THE EVOLVING EARTH AND OROGENIC GOLD

Orogenic gold deposit formation is inherent to juvenile crust formation throughout Earth history, thus reflecting an evolution from dominantly greenstone-hosted ores in the Precambrian to essentially identical ores in metasedimentary host rocks in the Phanerozoic. Processes that controlled the generation of ore-forming fluids varied little over geological time. These basically reflected synorogenic thermal events that mobilized fluids and metals during prograde metamorphism along active continental margins. However, evolving styles of plate tectonics on a cooling Earth affected the overall preservation potential of orogenic gold deposits and controlled the now well-documented temporal distribution of the ores. The secular decrease in global heat flow provided the ultimate control on these long-term changes in tectonic processes.

A scarcity of crust from the first few billion years of Earth history reflects the extremely rapid recycling of juvenile material on a very hot early Earth. Large blocks of continental crust evolved into well-defined cratons between mainly ca. 2.8 and 1.7 Ga. Reflecting the importance of episodic plume events on controlling early plate tectonics and thus periods of crustal growth, orogenic gold formation was associated with greenstone belt development between 2.8-2.5 and 2.1-1.8 Ga. Relatively buoyant Late Archean-Paleoproterozoic subcontinental lithosphere mantle (SCLM) was essential for cratonization of the new crust and thus preservation of orogenic gold deposits in continental interiors for billions of years. These gold-bearing cratons have survived numerous break-up and collisional episodes, with their contained gold endowment still intact. Beginning in the Mesoproterozoic, crustal growth was less episodic as Cordilleran-style plate tectonics dominated accretionary continental margins. Negatively buoyant, relatively thin Phanerozoic SCLM favored uplift and erosion of accretionary orogenic belts, such that middle to upper crustal rocks that host the gold deposits are only preserved in belts originating during the most recent 600 m.y. Gold ores show a general correlation with the growth phases of the global supercontinent cycle, and may form in both the deforming forearc and backarc regions of an active continental margin.