GEOCHEMISTRY OF TH-U-REE MINERALIZING MAGMATIC FLUIDS, CAPITAN MOUNTAINS, NEW-MEXICO

The Capitan Mountains in south-central New Mexico are formed by an eas t-west-trending Tertiary (28.3 Ma) alkali-feldspar granitic pluton whi ch intruded Permian sedimentary rocks of the Yeso and San Andres Forma tions. Several widely distributed Th-U-REE veins which occur along the western end and flanks of the pluton are characterized by smoky and c lear quartz, fluorite, adularia, hematite, calcite, thorite, titanite, allanite, chlorite, and clay minerals. Four main types of fluid inclu sions representing a large range in temperature and salinity occur in the quartz and fluorite. Type 1 inclusions are the dominant type and r epresent high temperature-high salinity fluids with T-h up to 600 degr ees C and bulk salinities up to 84 wt percent NaCl + KCl equiv: Inclus ion types 2, 3, and 4 have decreasing Th and salinities. Type 1 and 2 inclusions define a halite trend when plotted in the NaCl-KCl-H2O syst em. The observed T-h-salinity trends suggest a fluid evolution from th e 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 chromato graphy. The cations are dominated by Na, K, Ca, Mn, and Fe. Other cati ons analyzed include Mg, Al, Ba, Li, Sr, Rb, Cs, Pb, Zn, and REE. Cu w as below detection. The dominant anions are Cl and SO4. Levels of I an d Br were usually low. The main controls on fluid compositional variat ions are thought to be partitioning during vapor exsolution from the m agma and mineral precipitation in the veins. The delta(18)O (from quar tz) 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 r elations, fluid inclusion microthermometry, stable isotope data, petro graphic data, and inclusion fluid chemistry, the fluids responsible fo r the Th-U-REE mineralized zones in the western half of the Capitan Mo untains are most likely of magmatic origin. As a result of cooling-ind uced fracturing of the outer carapace of the Capitan pluton, the exsol ved magmatic fluids were introduced into brecciated zones. Halogen rat ios and sulfate concentrations are suggestive of evaporite assimilatio ns into the magma before fluid exsolution.