Tin-bearing Rhyolites, Black Range, New Mexico
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 zones in widely spaced cross-cutting veinlets of cassiterite ± wood tin (colloform cryptocrystalline cassiterite), and (3) most abundantly as widespread placer accumulations of wood tin ± cassiterite in drainages within or marginal to the domes. Veinlet minerals include early quartz, K-rich sanidine and topaz, followed by hematite, cassiterite ± wood tin, and cristobalite. Late chalcedony, fluorite, durangite, clays, zeolites, and a complex suite of unusual minerals are locally present in and near veinlets.
The δ 18O values of most individual minerals are remarkably uniform throughout the area (cassiterite, 4.3 to 3.2 ‰; wood tin, 3.0 to 1.2 ‰; hematite, 3.5 to 2.6 ‰). Within these narrow ranges the δ 18O values for wood tin and hematite are distinctive in each of the three types of tin occurrences and they generally decrease paragenetically in individual veinlet and placer samples. The δ 18O values of temporally overlapping cristobalite in veinlets, however, range from 9.7 to 11.8 ‰ and increase paragenetically. The δ 18O values for quartz in miarolitic cavities containing cassiterite are nearly the same as those for adjacent rhyolite quartz phenocrysts (7.5 ‰).
A consistent Δ18O of 1.0 ‰ for quartz-sanidine phenocrysts indicates isotope equilibration temperatures of ~700°C for rhyolite during dome emplacement. Temperatures based on quartz-hematite 18O fractionations range from ~800°C (miarolitic cavities) to ~485°C (wood tin–bearing veinlets). Late cristobalite in the veinlets precipitated at a maximum temperature of only ~230°C. Cassiterite-hematite Δ18O values are uniform (0.7 ± 0.1 ‰), suggesting that the cassiterite-water curve is parallel to the hematite-water curve between 800° and 400°C. An empirical 18O fractionation curve for cassiterite-water suggests that most wood tin precipitated above ~350°C. The δ 18O and δD values of smectite from pervasive argillic alteration range from 10.9 to 15.1 ‰ and −73 to −107 ‰, respectively, and along with whole-rock data on altered rocks, imply alteration temperatures of ⩽235°C. The δ 18O values for chalcedony range from 15.9 to 34.4 ‰ and imply temperatures generally ⩽160°C for local silicification of rhyolite.
The δ 18OH2O of the tin-bearing fluids was 8.0 ±0.5 ‰, indicating that the fluids equilibrated with a magma or high-temperature rhyolite throughout most of the time-space milieu of mineralization. However, limited data indicate that the δD H2O of the fluids ranged from −60 to at least as low as −101 ‰ and may have decreased with successive stages of mineralization, even though there is no evidence of mixing of the fluids with unexchanged meteoric water. Isotopically similar fluids may have been responsible for the pervasive alteration of the host rock.
Integrated stable-isotope, fluid-inclusion, and petrographic and geologic data suggest that tin mineralization resulted from NaCl-saturated fluids that derived from shallow magmas or that equilibrated with high-temperature rhyolite. Tin mineralization resulted largely from rapid temperature decrease and decompression of vapor-rich fluids, which produced cassiterite in fractures at deeper levels and wood tin in the carapace of the domes. At shallow levels, HCl from the hydrolysis of NaCl permeated the wall rock adjacent to the widely spaced veinlets and contributed to the pervasive alteration of the glassy groundmass of the rhyolite. This model suggests that the tin resource potential of the area has already been realized with the mining of the placer wood tin.