A scale-integrated exploration model for orogenic gold deposits based on a mineral system approach

Concept-based orogenic gold exploration requires a scale-integrated approach using a robust mineral system model. Most genetic hypotheses for orogenic gold deposits that involve near-surface or magmatic-hydrothermal fluids are now negated in terms of a global mineral system model. Plausible models involve metamorphic fluids, but the fluid source has been equivocal. Crustal metamorphic-fluid models are most widely-accepted but there are serious problems for Archean deposits, and numerous Chinese provinces, including Jiaodong, where the only feasible fluid source is sub-crustal. If all orogenic gold deposits define a coherent mineral system, there are only two realistic sources of fluid and gold, based on their syn-mineralization geodynamic settings. These are from devolatilization of a subducted oceanic slab with its overlying gold-bearing sulfide-rich sedimentary package, or release from mantle lithosphere that was metasomatized and fertilized during a subduction event, particularly adjacent to craton margins. In this model, CO2 is generated during decarbonation and S and ore-related elements released from transformation of pyrite to pyrrhotite at about 500
C. This orogenic gold mineral system can be applied to conceptual exploration by first identifying the required settings at geodynamic to deposit scales. Within these settings, it is then possible to define the critical gold mineralization processes in the system: fertility, architecture, and preservation. The geological parameters that define these processes, and the geological, geophysical and geochemical proxies and responses for these critical parameters can then be identified. At the geodynamic to province scales, critical processes include a tectonic thermal engine and deep, effective, fluid plumbing system driven by seismic swarms up lithosphere-scale faults in an oblique-slip regime during uplift late in the orogenic cycle of a convergent margin. At the district to deposit scale, the important processes are fluid focussing into regions of complex structural geometry adjacent to crustal-scale plumbing systems, with gold deposition in trap sites involving complex conjugations of competent and/or reactive rock sequences and structural or lithological fluid caps. Critical indirect responses to defined parameters change from those generated by geophysics to those generated by geochemistry with reduction in scale of the mineral system-driven conceptual exploration.