Distal magmatic-hydrothermal origin for the Camaqua Cu (Au-Ag) and Santa Maria Pb, Zn (Cu-Ag) deposits, southern Brazil

The Camaqua Cu (Au, Ag) and Santa Maria Pb-Zn (Cu, Ag) deposits are the lar gest base-metal deposits discovered so far in the sedimentary elastic seque nces of the Neoproterozoic-age Camaqua Basin. The origin of the Camaqua-San ta Maria deposits has been the centre of dispute, with three alternative ge netic hypotheses proposed: a syngenetic model, a diagenetic model, and a ma gmatic hydrothermal model. In detail, this mineralization has been suggeste d to be related to sedimentary clastic-diagenetic processes, volcanic-relat ed processes, or deep granitic magmatism. Reevaluation of previous data and new studies in the area yield the followi ng conclusions: (1) mineralization is fracture-controlled and magmatic-hydr othermal in origin rather than stratiform syngenetic or diagenetic; (2) the temperature of deposition of the main ore minerals was 210 to 300 degrees C; (3) the partial derivative(34)S of sulphides of around 0 parts per thous and indicates an external magmatic-hydrothermal source of sulphur; (4) Pb i sotope 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 SHRIM P (Sensitive High Resolution Ion Microprobe) determinations on zircons of t he Lavras Granite. Thus, the Camaqua and Santa Maria deposits are interpreted to be of magmati c-hydrothermal origin, with the metals derived from an old crustal-basement source during the end of Dom Feliciano Collisional Orogeny, at 594 Ma, lat e in the Brasiliano Cycle. The interpretation above is critical for base-metal exploration in the Sul Riograndense Shield. Previous exploration methodologies mainly followed mod els based on a sedimentary hypothesis for the origin of the deposits. Howev er, the occurrence of mineralization along fractures within specific wall-r ocks requires consideration of alternative exploration parameters. These in clude: (1) ancient EW- and MW-trending regional fractures and their interse ctions, which are potential structural sites for base-metal mineralization, and (2) preferential wall-rock sites, either rocks with high initial poros ity or secondary fracture-induced porosity or soluble rocks which are susce ptible to replacement processes. Postcollisional plutonism of the Dom Felic iano Orogeny was the most likely heat source, and also the source of sulphu r and base metals. Gravity surveys and alteration studies would be useful t o determine the presence of intrusive bodies at depth which may have the po tential to host porphyry-style Cu-Au deposits.