Orogenic gold and the mineral systems approach: Resolving fact, fiction and fantasy
Abstract The fact that mineral deposit attributes such as the size frequency of orogenic gold deposits in specific provinces exhibit power law distributions similar to forest fires, earthquakes, and fault size populations, is a compelling motivation to examine their genesis from a systems context. Based on well studied Earth systems such as climate, the systems related to mineral deposits are likely to be complex and potentially include sensitive dependent components that vary simultaneously and in subtly interconnected ways. Although a "systems approach" was enunciated for mineral exploration by Fyfe and Kerrich as early as 1976, it is yet to be fully embraced by the geosciences community that commonly retain models dependent primarily on deposit scale characteristics. Orogenic gold deposits are well studied and widely considered to represent a single class of deposit that has formed over much of Earth history in settings ranging from Archean granite greenstone belts to Phanerozoic turbidite sequences. Accordingly, the deposit type is well suited for assessment within a systems context. If orogenic gold deposits do in fact represent a single class of deposits, then the simplest application of a systems approach highlights the fact that the nature of the host upper crustal succession cannot be a fundamental control, with specific granite suites and pyritic sediments not universal, or at least not essential, components of the system. Furthermore the scale of orogenic gold systems implicates processes capable of tapping sub crustal source regions. Increasingly, advances in orogenic gold systems, and mineral systems in general, are linked to application of systems science that emphasize importance of system driven criticality. Orogenic gold systems and other mineral systems are typically short in duration and linked in time and space to tectonic triggers. The latter promote a rapid release of energy 'avalanches' that overcome system thresholds and are strong indicators of complex systems that may show power law behavior. Only a rigorous application of a systems approach can cut through the confusion that arises from conflicting models based on local deposit studies. Only a systems approach can evaluate the significance of rare or anomalous features in a small number of deposits. Truly predictive models for mineral exploration will ultimately be developed by workers who adhere to the systems approach.