Distal Magmatic-Hydrothermal Origin for the Camaquã Cu (Au-Ag) and Santa Maria Pb, Zn (Cu-Ag) Deposits, Southern Brazil

Abstract

The Camaquã Cu (Au, Ag) and Santa Maria Pb-Zn (Cu, Ag) deposits are the largest base-metal deposits discovered so far in the sedimentary clastic sequences of the Neoproterozoic-age Camaquã Basin. The origin of the Camaquã-Santa Maria deposits has been the centre of dispute, with three alternative genetic hypotheses proposed: a syngenetic model, a diagenetic model, and a magmatic hydrothermal model. In detail, this mineralization has been suggested to be related to sedimentary clastic-diagenetic processes, volcanic-related processes, or deep granitic magmatism. Reevaluation of previous data and new studies in the area yield the following conclusions: (1) mineralization is fracture-controlled and magmatic-hydrothermal in origin rather than stratiform syngenetic or diagenetic; (2) the temperature of deposition of the main ore minerals was 210 to 300°C; (3) the ∂ 34S of sulphides of around 0% indicates an external magmatic-hydrothermal source of sulphur; (4) Pb isotope ratios of sulphides indicate that metals were derived at the end of the Brasiliano Cycle from a large crustal source with very primitive Pb and that (5) the age of mineralization is 594 Ma, as constrained by U/Pb SHRIMP (Sensitive High Resolution Ion Microprobe) determinations on zircons of the Lavras Granite. Thus, the Camaquã and Santa Maria deposits are interpreted to be of magmatic-hydrothermal origin, with the metals derived from an old crustal-basement source during the end of Dom Feliciano Collisional Orogeny, at 594 Ma, late in the Brasiliano Cycle. The interpretation above is critical for base-metal exploration in the Sul Riograndense Shield. Previous exploration methodologies mainly followed models based on a sedimentary hypothesis for the origin of the deposits. However, the occurrence of mineralization along fractures within specific wall-rocks requires consideration of alternative exploration parameters. These include: (1) ancient EW- and NW-trending regional fractures and their intersections, which are potential structural sites for base-metal mineralization, and (2) preferential wall-rock sites, either rocks with high initial porosity or secondary fracture-induced porosity or soluble rocks which are susceptible to replacement processes. Post-collisional plutonism of the Dom Feliciano Orogeny was the most likely heat source, and also the source of sulphur and base metals. Gravity surveys and alteration studies would be useful to determine the presence of intrusive bodies at depth which may have the potential to host porphyry-style Cu-Au deposits.