The mineralogy and processing potential of the Commonwealth project in the Molong Volcanic Belt, central eastern New South Wales, Australia

Abstract

The Commonwealth prospect area comprises the base metal-gold Commonwealth Mine, Commonwealth South gold deposit and the Silica Hill deposit located 100 km north of Orange in New South Wales, Australia. Impact Minerals Limited has discovered high grade mineralization of gold, silver, zinc, lead and copper which occurs in massive sulfides with extensive pyrite, veins of sulfide and quartz, and disseminated sulfide in a variety of sedimentary and volcanic host rocks. The Inferred Resource comprises 720,000 tonnes at 2.8 g/t gold, 48 g/t silver, 1.5% zinc, 0.6% lead and 0.1% copper. The overall aim was to establish the metallurgical characteristics of these ores and develop an appropriate flow sheet to allow future mining operations at this prospect. The mineralogy of a suite of samples representative of the various ore types were characterised using advanced analytical microscopy and mass spectrometry techniques to identify metal deportment, mineral associations and liberation characteristics of both ore-bearing and gangue minerals. Leaching tests were carried out to determine the reactivity of minerals and the availability of metals to extraction processes by both conventional cyanide leaching and Curtin University's patented glycine leach (GlyCat™) technologies. The benign nature of glycine and atmospheric leaching conditions make it a potential favourable leaching option in treating such ores, particularly in sensitive areas where cyanide usage is discouraged. The samples from three areas studied consisted of: (a) the massive sulfide deposit from Commonwealth Main with high gold (2.41–11.6 g/t), silver (545–1080 g/t), zinc (6.6–10 wt%) and lead (2.3–3.9 wt%) mineralisation associated with intergrowths of predominately pyrite with sphalerite, arsenopyrite and galena, barite and zincian siderite; (b) Commonwealth South porphyry rhyolite-rhyodacite with moderate gold (1.31, 10.9 g/t Au) and silver (29.4, 32.2 g/t Ag) grades consisting of a mixture of different mineral assemblages ranging from sulfide barren quartz and muscovite/sericite grains, to particles with enriched zones of sphalerite, pyrite and galena and a sulfide-rich sandstone-shale, which is predominately quartz with moderate amounts of pyrite, muscovite, baryte and quartz/mica mixture; (c) Silica Hill feldspar porphyry rhyolite-rhyodacite samples consist of low gold (0.58, 7.65 g/t Au) and medium silver (30.7, 172 g/t Ag) grades and made up of predominately siliceous intergrowths with disseminated pyrite and arsenopyrite as both coarse euhedral aggregates and fine discrete grains (10–200 µm). The massive sulfide samples were largely refractory to gold and silver extraction under all leach test conditions due to precious metals associated mainly with pyrite. With the exception of zincian siderite, the practical separation of finely disseminated sphalerite and galena for zinc and lead contents will be a challenge. Pressure oxidation or chloride based processes may be economical if further resources can be defined. Direct leaching for precious metal extraction by both cyanide and Glycat™ is possible for the porphyry rhyolite-rhyodacite and the sulfide-rich sandstone-shale samples where gold extraction ranged from 75% to 90%. Silver extraction ranged from 40 to 60% and is largely from leaching of fine grained electrum. Minor silver-bearing minerals, tetrahedrite, acanthite, pyrargyrite, and freibergite have limited solubility in cyanide extraction processes and will require more intensive conditions to leach. GlyCat™ show potential as a heap leach or tank leach process option where low cyanide additions can be managed and removed.