The gold content of some Archaean rocks and their possible relationship to epigenetic gold-quartz vein deposits

Abstract

Gold mineralization in Archaean granite-greenstone environments, especially gold-quartz veins, contributes considerably to the world's gold production. The formation of epigenetic gold mineralization in greenstone belts is generally explained by the metamorphic secretion theory. This theory is based on the assumption that the source of the gold may be komatiitic or tholeiitic lavas, pyritic chemical or clastic sediments and even granitic rocks from which, as a result of regional metamorphic overprinting, gold was extracted and concentrated in suitable structures. It has been shown that in proposed potential source rocks, gold is predominantly associated with sulfide minerals and thus relatively easily accessible to secretion and reconstitution processes. A large number of various rock types originating from granite-greenstone terranes of the Kaapvaal and the Rhodesian cratons were geochemically investigated, and the following ranges for gold determined: volcanic rocks (komatiitic and tholeiitic): 0.1–372 ppb granitic rocks of the basement: 0.3–7.8 ppb iron-rich chemical sediments: 1.0–667 ppb Statistical treatment of the data reveals that volcanic rocks as well as iron-rich chemical sediments are favorable sources for epigenetic gold mineralization formed by metamorphic secretion, while the granitic rocks make less suitable primary gold sources. This finding explains the close spatial relationship which is common between gold-quartz veins and greenstone belts. The conspicuous abundance of epigenetic gold mineralization in the Archaean, however, is attributed to the unique geologic and metamorphic history of the granite-greenstone terranes.