TOPAZ CRYSTALS, BREWER MINE, CHESTERFIELD COUNTY, SC

Reactivation of mining and deepening the gold bearing ore pits at the Brewer Mine in 1987 by Brewer Gold Co., a subsidiary of Westmont Mining, Denver, Colorado provided a unique opportunity to briefly study weathered material placed on the heap leach pads. The mine is located within the Carolina Slate Belt about 80 km southeast of Charlotte, NC, along the SE flank of a east-northeast plunging anticlinorium. The Brewer was mined for gold, first as a placer, then as an open pit and underground mine; massive topaz was produced from 1941-1948 from residual accumulations and finally low grade cyanide treated heap leach gold was mined.

Ore is present in host rocks that appear to be mixed topaz meta-rhyolite, meta-rhyodacite flows, multi-episodic pyroclastic volcanic conduit breccia and tuff pipe with extensive alteration. Rocks are silicified, argillitized, prophyllitized and sericitized.

Massive pieces and fragments of topaz had been described earlier; prior to 1937, the gray fragments in the ore were assumed to be massive quartz (hornstone), not topaz. Topaz, an aluminum silicate mineral [Al₂SiO₄(F,OH)₂], occurs as late stage disseminated fine grains, as patches and streaks of aggregate, as irregular veins and as masses of microcrystalline rounded individual grains, microns in diameter; only a few grains had euhedral form. Massive cryptocrystalline topaz replaces quartz and andalusite in a NW trending fault zone (Stonehouse and Zwaschka,1991); cross cutting relationships show topaz to be a later feature. Petrographic study showed evidence for two topaz events: (a.) finely disseminated topaz occurring throughout a hydrothermal andalusite-quartz zone; (b.) massive fine grained topaz replacing andalusite, quartz and kyanite related to regional or contact metamorphism.

Topaz found in 1987 is dark gray to gray, or mottled blue-gray to light green-gray, and consists of cleavage fragments (001) and suhedral and euhedral crystals up to 7.5 cm long and 3.0 cm wide, and rounded crystals present in saprolite and residuum. The large crystals and cleavage fragments (7.5 -10.0 cm.) were found in stockpiled ore after rains.