Chemical and textural relations of britholite- and apatite-group minerals from hydrothermal REE mineralization at the Rodeo de los Molles deposit, Central Argentina

Abstract

Britholite group minerals (REE,Ca)5[(Si,P)O4]3(OH,F) are rather widespread rare-earth minerals in alkaline rocks and their associated metasomatic zones, where they usually form tiny accessory phases. In the REE deposit Rodeo de los Molles (RdM), Central Argentina, fluorbritholite-Ce (FBri) is the main carrier of REE and closely intergrown with fluorapatite (FAp). These minerals reach an exceptional abundance of locally up to 75 modal % (FBri) and 5 modal % (FAp) in veins. The RdM deposit is hosted by fenitized monzogranite of the Middle Devonian Las Chacras-Potrerillos batholith. The REE mineralization consists of fluorbritholite-(Ce), fluorapatite, allanite-(Ce) and REE carbonates (bastnaesite), and is associated with hydrothermal fluorite, quartz, albite, zircon and titanite. The REE mineralization takes two forms: irregular patches and discrete cross-cutting veins. The patches are more common, but here fluorbritholite-(Ce) is mostly replaced by REE carbonates. The REE mineralization in veins has more abundant britholite phases and less replacement textures. The majority of britholite grains at RdM are hydrothermally altered, and alteration is strongly enhanced by metamictization, which is indicated by darkening of the mineral, loss of birefringence, porosity and volume changes leading to polygonal cracks in and around altered grains. A detailed electron-microprobe study of apatite-britholite minerals from RdM reveals the dominant coupled substitution of REE3+ + Si4+ = Ca2+ + P5+ for the formation of fluorapatite and fluorbritholite-(Ce) and a compositional gap of ~4 apfu between the two phases. The latter is interpreted as a miscibility gap between the two phases. Exsolution textures of apatite in britholite and vice versa are presented for the first time and interpreted to have formed in order to maintain equilibrium at P-T conditions of the possible solvus.