, appear to closely coincide with the margins of ore lens. For instance the first appearance of the main 'zinc lodes' occurs immediately to the south of an abrupt local steepening of the plunge of the Eastern Synform, while the main No. 3 Lens terminates immediately north of a plunge reversal interval. Low grade intervals within the 'zinc lodes' also correspond to intervals where the plunge reversals offset the overall detailed trend of the ore zone. These plunge reversals correspond to oblique cross shears.
However as Carruthers (1965) pointed out there is a very strong stratigraphic control on the ore, with individual ore lenses faithfully following a single stratigraphic position over the whole length of the line of lode. In addition each of the lodes/lenses occupying a given stratigraphic position has a unique suite of gangue minerals and Zn:Pb ratio. The proportion of gangue minerals and metal ratios however may vary progressively within a given ore lens.
Haydon and McConachy (1987) have shown that the ore is contained within predominantly psammitic beds of Unit 4.7 of Suite 4, the Broken Hill Group, with lower grade mineralisation in internal and adjacent pelitic gneisses. This imposes the close stratigraphic control on the ore lenses that is observed, with the different psammite bands having different gangue and ore mineral assemblages.
Away from the structural positions detailed above there are substantial tonnages of low grade mineralisation, examples of which are the Western and Centenary and White Leeds mineralisation listed above. Lower grade dispersed to strongly geochemically anomalous base metals are found between the higher grade zones along strike and up and down dip, as reflected by the regional distribution of the 'lode rocks'.
Weakly mineralised surface exposures are evident for several km to both the NE and SW of the economic lodes at Broken Hill. This mineralisation consists of stratigraphically controlled layers of quartzite and siliceous gneiss containing persistent traces of Pb and Zn with some or all of the minerals pyrrhotite, manganese garnet, blue quartz, gahnite, green K feldspar and rhodonite (Carruthers 1965).
A significant number of small, concordant, high grade 'Broken Hill' like occurrences are distributed throughout the Broken Hill Block, confined largely to the Purnamoota Subgroup of the Broken Hill Group, while lode rocks similar to those described along strike from Broken Hill are also widespread, locally extending semi-continuously over strike lengths of 1 to 20 km and embracing these small high grade occurrences (Carruthers 1965). Johnson and Gow (1975) state that the 'lode horizon' is known over a cumulative strike length of 300 km, and commonly outcrops over widths of 1 to 200 m. This includes the lode rock developments along strike from the Broken Hill mines and the other areas away from the mine mentioned above. While a number of the concordant high grade sulphide masses have been exploited in the past, they generally only individually totalled a few hundred tonnes of ore. All of these occurrences lie within the southern and eastern two thirds of the exposed Willyama Inlier.
One of the other more significant styles of mineralisation within the district is known as the Thackaringa type and comprises transgressive galena-siderite veins developed within D3 retrograde shear zones. The largest example was the Umberumberka mine at Silverton which produced 0.04 Mt @ 20 to 30% Pb, 2000 g/t Ag.
At a number of localities mineralisation is associated with quartz-magnetite units, e.g., the Pinnacles Mine as described above.
Within the Broken Hill orebody markedly transgressive high grade ore extend upwards and downwards from the orebody. These comprise the 'droppers' described above. One of the best developed is the Zinc Lode Dropper which occurs immediately to the west of the keel synform in 'B' Lode and extends upwards to cut 'C' Lode and downwards through 'A' Lode. It has a continuous strike length of 1.6km, is 15m thick at its broadest, but may be as thin as 0.1m, with the thickness varying irregularly giving the appearance of pinch and swell structure. It is generally found in close proximity to the hinge zone of a major fold and in many cases occupies the axial plane of the Western Antiform. Stockfeld (1992) records that the Zinc Dropper is represented by a variety of rock types, ore textures and mineral assemblages which differentiate it from the main lode horizon. The most common style of mineralisation is a high grade band of ore which generally consists of rounded quartz clasts contained in a matrix of stained sulphide, at times resembling pebbly ore. Associated large very coarse grained quartz-sulphide veins are sporadically developed, comprising very coarsely crystalline white quartz, with lesser galena, sphalerite, pyrrhotite and chalcopyrite which are also coarsely crystalline. Maiden et al., (1986) describes the 'lead lode' droppers as comprising fragments of wall rock and silicate rich ore (if competent) set in a matrix of recrystallised sulphides, yielding grades enriched in Ag relative to the normal 'lead lodes'. Within the 'lead lode' droppers there is commonly a foliation developed parallel to the dropper margins. Sulphide bearing quartz veins similar to those described by Stockfeld, are developed from the extremities of the 'lead lode' droppers (Maiden et al., 1986). Droppers, smaller piercement massive sulphide 'veins' and cusps are commonly developed within more competent wall rocks adjacent to massive sulphides. These transgressive features are interpreted as being late stage (D3) structurally induced, chemically and physically remobilised sulphides. The ore within the droppers exhibits "Durchbewegung" textures.
O'Driscoll (1953) reports that the percentage of sulphides is particularly high in the centres of the large masses of sulphide within the main 'lead lodes', where ore assaying around 25% Pb, together with up to 20% Zn is commonly found. In such places the structure, elsewhere interpreted by calcitic banding, is obliterated or is revealed only by highly contorted unconnected remnants, and at times a brecciated texture with clasts of other more competent ore/gangue types, such as 1 to 2 cm rhodonite fragments. This implies substantial recrystallisation and remobilisation of ore within the main lodes as well as in the droppers described above.
Another of the common features of high grade 'lead lode' mineralisation is for the margins of the ore to be defined by narrow shears marking an abrupt change over <1 m from high grade ore to completely barren gneiss. This implies differential shear movement between rock types of different competence during deformation and/or mechanical mobilisation of ore relative to the embracing country rock.
On the north-eastern end of the mine the orebody plunges to the NE between the east dipping British and de Bavay shears, but also approaches the Globe Vauxhall Shear zone obliquely. It encounters the latter before reaching the de Bavay Shear, and is displaced by the barren shear zone some 200 m downward and 500 m to the north. The orebody has been located again, occurring as a smaller pod of high grade ore known as the Fitzpatrick Area, which is sandwiched between the main Globe Vauxhall Retrograde Shear and the parallel Western Shear 200 to 300 m to the NW. This pod occupies the tight synformal keel zone of the Eastern Synform between the two shear zones and comprised 4.8 Mt @ 9.5% Zn, 11.5% Pb, 180 g/t Ag (Leyh and Hinde 1990).
Origin - Large and Crawford (2010) note that the Broken Hill Pb-Zn-Ag deposits are hosted by high metamorphic grade arkosic sediments within a regional sequence that varies from quartzo-feldspathic to pelitic dominant. Tholeiitic metagabbroic sills form a significant component of the regional host package. In the immediate deposit area, the meta-arkosic sediments are accompanied by amphibolite, felsic gneiss, minor units of BIF and quartz-garnet-gahnite lithologies. They note that the deposit has a pronounced alteration halo that extends over at least 3 to 4 km along strike beyond the line of lode, and may be up to 500 m thick, defined by garnet spotting and increasing enrichment in K-Rb-Fe-Mn-Pb, and depletion of Na-Ca-Mg-Sr, towards mineralisation. Early interpretations favoured syn-kinematic or post-kinematic, often replacement-related origin for the mineralisation, in which ore fluids were closely linked with the granulite to amphibolite facies metamorphism and deformation. However, subsequent studies indicated that both the ores and their surrounding pronounced stratabound alteration halo existed prior to peak metamorphism, and that mineralisation was present some 80 m.y. before the major deformation event. Since then, models evolved from an essentially syn-sedimentary exhalative process (SEDEX), to a dominantly inhalative processes just below the seafloor, forming the stacked ore lenses over a 5 to 6 m.y. period, synchronous with or shortly after deposition of the host sediments (cf., Laisvall in Sweden). Neither of these models is consistent with the restricted range of δ34S isotopic values from the Broken Hill lode sulphides, which cluster around 0, and magmatic S/Se values. These values are homogeneous, and totally mantle-like, with minimal evidence for seawater-derived S, suggesting an igneous source for the S in the deposit.
A detailed study of the trace element and isotopic geochemistry of the extensive mafic sills in the Broken Hill Block (Crawford and Maas, 2009) has resulted in the development of an alternative model. Crawford and Maas (2009) argue that the Broken Hill Pb-Zn ore metals and S were ultimately derived from very strongly fractionated rift ferrotholeiite magmas emplaced along a major detachment around 1685 Ma that was associated with the extension and commencement of breakup of the Nuna-Columbia super-continent. They propose that highly evolved Fe-rich melts were forced out of intercumulus sites in gabbros up the detachment by synkinematic fractionation, to crystallise as very Fe-rich oxide gabbros. They also state that fractional crystallisation very effectively separates Cu from Pb and Zn, since Cu partitions strongly into a magmatic vapour phase from the early stages of fractionation, whereas lithophile Pb and Zn remain in the magma as fractionation proceeds. They suggest late magmatic fluid evolution from these Fe-rich magmas which transported large amounts of Pb and Zn via a saline-rich hydrothermal fluid that ultimately deposited those metals as massive and/or disseminated sulphide lenses beneath the seafloor, higher in the section along the trace of the extensional shear system. The abundant Mn, a characteristic of the Broken Hill ores and alteration halo, is ascribed to alteration of ilmenite (typically 0.5 to 2% MnO), a major phase in oxide gabbros. The major Thorndale gravity anomaly, centred east of, and bordered on its western margin by the Broken Hill Line of Lode, probably reflects the presence of these ferrogabbros on a major east-dipping crustal structure, the outcrop of which may define the line of lode. This model does not require the involvement of seawater-derived fluids, nor of metal extraction from the metasedimentary pile. New Pb isotope data for the metagabbroic sills (amphibolites) in the Broken Hill Block (Crawford and Maas, 2009) strongly supports this model. (The preceding paragraphs are paraphrased from Large and Crawford, 2010).
In addition, however, the structural relationships within the lode suggest structural remobilisation of ore, while Frost et al., 2011, observe that the massive sulphides exhibit strong evidence of melting, and suggest that during peak metamorphism, at temperatures of >750°C and 5 kbars pressure, the main Broken Hill orebody was partially molten, with much of the Pb, Cu, Ag, Sb and As in the melt, and the Fe and Zn residing in the restite. They also suggest that the main sulphide melt froze after the last penetrative deformation at temperatures of <720°, with a geometry and distribution determined by the structural dynamics during peak metamorphism.
History - The main Broken Hill deposit was recognised by Charles Rasp in 1883 as a gossan outcrop, following the discovery in 1882 of high grade Ag and Pb-Ag at Thackaringa and Silverton 20 to 30 km to the west. Charles Sturt had first collected samples from the Broken Hill ridge in 1844, but abandoned them when in danger of perishing in the desert later in the expedition. Mining has been continuous from 1883 to the present. Early mining was concentrated on the exposed gossans of the central hump of the line of lode by the company that became the Broken Hill Proprietry Company Limited (BHP). The northern down-plunge limb of the deposit was worked by a series of mines, which in 1885 were consolidated into the North Mining Company, later to become North Broken Hill Ltd (NBH). In 1885, Broken Hill South Limited (BHS) acquired leases adjacent to the BHP mines, and by 1940 had purchased the BHP leases to control the central part of the line of lode, the Central Leases, between the NBH and the Southern Leases. BHP had mined 13.43 Mt @ 12.5% Pb, 470 g/t Ag, 5.0% Zn before their departure from the field in 1940. The other early mines acquired by BHS in the Central Leases are estimated to have mined around 13.21 Mt of high grade ore.
Zinc Corporation Limited commenced mining on the Southern Leases over the southern down-plunge extensions of the line of lode in 1911. The associated New Broken Hill Consolidated (NBHC) mine started operations in 1937 on the continuation of the southern limb of the line of lode immediately to the south of the Zinc Corporation leases. A third shaft complex, the Southern Cross mine, was established as part of the ZC-NBHC operation as the southern continuation of the NBHC mine in the 1970's.
The four mines, NBH, BHS, ZC and NBHC, worked the line of lode on a large scale until BHS ceased operation in 1972. The Central Leases of BHS were acquired by Minerals, Mining and Metallurgy Limited (MMM) in 1972, who commenced an open pit to extract lower grade ore, remnants and mineralised dumps until 1990. Until it ceased operations, BHS had extracted 19.77 Mt @ 12.8% Pb, 170 g/t Ag, 7.7% Zn. By 1990, the total production on the Central Leases, including that of BHP, BHS, MMM and other early mines in the Central Leases had totalled 51.9 Mt of ore. The Central leases were taken acquired from MMM by Consolidated Broken Hill Limited but closed in mid-2008. Operations were restarted as the new CBH Resources Limited 0.65 Mtpa Rasp mine in 2012. Following a takeover in late 2010, CBH Resources Limited became a wholly owned subsidiary of Toho Zinc Co., Ltd, a Japanese company listed on the Tokyo Stock Exchange.
In 1988, the operations of NBH, and the ZC-NBHC mines of ZC Mines Pty Ltd were amalgamated, with other operations of the same companies to form Pasminco Mining Ltd. Until its closure in 1993, the North Mine, operated initially by NBH and later by Pasminco, had mined approximately 34 Mt @ 14% Pb, 230 g/t Ag, 11.5% Zn to a depth of 1700 m. By 1987, just prior to the formation of Pasminco the Southern Mines of ZC-NBHC had produced more than 73.5 Mt @ 11.1% Zn, 10.2% Pb, 78 g/t Ag.
In late 2001, Pasminco was liquidated and the company's Broken Hill operations were purchased by Perilya in early 2002 who have continued the operation until the present (2013). On 19 December 2013 Perilya Limited was 100% acquired by Shenzhen Zhongjin Lingnan Nonfemet Company Limited and delisted from the ASX. Perilya Limited continues operations at Broken Hill as a wholly owned subsidiary of Zhongjin Lingnan.
The 3.8 km long Central Leases held by CBH Resources Ltd contain the Kintore open pit mine developed on the main upper sections of the line of lode, and the down dip 'Western Mineralisation' zone that was too low grade to have been exploited when held by BHS.
The original pre-mining ore deposit (as quoted by Morland and Webster, 1998) consisted of 185 Mt of mineable ore, including 150 Mt @ >20% combined Pb+Zn. It has been estimated that approximately 60 Mt of ore was eroded from the original ore deposit. In addition there are ~100 Mt of >3% Pb+Zn mineralisation.
In late 1997 proven + probable reserves totalled 33.5 Mt @ 8.2% Zn, 5.1% Pb, 51 g/t Ag.
In mid 2007 the reserves and resources quoted by Perilya (2007) for the Northern and Southern Leases sections of their operation were:
Southern Operation - measured + indicated + inferred resource: 11.47 Mt @ 9.9% Zn, 7.6% Pb, 73 g/t Ag,
proved + probable reserve: 11.198 Mt @ 6.7% Zn, 4.9% Pb, 49 g/t Ag,
North Mine - measured + indicated + inferred resource: 0.165 Mt @ 6.6% Zn, 6.5% Pb, 108 g/t Ag,
proved + probable reserve: 0.046 Mt @ 4.5% Zn, 4.0% Pb, 87 g/t Ag,
North Mine Deeps - measured + indicated + inferred resource: 3.7 Mt @ 11.3% Zn, 13.5% Pb, 219 g/t Ag,
proved + probable reserve: 0.046 Mt @ 4.5% Zn, 4.0% Pb, 87 g/t Ag,
Potosi North - indicated + inferred resource: 0.304 Mt @ 12.7% Zn, 7.2% Pb, 64 g/t Ag,
Potosi Extended - inferred resource: 1.729 Mt @ 12.6% Zn, 2.9% Pb, 45 g/t Ag,
Flying Doctor - inferred resource: 0.52 Mt @ 5.4% Zn, 7.1% Pb, 72 g/t Ag,
Silver Peak - inferred resource: 0.335 Mt @ 5.9% Zn, 11.1% Pb, 91 g/t Ag,
Total Resource - 18.223 Mt @ 10.2% Zn, 8.4% Pb, 101 g/t Ag,
Total Reserve - 11.224 Mt @ 6.7% Zn, 4.9% Pb, 50 g/t Ag.
In mid 2007 the remaining resources of the Central Leases to a depth of 600 m were quoted by CBH Resources (2007) as:
10.1 Mt @ 4.9% Zn, 3.5% Pb, 43 g/t Ag.
The last JORC compliant publicly announced ore reserve and mineral resource figures for all Broken Hill operations, as at 30 June, 2012, released by Perilya Limited prior to the Zhongjin Lingnan takeover and delisting in 2013, were (Perilya ASX release, December 2012):
Southern Operation - measured + indicated + inferred resource: 11.7 Mt @ 9.3% Zn, 6.8% Pb, 72 g/t Ag,
proved + probable reserve: 11.7 Mt @ 6.2% Zn, 4.8% Pb, 50 g/t Ag,
North Mine Uppers - measured + indicated + inferred resource: 1.00 Mt @ 7% Zn, 9% Pb, 140 g/t Ag,
North Mine Deeps - measured + indicated + inferred resource: 3.3 Mt @ 11.5% Zn, 13.8% Pb, 239 g/t Ag,
Potosi - inferred resource: 1.60 Mt @ 14% Zn, 3% Pb, 46 g/t Ag,
Silver Peak - inferred resource: 0.400 Mt @ 5% Zn, 9% Pb, 77 g/t Ag,
Central Blocks - inferred resource: 0.700 Mt @ 5% Zn, 4% Pb, 43 g/t Ag,
Flying Doctor - indicated + inferred resource: 1.50 Mt @ 3% Zn, 4% Pb, 44 g/t Ag,
Henry George - inferred resource: 1.30 Mt @ 8% Zn, 1% Pb, 14 g/t Ag,
1130 - inferred resource: 0.20 Mt @ 12% Zn, 1% Pb, 7 g/t Ag,
Total Resource - 21.700 Mt @ 9.2% Zn, 7.0% Pb, 89 g/t Ag,
Total Reserve - 11.700 Mt @ 6.2% Zn, 4.8% Pb, 50 g/t Ag.
Olympic Dam - IOCG style copper-gold-uranium ...................... Monday 22 September, 2008.
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The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published. Some of these sources are listed on the "Tour Literature Collection" available from the Australia 2008 Tour options page.
The Olympic Dam copper-gold-uranium-REE ore deposit is located some 550 km NNW of Adelaide and 275 km NNW of Port Augusta, in northern South Australia (#Location: 30° 26' 24"S, 136° 53' 22"E).
Olympic Dam and all of the other significant known IOCG mineralised systems of the Mesoarchaean to Mesoproterozoic Gawler Craton are hosted within Palaeo- to Mesoproterozoic rocks, and are distributed along the eastern rim of the currently preserved craton to define the Olympic IOCG Province (Skirrow et al., 2007). Olympic Dam lies below the Neoproterozoic Stuart Shelf, where >300 m of flat lying, barren, Neoproterozoic to lower Palaeozoic sedimentary rocks unconformably overlie both the craton and the ore deposit. Some 75 km to the east, this cover sequence expands over the major NNW trending Torrens Hinge Zone at the edge of the craton, into the thick succession of the north-south aligned Neoproterozoic Adelaide Geosyncline rift basin, that masks the mid- to late-Palaeoproterozoic suture between the Gawler craton and Palaeo- to Mesoproterozoic Curnamona Province to the east.
The oldest basement rocks in the Gawler craton are Meso- to Neoarchaean gneisses (to the west) and metasedimentary and meta-volcanosedimentary rocks, and deformed granites correlated with the Palaeoproterozoic 1.96 to 1.85 Ga Hutchison Group, the 1.79 to 1.74 Ga Wallaroo Group, and the 1.85 to 1.69 Ga Lincoln Complex (Donington Suite) granitoids, respectively. These rocks are intruded by the widespread Mesoproterozoic A- and I-type granitoids of the ~1.59 Ga Hiltaba Suite (with the former dominating in the Olympic IOCG Province) and are overlain by comagmatic bimodal volcanic rocks of the areally extensive Gawler Range Volcanics (GRV).
Mineralisation at Olympic Dam is hosted by the 50 km2 Olympic Dam Breccia Complex (ODBC) that is developed within the Mesoproterozoic (1600 to 1585 Ma) Roxby Downs Granite. The Roxby Downs Granite is a pink to red coloured, undeformed, unmetamorphosed, coarse to medium grained, quartz-poor syenogranite with A-type affinities that is petrologically and petrochemically similar to granitoids of the Hiltaba Suite. Other lithologies within the ODBC comprise a variety of granite- to hematite-rich breccias, sedimentary facies, felsic/mafic/ultramafic dykes, volcaniclastic units, basalts and their altered/mineralised equivalents. The ODBC and the surrounding Roxby Downs Granite form a local basement high on a broader regional basement uplift.
Within the overall alteration envelope, the distribution of mineralisation and alteration exhibits a downward and outward zonation, while the ODBC correspondingly comprises a downward narrowing, funnel-shaped body of fractured, brecciated and hydrothermally altered granite which has resulted in a great variety of granitic, hematitic and siliceous breccias. The complex has a conical, downward tapering, central "core" of barren, but intensely altered hematite-quartz-breccia, passing outwards through concentrically zoned breccia types, including heterolithic hematite breccias (with clasts dominantly of granite and recycled hematite breccias, and domains where abundant sedimentary and volcaniclastics rocks predominate locally), to monoclastic granite breccias with a magnetite/hematite matrix, to weak incipient microfracturing of the RDG on the outer margins. A halo of weakly altered and brecciated granite extends out approximately 5 to 7 km from the core in all directions to an indistinct and gradational margin with the host granite. This progression represents an outward decrease in the degree of brecciation and intensity of iron metasomatism away from the core of the complex. The quantity of recycled hematite breccia, GRV and sedimentary rock clasts within the heterolithic hematite breccias decreases from shallow to deep levels (Ehrig, 2010; McPhie et al., 2010). The areal extent of more intensely hematite altered breccias within the complex is >5 km in a NW-SE direction, up to 3 km across, and is known to extend to a depth of at least 1400 m.
The development of the ODBC, which shows textural evidence of polycyclic alteration and brecciation events, can be considered as having formed by the progressive hydrothermal brecciation and iron metasomatism of the host granite. In detail, alteration assemblages are highly variable with complex mineral distribution patterns resulting from the polycyclic nature of the hydrothermal activity. Never-the-less, there are systematic patterns of alteration that are recognised across the deposit as a whole, and at the scale of individual breccia zones, with the degree of alteration intensity being directly related to the amount of brecciation.
The bulk of the mineralisation within the Olympic Dam deposit is associated with an assemblage of hematite-sericite-fluorite-barite-chalcopyrite-bornite-chalcocite (djurleite), the outer margin of which largely corresponds to the limits of the ODBC, where a deeper magnetite-carbonate-chlorite-pyrite±chalcopyrite zone marks the transition to the more regional magnetite-K feldspar±actinolite±carbonate assemblage (Ehrig, 2010). No associated sodic metasomatism has been observed.
The better mineralisation and strongest alteration outside of the barren core corresponds to the best-developed hematite-granite breccias. The concentric, moderate to steeply inward dipping breccia zones of the ODBC are cut by a convoluted, but overall roughly horizontal, ~50 m thick layer characterised by chalcocite and bornite, ~100 to 200 m below the unconformity with the overlying Neoproterozoic cover sequence. Both the upper and lower margins of this zone are mappable. Above the upper margin, sulphides are rare and little copper mineralisation is found in the same hematitic breccias. The lower margin marks a rapid transition to chalcopyrite, which decreases in copper grade downwards, corresponding to an increase in the pyrite:chalcopyrite ratio. While this zone is largely horizontal, as it approaches the central barren core it steepens markedly, but is still evident at depths of >1 km below the Neoproterozoic unconformity (Reeve et al., 1990; Reynolds, 2000; Ehrig, 2010). The geometry of this mineral zonation, strongly suggest interaction between upwelling and downward percolating fluids. For all fluids related to hematite alteration, fluid inclusion homogenisation temperatures are mostly between 150 and 300°C and salinities range from ~1 to ~23% NaCl equiv. (Knutson et al., 1992;
Oreskes and Einaudi, 1992; Bastrakov et al., 2007).
The higher grade underground resource occurs as up to 150 separate bodies distributed within an annular zone up to 4 km in diameter surrounding the central barren hematite-quartz breccia. These bodies correspond to the overlap of the flat-lying chalcocite-bornite layer and the steeper, inwardly dipping ring of hematite-granite breccias.
The principal copper-bearing minerals are chalcopyrite, bornite, chalcocite (djurleite-digenite), which on the basis of Nd isotopic data, textural and geochemical features appear to have precipitated cogenetically. Minor native copper and other copper-bearing minerals are locally observed. The main uranium mineral is uraninite (pitchblende), with lesser coffinite and brannerite. Minor gold and silver is intimately associated with the copper sulphides. The main REE-bearing mineral is bastnaesite. Copper ore minerals occur as disseminated grains, veinlets and fragments within the breccia zones. Massive ore is rare.
At the end of 1989, after commencing mining operations in mid-1988, reported resources and reserves (Reeve et al., 1990) amounted to:
Measured + indicated resource = 450 Mt @ 2.5% Cu, 0.6 g/t Au, 6.0 g/t Ag, 0.8 kg/tonne U3O8,
Inferred resource = 2000 Mt @ 1.6% Cu, 0.6 g/t Au, 3.5 g/t Ag, 0.6 kg/tonne U3O8,
Proved reserve = 13 Mt @ 3.0% Cu, 0.3 g/t Au, 10.2 g/t Ag, 1.1 kg/tonne U3O8,
Proved gold reserve = 2.3 Mt @ 1.6% Cu, 3.6 g/t Au, 2.9 g/t Ag, 0.3 kg/tonne U3O8.
At December 2004, published (BHP Billiton, 2005) reserves and resources were:
Proved+probable reserves totalled 761 Mt @ 1.5% Cu, 0.5 g/t Au, 0.5 kg/tonne U3O8,
within a total resource of 3810 Mt @ 1.1% Cu, 0.5 g/t Au, 0.4 kg/tonne U3O8.
At 30 June 2012, the published resources (BHP Billiton, September, 2012) amounted to:
Measured resource = 1474 Mt @ 1.03% Cu, 0.35 g/t Au, 1.95 g/t Ag, 0.30 kg/tonne U3O8,
Indicated resource = 4843 Mt @ 0.84% Cu, 0.34 g/t Au, 1.50 g/t Ag, 0.27 kg/tonne U3O8,
Inferred resource = 3259 Mt @ 0.70% Cu, 0.25 g/t Au, 0.98 g/t Ag, 0.23 kg/tonne U3O8,
Total resource = 9576 Mt @ 0.82% Cu, 0.31 g/t Au, 1.39 g/t Ag, 0.26 kg/tonne U3O8.
This resource includes a total proved + probable reserve of:
629 Mt @ 1.76% Cu, 0.73 g/t Au, 3.36 g/t Ag, 0.57 kg/tonne U3O8.
At the same date, the separate non-sulphide gold resource was 364 Mt @ 0.75 g/t Au, comprising:
Measured resource = 73 Mt @ 0.85 g/t Au; Indicated resource = 255 Mt @ 0.73 g/t Au; Inferred resource = 36 Mt @ 0.70 g/t Au.
At 30 June 2015, the published resources (BHP Billiton Annual Report, 2015) amounted to:
Measured resource = 1.330 Gt @ 0.96% Cu, 0.40 g/t Au, 2.0 g/t Ag, 0.29 kg/tonne U3O8,
Indicated resource = 4.610 Gt @ 0.79% Cu, 0.32 g/t Au, 1.0 g/t Ag, 0.24 kg/tonne U3O8,
Inferred resource = 4.120 Gt @ 0.71% Cu, 0.24 g/t Au, 1.0 g/t Ag, 0.25 kg/tonne U3O8,
Total resource = 10.060 Gt @ 0.78% Cu, 0.30 g/t Au, 1.0 g/t Ag, 0.25 kg/tonne U3O8.
This resource includes a total proved + probable reserve of:
484 Mt @ 1.95% Cu, 0.74 g/t Au, 4.0 g/t Ag, 0.59 kg/tonne U3O8.
Stockpile - 44 Mt @ 0.99% Cu, 0.51 g/t Au, 2.0 g/t Ag, 0.37 kg/tonne U3O8.
At 30 June 2015, a separate non-sulphide gold resource was 283 Mt @ 0.84 g/t Au, which was not reported in 2015.
At 30 June 2017, the published resources (BHP Annual Report, 2017) amounted to:
Measured resource = 1.460 Gt @ 0.96% Cu, 0.41 g/t Au, 2.0 g/t Ag, 0.30 kg/tonne U3O8,
Indicated resource = 4.680 Gt @ 0.79% Cu, 0.34 g/t Au, 1.0 g/t Ag, 0.25 kg/tonne U3O8,
Inferred resource = 3.920 Gt @ 0.71% Cu, 0.28 g/t Au, 1.0 g/t Ag, 0.24 kg/tonne U3O8,
Total resource = 10.100 Gt @ 0.78% Cu, 0.33 g/t Au, 1.0 g/t Ag, 0.25 kg/tonne U3O8.
This resource includes a total proved + probable reserve of:
508 Mt @ 1.99% Cu, 0.72 g/t Au, 4.0 g/t Ag, 0.58 kg/tonne U3O8.
Low grade Stockpile - 37 Mt @ 1.13% Cu, 0.51 g/t Au, 3.0 g/t Ag, 0.36 kg/tonne U3O8.
Production in 2011-12 totalled 192 600 tonnes of Cu, 3.66 t Au, 28.21 t Ag, 3885 tonnes U3O8.
Production in 2014-15 totalled 124 500 tonnes of Cu, 3.26 t Au, 22.52 t Ag, 3144 tonnes U3O8.
The mine is owned and operated by a subsidiary of BHP Billiton Ltd.
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