Petrology and geochemistry of rhyolites associated with tin mineralization in northern Mexico.

Tin deposits of the Mexican tin belt consist of cassiterite and wood tin filling fractures and faults in rhyolite flows and domes (host rhyolite), and coating fragments and disseminated in the matrix of rhyolitic ignimbrites overlying the host rhyolites. The host rhyolites are metaluminous or slightly peraluminous high- silica rhyolites, contain more Sn and F than associated rhyolites lacking Sn deposits, and are more radioactive than average for the Sierra Madre Occidental volcanic rocks. Chemical data indicate that the 'tin rhyolites' formed as extreme differentiates in high-level magma chambers closely related to caldera development. Crustal contribution may account for some Sn or F enrichment in the parent magmas, but differentiation was the major factor in concentrating Sn in the host rhyolites. The Mexican tin belt clearly formed from highly differentiated magmas that were an integral part of the Oligocene Sierra Madre Occidental volcanic pile, but restricted to its eastern flank. -G.J.N.