Towards a universal model for orogenic gold systems: A perspective based on Chinese examples with geodynamic, temporal, and deposit-scale structural and geochemical diversity

Although the term orogenic gold has become widely accepted over the past 20 years for disseminated- to lode-style gold deposits that formed in a variety of tectonic environments within convergent margins, models for orogenic gold systems have remained controversial. The main debates include interpretation of stable isotope and fluid inclusion data that are particularly equivocal for orogenic gold systems, the nature of lithospheric control on the generation of orogenic gold belts, and more critically, the role of deep crustal regional metamorphism or alternatively metasomatized and fertilized mantle lithosphere as sources of ore metals and fluids. The giant to world-class orogenic gold systems of China, now the premier gold producer globally, are in gold provinces or belts of several ages that are mostly marginal to Precambrian cratons or blocks, including the North China Craton and South China Block, an amalgamation of the Yangtze Craton and Cathaysia Block. Deposits from these gold provinces provide crucial evidence to constrain genetic debates. Importantly, following the acceptance that the giant Jiaodong gold province formed over 1700 million years after regional metamorphism in the host terrane, an increasing number of deposits, including the Phanerozoic hypozonal deposit at Danba, are shown to have postdated regional metamorphism and formed from components that were not derived from the host crustal rock sequences. A subcrustal model involving devolatilization of fertilized mantle lithosphere, that was metasomatized via earlier subduction processes involving oceanic sediments and/or altered oceanic crust, is compatible with a variety of data. This includes 40Ar/36Ar vs. 3He/4He mantle-like plots for gold-related pyrite, sulfur isotope compositions of ore-related sulfides that are incompatible with a crustal source, and PGE distribution patterns of spatially related mafic dykes that implicate elevated sulfide contents of underlying mantle lithosphere. Chinese orogenic gold deposits formed in a variety of tectonic regimes from syn-subduction slab rollback, post-subduction slab breakoff, and lithosphere thinning along craton margins, to continent collision. In all cases, lithosphere-scale shear zones and faults were the conduits for ore fluids and metals derived episodically from devolatilization of a subducted slab and/or long-lived metasomatized and fertilized mantle. These fluids deposited gold mineralization in rock sequences adjacent to subsidiary shear zones and faults from crustal depths of >20 km to <5 km, with hydraulic fracturing and fluid phase separation potentially the most important depositional process for lode deposits and fluid-rock reaction dominant in disseminated deposits. As the H2O-CO2 ore fluid containing H2S infiltrated into structural-lithological traps, it reacted with the wall rocks under P-T conditions related to depth of gold deposition to produce a continuum of broadly depth-related alteration mineral assemblages. The characteristics of the widespread orogenic gold systems of China strongly implicate a sub-crustal fluid and metal source for orogenic gold in a variety of gold provinces. Based on deposits from outside of China, where gold provinces are more distal to Precambrian cratons, this source is most likely from the devolatilization of down-going subduction zones and sediment wedges in addition to devolatilization of previously fertilized mantle lithosphere as implied by the Chinese examples.