New Mexico Geological Society Annual Spring Meeting — Abstracts

The capitan Mountains, Lincoln County, New Mexico, are composed of an east-west trending Tertiary (26.5 Ma, K-Ar biotite; Allen, 1988) granitic pluton which intrudes the Permian Yeso and San Andres Formations. The capitan pluton (CP), a large (37 km x 7 km) Tertiary intrusive in New Mexico, is aligned along the eastwest trending Capitan lineament. The CP is cut by several faults and numerous brecciated and mineralized zones occur along the flanks and west end.

The CP is an alkali granite which exhibits the following chemical composition: SiO₂ 70-76%, TiO₂ 0.1-0.4%, Al₂O₃ 13-15%, Fe₂O₃ 0.6-2.5%, MgO 0.0-0.3%, CaO <0.5%, Na₂O 4.0-6.0%, K₂O 4.07.0%. The CP is compositionally zoned, exhibiting west (apex) to east (core) trends of increasing Fe₂O₃, CaO, TiO₂, Al₂O₃, Zr and Sr and decreasing sio , Nb and posslbly Th.

The mineral deposits of the Capitan Mountains consist of: 1) quartz veins with REE; 2) Th-U-REE quartz veins; 3) feldspar; 4) iron skarns near limestone-pluton contacts and magnetite veins within the pluton; and 5) manganese veins. The quartz veins with REE (Mina Tiro Estrella prospect-MTE) consist of quartz, allanite, titanite, adularia, chlorite and clay minerals. Allanite and titanite are the host minerals for REE and both exhibit complex zonation. The Th-U-REE quartz veins, characteristic for most of the vein occurrences, consist of smoky to clear quartz, fluorite, adularia, hematite, allanite, thorite, ±calcite, ±magnetite and ±apatite.

Quartz and fluorite from the mineralized veins contain abundant fluid inclusions (FI). The FI consist of a multi-phase system composed of vapor, liquid and daughter minerals. Some of the daughter minerals have been identified optically, by SEM and laser Raman spectroscopy as halite, sylvite, anhydrite, hematite, titanite and magnetite. Optical microscopy and FI microthermometry of 300 inclusions in quartz and fluorite indicate the quartz veins were derived from fluids that were: 1) high temperature, with vapor homogenization and halite
dissolution temperatures averaging 550-610°C; 2) highly saline, up to 72 eq wt.% NaCI; 3) dense, 1.3-1.5 g/cm³ ; 4) complex, with the inclusion fluids containing between 6-15 daughter minerals.

Stable isotope data on the inclusion fluid and quartz from the MTE vein has δD values from inclusion fluid of -56 0/00 to -58 0/00 and δ¹⁸O values from quartz of 8.8 to 9.1 0/00 corresponding to water values of 7.8 to 8.1 0/00. δ¹⁸O values of quartz (smoky and clear) from other quartz veins range from 9.0 to 9.3 0/00, corresponding to water values of 8.0 to 8.3 0/00.

Field relationships, petrographic and geochemical data suggest that the mineral zones are of magmatic origin and were derived as a result of cracking of the pluton during cooling, followed by injection of the mineralizing fluids into the brecciated zones.