New Mexico Geological Society Annual Spring Meeting — Abstracts

During the past decade, worldwide demand for rare earth elements (REE) has surged; however, the genesis and geochemical characteristics of hydrothermal REE deposits associated with alkalic igneous systems remains poorly understood. This study addresses the genesis of bastnaesite-fluorite breccia deposits and their association with Ag-Pb-Zn sulfide vein and breccia mineralization in the GallinasMountains. The spatial and chemical association between these two types of deposits and the amenability of many ore and gangue minerals to stable isotopic study provides a unique opportunity to examine the history of hydrothermal fluids responsible for mineralization.

Mineral associations, especially products of sulfide weathering (i.e. agardite, an REE bearing mineral), indicate a genetic link between REE and sulfide mineralization in the deposits near RoughMountain. δ³⁴S isotopic ratios of sulfide and sulfate species vary from -21.1 to 1.5‰ and 9.6 to 13.3‰ vs. CDT, respectively. δ³⁴S and δ¹⁸O_SO4­ data, barite and galena trace element chemistry, and the overall mineralogy of the Rough Mountain deposits suggest an the presence of an oxidized hydrothermal fluid and that cooling was responsible for large isotopic changes over space and time. These δ³⁴S and δ¹⁸O_SO4­ data are interpreted as a mixing line between magmatic and  regional groundwaters in isotopic equilibrium with the overlying Yeso Formation. Stable isotope geothermometry (bar-gn) yields a temperature of formation of ~380°C in the deeper parts of the system and fluid inclusion temperatures from previous studies indicate a temperature of ~200°C for late sulfide formation in the upper reaches of the system. A model developed from this data suggests that Rough Mountain REE and sulfide deposits were derived from a magmatic source. Stable isotope and fluid inclusion evidence suggests that REE mineralization occurred due to fluid cooling and, in the vicinity of Rough Mountain, mixing occurred. Base and precious metal mineralization continued into the upper reaches of the system until solutions finally became relatively cool near the Yeso contact.

Whole rock and mineral trace element abundances, metal ratios, and mineralogy suggests that the M and E No. 13 is genetically different than the RoughMountain deposits and that mineralization may have been derived from a carbonatite source. Isotopic ratios of carbonate species (δ¹³C and δ¹⁸O) vary from -11.1 to -3.4‰ vs. PDB and 15.0 to 23.8‰ vs. VSMOW, respectively. δ¹³C data from the M and E No. 13 prospect fall within the range of primary carbonatites but δ¹⁸O data fall outside the expected range. These data suggest that calcite mineralization, particularly in the vicinity of the M & E No. 13 prospect was late stage fracture filling and precipitation occurred at low temperatures from a cooled magmatic fluid. Alternatively, dissolution of Yeso carbonate could cause this magnitude of δ¹⁸O enrichment.

This study suggests that the association between bastnaesite (REE) and sulfide mineralization can occur in deposits associated with alkalic igneous activity. The role of a carbonatite magma in the district is less well defined.