South Australia, SA, Australia
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The White Dam gold deposit is located in South Australia, 31 km NE of Olary and ~80 km west of Broken Hill (#Location: 32° 5' 37"S, 140° 34' 34"E).
Mineralisation at White Dam was discovered by MIM as the culmination of a program of regional geophysical and geochemical data collection and drilling between 1989 and 1997. Mining from two open pits, Hannaford and Vertigo (1.5 km to the WSW), commenced in 2010 and ended in 2012, with ore being stacked on pads for leaching.
The deposit is situated within the Olary Domain of the Paleo- to Mesoproterozoic Curnamona Province. The Olary Domain encompasses two major groups of rocks, the eastward thickening >4.5 km thick, 1720 to 1640 Ma Willyama Supergroup and the younger 1600 to 1580 Ma Ninnerie Supersuite. The Willyama Supergroup is divided into three lithostratigraphic packages, the lower Curnamona and overlying Saltbush and Strathearn groups. In the Olary Domain, the Curnamona Group, was deposited on the thinner western margin of a large rift basin centred on Broken Hill. It is predominantly composed of psammitic protoliths (now, quartzites, granofels, psammitic schists and schists), and includes metamorphosed ~1715 to ~1700 Ma mafic and A-type felsic volcanic rocks, and comagmatic intrusions. It is divided into the bi-modal volcanic and coarse clastic rocks of the Wiperaminga Subgroup and psammitic to pelitic protoliths of the overlying Ethiudna Subgroup. The Curnamona Group is overprinted by an ~1630 Ma regional oxidised alkali-magnetite/hematite alteration event, resulting in Na-rich felsic igneous rocks, albitic psammopelites, massive to laminated albitite, quartzo-feldspathic gneisses and iron formation. This large-scale alteration is characteristic of Proterozoic iron oxide copper-gold provinces (Williams and Skirrow, 2000; Skirrow and Ashley, 2000).
A regional redox boundary occurs at the top of the Curnamona Group. To the north in the Mulyungarie Domain, this takes the form of the ~250 m thick, ~1705 Ma Portia Formation at the top of the upper Ethiudna Subgroup. In the Olary Domain it is represented by the <50 m thick, ~1700 Ma Bimba Formation that followed a local ~20 m.y. hiatus and occurs at the base of the Saltbush Group. Both comprise sulphidic carbonate/calc-silicate rich, sodic altered pelitic units (possibly after evaporites) and may either be equivalents or diachronous members of the same lithofacies. The Bimba Formation is overlain by the graphitic schists of the Plumbago Formation. The several thousand metres of overlying Saltbush and Strathearn groups represent a renewal of rifting and magmatism, dominated by metamorphosed psammopelitic and pelitic lithologies, with local carbonaceous and aluminosilicate-bearing strata and banded iron formation. Restricted slivers of the Broken Hill Group, a temporal equivalent to the lower section of the Saltbush Group, are preserved above the Curnamona Group to the east and south-east of White Dam.
The Ninnerie Supersuite followed a break related to the 1620 to 1580 Ma Olarian Orogeny and ~1600 Ma peak metamorphism. It is largely composed of the 1590 to 1580 Ma, dominantly potassic S-type biotite and muscovite monzogranite of the Bimbowrie Suite, with comagmatic volcanic rocks further to the north in the Mulyungarie Domain. For more detail of these sequences and intrusions, see Kalkaroo record.
Most of the significant Cu-Au deposits of the Curnamona Province (e.g., North Portia and Kalkaroo , 75 and 40 km to the north respectively in the Mulyungarie Domain) are found close to, or within the Portia Formation redox transition. The White Dam mineralisation (Re-Os, molybdenite; e.g., Williams and Skirrow, 2000) lies within the same ~1632 to 1612 Ma age bracket as the other Cu-Au deposits of the Province, closely following the regional alkali-iron oxide alteration event, but predating the peak Olarian Orogeny and Ninnerie Supersuite. However, it differs in that it is located lower in the regional sequence, has a higher Au:Cu ratio and a very low iron oxide content. It is hosted by upper amphibolite facies quartz-feldspar-biotite migmatitic gneiss after a psammopelitic protolith, interpreted to belong to the George Mine Formation of the upper Wiperaminga Subgroup. It also occurs immediately adjacent to, and within an embayment of the contact with an extensive (>2000 km2) Bimbowrie Suite pluton composed of medium- to coarse-grained, massive to foliated, predominantly monzogranitic rocks. Mineralisation is stratabound, hosted by an open-folded, sub-horizontal, 30 to 50 m thick unit of leucocratic, banded, biotite altered, quartz-feldspar gneiss. It is associated with abundant thin veins to thicker (tens of cm) segregations of pegmatitic, leucocratic, K feldspar-albite-biotite-quartz, which are syn- to post-deformation and have biotite selvages.
Gold and disseminated molybdenite, chalcopyrite and pyrite are localised partly within the quartz-feldspar veins and segregations, and in the enveloping biotitic quartz-feldspar gneiss. Two populations of mineralisation are recognised. The dominant style is a subhorizontal zone of lower grade (~1 g/t Au) distributed throughout the host quartz-feldspar gneiss. Within this larger envelope, higher grade zones of >2 g/t Au have been delineated, apparently localised in both vein stockwork zones and local antiformal closures. Individual veins contain very high gold grades, including values of >100 g/t Au, while the intervening low grade gneiss envelope is generally consistently mineralised at a lower tenor of 0.5 to 1 g/t Au. The highest gold grades correspond to the densest veining. Sulphide minerals have been oxidised to depths of up 50 m, underlain by a thin supergene blanket containing minor native copper, chalcocite and covellite. Within the oxide zone, gold has been redeposited into the biotite lamellae associated with the hypogene veining (Cooke, 2003). Mineralisation has been mined over a strike length of ~500 m at each of the Hannaford and Vertigo pits.
The observations summarised herein suggest the Curnamona Group was affected by a crustal-scale heat event, most likely related to extension associated mafic underplating. This heat event both melted and mingled with the lower crust to generate bimodal volcanism and also drove large scale convection of hydrothermal fluids of magmatic and/or formational brine origin. Heating became more intense at the onset of inversion of the rift basin and the Olarian Orogeny, culminating in large scale crustal melting to produce the Ninnerie Supersuite (Burtt et al., 2004). The convecting hydrothermal fluids altered crustal rocks, particularly psammites, to produce widespread alkali-iron oxide alteration, and scavenge metals to form a solution in equilibrium with the oxidised regime. Interaction with a reducing environment and associated sulphides/sulphates, such as the capping Portia/Bimba formations, led to precipitation of metal sulphides as at North Portia and Kalkaroo. The same circulating fluids may also precipitate metals at other levels within the sequence, as at White Dam, in structurally or chemically permissive locations as part of the larger hydrothermal system. Whilst the molybdenite dated at White Dam is pre-peak metamorphism, mineralisation in syn- to post-peak pegmatitic veins and segregations hosted by migmatitic gneisses implies remobilisation both during peak metamorphism and the later emplacement of the immediately adjacent Bimbowrie Suite granitic pluton.
Mineral Resources estimates are as follows (Exco Resources Ltd reports to the South Australian Department of State Development, 2015; Exco Resources website, 2017):
Pre-mining Indicated + Inferred Mineral Resources
Hannaford Pit - 2010, cutoff 0.3 g/t Au:
Oxide - 0.690 Mt @ 0.96 g/t Au,
Hypogene - 2.281 Mt @ 0.90 g/t Au,
Vertigo Pit - 2011, cutoff 0.4 g/t Au:
Oxide - 1.711 Mt @ 0.95 g/t Au,
Hypogene - 0.738 Mt @ 1.26 g/t Au,
TOTAL Resources - 5.420 Mt @ 0.97 g/t Au,
Remaining Indicated + Inferred Mineral Resources
Hannaford Pit - 2015,
Transitional - 1.870 Mt @ 0.97 g/t Au,
Vertigo Pit - 2015,
Oxide - 3.0 Mt @ 0.71 g/t Au,
Transitional - 0.875 Mt @ 0.98 g/t Au,
Hypogene - 0.639 Mt @ 1.24 g/t Au,
TOTAL Resources - 3.387 Mt @ 1.02 g/t Au,
This summary has been drawn from the sparse information available in a number of published sources, including Williams and Skirrow (2000), Skirrow and Ashley (2000) and Cooke (2003) and various statutory reports by Exco Resources Ltd, the current owner of the operation.
The most recent source geological information used to prepare this summary was dated: 2003.
This description is a summary from published sources, the chief of which are listed below.
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