Abstract

It is generally thought that orogenic gold deposit formation is associated, in many cases, with metamorphic fluids. These fluids are mainly liberated by chlorite breakdown in the upper greenschist facies, and there is some debate as to whether the pyrite-to-pyrrhotite desulfidation reaction precedes or coincides with this fluid release. Since pyrite is an important metal host in metamorphic rocks, its breakdown to pyrrhotite may allow complexation of Au with liberated sulfur. Thus, the question of the temperature-pressure conditions required for pyrite desulfidation is important because it determines which tectonic processes and which parts of the crust are relevant to gold deposit genesis. Here, we investigate metal redistribution during pyrite breakdown using laser ablation-inductively coupled plasma-mass spectrometry and bulk-rock analysis on greenschist to amphibolite facies rocks of the Ballachulish metamorphic aureole, Scotland. Results suggest that the pyrite-to-pyrrhotite transition field spans a broader temperature range than indicated by recent modeling, and that pyrite breaks down simultaneously with chlorite. Gold, As, Bi, Sb, Mn, and W were likely removed from the rock in chlorite-derived fluid. Nickel, Co, Ga, Ge, Mo, Tl, V, and Cr are retained in the rock by uptake in pyrrhotite. Copper is retained by growth of chalcopyrite as temperature increases, whereas Pb and Zn are retained by progressive uptake in nonsulfide minerals; these elements are not depleted during amphibolite facies metamorphism. The tendency of S to react with silicate-hosted Fe and be retained in the rock implies that Fe-poor pyritic sedimentary rocks may produce more S- and metal-rich fluids in source regions.

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