GOLD MINERALIZATION IN AS-RICH MESOTHERMAL GOLD ORES OF THE BOGOSU-PRESTEA MINING DISTRICT OF THE ASHANTI GOLD BELT, GHANA - REMOBILIZATIONOF INVISIBLE GOLD

The Bogosu-Prestea mining district of southwestern Ghana is a 33 km se ction of the Early Proterozoic Ashanti Gold Belt. Greenschist facies c arbonaceous and carbonate-bearing turbidites and greywackes, and mafic dikes host numerous economic mesothermal gold deposits. Structurally higher ores in the Bogosu concession have brittle deformation and cons ist of disseminated-sulphide lodes in tectonically-disrupted sedimenta ry rocks and carbonate-altered mafic dikes. Most gold occurs as microm etre-size particles in arsenian pyrite, and as ''invisible'' gold in a rsenian pyrite and arsenopyrite. The structurally deeper ores of the a djoining Prestea concession are associated with brittle-ductile deform ation and consist of extensive crack-seal quartz-veins and graphitic s hear zones. Only minor amounts of ''invisible'' gold were detected; in these deeper lodes, gold occurs dominantly as abundant microscopic an d larger particles in sulphide/arsenide minerals and in gangue. The go ld distribution patterns revealed by SIMS microprobe analysis and ion maps, EMP and colour staining suggest that most of the primary gold in the Bogosu-Prestea system precipitated in solid-solution with sulphid e/arsenide minerals. However, postdepositional concentration and redis tribution occurred, in increasing degree with: 1) increase in metamorp hic/hydrothermal gradients in the gold system (depth), 2) decrease in the refractory properties of the host mineral, and 3) increase in the amount of post-depositional, host-mineral recrystallization and deform ation. Gold evolved from primary solid-solution within sulphide/arseni de minerals, to colloidal and micrometre-size particles concentrated i n voids, fractures and internal grain boundaries, and finally to micro scopic and larger particles at sulphide/arsenide grain margins and in the gangue assemblage. The general conclusions presented here are appl icable to As-rich gold deposits of all ages, worldwide. The presence o f gold in late fractures is insufficient evidence for late-stage intro duction of gold in mesothermal gold systems.