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Ballarat, Ballarat East |
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Victoria, Vic, Australia |
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Au
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Super Porphyry Cu and Au
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The Ballarat gold field is located beneath the town of Ballarat, approximately 120 km west of Melbourne in Victoria, Australia.
The Ballarat district has been an important historic gold producer. Gold was first discovered in 1851 with the initial mining of the Deep Lead buried palaeo-channel alluvial deposits, followed from 1880 by exploitation of the basement quartz reefs. Two north south lines of mineralisation have been exploited, the West and East Ballarat lines, approximately 2 km apart. Production was principally from 9 major mines distributed over a strike length of 4 km. The last significant mine was closed in 1916. A total of 65 t of hard-rock gold was produced from the Ballarat field between 1856 and 1918.
The Ballarat East line extends over a length of approximately 14 km and a width of up to 500 m and was worked to depth of up to 550 m. Approximately 47 t (at an average grade of 10 g/t Au) has been produced from the Ballarat East line. The line of deposits are largely concentrated in the sub-vertical limb of the mesoscopic Last Chance anticline, an asymmetric, west-verging parasitic fold, close to the hinge of the Ballarat Anticlinorium which lies just to the west. The bulk of production came from moderately west-dipping "leather-jacket" reefs (see description below), stacked 50 to 100 m apart as they cross the sub-vertical eastern limb of the Last Chance anticline. Individual reefs are lenticular quartz masses up to 25 m thick and hundreds of metres in length, often accompanied by horizontal tension gashes which extended outwards for up to 30 m. These lodes rarely extend across into the adjacent west dipping limbs of the structure where they become bedding parallel. Large sub-vertical quartz reefs also occur along some of the anticlinal axial planes, termed 'breached anticline reefs'.
The Ballarat West line is largely concealed below multiple flows of Quaternary basalt flows with an aggregate thickness of up to 150 m and were found during the mining of deep lead accumulations. Approximately 26 t of gold has been produced from Ballarat West at an average grade of 15 g/t Au, from three discreet mineralised faults distributed over a 7 km strike length, 1050 m vertical interval and 1 km width. These three faults are located on the western limbs of three west vergent anticlines which are variable developed along strike and define the eastern limb to the hinge zone of the Albion Anticlinorium. The lodes are thin (1 to 5 m) laminated quartz reefs, sub-parallel to bedding on the est dipping limbs and are interpreted to be 'crack-seal' veins developed in bedding parallel faults, possibly following the same black shale unit.
The host sequence is a tightly folded, weakly metamorphosed sequence of lower Ordovician turbidites, comprising cyclic sandstones and mudstones which have been cut by west dipping reverse faults. Veins are present in three principal sets, namely:
i). Leather jackets - which occur as west- and flat-dipping reverse faults trending at 5 to 10° east of bedding and dipping at up to 45°W, with associated brecciation, quartz veining and spurring, stacked in a vertical succession at approximately 100 m intervals. When these reverse faults traverse eastern dipping slates (usually carbonaceous and/or pyritic), the intersection is at a flat angle to bedding, producing dilational structures which focussed the development of quartz lodes, and the precipitation of gold, accompanied by pyrite, arsenopyrite, sphalerite, galena, marcasite, etc. These produced the bulk of the gold.
ii). Quartz-sulphide breccias - bedding parallel subvertical breccias on the eastern limb of the First Chance anticline, which were significant producers in some mines.
iii). Crosscourses - which are post mineralisation faults with strong sinistral displacement, trending NE and dipping from 55°SE to 55°NW.
On the eastern limbs of anticlines where the Leather jackets cut across bedding at an open angle breccia bodies with quartz-arsenopyrite are formed, while on the western limb where the fault-bedding angle is more acute quartz veins parallel bedding without the brecciation.
The gold is generally coarse, nuggetty and patchily distributed in veins, so drilling cannot readily define reserves. Wall rock alteration comprises a weak halo of silica, carbonate, chlorite and sericite which gives a bleached appearance
At the current (2007) Ballarat Goldfields development, mineralisation is associated with vertical shear zones which are typically developed in slate beds or on the margins of large rigid bodies such as thick sandstone units. Narrow slate beds typically localise narrow shear zones which are interpreted to have focused the movement of fluids and produced small but highly concentrated gold accumulations. Areas containing thick sand units tend to develop large shear zones which enabled movement around a large competent body as folding took place. These shear zones can vary by up to 10 m in thickness and carry low grade to high grade gold mineralisation throughout. The majority of the 'gold traps' at Ballarat East are located at the intersection of the narrow and large shear zones with the 'leather jacket faults' (Lihir Gold, 2007).
Production and ore reserve/resource figures include:
Historic production, Ballarat Gold Field, 1851 to 1916 - 65 t Au @ 8.7 g/t Au
Inferred resource, Ballarat Goldfields, 1988 - 0.40 Mt @ 9 g/t for 3.6 t Au (Taylor, 1998)
Inferred resource, Ballarat Goldfields, 1997 - 3.3 Mt @ 9.5 g/t for 31 t Au (Taylor, 1998)
Inferred, Ballarat Goldfields, 2007 - 3.1Mt @ 11g/t Au for 34.1 t Au (Lihir Gold, 2007)
Probable resource, William Australia deposit (3 km south of Ballarat Goldfields) 1996 - 4.436 Mt @ 5 g/t Au (Taylor, 1998)
William Australia open pit production, 1997 - 0.4 Mt @ 0.91 g/t Au for 0.364 t Au (Taylor, 1998).
Based on the known extent of veining, testing and the historical distribution of grade, the total inferred resource + exploration potential is estimated at 14.5 Mt @ 11g/t Au for 160 t Au (Lihir Gold, 2007)
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1990.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Arne D C, Bierlein F P, McNaughton N, Wilson C J L and Morand V J, 1998 - Timing of gold mineralisation in western and central Victoria, Australia: New constraints from SHRIMP II analysis of zircon grains from felsic intrusive rocks: in Ore Geology Reviews v13 pp 251-273
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Bierlein F P and Maher S 2001 - Orogenic disseminated gold in phanerozoic fold belts - examples from Victoria, Australia and elsewhere : in Ore Geology Reviews v18 pp 113-148
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Bierlein F P, Christie A B, Smith P K, 2004 - A comparison of orogenic gold mineralisation in central Victoria (AUS), western South Island (NZ) and Nova Scotia (CAN): implications for variations in the endowment of Palaeozoic metamorphic terrains: in Ore Geology Reviews v25 pp 125-168
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Bierlein F P, Fuller T, Stuwe K, Arne D C and Keays R R 1998 - Wallrock alteration associated with turbidite-hosted gold deposits. Examples from the Palaeozoic Lachlan Fold Belt in central Victoria, Australia: in Ore Geology Reviews v13 pp 345-380
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Boucher, R.K., Rossiter, A.G., Fraser R.M. and Turnbull, D.G., 2015 - Review of the structural architecture of turbidite-hosted gold deposits, Victoria, Australia: in Trans. IMM (incorp. AusIMM Proc.), Section B, Appl. Earth Sc. v.124, pp. 136-146.
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Cayley, R.A., Korsch, R.J., Moore, D.H., Costelloe, R.D., Nakamura, A., Willman, C.E., Rawling, T.J., Morand, V.J., Skladzien, P.B. and O Shea, P.J., 2011 - Crustal architecture of central Victoria: results from the 2006 deep crustal reflection seismic survey: in Australian J. of Earth Sciences v.58, pp. 113-156.
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Cox S F, Wall V J, Etheridge M A and Potter T F, 1991 - Deformational and metamorphic processes in the formation of mesothermal vein-hosted gold deposits - examples from the Lachlan Fold Belt in central Victoria, Australia : in Ore Geology Reviews v6 pp 391-423
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d Auvergne P B 1990 - Ballarat East Gold deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp1277-1278
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Fairmaid, A.M., Kendrick, M.A., Phillips, D. and Fu, B., 2011 - The origin and evolution of mineralizing fluids in a sediment-hosted orogenic-gold deposit, Ballarat East, southeastern Australia: in Econ. Geol. v.106 pp. 653-666
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Forde A and Bell T H 1994 - Late structural control of mesothermal vein-hosted gold deposits in central Victoria, Australia: Mineralization mechanisms and exploration potential : in Ore Geology Reviews v9 pp 33-59
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Foster D A, Gray D R, Kwak T A P and Bucher M 1998 - Chronology and tectonic framework of turbidite-hosted gold deposits in the Western Lachlan Fold Belt, Victoria: 40Ar-39Ar results: in Ore Geology Reviews v13 pp 229-250
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Hughes, M.J. and Phillips, G.N., 2015 - Mineralogical domains within gold provinces: in Trans. IMM (incorp. AusIMM Proc.), Section B, Appl. Earth Sc. v.124, pp. 191-204.
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Phillips G N and Hughes M J, 1996 - The geology and gold deposits of the Victorian gold province : in Ore Geology Reviews v11 pp 255-302
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Phillips, G.N. and Hughes, M.J., 1998 - Victorian gold province: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne Mono 22 pp. 495-506.
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Ramsay W R H, Bierlein F P, Arne D C and VandenBerg A H M, 1998 - Turbidite-hosted gold deposits of Central Victoria, Australia: their regional setting, mineralising styles, and some genetic constraints : in Ore Geology Reviews v13 pp 131-151
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Sibson R H and Scott J 1998 - Stress/fault controls on the containment and release of overpressured fluids: Examples from gold-quartz vein systems in Juneau, Alaska; Victoria, Australia and Otago, New Zealand: in Ore Geology Reviews v13 pp 293-306
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Taylor D H 1998 - Ballarat gold deposits: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne Mono 22 pp 543-548
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Wilson, C.J.L., Osborne, D.J., Robinson, J.A. and Miller, J. Mcl., 2016 - Structural Constraints and Localization of Gold Mineralization in Leather Jacket Lodes, Ballarat, Victoria, Australia: in Econ. Geol. v.111, pp. 1073-1098.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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