New Mexico Geological Society Annual Spring Meeting — Abstracts

Tertiary age (28 m.y.) high-silica, high-potassium rhyolite lavas host a number of tin occurrences in the northern Black Range and Sierra Cuchillo of southwestern New Mexico. These lavas are typically crystal rich, white, flow banded rocks that form thick flows and domes and contain >75 percent SiO₂, >4.5 pe rcent K₂O, <0.5 percent CaO, and 2.5 to 4 percent Na₂O. Rb-Sr ratios are typically 10 to 20 with Sr values about 2 to 40 ppm and high initial 87Sr/86Sr ratios. Rare earth element patterns are flat with .180 of about +8 per mil (SMOW).

Vapor phase recrystallization due to fluids evolved during cooling bleached the rhyolites and locally produced a punky, friable rock consisting of the phenocrysts and small amounts of remnant groundmass. The tin deposits are always associated with vapor phase recrystallized zones, but a number intensely recrystallized zones do not have obvious associated tin. The tin occurrences are at the margin of the most intensely recrystallized areas and near the tops and sides of the rhyolite domes, just outboard from vent areas, as evidenced by the Taylor Creek occurrence in which the tin is just below the carapace breccia of the dome.

The primary mineralization consists of fine to coarse, intergrown crystalline hematite and cassiterite. Wood tin (amorphous SnO₂) is found in placers and rarely in veins. The general paragenesis is minor quartz; hematite with no cassiterite: hematite and cassiterite in varying proportions; calcite; quartz, fluorite, and hematite in varying proportions; and last, zeolites. This sequence is rarely seen in a single occurrence. Locally, hematite and cassiterite have replaced magneite. The position in the paragenetic sequence of the magnetite is problematic at this time. The origin of the tin deposits is uncertain, but temporal and spatial relationships of the rhyolite lavas and the tin deposits suggest that the tin is related to magmatic and/or hydrothermal activity during cooling of the lavas.