Th-U-REE quartz/fluroite veins, Capitan Pluton, New Mexico: Evidence for a magmatic/hydrothermal origin
— Randall S. Phillips, Andrew R. Campbell, and Virginia T. McLemore

The Capitan Mountains, south-central New Mexico, contain several widely distributed Th-U-REE vein deposits near the western end and along the flanks of an east-west-trending Tertiary (26.5 Ma) granitic pluton. Mineralization associated with the Th-U-REE veins consists of quartz, fluorite, adularia, hematite, calcite, thorite, titanite, allanite, chorite and clay minerals. Fluid-inclusion microthermometry and optical microscopy of 638 fluid inclusions in quartz and fluorite from 14 vein systems indicates four main types. The most dominant type of inclusion is primary/pseudosecondary in origin and high-temperature, high-salinity (Th and Tm NaCI up to and greater than 600°C; bulk salinities as high as 84 eq wt% NaCI + KCI). Other types show a gradational trend from the high-temperature, high-salinity inclusions to low-temperature, low-salinity inclusions (Th as low as 110 °C; salinities as low as 11 eq wt% NaCI). Most fluid-inclusion types define a trend when plotted on a Th vs. salinity diagram. The observed trends suggest evolution of the original parent fluid by cooling and crystallization of halite. ¹⁸0 (from quartz) and SD (from inclusion fluids in quartz) of the fluids are 7.08%o to 8.58%c and –22%c to –86%c, respectively. These isotopic data suggest that magmatic water was an important and possibly dominant source for the mineralizing fluids. Field relations, petrographic data, fluidinclusion microthermometry and stable isotope data indicate that the fluids responsible for the Th-U-REE mineralized zones in the western half of the Capitan Mountains were of magmatic origin and were derived as a result of contraction and cracking of the Capitan pluton during cooling, followed by injection of the exsolved magmatic fluids into the brecciated zones. A late-stage, post-mineralization fluid, characterized by the lower temperature inclusions, may represent a mixture of an evolved magmatic fluid with meteoric water. This laterstage fluid appears to be restricted to the west end (apex) of the Capitan pluton.