Geochemistry of Th-U-REE mineralizing magmatic fluids, Capitan Mountains, New Mexico

Abstract

The Capitan Mountains in south-central New Mexico are formed by an east-west-trending Tertiary (28.3 Ma) alkali-feldspar granitic pluton which intruded Permian sedimentary rocks of the Yeso and San Andres Formations. Several widely distributed Th-U-REE veins which occur along the western end and flanks of the pluton are characterized by smoky and clear quartz, fluorite, adularia, hematite, calcite, thorite, titanite, allanite, chlorite, and clay minerals. Four main types of fluid inclusions representing a large range in temperature and salinity occur in the quartz and fluorite. Type 1 inclusions are the dominant type and represent high temperature-high salinity fluids with Th up to 600 degrees C and bulk salinities up to 84 wt percent NaCl + KCl equiv. Inclusion types 2, 3, and 4 have decreasing T h and salinities. Type 1 and 2 inclusions define a halite trend when plotted in the NaCl-KCl-H 2 O system. The observed T h -salinity trends suggest a fluid evolution from the original parent fluid by cooling and crystallization of halite. The chemical composition of the inclusion fluid was determined by a crush and leach extraction and analysis by ICP-MS, ICP-AES, and ion chromatography. The cations are dominated by Na, K, Ca, Mn, and Fe. Other cations analyzed include Mg, Al, Ba, Li, Sr, Rb, Cs, Pb, Zn, and REE. Cu was below detection. The dominant anions are Cl and SO 4 . Levels of I and Br were usually low. The main controls on fluid compositional variations are thought to be partitioning during vapor exsolution from the magma and mineral precipitation in the veins. The delta 18 O (from quartz) and delta D (from inclusion fluids in quartz) values of the fluids are 7.1 to 8.0 and -54 to -80 per mil, respectively. Based on field relations, fluid inclusion microthermometry stable isotope data, petrographic data, and inclusion fluid chemistry, the fluids responsible for the Th-U-REE mineralized zones in the western half of the Capitan Mountains are most likely of magmatic origin. As a result of cooling-induced fracturing of the outer carapace of the Capitan pluton, the exsolved magmatic fluids were introduced into brecciated zones. Halogen ratios and sulfate concentrations are suggestive of evaporite assimilations into the magma before fluid exsolution.