RAPID DURATIONS OF ORE DEPOSIT FORMATION AND SELF-ORGANISING PHENOMENA: IMPLICATIONS FOR TARGETING?

This talk briefly reviews the physical hydrogeology of epithermal- and orogenic-type gold deposits and their linkages to self-organizing phenomena, such as earthquake-aftershock sequences, seismic swarms and fluid pressure driven failure. It is shown how these fundamental concepts led to the development of exploration targeting tools based on Coulomb failure stress changes and earthquake mechanics. Building on these developments we use a combination of field observations and boundary element modelling to explore the relationship between fault stepovers, fluid flow and mineralization. It is shown that underlapping stepover geometries are typically rare in fault systems but anomalously associated with gold deposits. Further, the along-strike and across-strike dimensions of both underlapping and overlapping fault stepovers fit, to a first-order approximation, the same self-similar trend. We find that a larger region of damage and permeability enhancement is created around underlapping stepovers than around overlapping stepovers. By taking into account both the enhancement and decay of permeability during the seismic cycle it is estimated that a 5 Moz goldfield could feasibly form in 1-16 earthquake-aftershock sequences; potentially representing durations of just 10-8000 years. Similar rapid durations for goldfield formation have been inferred from different environments, including volcano sector collapse (e.g. Lihir, Papua New Guinea) and convective circulation in geothermal regions (e.g. Taupo Volcanic Zone, New Zealand). The result that deposits form extremely rapidly, even though connected to fault systems with million-year lifespans, may suggest that economic grade mineralization requires several different self-organizing phenomena to overlap in time and space. Such a concept potentially explains why deposits cluster in metal-rich provinces but with multiple episodes of mineralization, sometimes millions of years apart.