Genesis and New Mineral Chemistry Data of Carlosbarbosaite, a Potential U and Nb Ore Source from Miarolitic-, A-Type Granites and NYF Pegmatites of the La Chinchilla Pluton, Velasco Ranges, La Rioja, Argentina

Abstract

Abstract The La Chinchilla granite is a ∼3.75 km2 epizonal pluton of Lower Carboniferous age located in Sierra de Velasco, Sierras Pampeanas, Argentina. Equigranular micropegmatitic and porphyritic main granite types host abundant millimeter- to <2 m-sized miarolitic pegmatites and pockets of simple major mineralogy (±beryl). Both granite types host micrometer-sized accessory species [i.e., monazite-(Ce), several high field strength element oxide species, ilmenite, cassiterite, fluorapatite] and fluorite. A F-Na-rich fluid phase promoted strong albitization at late-miarolitic stages, along with crystallization of extremely F-rich polylithionite and fluorite, and the formation of replacing pyrochlore group species associated with a second generation of cassiterite. The increase of the Ta# from hydroxycalciopyrochlore to hydroxycalciomicrolite and from micromiarolitic cassiterite (cassiterite 1) to hydrothermal cassiterite (cassiterite 2) supports Nb-Ta fractionation at hydrothermal temperatures. Carlosbarbosaite [(UO2)2Nb2O6(OH)2·2H2O] occurs as a pseudomorphic or short-range transported phase. Low-T, hydrothermal carlosbarbosaite formed after the replacement of columbite-(Fe), U-free Nb-bearing ilmenite, and likely after U-bearing pyrochlore supergroup species and a columbite group mineral, plausibly due to interaction with a hydrothermal, U6+(±Nb±Ta)-enriched fluid, in some cases a SiO2-bearing fluid. This fluid likely represents a lower T, less alkaline, and more oxidizing fluid that evolved from the higher T F-Na-rich fluids active during the late-miarolitic hydrothermal stage. Low-T, hydrothermal carlosbarbosaite has the ideal U-,Nb-rich endmember composition, though it is significantly richer in Ca and poorer in total Nb+Ta but with higher Nb# than that from the type locality. Supergene fluids deposited the transported type, which attained economic concentrations in a fault zone where restricted, likely alkaline oxidizing conditions could have favored Nb mobility.