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

Orogenic gold mineralisation in the Lachlan Orogen of southeastern Australi a and in other Phanerozoic fold belts has generally been assumed to be conf ined entirely to quartz veins. However, an improved understanding of the te ctonic, structural and geochronological framework in which these occurrence s form, and recent advances towards the extent of wallrock alteration aroun d these veins, cast doubt on the validity of this deeply entrenched 'lode-g old paradigm'. Ore-grade orogenic disseminated gold mineralisation has been recognised in a number of fold belts that are known to host significant or ogenic lode gold deposits (Lachlan Orogen, Australia; Buller Terrane and Ot ago Schist, New Zealand; Meguma Terrane, Canada, Tien Shan Mountains in Uzb ekistan and Kyrgyzstan; Yana-Kolyma province, Baikal, Verhoyansk and Allakh -Yun fold belts in north-central Mongolia and Far East Russia). Disseminate d mineralisation in these collisional belts is invariably hosted by pervasi vely 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 stru ctural relationships, fluid composition, mineral paragenesis and a common a ssociation of carbonaceous matter with mineralisation. Features of alterati on, physico-chemical characteristics of the ore-bearing fluids and strong s tructural control in all of the studied occurrences point to a close geneti c association with lode gold mineralisation. The two styles are considered to represent end-members of a crustal continuum of orogenic gold emplacemen t in Phanerozoic fold belts, with disseminated mineralisation more likely t o develop at shallower levels and within more permeable units during the wa ning stages of uplift and exhumation, These conditions facilitate ground pr eparation via brittle fracturing and the development of intricate stockwork systems. Efficient gold segregation from bisulphide complexes is largely c ontrolled by fluid mixing of an evolved metamorphic fluid with either more oxidised ascending fluids or meteoric fluids, and accelerated by the presen ce of carbonaceous matter at the site of deposition. (C) 2001 Elsevier Scie nce B.V. All rights reserved.