CONTROLS ON REE-Y-ZR MINERALIZATION IN THE PAJARITA MOUNTAIN PERALKALINE SYENITE COMPLEX, MESCALERO APACHE RESERVATION, OTERO COUNTY, NEW MEXICO

The Pajarita Mountain peralkaline syenite complex is a 1200-1100 Ma layered cumulate intrusion that hosts elevated concentrations of HREE, Y, and Zr in silicates of the eudialyte group. The complex, at least 300 m thick, is exposed through an erosional window in Permian sedimentary rocks at the eastern margin of the Rio Grande Rift. This study aims to elucidate the major controls on REE-Y-Zr distribution at Pajarita Mountain with emphasis on the magmatic evolution of the complex.

Emplacement involved at least three stages of silica-oversaturated, eudialyte-bearing, syenitic magmatism that show progressively better developed cumulate textures and modal and textural layering. Fine-grained eudialyte ± Fe-Ti silicate-bearing syenites of the first two stages show some textural evidence for accumulation and occur only as lithic inclusions in third stage syenites. These inclusions locally have ductile morphologies suggesting incomplete crystallization prior to intrusion of the third stage.

Third stage syenites show well-developed cumulus features with one roughly 30 m thick interval showing consistent vertical trends in cumulus mineralogy (eudialyte, alkali feldspar, Na amphibole, and quartz) and texture. Intercumulus phases are Na amphibole and quartz. The most richly mineralized rocks studied are third stage syenites containing up to 50 vol% cumulus eudialyte, 3.80 wt% [TREO+Y₂O₃], 7.47 wt% ZrO₂, 2210 ppm Nb, 165 ppm Ta, and 1330 ppm Hf. Strongly mineralized syenites occur at the base of the aforementioned interval; generally, well-developed cumulus textures correlate with strong REE-Y-Zr mineralization.

Aspects of the evolution of the Pajarita Mountain peralkaline magma can be inferred from petrography and EDS semi-quantitative data. Notably, EDS analyses of cumulus amphiboles with intercumulus overgrowths indicate that Mg# and Ca availability decreased with time. Alkali feldspars showing complex exsolution, resorption, and overgrowth textures and striking near-end-member albite and K feldspar compositions, and cumulus quartz megacrysts surrounded by intercumulus Na amphibole suggest compositional and/or thermal perturbations of the system believed to be related to melt injection events. EDS analyses of cumulus eudialytes suggest a possible post-crystallization enrichment in HFSE.