Origin of the CO2-only fluid inclusions in the Palaeoproterozoic Carará vein-quartz gold deposit, Ipitinga Auriferous District, SE-Guiana Shield, Brazil: Implications for orogenic gold mineralisation

Abstract

The Carará gold deposit, located in the Ipitinga Auriferous District, south-eastern portion of the Guiana Shield, northern Brazil, is a typical orogenic, greenstone-hosted, auriferous quartz vein. Mineralisation was post-metamorphic and syn-tectonic in relation to the host Palaeoproterozoic (ca. 2.03 Ga) shear zone developed close to the tectonic boundary between a Palaeoproterozoic continental arc and an Archaean block. The deposit style is very simple, consisting of a quartz vein and its hydrothermal envelope, which is composed of muscovite and tourmaline; sulphides are rare. Muscovite and tourmaline, in addition to gold,fill small fractures in the quartz vein. The fluid inclusion assemblage trapped in high- and low-grade portions of the Au–quartz vein is rather enigmatic, consisting of one-phase CO2 inclusions with no visible water at room or sub-zero temperatures, although small amounts of water have been detected by micro-Raman analysis. In this aspect Carará differs from the other gold showings in the same district, which are characterized by abundant aqueous-carbonic fluid inclusions. The carbonic fluid is composed predominantly of CO2 in addition to b2 mol.% N2 and traces of CH4 and C2H6. The carbonic fluid show very variable densities, which is interpreted to result from post-entrapment reequilibration. Inclusions in the high-grade quartz are the densest (0.89 to 1.07 g/cm3) and with less effects of re-equilibration. These inclusions approximate the physico-chemical characteristics of the parental fluid that started to be trapped at least around the amphibolite facies metamorphic conditions and then followed a retrograde path. Most of the inclusions appear to have been trapped and/or re-equilibrated at 350 to 475 °C and 1.8 to 3.6 kbar, which implies a 7 to 12 km depth of vein formation and gold mineralisation. Both phase separation of a carbonic-aqueous fluid (XCO2N0.8) and the existence of an originally CO2-dominated fluid could account for the observed fluid inclusion properties and the absence of H2O-bearing inclusions in the mineralised vein. The fluid inclusion characteristics, combined with published geological and isotopic information, indicate a deep-seated source for CO2 that could be mantle, magmatic or metamorphic in origin. We suggest that the likely sources are fluid produced by the 2.07 Ga-old charnockites that occur in the region and/or the coeval high grade metamorphism that is widespread in the Guiana Shield