Mineralogy of lode and alluvial gold from the western Capricorn Orogen, Western Australia

Abstract

There are significant differences in the mineralogy of lode and alluvial gold from the Egerton Mining Centre (EMC) compared with that of gold from the Bangemall Mining Centre (BMC) in the western Capricorn Orogen of Western Australia. In contrast, within each mining centre, lode and alluvial gold are very similar, with minor differences being due to weathering and transport processes. Lode and alluvial gold from the EMC, associated with low-grade Paleoproterozoic metasedimentary rocks of the Padbury Group, have diverse grain morphologies and low silver concentrations (0.5–3.5 wt% Ag). They exhibit a complex polycrystalline internal structure comprising partly deformed and recrystallised outer rims, coherent and incoherent twins that are commonly bent, intergranular high-purity veinlets and segregations of secondary gold. Lode and alluvial gold from the BMC are hosted by Mesoproterozoic dolerite and very low-grade sedimentary rocks of the Edmund Group. They have moderately diverse morphologies, a relatively simple polycrystalline internal structure and high, though variable, silver concentrations (5–23 wt% Ag). High-purity intergranular veinlets and polysynthetic twin lamellae occur in several grains. Low-silver outer rims are also present. Features observed in EMC gold are characteristic of a relatively high-pressure and high-temperature hypogene origin, with significant secondary alteration. During Phanerozoic weathering, gold in brecciated ferruginous quartz underwent mechanical deformation and chemical leaching that resulted in the localised depletion of silver and the development of late-stage internal dislocations and high-purity veinlets. High diversity in the alluvial gold morphology at the EMC Hibernian workings could indicate the presence of different gold sources, but lode gold showing similar variation in morphology and with a low silver content, is present near the alluvial sample site. Most primary gold mineralisation at BMC occurred in a shallow, low-pressure, low-temperature environment, although the presence of a single complex polycrystalline gold relict with a very low silver content could imply that there was an earlier hypogene gold crystallisation event. A single source for McCarthy's Patch alluvial gold at the BMC is likely, but the limited extent of the current drainage system suggests this gold probably underwent several transport cycles before being incorporated into the present-day alluvium.