Titanite: A source, thermodynamic, and temporal proxy for W-Mo-Cu Skarn mineralization at the Seridó Mineral Province, Northeastern Brazil

Abstract

New zircon U-Pb geochronology (SHRIMP) and geochemistry combined with geochronology (LA-ICP-MS) of titanite data are provided to investigate the metallic sources (whether igneous or sedimentary) and the temporal evolution of one of the most relevant Precambrian metallogenic provinces of the South American Platform, the Seridó Mineral Province. The granites of Serra de João do Vale pluton, calc-silicate “skarnoid” rocks and paragneiss related to the Bonito W-Mo-Cu exoskarn deposit were investigated. It was analyzed titanites of magmatic (Serra de João do Vale), hydrothermal (calc-silicate rocks) and metamorphic (paragneiss) origin. The magmatic titanite is wedge-shaped with larger grain size (up to 250 μm), richer in HFSE elements, Al and Fe, and manifests negative Eu anomalies (Eu/Eu × averaging 0.31–0.60). The hydrothermal titanite is found alongside epidote and calcite, have subhedral shapes and medium grain size (up to 150 μm). The metamorphic titanite is anhedral, generally smaller than 100 μm and appear as thin coronae in ilmenite or as inclusions in biotite. Both metasomatic and metamorphic titanites manifest positive Eu anomalies (Eu/Eu × averaging 1.20 and 1.06, respectively) and are enriched in Ti, Rb and U in relation to the magmatic. Conventional mineral thermobarometry (Al-in-titanite independent barometer, and Zr-in-titanite thermobarometer) provided pressures between 400 and 550 MPa and temperatures between 720 and 787 °C for the crystallization history of the granite, coeval with the regional peak metamorphism at 568–624 °C and 300–500 MPa (Ti in biotite independent barometer and biotite-muscovite thermobarometer). Conditions for the titanite-epidote calc-silicate assemblage were constrained through thermodynamic modelling in NCKFMAST + COH pseudosection at 439 ± 35 °C and 190 ± 50 MPa. The zircon U-Pb age registered the magmatism between 585.5 and 580.5 Ma. Titanite U-Pb ages from granite and calc-silicate rocks timed the closure of U-Pb system at the isotherm of 550 °C between 562 and 557 Ma. (20 m.y. after the intrusion), which marks the metasomatic process. The average W content in paragneisses and calc-silicate titanites (35.8–35.5 ppm) is 4 times higher than in the granite (6.76 ppm), reaching a maximum of 328 ppm. Mo and Cu mirrors this behavior, with concentrations that exceed nearly double that of titanites from Serra de João do Vale granites, peaking at 308 ppm of Mo and 162 ppm of Cu. Metal contents in titanite disclose the metasiliciclastic rocks of the Jucurutu Formation as, at least partially, a source of metals (mainly for W, but also Cu and Mo) in the skarn mineralization during the terrane exhumation at the end of Ediacaran period.