Mass balance and origin of fluids associated to smectite and chlorite/smectite alteration in Seival Mine Cu–Mineralization – Camaquã Basin – Brazil (Part II)

Abstract

The Seival Mine, located in the Camaquã Basin, hosts several small Cu and Ag ore deposits in volcano–sedimentary sequence surrounding the Lavras do Sul Intrusive Complex. Alteration minerals consist of calcite, sulfate, smectite and chlorite/smectite associated with Cu–rich sulfides. The mass balance calculations from less altered to altered volcanic rocks show large chemical changes with the mobility of mettallic elements in major or trace amounts. Mass balance calculations for similar volcanic protoliths, with different degrees of alteration or textural variations, indicate that increasing proportions of clay minerals reflects higher degrees of alteration. This alteration did not mobilize Si, Al, Fe, and Mg and had a small effect on other metallic elements such as Cu, Zn, Ni, and Au. Consequently, fluids associated with propylitic to argillic alteration with chlorite–smectite precipitation did not participate in the transport of base metals and ore deposition. Interpretations of oxygen stable–isotopes for magmatic, late magmatic and hydrothermal minerals show decreasing proportion magmatic fluid (δ18Ofluid of andesine labradorite ca. 5 to 7‰), toward a predominance of meteoric fluid (δ18Ofluid of quartz and calcite) between −12 and 0‰ for the low–temperature alteration (250 to 80 °C). Carbon stable isotopes result suggest a mixture of inorganic carbon ca. −7 to −5‰ from volcanic CO2 with late carbonate precipitation with atmospheric signatures ca. −4 to −1‰. These mass balance and stable isotopes interpretations suggest that high–temperature late–magmatic fluid/rock interaction (650 to 350 °C) has promoted the base metal enrichment.