Tin mineralization in the Black Range is present in a series of Oligocene rhyolite domes that cover an area of approximately 200 km² in southwestern New Mexico. These rhyolites are typically high silica (76 to 78 percent), peraluminous, and topaz bearing. The δ ¹⁸O values (~7.0 ‰) of unaltered rocks are typical of normal I-type granites.

Locally along dome margins, the glassy groundmass of the rhyolite is pervasively altered to smectite, although sanidine phenocrysts are usually fresh, even in contact with tin-bearing veinlets. Tin occurs (1) rarely as cassiterite in miarolitic cavities near tin-bearing veinlets, (2) within altered...