CONTROLS OF REE MINERAL DEPOSITS IN THE GALLINAS MOUNTAINS (GALLINAS DISTRICT), CENTRAL NEW MEXICO, USA

Many rare earth elements (REE) deposits are found in North American Cordilleran igneous belt, a zone of Cenozoic alkaline igneous rocks and associated mineral deposits that extends from Canada, western U.S., including New Mexico, and into eastern Mexico. This zone has been explored and exploited for numerous types of mineral deposits. The Gallinas Mountains is one such district with established REE potential, where a small amount of bastnäsite was recovered during fluorite processing from 1954-1956 and 1980. In addition, Pb, Cu, Zn, Ag, Au, and Fe have been produced. Host rocks include Proterozoic granite and gneiss, overlain by Permian sedimentary rocks that are intruded by 38.5-24.4 Ma rhyolite, trachyte, and syenite laccoliths, sills and dikes. Seven types of mineral deposits are found in the district, distinguished by mineralogy, chemistry, form, and host rocks: (1) Fe skarn-contact replacement deposits, (2) hydrothermal breccia and fissure veins, (3) F replacements/disseminations, (4) magmatic, intrusive breccia pipes, (5) carbonate breccias, (6) hypogene oxidation, and (7) supergene oxidation. The hydrothermal breccia and fissure veins are controlled by minor faults, fractures and bedding planes, and subdivided according to mineralogy and chemistry. The intrusive breccia pipes form a 2 km-NE-trending belt and locally host REE-F and F veins and anomalously high Au (<222 ppb). Fe skarn-contact replacement deposits consist of hematite and magnetite, with local elevated REE, replacing limestone and sandstone. Carbonate breccias/veins are found near limestone and gypsum beds and many contain F, local Au (<178 ppb), and variable amounts of REE. Different alteration styles are associated with the intrusions and mineral deposits. The origin of the mineralizing fluids is uncertain but geochemical analyses, mineralogy, texture, and field relationships are consistent with previous conclusions that the REE deposits are derived from magmatic-hydrothermal fluids from either an alkaline magma, such as the trachyte and syenite exposed in the area or from a deep-seated, buried carbonatite. New mapping reveals that many of the magmatic intrusive breccia pipes were emplaced along the margins of syenite intrusions, suggesting a connection between fluids associated with the intrusions and breccia pipe formation.