The genesis of hydrothermal fluorite-REE deposits in the Gallinas Mountains, New Mexico

Breccia-hosted fluorite-bastnaesite deposits associated with epizonal quart z syenite intrusions in the Gallinas Mountains, New Mexico, provide an impo rtant example of hydrothermal rare earth element (REE) mineralization, and an excellent opportunity to investigate the transport and deposition of the REE under hydrothermal conditions. Fluid inclusion studies show that miner alization commenced with the deposition of quartz at a temperature of appro ximately 400 degrees C from sulfate-rich NaCl-KCl brines having a salinity of similar to 15 wt percent NaCl equiv. These fluids are interpreted to hav e been of orthomagmatic origin. Fluorite and bastnae- site deposition occur red at slightly lower temperatures, and coincided with the introduction of separate CO2-bearing and sulfate-poor NaCl brines into the system. These fl uids are considered to have been of external origin, the latter being a for mation water that circulated in surrounding Permian sandstones and limeston es. Evaluation of the available thermodynamic data on the aqueous speciatio n of REE, and the preliminary data on the chemistry of the fluids obtained in this study, suggest that the REE were dissolved primarily as fluoride co mplexes. A model is proposed in which the REE were transported by the ortho magmatic fluids, and deposited as bastnaesite owing to destabilization of R EE-fluoride complexes. The latter occurred in response to the sharp reducti on in F- activity that accompanied large scale deposition of fluorite as a result of mixing of the orthomagmatic fluids and formation waters. This mix ing also led to the deposition of anhydrite and barite owing to the interac tion of externally introduced calcium and barium ions with orthomagmatic su lfate ions. The later stages in the evolution of the Gallinas Mountains hyd rothermal system were marked by increased incursion of external fluids, and the alteration of bastnaesite to Ca bastnaesite and minor deposition of Ca bastnaesite and parisite owing to lowered F-/CO32- ratios and/or increased Ca2+ activity. The study suggests that Ca-free, REE fluoride-bearing fluid s are an important ingredient for REE mineralization, that mixing of such f luids with Ca-, carbonate-bearing fluids or interaction with carbonate rock s is a major control of REE-fluorocarbonate mineral deposition, and that th e occurrence of fluorite could provide an important guide in the exploratio n for economic REE deposits.