Electronic supplementary material S1. Geology of selected gold deposits in central Victoria
Central Victoria’s largest gold deposits are hosted by turbidites and include Stawell – Magdala, Bendigo, Wattle Gully and Ballarat (Fig. 1A). These all formed at ca. 440-420 Ma close to peak metamorphism and have previously been classified as mesozonal ‘orogenic’ lode gold deposits. These gold-only ore deposits commonly occur as reefs, which are typically centimetre to metre thick quartz veins with visible gold.
Fosterville in the Bendigo Zone and Costerfield in the Melbourne Zone are smaller disseminated-stockwork gold (-Sb-As) deposits that may have formed as late as 380-370 Ma. These deposits have previously been classified as high level epizonal ‘orogenic’ gold deposits (e.g., VendenBerg et al. 2000, 2006; Phillips et al. 2003). Gold deposits of the same age (ca. 370 Ma) at Woods Point and Walhalla from the Melbourne Zone are characterised by gold-stibnite-arsenic (Au-Sb-As) ore assemblages, and many of them are directly hosted by the Woods Point Dyke Swarm (Green et al. 1982).
In contrast, other disseminated and stockwork hosted gold-dominated, polymetallic deposits including Mount Piper and Malmsbury – Leven Star have a complex metallurgy (e.g., Au ± Mo-W-Bi-Te-Cu) and appear to be associated with post-metamorphic felsic intrusions, and in these cases a possible genetic link to ca. 380-370 Ma magmatism has been emphasised (Bierlein and McKnight 2005; Whittam et al. 2006). Some gold in the Stawell – Wonga and Maldon deposits may have been remobilised or metamorphosed during the emplacement of these late-orogenic granites (e.g., Foster et al. 1998; Wilson et al. 1999; Bierlein et al. 2001a).
‘Orogenic’ gold deposits
The Bendigo goldfield in the Bendigo Zone is Victoria’s largest historic gold producer (540 t non-placer gold). It includes the Hustlers, Garden Gully, Deborah, Sheepshead and New Chum deposits, and there are three different types of reef: saddle, fault/neck and spur reefs, being classic vein-hosted deposits (Schaubs and Wilson 2002). The Bendigo goldfield occurs within Early Ordovician turbidites, consisting of sandstone, siltstone, slate and graphitic slate, all metamorphosed to greenschist-facies.
Non-placer gold production (27 t Au) in the Castlemaine – Chewton goldfield is mainly from auriferous quartz veins in the Wattle Gully mine. Alteration assemblages near quartz veins include muscovite, carbonates, chlorite and albite, and the ore contains arsenopyrite and pyrite with gold.
The Fosterville goldfield (>45 t Au) is located approximately 25 km east of Bendigo, near the eastern margin of the Bendigo Zone (Leader et al. 2010). Two mineralised trends occur along structures known as the Fosterville Fault and O’Dwyers Line. The host rocks are composed of Ordovician turbidites and have undergone three distinct episodes of deformation (Leader et al. 2010). The majority of the mineralisation occurs within sandstone units, adjacent to the Fosterville Fault, as disseminated arsenopyrite-pyrite-gold accompanied by numerous quartz-carbonate veins, which also cross-cut Late Devonian felsic dykes. This early phase of mineralisation is cross-cut by gold bearing quartz-stibnite veins that represent a second mineralisation event. There is an absence of quartz reef development typical of the majority of the other gold deposits in the Bendigo Zone (Schaubs and Wilson 2002; Willman 2007).
The Maldon goldfield (56 t Au) shares many similarities to other goldfields such as Bendigo and Ballarat, including host rock lithologies and structural style of mineralisation (e.g., Phillips et al. 2003). However, the Maldon goldfield is situated in the contact aureole of the Late Devonian Harcourt Granite and quartz veins in the pelitic and psammopelitic rocks have been variably recrystallised during contact metamorphism. Mineralisation is found as native gold and minor maldonite (Au2Bi) and is accompanied by sericite-chlorite-carbonate alteration. Other ore minerals include pyrite, arsenopyrite, chalcopyrite, sphalerite, galena, molybdenite, scheelite, bismuth, native antimony and stibnite.
The Stawell goldfield (~100 t Au) is located in the Stawell Zone and has recently been Victoria’s main gold producer; it includes the Magdala and Wonga deposits (Mapani and Wilson 1998). The Magdala deposit is hosted by the Moornambool Metamorphic Complex, which forms part of Cambrian Delamerian Orogen and was reworked during the Ordovician Lachlan Orogeny (Miller et al. 2006). There may be three distinct stages of gold mineralisation in the Stawell goldfield. Early ‘orogenic’ gold mineralisation in the Magdala deposit has been dated at ca. 440 Ma (Miller et al. 2005). Syn- or post-magmatic gold mineralisation in the Wonga deposit was generated during the emplacement of Late Silurian to Early Devonian felsic dykes (ca. 410 Ma; Bierlein et al. 2001b). In addition, some gold may have been remobilised during emplacement of the Stawell Granite (ca. 400 Ma; Wilson et al. 1999).
The Woods Point – Walhalla goldfield (108 t Au) has been the largest gold producer in the Melbourne Zone, and comprises gold deposits hosted in and/or near dykes of the Woods Point Dyke Swarm (e.g., Green et al. 1982; Fleming 1987). The Woods Point Dyke Swarm is the most extensive system of Paleozoic dykes and sills very unevenly distributed in the central Victoria gold province, and rock types including quartz diorite, quartz monzonite, gabbro, peridotite, quartz-feldspar porphyry and granophyre. These gold deposits, which include the Morning Star (formerly Hope mine; 19.6 t Au) and A1 mines, are characterised by gold-stibnite-arsenic (Au-Sb-As) ore assemblages.
The Walhalla deposit is located 180 km east of Melbourne in Victoria’s Gippsland region. Mineralisation such as at Cohen’s Reef (46 t @ 32.2g/t Au) is within the Walhalla Group, a sequence of Lower Devonian turbidites. Mineralisation (e.g., at the Long Tunnel mine) is controlled by the steeply west-dipping, 20 to 50m wide, Cohen’s Shear Zone which appears to be a reactivated reverse fault. Mineralised faults displace several dykes and postdate the major Middle Devonian deformation event. Laminated veins within Cohen’s Shear Zone are boudinaged, resulting from extension in the direction of earlier reverse fault movement. En echelon veins are associated with shear zones and ptygmatically folded veins are also present. By contrast, gold mineralisation at the Eureka mine as well as Tubal Cain, predominantly dyke-hosted, is associated with pyrite, arsenopyrite, galena, chalcopyrite and sphalerite (Hough et al. 2009).
‘Intrusion-related’ gold deposits
The Mount Piper goldfield (>10 t Au) is situated in the western part of the Melbourne zone, 10 km southwest of the ca. 370 Ma Strathbogie batholith (e.g., Bierlein et al. 2001c). The host turbidites and disseminated plus stockwork-style gold mineralisation are overprinted by regional biotite-cordierite grade metamorphism and later hydrothermal alteration associated with a complex gold-scheelite-fluorite-cassiterite-tetradymite-stibnite mineral assemblage, and later skarn assemblages (Bierlein and McKnight 2005). These characteristics are distinctive compared to more typical ‘orogenic’ lode gold deposits in the region. Furthermore, the mineralised system at Mount Piper comprises a linear stockwork breccia with a north-easterly trend. This contrasts with the NNW trend observed in the nearby Bendigo goldfield.
References
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Bierlein FP, Arne DC, Foster DA, Reynolds P (2001a) A geochronological framework for orogenic gold mineralisation in central Victoria, Australia. Mineral Deposit 36: 741–767
Bierlein FP, Hughes M, Dunphy J, McKnight S, Reynolds P, Waldron H (2001b) Tectonic and economic implications of trace element, 40Ar/39 Ar and Sm-Nd data from mafic dykes associated with orogenic gold mineralisation in central Victoria, Australia. Lithos 58: 1–31
Bierlein FP, Arne DC, Keay SM, McNaughton NJ (2001c) Timing relationships between felsic magmatism and mineralisation in the central Victorian gold province, Southeast Australia. Aus J Earth Sci 48: 883–899
Fleming G (1987) The Tubal Cain Gold Mine, Walhalla, Victoria: A Possible Hybridized Camptonitic Basalt Diatreme-Hosted Metamorphic Vein-Gold Deposit. Unpublished BSc Thesis, New South Wales Institute of Technology
Foster DA, Gray DR, Kwak TAP, Bucher M (1998) Chronology and tectonic framework of turbidite-hosted gold deposits in the Western Lachlan Fold Belt, Victoria: 40Ar-39Ar results. Ore Geol Rev 13: 229–250
Green AH, Donnelly TH, Jahnke FM, Keays RR (1982) Evolution of gold-bearing veins in dykes of the Woods Point dyke swarm, Victoria. Mineral Deposit 17: 175–192
Hough M, Ailleres L, Bierlein F (2009) Tectonic controls on orogenic gold mineralization of Walhalla-Woods Point goldfields, south eastern Australia. In: Williams PJ et al. (eds) Smart Science for Exploration and Mining: Proceedings of the 10th Biennial SGA Meeting 2009. pp. 936–938. Tonwsville, Australia
Leader LD, Robinson JA, Wilson CJL (2010) The role of faults and folding in controlling gold mineralisation at Fosterville, Victoria. Aus J Earth Sci 57: 259–277
Mapani BES, Wilson CJL (1998) Evidence for externally derived vein forming and mineralising fluids: an example from the Magdala gold mine, Stawell, Victoria, Australia. Ore Geol Rev 13: 323–343
Miller JMcL, Phillips D, Wilson CJL, Dugdale LJ (2005) Evolution of a reworked orogenic zone: the boundary between the Delamerian and Lachlan Fold Belts, southeastern Australia. Aus J Earth Sci 52: 921–940
Miller JMcL, Wilson CJL, Dugdale LJ (2006) Stawell gold deposits: a key to unraveling the Cambrian to Early Devonian structural evolution of the western Victorian goldfields. Aus J Earth Sci 53: 677–695
Phillips GN, Hughes MJ, Arne DC, Bierlein FP, Carey SP, Jackson T, Willman CE (2003) Gold historical wealth, future potential. In: Birch WD (ed) Geology of Victoria. Geological Society of Australia Special Publication 23, pp 377–433
Schaubs PM, Wilson CJL (2002) The relative roles of folding and faulting in controlling gold mineralization along the Deborah Anticline, Bendigo, Victoria, Australia. Econ Geol 97: 351–370
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Electronic supplementary material S2. List of quartz samples from Victorian gold deposits and abundance of different compositional fluid types observed.
(see a separate Excel file).
Electronic supplementary material S3. Microthermometric data and laser Raman results for representative non-aqueous fluid inclusions in Victorian gold deposits.
(see the Excel file above).
Electronic supplementary material S4. Irradiation parameters and correction factors.
(see the Excel file above).
Electronic supplementary material S5. Detailed fluid inclusion noble gas and halogen data for quartz samples from Victorian gold deposits analysed in this study.
(see the Excel file above).
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