gems, Brazil, topaz, imperial topaz, Capão, Minas Gerais

Topaz deposition near Ouro Preto, Minas Gerais, Brazil, took place in the Sabará Formation, a Precambrian phyllite of considerable compositional range. Topaz and other minerals were deposited in and adjacent to a right-lateral fault and associated tension fractures in the Sabará. The tension fractures occur where the Sabará had a relatively low sericite content; elsewhere the formation deformed by flowage, and little or no topaz was deposited. Topaz, specularite and euclase were deposited by hydrothermal solutions that altered and replaced the Sabará along fractures and permeable beds intersected by them. Later hydrothermal solutions contained less fluorine, as indicated by feldspar deposition concurrent with and slightly later than the topaz. Quartz and muscovite, which are consistently present in the topaz-bearing rock, together with orthoclase subsequently altered to kaolin, were derived from the host silicates by hydrothermal alteration. Weathering has reduced the original hydrothermal assemblage to a mixture of quartz, topaz, sericite and kaolinite.