VCMSM Report 9 - Mineralisation, textures and timing of select gold-antimony occurrences from the Melbourne Zone, central Victoria
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Product description:Executive Summary:There are currently 254 gold-antimony occurrences in Victoria, of which 124 have historically produced gold-antimony ore in some form. Almost all occur in the Melbourne and Bendigo zones of central Victora, with the cluster of occurrences at Costerfield forming the largest gold-antimony deposit by ore tonnes, gold, and antimony metal produced. Costerfield also has the largest mineral resources and reserves, although recent exploration success at Sunday Creek and Nagambie, in combination with historically significant deposits at Coimadia and Ringwood, demonstrate widespread potential for unrealised gold-antimony endowment in central Victoria. As part of the Developing Victoria’s Critical Minerals Initiative, new Maia X-ray fluorescence maps and TIMA modal mineralogy data were acquired from gold-antimony occurrences at Costerfield, Sunday Creek, Whroo, Nagambie and Enochs Point. This work was complemented by a review and update of the database of gold-antimony occurrences in Victoria. The new Maia and TIMA datasets reveal a remarkable similarity in mineral paragenesis, which comprises an early phase of gold-arsenopyrite precipitation, overprinted by gold-stibnite. In the analysed samples, gold-arsenopyrite mineralisation is confined mostly to the Siluro-Devonian sedimentary wall rocks to quartz-carbonate veins, while gold-stibnite mineralisation is hosted almost exclusively within the veins. At the deposit scale, mineralised veins mostly trend north-south and dip steeply and, whilst the widths of individual veins vary from centimetre to low metre scale, they commonly persist for hundreds of metres along strike. Individual gold-antimony occurrences are also commonly comprised of multiple vein sets. Whilst stibnite is by far the most common ore mineral for antimony, antimony is also present in several sulphosalts of lead, copper, and zinc. In some instances it can also be found as native antimony. Native gold and aurostibite are the most common gold minerals. Although probably oversimplifying the mineralisation history the Melbourne and Bendigo zones, a review of the overprinting relationships from other gold-antimony occurrences suggests the mineralisation history observed here — gold-arsenopyrite mineralisation preceding gold-stibnite mineralisation — is replicated widely. In the Melbourne and Bendigo zones most late Devonian gold-arsenopyrite mineralisation and all gold-stibnite mineralisation appear to post-date the Tabberabberan Orogeny (c. 385 Ma); mineralisation is thus largely decoupled from a classical “compressional” tectonic setting. Instead late Devonian gold-antimony occurrences have a close spatial and temporal relationship with igneous activity. The tectonic regime under which late Devonian gold-antimony mineralisation occurred is thus starkly different from the syn-rogenic and amagmatic environment in which late Ordovician (c. 440 Ma) gold-arsenopyrite mineralisation — the principal mineralising event of the Bendigo and Stawell zones — occurred. In detail, the texturally older phase of late Devonian gold-arsenopyrite mineralisation (c. 381–377 Ma) slightly predated the most voluminous phases of magmatic activity (c. 378-370 Ma), while the limited direct constraints on the timing of gold-stibnite precipitation suggest this stage of mineralisation largely post-dates magmatism and occurred sometime after c. 370 Ma. Late Devonian gold-antimony occurrences also occur mostly above the sub-surface extent of the Selwyn Block, a region of Proterozoic and Cambrian aged crust that underlies central Victoria. Although the late Ordovician and late Devonian mineralising events formed in very different tectonic environments, their mineralising fluids are compositionally similar and characterised by low-salinities and high dissolved concentrations of CO2, H2S and CH4. This suggests both mineralising events developed in response to fundamentally the same process in the middle to lower crust, with increasing temperature and metamorphic driven devolatisation the likely process by which metalliferous fluids were formed. For the late Ordovician event metamorphism was driven by crustal thickening, while in the late Devonian devolatilization occurred due to heating of the crust via magmatic underplating. This underplate drove both the formation of metalliferous fluids in advance of broadly coeval lower crustal partial melting, manifest as voluminous shallow-level granitoid plutonism and widespread subaerial volcanism. In the Bendigo Zone many gold-antimony occurrences occur along and in the hanging wall regions of north-trending faults formed during late Ordovician orogenesis. Whilst there are indications that the gold-antimony mineralisation associated with this setting is late, the timing remains unconstrained and could be either of late Ordovician or late Devonian age. The close spatial relationship between north-trending faults and mineralisation extends into the westernmost edge of the Melbourne Zone, where faults are younger and either Bindian or Tabberabberan in age. In the wider Melbourne Zone there is a weaker spatial link between major faults and mineralisation, with gold-antimony occurrences instead clustering within two corridors that broadly parallel the structural grain of the underlying geology. The westernmost corridor has an arcuate form that extends from the eastern suburbs of Melbourne to north-central Victoria where its continuation to the north is obscured by the Cenozoic Murray Basin. This corridor includes the gold-antimony occurrences found at Templestowe, Diamond Creek and Ringwood in the south, as well as Sunday Creek, Costerfield, Redcastle, Whroo and Nagambie further to the north. The second more easterly corridor closely parallels the Walhalla-Woods Point-Tallangallook goldfield and includes the gold-antimony occurrence at Enochs Point. A notable gap separates these two corridors, with the region of apparent limited gold antimony mineralisation overlying a low-wavelength and thus deep-seated magnetic body in the middle crust. The origin of this anomaly is uncertain, but the spatial correlation between it and the apparent lack of gold-antimony occurrences may point to a basement influence on the distribution of mineralisation. Limited erosion in central Victoria has been key to the preservation of gold-antimony mineralisation, much of which has been discovered at or near surface. Present mining operations and mineral exploration drilling indicate that antimony mineralisation persists to depths of between 500-1300 m below the current surface, after which the presence of antimony decreases significantly. Models for gold mineralisation suggest these systems transition from shallow gold-antimony mineralisation to deeper mineralisation dominated by gold. If antimony is encountered near surface, its presence is likely to indicate a greater gold system at depth. The considerable metal endowment at Costerfield and the recent re-emergence of Sunday Creek and Nagambie, combined with the large number of gold-antimony occurrences that remain untested, suggests central Victoria holds potential for further gold-antimony discoveries. Further work is required to better understand the influences on prospectivity, to improve mineral exploration targeting at the camp and deposit scale, and to ultimately better understand the full potential of this region. Bibliographic Reference: Cairns, C.P., Boger, S.D., Waugh, S.A.F., Pinter, Z., Farmer, N., Travers, S.J., Andrews, T.M. & Cayley, R.A., 2026. Mineralisation, textures and timing of select gold-antimony occurrences in central Victoria. Victoria's Critical Minerals and Strategic Materials Report 9. Geological Survey of Victoria, Department of Energy, Environment and Climate Action, Melbourne, 100 pp. Download: The downloadable version of this report is supplied as (PDF 154 MB), Attachment A1 (XLSX/ODS 92 KB), Attachment A2 (XLSX/ODS 119 KB) & Attachment A3 (XLSX/ODS 30 KB). Related products:
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