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 late-stage 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 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.
Highlights
The São Luís cratonic fragment in north-northeastern Brazil hosts one gold mine (Piaba), a few deposits, and a series of showings (Fig. 1) that have been mined by artisanal workers since the 17th century
In this work, the U-Pb sensitive high-resolution ion microprobe (SHRIMP) technique has been used to date zircon crystals from the same sample dated by Klein et al (2002), along with crystals from a strongly altered and mineralized sample of the same microtonalite
Stable isotopes, and chlorite chemistry (Klein & Koppe 2000, Klein et al 2000, 2005b) revealed that gold mineralization occurred at 262 – 307°C and 1.6 – 4.6 kbar from a reduced, low-salinity (
Summary
The São Luís cratonic fragment in north-northeastern Brazil hosts one gold mine (Piaba), a few deposits, and a series of showings (Fig. 1) that have been mined by artisanal workers since the 17th century. Geological mapping has been undertaken by the Geological Survey of Brazil in the cratonic area (Klein et al 2008a), followed by high-resolution U-Pb zircon geochronology and wholerock geochemical studies These studies have shown that the evolution of the São Luís cratonic fragment took place between about 2240 and 2056 Ma (Fig. 2), comprising rock associations that indicate early arc/back-arc settings (2240 Ma), intensive subduction-related calc-alkaline magmatism in oceanic to continental arc settings (2172 – 2143 Ma), collision-related peraluminous granitic magmatism (2100 Ma), and late- to post-orogenic, evolved volcanic and granitic rocks The postulated age of the gold-hosting Caxias Microtonalite does not fit in this relatively well-constrained tectonic-geochronological framework, and the role of this rock in the geological evolution of the São Luís cratonic fragments was never understood (Klein et al 2002, 2008a). This fluid was interpreted to be metamorphic in origin, and the Caxias deposit was included in the class of orogenic gold deposits (Klein et al 2005b, 2008a)
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