Extreme halogen abundances in tin-rich magma of the Taylor Creek Rhyolite, New Mexico

Abstract

Crystal-bearing glass inclusions are hosted by quartz phenocrysts in eruptive units of the tin-mineralized Taylor Creek Rhyolite, New Mexico. Inclusions from four units were refused at 2 to 4 kbars and temperatures > or = 890 degrees C and analyzed for major and minor elements by electron microprobe and for H, Li, Be, B, Rb, St, Y, Nb, Mo, Sn, Cs, Ce, Th, and U by ion microprobe. Average glass inclusion compositions are chemically representative of Taylor Creek melt, and comparison of inclusion compositions with their whole-rock equivalents suggests that Li, F, Cl, Ca, and H 2 O degassed from cooling lava and/or magma after inclusion entrapment.Taylor Creek magma was strongly and variably enriched in F and Cl prior to eruption; average F concentrations of melt apparently ranged from 0.15 to 3.9 wt percent. Eruptive units containing the most F-enriched glass inclusions are characterized by comparatively high average molar (Na 2 O/Na 2 O + K 2 O) ratios, low Li concentrations, and relatively low phenocryst contents which are compatible with high preeruptive concentrations of F in magma. Average C1 concentrations were more uniform, ranging from 0.23 to 0.37 wt percent. High concentrations of F and Cl are in accord with the style and extent of alteration in vapor-phase altered parts of Taylor Creek Rhyolite and are consistent with alkali chloride-rich fluid inclusions in vapor-phase minerals of altered rhyolites. Preeruptive enrichments in other volatiles were not as significant; B in melt was <100 ppm and the apparent H 2 O content of melt was <2 wt percent.Geochemical studies of other tin-topaz rhyolites and molybdenite-bearing, Climax-type, porphyritic granites imply that their magmatic F concentrations were highly variable and that some fractions of magma were strongly enriched in halogens. We suggest that such high levels of halogen enrichment may be generally representative of other highly evolved granitic magmas genetically associated with lithophile mineralization.