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.
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Broken Hill Field Workshop
The Tectonics, Geology and Mineralisation of the Broken Hill District
The topics covered by the workshop included:
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- 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 parts of the host sequence, gossan and lode horizon at Broken Hill.
The Olympic Dam ore deposit lies within the outer margins of the Lower Proterozoic Gawler Craton in northern South Australia. It falls within the Stuart Shelf where 300 m of flat lying, barren, Upper Proterozoic to lower Paleozoic sediments unconformably overlie both the craton and the ore deposit. Mineralisation is hosted by the Olympic Dam Breccia Complex which is developed within the Mid Proterozoic (1600-1585 Ma) Roxby Downs Granite. The Olympic Dam Breccia Complex 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 outer margins of the complex are diffuse, grading outwards from heterolithic breccia, through granite breccia, to crackle breccia to fractured granite. Minor volcaniclastics are found in diatreme structures at the top of the complex. The resource occurs as up to 150 separated bodies distributed within an annular zone up to 4 km in diameter surrounding the central barren hematite-quartz breccia. The highest grade mineralisation occurs within an up to 40 m thick, shallowly inward dipping, irregularly developed, chalcocite-bornite 'layer', where it cuts hematite rich breccia zones. This undulose layer grades down and outwards into chalcopyrite and then pyrite rich zones. As at June 1997 Proven reserves totalled 82 mt @ 2.4% Cu, 0.6 g/t Au, 0.7 kg/t Uranium oxide; Probable reserve of 484 mt @ 2.0% Cu, 0.7 g/t Au, 0.6 kg/t Uranium oxide; and Inferred resource of 1620 mt @ 1.1% Cu, 0.4 g/t Au, 0.4 kg/t Uranium oxide. The current expansion should result in an annual production rate of 200 000 t of contained copper.
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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 km2 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, 65 g/t Ag, with resources of 102 mt @ 14% Zn, 6.4% Pb, 64 g/t Ag. Production in the 1996/97 year totalled 98000 t of contained Zn, at below expected rates.
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Mount Isa Zn-Pb-Ag
The Mt Isa lead-zinc orebody is 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 degrees. 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. Another major resource is currently being mined at the Hilton & George Fisher mine some 20 km to the north in the same hosts.
The Mt Isa Copper orebodies are a separate operation to the Zinc-Lead mine, and are contained entirely within a single large irregular, transgressive, and younger 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. Past production and reserves in the copper mine indicate a total resource of around 225 mt @ 3.3% Cu.
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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.
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The Osborne copper -gold deposit is located some 195 km south east of Mt Isa in a complex sequence of metamorphic, igneous and metasomatic rocks belonging to the Mid Proterozoic Eastern Fold Belt of the Mt Isa Inlier. In 1993, prior to mining, the total measured and indicated mineral resource was 11.2 mt @ 3.5% Cu, 1.5 g/t Au within a larger global resource of 36 mt @ 2% Cu, 1 g/t Au. The predominantly meta-sedimentary host sequence comprises feldspathic psammites ± thin layers of pelite, stromatitic migmatites and local pre-metamorphic banded ironstone units and schists. Sheet intrusions of amphibolite and post metamorphic pegmatites are also present. The psammites are characterised by sodic plagioclase comprising >95% albite and/or oligoclase + quartz. The ironstones are predominantly composed of varying proportions of magnetite , quartz and apatite. The deposit area is divided into two domains by the Awesome Fault. The bulk of the high grade Cu-Au mineralisation is focused along the contacts of the upper banded ironstone with feldspathic psammite in the Western domain. Both ironstones in this domain contain disseminated Cu-Au mineralisation along their 1.3 km strike lengths and the gross distribution of ore mirrors the shape of the ironstone. The Eastern Domain is largely devoid of ironstones but embraces a prolate high grade lens of ore in pegmatite, feldspathic psammite and meta-mafic intrusives. The majority of the Cu is present as chalcopyrite hosted by zones of coarse silica flooding. Within the ironstones mineralisation is present with secondary hematite-pyrite-magnetite, whereas well mineralised silica flooded zones outside of the ironstones contain only magnetite-pyrite. In the Eastern Domain ore is characterised by pyrrhotite-pyrite-magnetite. Peak amphibolite facies metamorphism is dated at 1595 to 1568 Ma, while the Cu-Au is 1538 Ma. Fluid inclusion studies indicate the orebody formed at around 300 degrees C from immiscible hyper-saline brines and carbon dioxide rich components. The Osborne mine is 100% owned by Placer dome Inc, through Placer Pacific Limited.
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For more information contact:
T M (Mike) Porter, of Porter GeoConsultancy
This tour was organised by:
T M (Mike) Porter of Porter GeoConsultancy Pty Ltd on behalf, and to the specification, of the client.
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