EVOLUTION OF GOLD MINERALIZATION IN THE ASHANTI GOLD BELT, GHANA - EVIDENCE FROM CARBONATE COMPOSITIONS AND PARAGENESES

Ankerite, siderite, calcite and magnesite occur in variable proportion s within all host and mineralized rocks of the Bogosu and Prestea mini ng districts of the Ashanti Gold Belt, Ghana. The compositions of coex isting ankerite-siderite grains establish that complex rhythmically zo ned growth banding and replacement textures are present. This composit ional variation is attributed to episodic fluctuation in the temperatu re and composition of fluids in the Bogosu-Prestea mesothermal gold sy stem. Temperatures derived from the ankerite-siderite composition geot hermometer are generally consistent with those from calcite-dolomite, arsenopyrite, carbon and oxygen stable isotope, and fluid inclusion ge othermometers, and are about 360 degrees C for the metamorphic peak, 4 00 to 350 degrees C for carbonate alteration of mafic dikes, and 340 t o 140 degrees C for gold deposition. The latter range occurs on a thin -section scale and represents separate pulses of fluid in the ore cond uit.