U-Pb SHRIMP and⁴⁰Ar/³⁹Ar constraints on the timing of mineralization in the Paleoproterozoic Caxias orogenic gold deposit, São Luís cratonic fragment, Brazil

Abstract

Caxias is an orogenic gold deposit in the São Luís 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 latestage magmatic event in the São Luís 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 40 Ar/ 39 Ar 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.

Rhyacian evolution of the eastern São Luís Craton: petrography, geochemistry and geochronology of the Rosário Suite

“…The Negra Velha Granite (2056-2,076 Ma) consists of small granitic bodies intruded into the Tromaí Granitic Suite and associated with felsic volcanic and pyroclastic rocks of the same age (Klein et al , 2009). The Caxias Microtonalite, with age of 2009 ± 11 Ma (Klein et al 2014), represents the youngest magmatic plutonic activity of this cratonic area.…”

“…The Negra Velha Granite (2056-2,076 Ma) consists of small granitic bodies intruded into the Tromaí Granitic Suite and associated with felsic volcanic and pyroclastic rocks of the same age (Klein et al , 2009). The Caxias Microtonalite, with age of 2009 ± 11 Ma (Klein et al 2014), represents the youngest magmatic plutonic activity of this cratonic area.The Gurupi Belt is interpreted as a Neoproterozoic-early Cambrian orogen with NNW-SSE orientation, developed in the south-southwestern margin of the São Luís Craton (Almeida et al 1976, Abreu et al 1980Costa 2000, Klein et al 2005a. The belt and its reworked basement include rock units of varied nature and ages ranging from Archean to Eocambrian (Klein et al 2005b, Palheta et al 2009).…”