Turbidite-hosted gold deposits of Central Victoria, Australia: their regional setting, mineralising styles, and some genetic constraints

Abstract

Victoria is a gold province of international stature with primary gold mineralisation richly developed in the pre-Carboniferous rocks of the Lachlan Fold Belt. Total recorded production is in the order of 2,460,000 kg gold of which some 60% was sourced from extensive alluvial deposits, both of deep and shallow lead origin. Eleven mining centres each produced over 30 tonnes of gold with the major producer being Bendigo (684,300 kg). The Palaeozoic sequence in central Victoria is characterised by Cambrian metabasites or greenstones with associated volcanogenic and pelagic sequences followed by a Cambro–Ordovician to Early Devonian sequence of silici-clastic sediments. This succession is characterised by north–south trending open to tight folds and west-dipping thrust faults. Extensive S- and I-type granites, were emplaced from Late Silurian to Late Devonian, with the latter group, in part, associated with acid volcanic complexes. Mineralising styles include mesothermal deposits, either turbidite-hosted or dyke-affiliated, and these account for the bulk of primary gold mineralisation in the State. Less significant epithermal prospects and deposits are associated with either Cambrian calc-alkaline volcanics or sediments, typically of Siluro–Devonian age. Other styles of minor importance include Cambrian volcanogenic exhalative Cu–Zn mineralisation with gold as a minor credit (0.15–0.6 g/t Au), granite-hosted gold, and a potentially important group comprising disseminated gold in sediments. Examples of this latter group may exhibit spatially-related coticule development and by analogy with Nova Scotian disseminated intrametallic deposits, may have implications for a hitherto unrecognised style of gold mineralisation in Victoria. A gradation in lode-Au mineralising styles occurs across central Victoria. To the west in the Stawell metallogenic province, mineralisation was emplaced structurally and stratigraphically lowest in the crust. Mesothermal mineralisation is shear-hosted in Cambrian meta-sediments and greenstones exhibiting repeated deformation, multiple foliations, and extensive shearing. Host sediments were dominated by greenschist grade regional metamorphic conditions. Within the Ballarat metallogenic province, mesothermal mineralisation is hosted by dilational-site's style, in Ordovician sediments which were subjected to upper prehnite–pumpellyite facies conditions to the west and zeolite facies conditions to the east adjacent to the Melbourne metallogenic province. Ore fluids were of the low salinity C–O–H type with 1–10 wt% NaCl equivalent. CO 2 and CH 4 contents of fluid inclusions tend to be low with variable CO 2/CH 4 ratios. Depth of deposition (based on evidence from Wattle Gully mine) was in the order of 7 km and isotopic data suggest that ore fluids were derived either from an evolved igneous or from a devolatilised metamorphic source. Towards the eastern margin of the Ballarat metallogenic province and within the adjoining Siluro–Devonian Melbourne metallogenic province, epithermal style Au mineralisation is commonly emplaced in structurally and stratigraphically higher rocks. Trapping temperatures are lower than in deposits to the west and ore fluids are of low salinity C–O–H–N type with N 2>15 vol%. Gangue pyrite has elevated Sb contents and ore fluids were dominated by evolved meteoric water. δ 13 C values from vein carbonates suggest their derivation from oxidised organic carbon from within the Siluro–Devonian marine sediments. Alteration styles in gold deposits across central Victoria include sericitisation, carbonatisation, sulphidation, chloritisation, and (de-) silicification. Owing to the primary mineral assemblages and the physico-chemical characteristics of the ore-bearing fluids, visible evidence for wall rock alteration is commonly limited to 5–10 m wide zones of discolouration containing disseminated arsenopyrite and pyrite porphyroblasts and carbonate spotting. However, current petrographical and geochemical studies have demonstrated that the effects of pervasive wall rock alteration are more pronounced and extensive than previously recognised. Structural deformation within the Stawell and Ballarat metallogenic provinces, both from field evidence and 40 Ar / 39 Ar dating of cleavage phyllosilicates, indicates an Early Silurian event which migrated progressively eastwards with time. This Silurian event probably dates accretion of the proto Lachlan Fold Belt with the Kanmantoo Group of foreland Australia. Preliminary dating of hydrothermal sericites and felsic dykes associated with mineralisation, suggests episodic mineralising events 20–70 million years after accretion. These events demonstrate a spatial and temporal relationship with low-pressure regional metamorphism and/or granite plutonism; the latter being concentrated in two main pulses (400 Ma and 365 Ma) in central Victoria. The source of gold is equivocal with both igneous and devolatilised metamorphic sources being suggested. One potential reservoir source is the basement Cambrian greenstones of both MORB and boninitic affinity with associated sulphidic interflow sediments. Initial geochemical data indicate mean values of 65 ppb Au in these sulphidic interflow sediments which could have reacted with CO 2-rich fluids developed during Siluro–Devonian regional metamorphism and magmatism. Subsequent deposition of gold from such fluids could have occurred in structurally higher level dilational traps.