The link between hydrothermal epigenetic copper mineralization and the Cacapava Granite of the Brasiliano Cycle in southern Brazil

Base-metal deposits in the Cacapava do Sul Copper Province are hosted by bo th volcanosedimentary rocks of the Bom Jardim Group and by metamorphic rock s of the Passe Feio Formation, and show a spatial relationship to the Cacap ava Granite. These associations have led to much controversy about the gene sis of the base-metal deposits, which has been at least partly resolved by precise dating using SHRIMP (Sensitive High Resolution Ion Microprobe) U/Pb zircon studies combined with S, Pb, and Sr isotope trace studies. The Passe Feio Formation is Neoproterozoic in age and was derived from a co mplex continental source, as shown by the presence of xenocryst zircons of Archaean, Paleoproterozoic, and Neoproterozoic ages. It was metamorphosed a t ca. 700 Ma. The syntectonic Cacapava Granite that intruded the supracrust al rocks of the Passe Feio Formation at 562 Ma was derived from an old sial ic basement. Lead-isotope data ale consistent with a 562 Ma age for the base-metal sulph ide deposits sited in the Passe Feio Formation. The least-radiogenic compos itions lie between the field of the isotopic compositions of the Cacapava G ranite and rocks of the Passe Feio Formation, suggesting that Pb in the sul phide deposits may have been derived from both sources. The Pb, like that i n the Cacapava Granite and Passe Feio Formation. was derived from a primiti ve crustal source. Sulphur isotope data from the base-metal sulphide deposi ts in the Passe Feio Formation are compatible with a mixed sedimentary and magmatic source. The most logical model for ore genesis, based on the isotopic data and spat ial relationships, is that magmatic metal-bearing fluids from Cacapava Gran ite leached metals from the Passe Feio Formation and that the deposited sul phides therefore show mixed isotopic signatures. However, there is also som e isotopic evidence from the Cacapava Granite itself that suggests assimila tion of S-bearing rocks of the Passe Feio Formation during emplacement. Thu s, isotopic signatures could have been inherited from assimilated metal sul phides at this stage, and deposition could have been entirely from Cacapava Granite-derived magmatic fluids. Importantly, the inferred 562 +/- 8 Ma age for the deposits in the Passe Fe io Formation is younger than the well-constrained age of 594 +/- 5 Ma for t he Camaqua/Santa Maria deposits. Thus, the epigenetic sulphides in the Pass e Feio Formation cannot be the source of these deposits as previously sugge sted. Other isotopic data also argue against such a model. (C) 2000 Elsevie r Science Ltd. All rights reserved.