New Mexico Geological Society Annual Spring Meeting — Abstracts

Two periods of alteration and possible mineralization have been defined in the north-central Black Range of New Mexico. Tin as cassiterite and wood-tin occurs with specular hematite in veins hosted by Tertiary rhyolite lava domes and flows. The host rock, the Taylor Creek Rhyolite, is a high-silica (76-78% of SiO₂), high-potassium (4-6% K₂O), and high fluorine (as much as 5000 ppm) rhyolite. The Taylor Creek Rhyolite is part of a bimodal assemblage of basaltic andesitic flows and high silica lavas and ash-flow tuffs erupted 22 to 28 m.y. ago. The volcanic rocks underlying the Taylor Creek Rhyolite are dominantly felsic ash-flow tuffs and mafic lava flows. Mafic lava flows, felsic-ash-flow tuffs, and basin-filling sedimentary rocks overlie the Taylor Creek.

Mineralization is restricted to the Taylor Creek Rhyolite as is the alteration, suggesting a genetic and temporal relationship. The mineralization consists of discontinuous hematite-cassiterite veins with minor wood-tin, quartz fluorite, and cristobalite. Locally, hematite and cassiterite are disseminated through the host rock. Preliminary ¹⁸O/¹⁶O determinations suggest that the fluids responsible for the alteration were cool magmatic water or enriched meteoric water.

Alteration ranging from propyllitic to quartz-sericite accompanies a group of rhyolite porphyry intrusives near the crest of the Black Range. Pyrite is locally present in the quartz-sericitic alteration. The intensity of the alteration increases toward the intrusives and has been noted 2 to 3 km from them. The age of these intrusives is unknown but it intrudes and alters rocks that are aboltt 24-28 m.y. old. As with the Taylor Creek alteration. preliminary ¹⁸O/¹⁶O determinations suggest cool magmatic water or enriched meteoric water.