Deposit-scale structural controls on orogenic gold deposits: an integrated, physical process–based hypothesis and practical targeting implications

Abstract

The application of deposit-scale structural targeting to orogenic gold deposits is primarily concerned with locating volumes of higher-grade mineralisation (ore shoots) within a broader, host structural system. Although empirical correlations between structural heterogeneities and ore shoots have long been noted and certain key physical processes elucidated, no integrated, system-based synthesis has been previously published. However, linking the existing understanding with important recent advances in understanding of the dynamic physical processes of emplacement of hydrothermal ore systems now provides the opportunity to develop such a synthesis. The most important of these new concepts includes the idea that deposits form in vertically extensive conduit systems linking source to sink and the related concept of the phenomena of injection-driven swarm behaviour. Integration of all of these elements leads to the proposal of a coherent physical process–based model for the structural emplacement of orogenic gold systems and the local formation of gold-enriched volumes. This model considers that an orogenic gold deposit must be viewed from the perspective of three different geometric elements, each with a particular meaning in terms of physical process, and which typically relate to different scales. These elements include (a) the integrated swarm volume (ISV), which is the rock volume which bounds the limits of the mineralisation-hosting structural system and is effectively the geological entity that defines a deposit; (b) fluid pathways, which are 3D pathways through the ISV where dynamic ore fluid emplacement has been repeatedly strongly focused; and (c) ore shoots, which are localised volumes of high-grade gold deposition that are interpreted to represent volumes of anomalous dilation or second-order valve sites\xa0 within host fluid pathways. Consideration of this new framework suggests significant potential to improve the effectiveness of structural targeting at the deposit scale. In particular, ore shoots are only expected within fluid pathways, and equivalent dilatant zones outside such pathways are predicted to be barren. A key theme is that the rheological architecture of the host rock mass is the most important predictive input to targeting, both at the scale of emplacement of the ore-hosting conduit system and the scale of localised ore shoots.