Post-collisional magmatic-hydrothermal mineralization in the São Gabriel Terrane (southern Brazil): Insights from mineral chemistry, stable isotopes, and sulfide trace-elements of the Caçapava do Sul skarns

Abstract

Prograde and retrograde magnesian skarns associated with the Caçapava do Sul Granitic Complex intrusion (central Dom Feliciano Belt, southern Brazil) host Cu–Fe (Mo–Au) mineralization. Vein and contact prograde skarns contain disseminated pyrite, pyrrhotite, molybdenite, and chalcopyrite formed from the interaction of mafic and felsic intrusions and dolomitic marbles at ca. 578 Ma. The prograde skarn mineralization is mainly localized in fold hinges and pinch-and-swell structures. Retrograde skarns host massive chalcopyrite and pyrite with chlorite and calcite associated with brittle structures at ca. 557 Ma. In this contribution, we provide mineral chemistry analyses of magmatic and skarn assemblages, calcite stable isotope analyses, and trace-element characterization of pyrite and chalcopyrite to assess ore-forming processes involved in the formation of the skarns. Amphibole-plagioclase geothermobarometry in granodiorite and monzonite apophyses of the Caçapava do Sul Granitic Complex yielded crystallization pressures between 4 and 5 kbar and temperatures from 680 to 750 °C. Biotite and amphibole composition of granodiorite, monzonite, and garnet-leucogranite apophyses suggest a trend of decreasing oxygen fugacity from early to late intrusions. Calcite-dolomite solvus thermometry in prograde skarns indicates temperatures between 580 and 630 °C whereas chlorite thermometry in retrograde skarn yields lower temperatures between 280 and 310 °C. Calcite from the retrograde skarns shows values of approximately −5.5‰ for δ13CVPDB and ∼9.0‰ for δ18OSMOW, indicating a major contribution of magmatic fluids during the retrograde stage whereas previous studies indicate a mixed igneous-sedimentary character of the calcite in prograde skarns. Our findings propose a multiphase magmatic-hydrothermal origin for the Cu–Fe (Mo–Au) skarn mineralization hosted by the Passo Feio Metamorphic Complex. The prograde and retrograde magnesian skarns of Caçapava do Sul were formed in the same age interval (578 - 557 Ma) as many other magmatic-hydrothermal base and precious metals mineralization in the São Gabriel Terrane. Hence, we suggest that the studied skarns belong to a multi-intrusive Cu-porphyry system in the São Gabriel Terrane formed during the post-collisional stage of the Dom Feliciano Belt.