TURBIDITE-HOSTED GOLD DEPOSITS OF CENTRAL VICTORIA, AUSTRALIA - THEIRREGIONAL SETTING, MINERALIZING STYLES, AND SOME GENETIC CONSTRAINTS

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 dep osits, both of deep and shallow lead origin. Eleven mining centres eac h produced over 30 tomes of gold with the major producer being Bendigo (684,300 kg). The Palaeozoic sequence in central Victoria is characte rised by Cambrian metabasites or greenstones with associated volcanoge nic and pelagic sequences followed by a Cambro-Ordovician to Early Dev onian sequence of silici-clastic sediments. This succession is charact erised by north-south trending open to tight folds and west-dipping th rust 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 accou nt for the bull; of primary gold mineralisation in the State. Less sig nificant epithermal prospects and deposits are associated with either Cambrian calc-alkaline volcanics or sediments, typically of Siluro-Dev onian age. Other styles of minor importance include Cambrian volcanoge nic 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 co mprising disseminated gold in sediments. Examples of this latter group may exhibit spatially-related coticule development and by analogy wit h Nova Scotian disseminated intrametallic deposits, may have implicati ons for a hitherto unrecognised style of gold mineralisation in Victor ia. A gradation in lode-Au mineralising styles occurs across central V ictoria. To the west in the Stawell metallogenic province, mineralisat ion was emplaced structurally and stratigraphically lowest in the crus t. Mesothermal mineralisation is shear-hosted in Cambrian meta-sedimen ts and greenstones exhibiting repeated deformation, multiple foliation s, and extensive shearing. Host sediments were dominated by greenschis t grade regional metamorphic conditions. Within the Ballarat metalloge nic province, mesothermal mineralisation is hosted by dilational-site' s style, in Ordovician sediments which were subjected to upper prehnit e-pumpellyite facies conditions to the west and zeolite facies conditi ons to the east adjacent to the Melbourne metallogenic province. Ore f luids were of the low salinity C-O-H type with 1-10 wt% NaCl equivalen t. CO2 and CH4 contents of fluid inclusions tend to be low with variab le CO2/CH4 ratios. Depth of deposition (based on evidence from Wattle Gully mine) was in the order of 7 km and isotopic data suggest that or e fluids were derived either from an evolved igneous or from a devolat ilised metamorphic source. Towards the eastern margin of the Ballarat metallogenic province and within the adjoining Siluro-Devonian Melboum e metallogenic province, epithermal style Au mineralisation is commonl y emplaced in structurally and stratigraphically higher rocks. Trappin g temperatures are lower than in deposits to the west and ore fluids a re 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. delta(13)C values from vein carbonates suggest their derivatio n 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 an d the physico-chemical characteristics of the ore-bearing fluids, visi ble evidence for wall rock alteration is commonly limited to 5-10 m wi de zones of discolouration containing disseminated arsenopyrite and py rite porphyroblasts and carbonate spotting. However, current petrograp hical and geochemical studies have demonstrated that the effects of pe rvasive wall rock alteration are more pronounced and extensive than pr eviously recognised. Structural deformation within the Stawell and Bal larat metallogenic provinces, both from field evidence and Ar-40/Ar-39 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 Kanm antoo Group of foreland Australia. Preliminary dating of hydrothermal sericites and felsic dykes associated with mineralisation, suggests ep isodic mineralising events 20-70 million years after accretion. These events demonstrate a spatial and temporal relationship with low-pressu re regional metamorphism and/or granite plutonism; the latter being co ncentrated in two main pulses (400 Ma and 365 Ma) in central Victoria. The source of gold is equivocal with both igneous and devolatilised m etamorphic sources being suggested. One potential reservoir source is the basement Cambrian greenstones of both MORE and boninitic affinity with associated sulphidic interflow sediments. Initial geochemical dat a indicate mean values of 65 ppb Au in these sulphidic interflow sedim ents which could have reacted with CO2-rich fluids developed during Si luro-Devonian regional metamorphism and magmatism. Subsequent depositi on of gold from such fluids could have occurred in structurally higher level dilational traps. 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