Orogenic disseminated gold in phanerozoic fold belts—examples from Victoria, Australia and elsewhere

Abstract

Orogenic gold mineralisation in the Lachlan Orogen of southeastern Australia and in other Phanerozoic fold belts has generally been assumed to be confined entirely to quartz veins. However, an improved understanding of the tectonic, structural and geochronological framework in which these occurrences form, and recent advances towards the extent of wallrock alteration around these veins, cast doubt on the validity of this deeply entrenched ‘lode-gold paradigm’. Ore-grade orogenic disseminated gold mineralisation has been recognised in a number of fold belts that are known to host significant orogenic lode gold deposits (Lachlan Orogen, Australia; Buller Terrane and Otago Schist, New Zealand; Meguma Terrane, Canada; Tien Shan Mountains in Uzbekistan and Kyrgyzstan; Yana-Kolyma Province, Baikal, Verhoyansk and Allakh-Yun fold belts in north–central Mongolia and Far East Russia). Disseminated mineralisation in these collisional belts is invariably hosted by pervasively altered greenschist to subgreenschist facies rocks. These occurrences also share a number of characteristics with some Carlin-type gold deposits in the Great Basin of western North America (e.g., Getchell) regarding structural relationships, fluid composition, mineral paragenesis and a common association of carbonaceous matter with mineralisation. Features of alteration, physico-chemical characteristics of the ore-bearing fluids and strong structural control in all of the studied occurrences point to a close genetic association with lode gold mineralisation. The two styles are considered to represent end-members of a crustal continuum of orogenic gold emplacement in Phanerozoic fold belts, with disseminated mineralisation more likely to develop at shallower levels and within more permeable units during the waning stages of uplift and exhumation. These conditions facilitate ground preparation via brittle fracturing and the development of intricate stockwork systems. Efficient gold segregation from bisulphide complexes is largely controlled by fluid mixing of an evolved metamorphic fluid with either more oxidised ascending fluids or meteoric fluids, and accelerated by the presence of carbonaceous matter at the site of deposition.