Evolution of gold mineralization in the Ashanti Gold Belt, Ghana: Evidence from carbonate compositions and parageneses

Ankerite, siderite, calcite and magnesite occur in variable proportions within all host and mineralized rocks of the Bogosu and Prestea mining districts of the Ashanti Gold Belt, Ghana. The compositions of coexisting ankerite-siderite grains establish that complex rhythmically zoned growth banding and replacement textures are present. This compositional variation is attributed to episodic fluctuation in the temperature and composition of fluids in the Bogosu-Prestea mesothermal gold system. Temperatures derived from the ankerite-siderite composition geothermometer are generally consistent with those from calcite-dolomite, arsenopyrite, carbon and oxygen stable isotope, and fluid inclusion geothermometers, and are about 360 degrees C for the metamorphic peak, 400 to 350 degrees C for carbonate alteration of mafic dikes, and 340 to 140 degrees C for gold deposition. The latter range occurs on a thin-section scale and represents separate pulses of fluid in the ore conduit.