Geology and mineralization of the Pajarita mountain layered peralkaline syenitic pluton-hosted REE-Zr prospect, Mescalero Apache Reservation, New Mexico

Abstract

Peralkaline plutons represent some of the best known future sources of rare earth elements (REE), and, specifically, the more valuable heavy (H)REE. These plutons are diverse in size, morphology and enrichment, and understanding the factors that control the distribution of REE and other desirable elements is critical to efficient exploration for potential resources. The Pajarita Mountain peralkaline syenitic pluton, located on the Mescalero Apache Reservation in southern New Mexico, is prospective for REE, including yttrium (Y), and zirconium (Zr), but has not previously been extensively studied. This study investigates the REE-Zr mineralization and the igneous processes involved in the genesis of the pluton through field mapping, logging of 762 m of new drilled core, whole-rock geochemistry, and detailed petrographic description of the principle lithologies predominantly via optical microscopy and FE-SEM mineral chemistry. Evidence is presented indicating that the Pajarita Mountain pluton is a layered intrusion that is composed of peralkaline syenitic cumulate rocks. REE-Zr mineralization dominantly occurs in the form of magmatic eudialyte group minerals, which are partly replaced by secondary minerals formed during late stage alteration. Layering is defined by phase, texture, and/or mode, and contacts between layers may be abrupt or gradational. Specific layered sequences are locally repeated within the studied sections, and their descriptions may have significance for REE and Zr exploration at Pajarita Mountain and in other similar peralkaline plutons. Based on detailed petrographic observations, field relations, whole-rock geochemistry, and interpretation of layered sequences, this study recognizes several processes in the genesis of the Pajarita Mountain pluton. These include emplacement of at least three major peralkaline magma pulses, crystal separation and accumulation, and interaction of the cumulus pile with evolving melts. The latter may involve magma mixing related to magma recharge. Each of these processes appears to have played a role in the genesis of the pluton and to have had an impact on the distribution of REE and Zr mineralization. The three major peralkaline magmatic episodes are represented by peralkaline syenitic rocks of Types I-III; minor pegmatites were also emplaced (Type IV). Cross-cutting relationships suggest that Type III rocks formed first, followed by Type II, then Type I cumulates, with Type IV forming last. Correlation of sections from two study areas enables division of the pluton into three layered zones, the Lower Layered, Central Mixed, and Upper Layered Zones. The Lower Layered Zone consists predominantly of layered Type I cumulates, while the Central Mixed Zone features rocks of all four types, including bands of Type I cumulates, and the Upper Layered Zone consists of a basal Type I layered sequence overlain by Type II rocks. Type I cumulates display the best developed cumulus textures and commonly show layering by mode, texture and/or phase. They contain the most abundant eudialyte and greatest REE-Zr concentrations, typically at the base of layered successions.