Unraveling the evolution of the Bodó W-skarn district in the Seridó Mineral Province (NE-Brazil): constraints from C-O stable isotopes, thermodynamic modeling, and geochronology

Abstract

Tungsten mineralization in the Bodó mineral district (9 Mt with an average grade of 2% WO₃) is in a sequence of metasomatized marbles and W-(Mo)-skarn lenses in the Seridó Belt, northern Borborema Province. The marble lenses have variable amounts of diopside, grossular, scapolite, phlogopite, and tremolite. The skarn lenses are mainly composed of massive grossular, diopside, vesuvianite, epidote, and quartz. A spatially related granite yielded a SHRIMP U‒Pb zircon date of 536.6 ± 3.4 Ma and a ⁴⁰Ar/³⁹Ar biotite date of 490.65 ± 0.67 Ma, whereas a nearby pegmatite yielded a ⁴⁰Ar/³⁹Ar muscovite date of 501.63 ± 0.59 Ma. Literature data for molybdenite in the skarn mineralization yielded a Re‒Os date of 510 ± 2 Ma, which is coeval with U‒Pb dates of columbite-tantalite from other regional pegmatites (515–509 Ma). Therefore, it is likely that pegmatite magmatism acted as the source of fluid and heat for the mineralization. The C–O stable isotope data for marbles and skarns are consistent with interaction of magmatic fluid and host marble at variable XCO₂ conditions. T-XCO₂ pseudosections define peak conditions of metamorphism/metasomatism at 650–600 °C over a wide range of XCO₂ (between 0.4 and 0.8), whereas the retrograde stage started at ~ 550 °C. Late garnet crystallization at low XCO₂ (< 0.2) indicates high H₂O influx, while scapolite crystallization required high XCO₂ (~ 0.8). Together with the interpretation of textural relationships, these observations indicate that skarn and marble formation occurred under open-system conditions with fluctuating XCO₂ fluid composition as a consequence of magmatic fluid infiltration.