Mineralogical domains within gold provinces

Abstract

Mineralogical domains use hypogene minerals (i.e. minerals not modified by weathering) and related geochemical characteristics of mineral occurrences, not only ore deposits, to subdivide large mineralised regions. Their use in the Victorian gold province is described using readily available historical data and field checking, and this is a scheme that has not required modification since 1997.The Victorian province is typical of sediment-hosted hydrothermal ores in metamorphic terrains (often termed orogenic gold deposits). Five distinctly different mineral assemblages are used to subdivide all Victorian gold occurrences into eight domains up to hundreds of kilometres in length and tens of kilometres in width. These parallel the regional structural trend and most are closely associated with, or sharply bounded by, major regional-scale faults. Seismic work has shown these faults to be listric thrusts, which flatten into a zone of duplexed greenstones overlying older basement rocks in the deeper crust.

Although not defined genetically or temporally, mineralogical domains provide an additional variable related to fluid flow to assist genetic interpretation such as the scale at which a combination of processes operates, permitting predictions as to the origin of the fluids and their pathways. The variations in mineralogy in Victorian gold occurrences indicate that ore fluid compositions differed significantly between adjacent domains, and between areas overlying different regions of deeper crust. The pattern of domains gives clues to the existence of multiple mineralising events and to the degree of overprinting of these events. Domains also assist genetic comparisons by projection into similar adjoining regions to create new domains, for example Tasmania (Mathinna domain and Lefroy sub-domain), NSW (Cobar domain) and New Zealand (Reefton domain). The domainal pattern has application to mineral exploration, metallurgy and environmental issues. Mineralogical domains could be applied elsewhere, particularly in the study of difficult-to-subdivide sediment-hosted gold ores and Archaean greenstone-hosted gold, and possibly for other commodities, especially those that occur as hydrothermal ores.