DESCRIPTIONS OF ORE DEPOSITS, SEMINARS/WORKSHOPS, ETC.
This tour was undertaken in conjunction with ADIMB of Brazil and was divided into two modules, the first of 9 days covering "Base Metals & Gold in the Proterozoic and Archaean Shield of South western Australia", in South and Western Australia, while the second of 11 days which followed on covered "Base Metals & Gold in the Proterozoic and Phanerozoic Mobile Belts of Eastern Australia". This module and deposits visited can be summarised as follows:
MODULE 2: EASTERN AUSTRALIA - 11 days, from Wednesday 19 to Saturday 29 May 1999
Image: Exposed gossan, Broken Hill, New South Wales
MODULE 2: EASTERN AUSTRALIA - Base Metals & Gold in the Proterozoic and Phanerozoic Mobile Belts of Eastern Australia
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Deposits were visited and workshops presented in the following order:
McArthur River Zn-Pb-Ag - Northern Territory
The HYC orebody at McArthur River contains a total geological resource of 227 Mt @ 9.2% Zn, 4.1% Pb, 41 g/t Ag, 0.2% Cu. It is a shallow dipping, shale hosted, stratabound deposit hosted by the Middle Proterozoic (1643±10 Ma) HYC Pyritic Shale Member of the McArthur Group, in the McArthur Basin, just south of the Gulf of Carpentaria in the Northern Territory. The McArthur Group is a sequence of interbedded dolostones (massive dolomite, stromatolitic dolomite, thinly bedded dolomite and dolomitic siltstones), gypsum rich beds, red beds and quartz arenite, with minor lutite, sedimentary breccia, siltstone and tuff. The ore occurs as seven conformable, well banded, semi massive sulphide units separated by thicker, barren sedimentary breccias with clasts up to 1 m across. The major sulphides are pyrite, sphalerite and galena, with lesser chalcopyrite, arsenopyrite and marcasite. The mineralisation covers an area of 2 sq. km and averages 55 m in thickness. It is elongated parallel to the major Emu growth Fault which is 1.5 km to the east, but is separated from the ore by carbonate breccias of the Cooley Dolomite Member. Current proven and probable reserves amount to 23 Mt @ 13.5% Zn, 6.4% Pb, 65g/t Ag, with resources of 102 Mt @ 14% Zn, 6.4% Pb, 64 g/t Ag. Production in the 1996/97 year totalled 98 000 t of contained Zn, at below expected rates. The McArthur River operation is owned by MIM Holdings Ltd.
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Century Zn-Pb-Ag - Queensland
The Century deposit is 250 km to the NNW of Mt Isa and 375 km SW of McArthur River. It is hosted by shales and siltstone of the Lawn Hill Formation, the youngest member in the Middle Proterozoic McNamara Group of the Lawn Hill Platform on the NW exposed portion of the Mt Isa Inlier. The 1595±6 Ma Lawn Hill Formation comprises between 1800 and 2200 m of shale, carbonaceous shale, siltstone and sandstone, with lesser tuff and tuffaceous siltstone and is near the top of Cover Sequence 3 of the Mt Isa Inlier. The immediate host unit is 850 m thick, with a basal 300 to 400 m of carbonaceous shale. This is followed by a 300 m package of interbedded siltstone, sideritic siltstone, shale and minor fine quartz-lithic sandstone which forms both the footwall and hangingwall of the 40 m thick mineralised sequence. Century differs from the deposits at McArthur River and Mt Isa in that it is hosted by a siliciclastic sequence rather than carbonate rich sediments. Economic grade mineralisation occurs as delicately banded, fine grained sphalerite, galena and very minor pyrite within carbonaceous shales, separated by distinctly less mineralised sideritic siltstone. While individual sulphide bands are conformable with bedding, the deposit overall transgresses stratigraphy. The deposit is immediately to the west of the major NW trending Termite Range Fault zone. It covers an area of 1200x1400 m and averages 40 to 50 m in thickness. The total in situ resource is 167.5 Mt @ 8.24% Zn, 1.23% Pb, 33 g/t Ag, with a higher grade resource of 105.1 Mt @ 12.10% Zn, 1.69% Pb, 46 g/t Ag at a 3.5% Zn cut-off. The Century mine is owned and operated by Pasminco Ltd.
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Mt Isa Pb-Zn-Ag - Queensland
The Mt Isa orebody and the Hilton & George Fisher Zn-Pb-Ag mines, which are 20 and 22 km north along strike respectively, are all within the Middle Proterozoic (1653 Ma) Urquhart Shale of the Mt Isa Group. The Mount Isa Group lies within the Leichhardt River Fault Trough, and belongs to Cover Sequence 3 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. The Mt Isa Group also underlies the McNamara Group which is the host to Century. In the Mt Isa mine area, the Mt Isa Group strikes north-south and has a persistent westerly dip of 65°. It is around 4 km 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 7 km thick Eastern Creek Volcanics. The host Urquhart Shale is a pyritic, dolomitic siltstone around 1000 m thick. Mineralisation at the Mt Isa mine is distributed over much of the thickness of the unit, although the bulk of the ore is within the upper 650 m, confined to a strike length of 1600 m and down dip extent of 1200 m. Individual sulphide bands of galena and sphalerite, with fine framboidal pyrite and pyrrhotite, are parallel to bedding and range from 1 mm to 1 m in thickness. Where these bands are grouped together in sufficient density to provide economic grades they constitute an orebody. Some 30 such 'orebodies' are defined over the upper 650 m of the Urquhart Shale. The Mt Isa mineralised system is developed adjacent to the major north-south trending Mt Isa Fault. Total economic resources + past production at Mt Isa amounts to more than 125 Mt of ore. Current proven reserves are 11.3 Mt @ 6.1% Zn, 5.9% Pb, 155 g/t Ag. Total reserves + resources in 1997 were around 43 Mt. The mine is operated by MIM Holdings Ltd.
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Mt Isa Cu - Queensland
The Mt Isa Copper orebodies are a separate operation to the Zinc-Lead mine. The individual copper orebodies are contained entirely within a single large irregular silica-dolomite mass which lies to the south of and overprints the Zn-Pb-Ag orebodies, but lie within the Urquhart Shale. 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. 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 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. The mine is operated by MIM Holdings Ltd.
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Mt Isa Workshop - "Geology & Mineralisation of the Mt Isa Inlier and McArthur Basin".
The aim of this one day workshop, presented by noted consultant geologist Dr Geoff Derrick, is to provide an overview of the Mt Isa Inlier and McArthur Basin, which is probably the premier zinc province of the world. In addition to Zn, Pb & Ag, the Mt Isa Inlier hosts major Cu, Cu-Au and U deposits. The workshop will comprise a major presentation on the tectonics, geology and metallogeny of the Inlier, as well as other significant deposits that will not be visited (such as Dugald River, Lady Loretta and Osborne), followed by field traverses around Mt Isa looking at the host sequence and the surface expression of the orebody.
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Ernest Henry Cu-Au - Queensland
The Ernest Henry ore deposit lies to the east of the Cloncurry Overthrust, within the Cloncurry-Selwyn zone of the Eastern Fold Belt, of the Mt Isa Inlier. It is concealed 35 to 60 m below 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 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 from hangingwall to footwall are a diorite intrusive, altered felsic volcanics, brecciated volcanic rocks which host the orebody, footwall siltstone, marble matrix breccia and more altered felsic volcanic rocks. The brecciated volcanic mass that hosts the ore forms a plunging elongate body, some 250 m thick, 300 m average width and at least 1000 m down plunge extent to the SSE where it has not been closed by drilling. The breccia ranges from unbrecciated volcanics, to crackle fracture veining to clast supported and matrix supported breccia to total clast digestion (massive matrix). The matrix is a magnetite-carbonate-sulphide assemblage with variable associated biotite, chlorite and barite. The sulphide assemblage is dominated by chalcopyrite within a magnetite-carbonate gangue. The bulk of the economic mineralisation is restricted to breccia zones with more than 10% matrix. The total reserve + resource in 1997 was 166 Mt @ 1.1% Cu, 0.54 g/t Au. The operation is managed by Ernest Henry Mining Pty Ltd for a joint venture between MIM Holdings Ltd 51% and Savage Resources Ltd 49%.
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Cannington Ag-Pb-Zn - Queensland
The Cannington ore deposit, which is 160 km to the SSE of Ernest Henry, lies below 10 to 60 m of Phanerozoic cover in the south eastern corner of the Mt Isa Inlier to the east of the Cloncurry Overthrust, within the Eastern Fold Belt. The hosts belong to the strongly metamorphosed 1677±9 Ma Fullarton River Group, which have been extensively intruded by the 1560 to 1480 Ma granitoids. The deposit is hosted by a sequence of garnetiferous psammite within a migmatitic quartzo-feldspathic gneiss terrain. The sequence strikes north-south and is cut by two major NW trending structures. Four periods of deformation are recognised. The host migmatite gneiss contains intercalated bands of fine grained schistose biotite-sillimanite-quartz bands and pegmatitic quartz-feldspar, while a thick sequence of quartz-garnet-sillimanite and foliated garnet psammite (almandine) is developed in the hangingwall. The sulphide mineralisation is associated with a diverse package of siliceous and mafic rocks. Gangue minerals include pyroxmangite, manganese-fayalite, fluorapatite, fluorite and hedenbergite in the mafic associations, and blue-quartz, feldspar and carbonate in the siliceous lodes. The sulphide assemblage comprises galena and sphalerite, with minor pyrrhotite, marcasite, arsenopyrite and chalcopyrite, and a range of silver minerals, predominantly freibergite. Magnetite is found in some lodes. A number of different strongly folded and faulted lode horizons have been delineated. The total resource comprises 43.8 Mt @ 11.6% Pb, 4.4% Zn, 538 g/t Ag. The mine is operated by BHP Limited.
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Broken Hill Workshop - "Tectonics, Geology & Mineralisation of the Broken Hill District"
The workshop was presented by Dr Barney Stevens and geologists of the NSW Department of Mineral Resources and covered:
- The tectonic setting of the Willyama-Olary Blocks within the Curnamona Nucleus and Australia,
- The tectonics/structure, geology/stratigraphy and metallogeny of the Willyama-Olary Blocks,
- The geology and mineralisation of the Broken Hill district,
- The data available, including maps, reports, magnetic images, etc.,
- Field reconnaissance across the host sequence at Broken Hill, as well as the gossan and lode horizon.
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Broken Hill Zn-Pb-Ag - New South Wales
The Broken Hill orebodies in far western New South Wales are hosted by the Middle Proterozoic (1820±60 Ma) Willyama Supergroup of the Willyama-Olary Blocks. The rocks of these blocks have been metamorphosed into assemblages ranging from andalusite-muscovite, through sillimanite-muscovite, sillimanite-potassium feldspar-garnet to hornblende-granulite facies. Peak metamorphism has been dated at 1660±10 Ma. 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 width of 300 to 1000 m. All lie within the Hores Gneiss of the Broken Hill Group. Each of the elongate lodes overlaps the lode above and below along the direction of elongation. They are steeply dipping, but plunge shallowly in a coat-hanger like structure that outcropped as a 30 to 40 wide manganiferous gossan for more than a kilometre along the central apex. Ore occurs as both massive sulphides and as 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. The original pre-mining ore deposit 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. The mine is operated by Pasminco Limited.
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Elura Zn-Pb-Ag - New South Wales
The Elura ore deposit is hosted by the early Devonian CSA Siltstone of the Amphitheatre Group, towards the northern margin of the Cobar Basin, which is in turn part of the Lachlan Fold Belt in central New South Wales. The Amphitheatre Group is a thick, monotonous sequence of turbiditic mudstone and siltstone, with minor fine grained sandstone beds. The sequence is characterised by a lack of intrusive bodies. At Elura the CSA Siltstone is a very dark grey to black, fine grained, finely bedded to laminated siltstone to shale with pale grey siltstone beds and occasional sandy bands. In the mine area the rocks have only undergone mild lower greenschist facies metamorphism. Unlike the Cu and Cu-Au deposits of the Cobar Basin, Elura is not associated with the intensely faulted basin margins. The ore deposit is a series of seven near vertical, pipe shaped bodies of mainly massive sulphides. These bodies are elliptical in plan, and crosscuts the deformed CSA Siltstone sediments at a high angle. Adjacent pipes are linked by massive, vein and stringer mineralisation. At depth the massive sulphides are enshrouded by a sub-vertical envelope of sulphide veining. Each of the pipes has a core of massive, banded, pyrrhotitic sphalerite ore, surrounded by siliceous pyrrhotitic and/or pyritic mineralisation which comprises a dense stockwork of pyrrhotite dominated sulphide veins at the contact between the massive ore and wallrocks. The main pipe has a vertical extent of more than 1000 m, and is 100 to 200 m in diameter. The deposit originally had an Identified Mineral Resource of 45 Mt. The combined proven and probable reserves in 1997 were 20.8 Mt @ 8.3% Zn, 5.2% Pb, 64 g/t Ag. The deposit is open at depth and may comprise more than 50 Mt. The mine is owned and operated by Pasminco Limited. The visit will include a regional overview and exploration perspective of the Cobar district session at the Pasminco Exploration office.
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Hellyer Zn-Pb-Cu-Au-Ag - Tasmania
The Hellyer and Que River deposits, which are 3 km apart, are located in western Tasmania. The host sequence belongs to the Mt Read Volcanic belt which is around 300 km long and 15 km wide. The immediate hosts to ore overlie the core Cambrian Central Volcanic Complex of that belt which contains the larger Rosebery-Hercules Zn-Pb-Cu-Ag-Au massive sulphide deposits 25 km to the SW and the even larger Mt Lyell Cu deposit a similar distance further to the south of Rosebery. Both Hellyer and Que River are interpreted to be at the same stratigraphic position within the host sequence. The succession at Hellyer commences with a 170 m thick lower feldspar phyric sequence composed of andesitic lavas, lava-breccia, lapilli and breccia volcani-clastics, and epi-clastics. The massive sulphide orebody occurs at the top of this unit, underlain by extensive alteration and stringer mineralisation. This unit is overlain by the hangingwall volcani-clastics ranging from a few to 40 m in thickness, composed of an inter-digitating polymictic, poorly sorted volcanic breccia and fine laminated basaltic ash. These are followed by 250 m of basaltic pillow basalts, the 150 m thick Que River Shale and the 700 m upper rhyolitic sequence. The massive sulphide body at Hellyer is restricted to a single, but faulted body with virtually no internal waste. It occurs over an 800 m length, 200 m width and average thickness of 43 m. The sub-economic footwall stringer mineralisation is 50 to 100 m wider than the ore, and continues un-interrupted to the north and south of ore. The massive sulphides average 54% pyrite, 20% sphalerite, 8% galena, 2% arsenopyrite, and 1% chalcopyrite, with minor tetrahedrite. The lower and inner regions are rich in Cu and Fe, while the upper and outer sections are higher in Pb, Zn, Ag, Au and As. Bands of crystalline barite occur above the massive sulphides. The original deposits comprised 16.9 Mt @ 13.8% Zn, 7.2% Pb, 0.4% Cu, 167 g/t Ag, 2.5 g/t Au at Hellyer, and 3.3 Mt @ 13.3% Zn, 7.4% Pb, 0.7% Cu, 195 g/t Ag, 3.3 g/t Au at Que River. The operation is owned by Western Metals Limited.
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T M (Mike) Porter, of Porter GeoConsultancy
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