Abstract

The Archean was the principal period for gold mineralization.These rocks compose ∼12% of the exposed crust but have produced more than half of the world's gold. Most major lode deposits formed at 2.7 ±0.2 Ga. This corresponds to the principal time of crustal thickening and stabilization, an essential part of which was formation of granulite, the anhydrous, refractory base for the crust. Upward streaming of mantle CO2 along shear zones in the lower crust is one means for dehydrating amphibolite to form granulite and is accompanied by removal of large ion lithophile (LIL) elements to higher crustal levels. Archean lode-gold deposits were formed along major shear zones at intermediate crustal depths. They are enclosed by zones of CO2 metasomatism, many of them kilometres in extent; δ13C data indicate that the CO2 is juvenile. Some major Archean deposits have been shown to have formed from relatively oxidized hydrothermal fluids. CO2 streaming with LIL-element depletion could only have occurred under relatively oxidized conditions that permitted a free CO2 vapor to exist in the lower crust. Such conditions would also have favored dissolution of gold and associated sulfide and their transfer to the mid-crust in a CO2-H2O fluid, the H2O coming from dehydration of amphibolite.

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