An International Study Tour to Order
Organised for ADIMB Brazil by Porter GeoConsultancy by Porter GeoConsultancy
ADIMB Australia 2008
Australian Gold & Base Metal Deposits
8 to 22 September 2008
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CONTENT and DESCRIPTIONS OF ORE DEPOSITS
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.
Click on image for details.
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.
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).
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St Ives - Gold ...................... Tuesday 9 September, 2008.
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.
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Cawse - Lateritic Nickel ...................... Wednesday 10 September, 2008.
The Cawse project area is located some 55 km to the north of Kalgoorlie in Western Australia (#Location: 30° 30'S, 121° 08'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.
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Kambalda (Juan-Otter & Miitel) - massive sulphide nickel-copper ...................... Thursday 11 September, 2008.
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 between the Tramway belt and Widgiemooltha Dome, some 40 km to the south, v). the Blair and Carnilya Hill mines at the Golden Ridge-Carnilya Hill Belt 40 km to the north. 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.
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Forrestania - massive and disseminated sulphide nickel ................ Friday 12 September, 2008.
The Forrestania group of Ni-Cu deposits are located approximately 400km 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.
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, 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 ulits. 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. Hangingwall 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 hangingwall, 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 mesocululate, 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 deined by a fault with the underlying BIF, while the upper margin is marked by a reduction in Ni grade to the backgound 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.
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Travelling from Southern Cross to Kalgoorlie to Perth to Brisbane to Mt Isa ...................... Saturday 13 and Sunday 14 September, 2008.
Ernest Henry - IOCG style copper-gold .................. Monday 15 September, 2008.
The Ernest Henry IOCG style Cu-Au deposit is located 35 km north-east of Cloncurry, 150 km east of Mt Isa and 750 km west of Townsville in north-west Queensland.
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 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 bounding shear zones and formed a pipe-like zone of dilation in the K-feldspathised metavolcanic rocks. 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 Na-Ca alteration, occurring mainly as albitic plagioclase-, magnetite-, clinopyroxene- and amphibole-rich veining and fault-related breccia-fill. ii). Pre-mineralisation 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. 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. iv). Post-ore multiple stage calcite-dolomite- and/or quartz-rich veining and alteration.
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 Xstrata Ltd.
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George Fisher - sediment hosted zinc-lead-silver ...................... Half Group on each of Tuesday 16 & Wednesday 17 September, 2008.
The George Fisher South (Hilton) and George Fisher North are located approximately 20 km north of the city of Mt Isa, in north-western Queensland, Australia and 975 km west of Townsville.
The George Fisher orebodies lie within the Mt Isa-McArthur basin system that are part of the Northern Australian Platform cover which is a 5 to 15 km thick volcano-sedimentary succession deposited during the period 1800 to 1580 Ma. Deposition took place in three super-basins which represent three nested cycles of deposition and exhumation, specifically the Leichhardt (1800 to 1875 Ma), Calvert (1735 to 1690 Ma) and Isa (1670 to 1580 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 the system, including George Fisher deposits, are hosted by the Isa super-basin.
Both of the George Fisher orebodies lie within the Middle Proterozoic (1653 Ma) pyritic, dolomitic siltstone of the approximately 1000 m thick Urquhart Shale, which is near the top of the Mt Isa Group. The Mount Isa Group was deposited within the Leichhardt River Fault Trough, and belongs to Cover Sequence 3 of the Isa super-basin in the Western Fold Belt of the Mt Isa Inlier.
The Western Fold Belt lies immediately to the east of the Lawn Hill Platform that embraces the Century deposit some 250 km to the NNW, hosted by the McNamara Group, also part of the Isa super-basin. The McNamara Group temporally overlaps the Mt Isa Group, but is generally younger than the Mt Isa Group. Both groups unconformably overlie the rift phase sediments and lesser volcanics of the Calvert super-basin.
Cover Sequence 3 platform and sag phase sediments of the Isa and Calvert super-basins unconformably overlies the thick mafic volcanics and quartzites of Cover Sequence 2 of the Leichhardt super-basin, which includes the 7 km thick Eastern Creek Volcanics.
In the George Fisher mine area, the Mt Isa Group strikes north-south and has a persistent moderate to steep westerly dip. It is around 4500 m in thickness and comprises a succession of siltstone and shale which are commonly dolomitic, calcareous or pyritic, with minor sandstone and conglomerate, mainly at the base, thickening to the east, and minor felsic tuff. The overall sequence is dated at approximately 1670 Ma.
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 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.
The George Fisher South (Hilton) deposits occur as 7 to 10 stacked ore lenses within a 250 m stratigraphic interval, which have been complicated by intense shortening associated with the Isa Orogeny. The George Fisher North deposit is present as 11 stacked lenses within a 350 m stratigraphic interval. Bedding-parallel, fine-grained pyrite is abundant, with from 10 to >50%, throughout the host stratigraphy, over a thickness of 800 m, enveloping both deposits and extending over a strike length of >10 km.
The Zn-Pb ores occur as individual stratabound, well banded sphalerite-galena-pyrite ore lenses separated into hangingwall and footwall orebody groups by a barren zone. Sub-economic mineralisation follows the strike of the Urquhart Shale for 4 km to the north and south of the economic orebodies.
Within the deposits there is a zonation such that the Zn/Pb ratio varies from 4 to 10 in the lowermost lenses to 1 to 4 in the uppermost ore lenses.
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 Xstrata Zinc.
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
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
Mine production in 2005 totalled 2.7 Mt @ 8.3% Zn, 5.0% Pb, 115 g/t Ag.
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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).
The 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.
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Travelling from Mt Isa to Brisbane to Sydney to Broken Hill ...................... evening of Wednesday 16 & Thursday 17 September, 2008.
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° 58'S, 141° 28'E).
The Broken Hill deposits lie within the Willyama block which extends from far western New South Wales into eastern South Australia and is composed of Paleo- 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 Willyama block is largely occupied by the metamorphics of the ~7 km thick, ~1720 to ~1640 Ma 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 a metamorphosed protolith 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 lower Willyama Supergroup consists of an oxidised and albitised succession including granitic gneisses, amphibolite sills, and psammitic, pelitic, and calc-silicate rocks. The upper succession is dominated by metapelites with graphitic layers deposited in a reducing environment.
Two periods of folding accompanied the prograde metamorphism in the Broken Hill and Euriowie Blocks, namely D1 which produced very large recumbent folds/nappes with a transport direction from the SE, and D2 which produced large folds with steeply dipping axial planes. Both D1 and D2 resulted in sillimanite grade axial plane schistosity. The peak metamorphism associated with these deformations appears to be at 1660±10 Ma. A third phase of deformation, D3, produced folds on all scales throughout the Broken Hill Block, generally with near vertical axial planes and narrow to broad zones of retrograde shearing, commonly parallel to the fold axes and usually coinciding with F3 anticlinal axial planes. Major D3 monoclines fold the sequence.
The D3 shear zones range from a few metres in width to several hundred metres, while in some cases they constitute complex retrograde belts several kms wide. All of these 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. These shear zones are generally aligned in a NE-SW or NW-SE direction, while some are E-W. They appear to have been generated early in the D3 phase, before 1570 Ma, but were reactivated during the Delamerian event at 500±20 Ma.
All of the economic mineralisation at Broken Hill is hosted by the Broken Hill Group (Suite 4), of the Willyama Supergroup, which is composed of the following, from the base, the:
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, 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 & 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.
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, 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.
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.
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 100m and in places below 200m, while the outcropping zone of Fe and Mn oxides extended to an average depth of 75m.
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.
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 & Maas, 2009) has resulted in the development of an alternative model. Crawford & 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 crystallize 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 E-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) 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.
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 (2008).
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 around 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.
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Travelling from Broken Hill to Adelaide & free weekend ...................... Saturday 20 & Sunday 21 September, 2008.
Olympic Dam - IOCG style copper-gold-uranium ...................... Monday 22 September, 2008.
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' 31"S, 136° 50' 9"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.
Production in 2011-12 totalled 192 600 tonnes of Cu, 3.66 t Au, 28.21 t Ag, 3885 tonnes U3O8.
The mine is owned and operated by a subsidiary of BHP Billiton Ltd.
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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.
This tour was organised by:
T M (Mike) Porter of Porter GeoConsultancy Pty Ltd on behalf, and to the specification, of the client.
Porter GeoConsultancy Pty Ltd|
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