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The variation in As content of pyrite from three gold deposits (Agnico-Eagle, Quebec; Fairview and Pilgrim’s Rest, South Africa) has been studied by color-staining polished surfaces with a solution of KMnO4 in 1:1 H2SO4 followed by electron microprobe analysis. Pyrite grains from strata-bound and stratiform gold deposits are commonly oscillatory zoned in As content on a fine scale, with band widths on pyritohedral growth surfaces as narrow as 0.5 (im, Microprobe analyses with a surface resolution frequently overlapping several bands of alternating high and low As content give maximum average As contents of 4 to 8 wt percent (2-5 at. %). The As appears to be incorporated in pyrite as a metastable solid solution of the type Fe(S,As)2. These pyrite grains indicate that the As and S contents of the fluids responsible for the gold mineralization varied episodically. Many pyrite grains are corroded: low As cores have been dissolved away either to leave atoll-like structures or to be filled in and overgrown by later low As pyrite. As-bearing pyrite also has a TEM microstructure of (100) planar faults which under high resolution are seen to be thin coherent lamellae of marcasitelike structure. Idiomorphic growth zones in pyrite grains reveal that As (and presumably Au) mineralization was an early feature of strata-bound-stratiform gold deposits.

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