The topaz-bearing rhyolites from the San Luis Potosi area (Mexico); characteristics of the lava and growth conditions of topaz

In Mexico, the topaz-bearing rhyolites occur mainly in the San Luis Potosi area (San Luis Potosi and Guanajuato States). These rhyolites appear as domes related to the Tertiary extensional tectonism. Three domes of the San Luis Potosi volcanic field were selected for study, according to the color, the habits and the size of the topaz they contain, in order to determine the characteristics of the lava and the growth conditions of the topaz. These rhyolites are high-silica, metaluminous to slightly peraluminous and alkali-rich lavas. They are enriched in fluorine and in incompatible lithophile elements such as Rb, Cs, Ta, U and Th and depleted in Ba, Sr, Ca, Mg, Ti and Ni. Therefore, they are very similar to the topaz-bearing rhyolites from western United States. In Cerro El Gato dome, both amber-colored and colorless topazes crystallized in voids and fractures. Comparing these different crystals with respect to their growth environment, habits, chemical compositions and EPR characteristics provides an explanation for the crystallization conditions and color of topaz. Colorless topaz from Cerro El Gato crystallized at a temperature above 500°C (lack of color centers) from fluids enriched in elements leached from the lava, whereas the amber-colored topaz crystallized below 500°C (based on the presence of color centers) and from fluids richer in volatile elements (As). In Cerro El Lobo the topaz have intermediate characteristics between the colorless and the amber-colored topazes of Cerro El Gato.