U-Pb SHRIMP and 40Ar/39Ar constraints on the timing of mineralization in the Paleoproterozoic Caxias orogenic gold deposit, Sao Luis cratonic fragment, Brazil

Caxias is an orogenic gold deposit in the Sao Luis cratonic fragment, which is correlated with the Rhyacian terranes of the West-African Craton. The deposit postdates peak metamorphism (estimated at 2100 +/- 15 Ma) and is hosted in a shear zone that cuts across schists of the Aurizona Group (2240 +/- 5 Ma) and the Caxias Microtonalite. The emplacement age of the microtonalite, as determined in this work by SHRIMP U-Pb zircon dating, is 2009 +/- 11 Ma and represents a late-stage magmatic event in the Sao Luis cratonic fragment. Older zircon age of 2139 +/- 10 Ma is interpreted as due to inheritance from the older granitoid or volcanic suites (magmatic sources?) or to contamination during emplacement. Lead isotope compositions indicate that the Pb incorporated in ore-related pyrite was probably sourced from regional, orogenic calc-alkaline granitoids of ca. 2160 Ma. Hydrothermal sericite from Caxias yielded a 40Ar/39Ar plateau age of 1990 +/- 30 Ma, which combined with the emplacement age of the Caxias Microtonalite brackets the age of gold mineralization between 2009 +/- 11 and 1990 +/- 30 Ma.