The Boring Billion: A key to resolving controversy on ore-fluid source models for orogenic gold deposits?

Orogenic gold systems are arguably the most variable mineral system globally in terms of an extreme range of depositional depths, corresponding P-T conditions and wallrock alteration assemblages, structural controls and styles, and element associations. This diversity has ignited controversy on genetic models for the two decades since orogenic gold became a widely accepted term. From the diverse genetic models proposed, the two groups of fluid-source models that meet most genetic constraints are the following: (1) deposition from crustal fluids via metamorphic devolatilization at the amphibolite-greenschist transition, or potentially even deeper under specific tectonic conditions, and (2) deposition from sub-crustal fluids either by direct devolatilization of subducted oceanic crust and overlying sediment wedge or of previously metasomatized and fertilized mantle lithosphere. Both models normally postulate gold deposition within a geodynamic system that evolves from extension through compression into syn-gold transpression. Crustal metamorphic models normally invoke subduction-driven geodynamic systems that involve advection of crustal metamorphic fluids up crustal-scale faults. In contrast, sub-crustal devolatilization models involve subduction-related processes as both geodynamic drivers and gold sources with fault-controlled fluid conduits extending to below the Moho. The overall lack of orogenic gold and other subduction-related mineral systems during the unique Boring Billion (1.8-0.8 Ga) period provides an important constraint on this genetic debate. Boring Billion orogens had varying geodynamic drivers, asthenosphere upwelling, and low-P metamorphic terranes with crustal-scale faults, all parameters consistent with formation of orogenic gold systems, during subduction-independent accordion-type tectonics. The absence of orogenic gold during the Boring Billion provides critical evidence against the crustal metamorphic model and furthers the sub-crustal model which requires subduction as both the geodynamic driver and auriferous fluid source.