Megan A. Hough, Laurent Ailleres School of Geosciences, Monash University VIC 3800, Australia Frank P. Bierlein Centre for Exploration Targeting, University of Western Australia WA 6000, Australia Adele Seymon Geoscience Victoria, GPO Box 4440, Melbourne VIC 3001, Australia Stuart Hutchin Goldstar Resources, Rawson VIC 3825, Australia
ABSTRACT: Comparisons are made between three gold deposits (Cohen's Reef, Eureka, Tubal Cain) to predict favourable sites of mineralisation throughout the historically important (>80 t Au produced) Walhalla-Woods Point Goldfield in southeastern Australia. The goldfield hosts anomously large deposits, in comparison to similar-aged goldfields in the region. In this regard, the ten highest yielding deposits within the Walhalla-Woods Point Goldfield are either hosted within, or spatially associated with, intrusions of the Devonian Woods Point Dyke Swarm. By comparing the three deposits, we hope to establish the principal structural, rheological and chemical controls on gold mineralisation in this region.
KEYWORDS: gold mineralisation, Walhalla, Devonian, dykes
The state of Victoria in southeastern Australia is host to one of the world's major orogenic gold provinces, having produced approximately 2,500 tonnes of gold since the 1850's (Ramsay et al. 1998; Bierlein and Maher 2001). The sixth largest gold-producing region in Victoria with over 400 known reef and alluvial gold deposits is the Walhalla-Woods Point Goldfield, situated 150 kilometres east of Melbourne. The focus of this project has been on three gold deposits (Cohen's Reef, Eureka and Tubal Cain), near the historical township of Walhalla, in conjunction with recent mapping by Geoscience Victoria and current exploration by Goldstar Resources. Cohen's Reef, historically the most productive gold deposit in the Walhalla-Woods Point Goldfield, is a predominantly sediment-hosted, shear zone-associated, laminated to massive auriferous quartz vein system, in close proximity to a thin (< 1.7 metres) hornblende-diorite dyke. The reef was mined to a depth of over one kilometre, and produced 46 tonnes of gold (Ramsay and Willman 1988). In contrast, the Tubal Cain and Eureka gold deposits are predominantly dyke-hosted. Visible and disseminated gold in these deposits is associated with parallel ladder veins. Eureka was mined from 1867 to 1915, to a depth of approximately 130 metres, and produced 70 kg gold (Vandenberg et al. 2006). Tubal Cain was worked from 1866 to 1911, to a depth of 100 metres, and produced 120 kg (Vandenberg et al. 2006). The aim of our research is to understand, and eventually predict, favourable sites of gold mineralisation in the three different gold deposits, and throughout the goldfield, from drill hole data, structural mapping and historical adit map plans.
Figure 1. Geological map of the three gold deposits within the Walhalla Goldfield (modified from Morand et al. 2006) Thirty-eight samples from Cohen's Reef, eleven samples from Eureka, and seventeen samples from Tubal Cain were collected from recent exploration drilling by Goldstar Resources NL, allowing detailed petrological analysis of the lithologies, the extent of alteration and mineralisation, and specific siting of gold. Preliminary results from petrological and geochemical analyses, including whole-rock and quantitative XRD, of the altered/ unaltered dyke and host metasediments from Cohen's Reef, have reinforced results from hand sample and thin section studies. 3D mapping of the gold system from drillhole data and adit maps allowed the structural and lithological context of the alteration and mineralisation to be constrained.
The Walhalla-Woods Point Goldfield lies within the Late Devonian Walhalla Synclinorium, near the eastern margin of the Western Lachlan Orogen. The Western Lachlan Orogen is one of three major thrust systems that formed due to plate convergence during the tectonic evolution of the Lachlan Orogen (450-340 Ma; Gray and Foster 2004). In regards to orogenic gold deposits, the Lachlan Orogen is comparable to other Phanerozoic orogens that formed by accretion of continental crust along the complex subduction zone of Gondwana (Gray and Foster 2004). The Woods Point Dyke Swarm intruded the Walhalla Synclinorium along reverse faults and shear zones, commonly parallel to the strike of the fold axis, at 378-376 Ma (Bierlein et al. 2001) shortly before hydrothermal activity and gold mineralisation at 374-372 Ma (Bierlein et al. 2001). The dyke swarm forms a 150-kilometre long belt of N to NW-trending dykes, and is associated with over 30% of the known quartz-gold ("reef") deposits in the field. Throughout the region, the dyke compositions vary from basic gabbros to hornblende diorites. The higher yielding dyke-hosted gold deposits within the Walhalla-Woods Point Goldfield, are hosted within west-dipping faults structures and/ or conjugate faults sets, such as Morning Star, A1 and Loch Fyne further to the north. Three phases of orogenic gold mineralisation are recognised in the Western Lachlan Orogen. The first phase was closely associated with peak metamorphism at 440-455 Ma (Bierlein et al. 2001) and produced most of the mesothermal, structurally controlled and economically important gold deposits in the Western Lachlan, such as Ballarat and Bendigo (Ramsay et al. 1998). The second phase occurred due to fault reactivation at 420-400 Ma (Gray and Foster 1998) and a thermal event at ~400 Ma (Gray and Foster 1998). The third phase, at 390-360 Ma produced mostly smaller deposits, with the notable exception of the Walhalla-Woods Point Goldfield (Bierlein et al. 2001; Gray and Foster 2004).
The three deposits were chosen due to the availability of drill core and safe mine access. The deposits differ in regards to host lithology – Cohen's Reef is sediment-hosted but dyke-associated, whereas Eureka and Tubal Cain are both dyke-hosted. The dykes at Tubal Cain and Eureka are uniquely different to most dykes in the goldfield, as they are fine-grained and have an uncharacteristic porphyritic texture. 3.1 Cohen's Reef The known extent of Cohen's Reef (Fig. 1) is approximately 1500 metres long and 1134 metres deep, along the valley of the township of Walhalla. The extensive and continuous reef is predominantly a laminated quartz-carbonate vein with some associated stockwork veins, and in places incorporates sheared dyke and meta-sedimentary rocks. Cohen's Reef is hosted within a steeply west-dipping reverse fault (Tomlinson et al. 1988). The reverse fault lies along the margin of an extensive but narrow dyke, within a complex fifty metre wide shear zone. The shear zone is associated with tight steeply plunging asymmetric folds, suggesting strike-slip movement, prior to the emplacement of the dyke. 3.2 Eureka The Eureka dyke (Fig. 1) is elongate, almost arcuate with a central bulge. The approximate north-south strike length of the dyke is 340 metres, with a known depth of 512 metres. At the bulge, the width of the dyke is 106 metres, although the average width is 56 metres. Three reefs were historically worked with Goldstar Resources recently discovering four more auriferous reefs at depth. The parallel ladder reefs are hosted within high-angle reverse faults, which dip uncharacteristically to the east. 3.3 Tubal Cain Tubal Cain (Fig. 1) is the largest known intrusion of the Woods Point Dyke Swarm. Recent drilling has defined the shape of Tubal Cain of a length of 200 metres, a width of 180 metres and depth of 600 metres (Goldstar Resources, unpublished data). Eight reefs were historically worked with Goldstar Resources recently discovering seven more auriferous reefs at depth. The parallel ladder reefs dip to the southwest.
Based on geochemical drillhole traverses, petrological analyses, and 3D mapping of the gold deposit systems, a number of comparisons can be made. Historical production along Cohen's Reef concentrated on visible gold within the laminated reef system. Recent assay results have highlighted the increased levels of visible gold, and also the presence of disseminated gold, especially along dyke margins. The ten highest gold assay results (5 – 12 ppm) from recent drilling in proximity to Cohen's Reef are found in laminated veins within or on the margins of dyke; or in laminated or brecciated veins within meta-sedimentary rock, predominantly shale. At Tubal Cain, elevated gold assay results are associated with laminated quartz veins, mineralised dyke with minor veins, and quartz breccia. Gold values greater than 10 ppm (15 – 211 ppm) are most commonly associated with the quartz breccia zones. Highest gold assay results at Eureka (5603 ppm) are also associated with quartz breccia. Elevated gold values (10 – 285 ppm) are found in laminated quartz reefs, whereas gold grades of 1 – 10ppm are commonly found in mineralised dolerite with quartz veins, laminated veins and quartz breccia. Based on observations from recent drillhole intersections, laminated veins are more common in the sediment-hosted Cohen's Reef, whereas brecciated quartz veins are more common in the dyke-hosted deposits such as Eureka and Tubal Cain, especially at depth. Hydrothermal alteration, in particular the extent of the sulphide mineralisation, also varies between deposit and vein type. At Cohen's Reef hydrothermal alteration is more extensive within the dyke. Visible carbonate bleaching varies from 4cm to over a 1 metre, depending on the width and concentration of the quartz veins. Visible sulphide alteration is localised to within a few centimetres of the mineralised vein, although elevated arsenic values can be detected up to 15 metres from the mineralised zone. Altered and bleached metasedimentary rocks exhibit an increase in SiO2 and CO2, and decrease in Al2O3, Fe2O3 and K2O. The highest gold and sulphur values are found within the dyke. Arsenopyrite is commonly a good indicator for gold mineralisation, although at Cohen's Reef elevated gold values do not correspond with presence of arsenopyrite. Instead, the elevated gold values (7 ppm) correspond with increased pyrite and arsenic (< 6000 ppb), therefore refractory gold may be found in the arsenic-rich pyrite rims. Within the Eureka and Tubal Cain, the extent of hydrothermal alteration surrounding the mineralised quartz veins is also very limited, from 10 cm to 2 m, depending on the width of the quartz veins. Commonly, the extent of the sulphides does not correspond with high gold values. Elevated values of arsenic are confined to quartz breccia zones, or mineralised dolerite, although the highest arsenic values are not associated with the highest gold values. Geochemical traverses along drill core from Eureka and Tubal Cain revealed local variation of a number of elements. Within quartz breccia with gold contents greater than 50ppm, elevated As, Bi, Mo, Cu, Sb, Pb and Zn were observed, and in conjunction with depletion of Co and Ni. With gold values less than 50 ppm, elevated As, Bi, Mo, Pb, Sb were observed, but decreased values of Co, Cu, Ni, Sn, and Zn. Anomously elevated contents of As (> 10000ppm), Pb (> 60 ppm), Mo (3ppm) and Sb (~200 ppm), related to elevated gold values within the dyke-hosted breccia zones, may be a signature of magma-related mineralisation (Ramsay et al. 1998), although further analysis is required.
Based on current research, favourable sites for gold mineralisation in the Walhalla-Wood's Point Goldfield are dyke-hosted quartz breccia zones. The ten highest-yielding gold deposits within the Walhalla-Wood's Point Goldfield are either dyke-hosted or dyke-associated/ sediment hosted. A number of hydrothermal phases are evident in all three deposits, from initial barren quartz or carbonate veins, to quartz-carbonate veins associated with sulphides, to later gold-rich quartz veins. The initial phases of hydrothermal activity formed the crack-seal laminated quartz-carbonate veins with minor sulphide and host rock selvages. The later hydrothermal events occurred at shallower levels under more brittle conditions, and are characterized by open space vein fill with vugs, brecciated quartz zones, and the highest gold values. The extent of hydrothermal alteration in wallrock is an exploration tool best suited to gold associated with laminated veins. Sulphide mineralisation associated with breccia zones is more localised, and not necessarily indicative of gold values. ACKNOWLEDGEMENTS ARC Linkage funding, along with direct and in-kind support from collaborative partners Geoscience Victoria and Goldstar Resources supported this research. Stafford McKnight is thanked for assistance in SEM and XRF analyses. REFERENCES Bierlein FP, Hughes M, Dunphy J, McKnight S, Reynold P, Waldron H (2001) Tectonic and economic implications of trace element, 40Ar/39Ar and Sm-Nd data from mafic dykes associated with orogenic gold mineralisation in central Victoria, Australia. Lithos 58: 1-31 Bierlein FP, Maher S (2001) Orogenic disseminated gold in Phanerozoic fold belts — examples from Victoria, Australia and elsewhere. Ore Geology Reviews 18: 113-148 Gray DR, Foster DA (1998) Character and kinematics of faults within the turbidite-dominated Lachlan orogen: implications for tectonic evolution of Eastern Australia. Journal of Structural Geology 20: 1691-1720 Gray DR, Foster DA (2004) Tectonic evolution of the Lachlan Orogen, southeast Australia: historical review, data synthesis and modern perspectives. Australian Journal of Earth Sciences 51: 773-817 Morand VJ, Wilman CE, Taylor DH, Quinn C, Vandenberg AHM, Seymon AR, Hough M (2006) Walhalla and parts of Seaton and Moe 1:50 000 geological map. GeoScience Victoria. Department of Primary Industries. Ramsay WRH, Bierlein FP, Arne DC, VandenBerg AHM (1998) Turbidite-hosted gold deposits of central Victoria, Australia; their regional setting, mineralising styles, and some genetic constraints. Ore Geology Reviews 13: 131-151 Ramsey WRH, Willman CE (1988) Economic Geology. In: Douglas JG, Fergusson JA (eds) Geology of Victoria. Geological Society of Australia (Victorian Division) Publication, pp 454-481 Tomlinson KM, Wilson CJL, Hazeldene R, Lohe EM (1988) Structural control on gold mineralisation at Walhalla, Victoria. Australian Journal of Earth Sciences 35: 421-444 VandenBerg AHM, Cayley RA, Willman CE et al. (2006) Walhalla – Woods Point – Tallangallook special map area geological report. Geological Survey of Victoria Report 127: pp455