A comparison of fluid inclusion data and mineralisation processes for Australian orogenic-gold and intrusion-related gold systems

摘要

There is considerable debate as to whether or not orogenic gold deposits and intrusion-related gold deposits should be regarded as separate classes of deposits or as variants of the same mesothermal gold system (e.g. Groves et al., 2003). Australia has a range of gold deposits that may be classified as either orogenic-gold or intrusion-related gold deposits. In this study we examine the fluid inclusion data and fluid chemistry of Australian gold deposits to determine if similar mineralisation processes apply to both styles of deposits. Orogenic gold deposits typically form along convergent margins during the latter part of the deformational-metamorphic history of terrane accretion (Groves et al., 2003). The deposits have a strong structural control involving faults or shear zones, folds, and/or zones of competency contrast. Host rocks are extremely variable, although there is an overall trend from volcanic- or intrusion-hosted deposits in Archean provinces to sedimentary rock-hosted deposits in Phanerozoic provinces. Although the deposits occur in rocks of metamorphic grade varying from subgeenschist to granulite facies, the majority are hosted in greenschist facies rocks. Quartz ± carbonate veins are ubiquitous and the deposits may extend to depths of up to 1 or 2 km, with only subtle metal zoning, and distinctive strong, lateral zonation of wall-rock alteration. A distinctive metal association (Au-Ag ± As ± B ± Bi ± Sb ± Te ± W) is characteristic of orogenic gold deposits, which are associated with near-neutral, low salinity fluids (typically < 10 wt.% NaCl equiv.) with CO_2 contents ranging from 10 to 25 mol.% (Ridley & Diamond, 2000).