Hydrothermal alteration of volcanic rocks in Seival Mine Cu–mineralization – Camaquã Basin – Brazil (part I): Chloritization process and geochemical dispersion in alteration halos
Résumé
The Seival Mines are situated in the NE portion of the Lavras do Sul mining district, southernmost Brazil. They are hosted by volcanic and sub-volcanic rocks of Neoproterozoic age, which are part of the post–collisonal Camaquã Basin volcano–sedimentary sequence. The mineralization occurs in shoshonitic volcanic and sub–volcanic rocks of the Lavras do Sul Shoshonitic Association. They are of intermediate composition and exhibit widespread hydrothermal alteration. The mineralization occurs primarily in the form of bornite, chalcocite, covellite, pyrite, and in supergene phases as malachite. Mineral occurrence is always controlled by fractures. Ore is associated with chloritization processes, which produced smectite, chlorite/smectite and corrensite clay minerals and gangue of carbonate, mostly calcite, and barite. In this study field mapping and drill core sampling, petrography with optical microscopy and electron microscopy, X–ray diffraction for clay size fraction characterization and whole–rock geochemistry are used to understand the spatial distribution and relative chronology of hydrothermal alteration products of different lithology in the mineralized zones. The post–magmatic fluid activity and hydrothermal lower–temperature alteration that produced smectite ➔ chlorite/smectite ➔ chlorite and corrensite ➔ carbonate have changed the major, minor, trace and rare earth element (REE) contents. Lavas and sub-volcanic rocks contain Cu–Fe sulfides. In alteration halos, Cu–sulfides with supergene influence is related to circulation of late hydrothermal fluids. The pH variations and sulfide materials are related to dispersion metals around vertical structures.