Fluid inclusion and isotopic constraints on the genesis of vein-quartz gold deposits of the Ipitinga Auriferous District, SE-Guyana Shield, Brazil

Abstract

The Ipitinga Auriferous District (SE-Guyana Shield, NW-Pará state) contains vein-quartz gold occurrences hosted in Paleoproterozoic metavolcano-sedimentary sequences and associated granitoids, especially near the contact between the granitoids and the supracrustal rocks. The structurally-controlled deposits are hosted in ductile shear zones and in foliation planes of the host rocks, have sericitization as the main important proximal alteration, and have small amounts of sulfi de minerals in the ore paragenesis. Fluid inclusion types and properties are similar in most of the deposits. The aqueous-carbonic inclusions, with small to moderate amounts of CH4 (locally C2H6) are the predominant types. These inclusions have low salinity (~5 wt. % NaCl equiv.) and homogenized between 290° and 355°C. Carará differs from this pattern in that it shows only water-free CO2-bearing inclusions, the origin of which is still uncertain. Oxygen and hydrogen isotope compositions overlap the fields of magmatic and metamorphic waters, except at Carará that clearly shows a metamorphic source for the hydrothermal fl uid. Integrated geologic characteristics, and fl uid inclusion and isotopic results are consistent with the class of orogenic gold deposits. The characteristics reported in the literature for the Serra do Ipitinga Au occurrence strongly differs from those presented by the studied deposits in the district and probably belong to another class of deposit. Pb-Pb and 40Ar-39Ar systematics have not succeed in determining the age of the mineralizing event in the Ipitinga Auriferous District, but bracketed a time window for mineralization between 1940 and 2030 Ma