The Paleoproterozoic Dorlin gold deposit, French Guiana: genetic constraints of the stable isotope geochemistry

This paper reports on oxygen, hydrogen, carbon and sulphur isotopic compositions of silicates, carbonates and sulphides from the Dorlin gold deposit in the Paleoproterozoic greenstone belt of central French Guiana. The Dorlin orebodies result from the polydeformation of a large tourmalinite-bearing stratabound sulphide deposit hosted by volcanic and sedimentary rocks of the Paramaca Series, metamorphosed in the greenschist facies. The mineralizing envelope would initially have consisted of an inner zone of tourmaline-sulphide-bearing ores (M2) surrounded by a zone of chlorite-sulphide-bearing mineralization (M1). The stratabound sulphide deposit was locally cut by a muscovite-sulphide–quartz stockwork (M3). The δ18O values of quartz and chlorite from the chlorite-sulphide mineralization are +12.2‰ and about +5.5‰ (SMOW), respectively; those of quartz and tourmaline from the tourmaline-sulphide mineralization are about +14‰ and +8.5‰ (SMOW), and those of quartz and muscovite from the discordant stockwork are +12.4‰ and +6.9‰ (SMOW). δD values of the chlorite, tourmaline and muscovite are about −45‰, −28‰ and −38‰ (SMOW), respectively. Carbonates (calcite, dolomite/ankerite and siderite) in the mineralizations have δ18O values between +10.8 and +15.7‰ relative to SMOW and δ13C values of −5.2 to −0.8‰ relative to PDB. δ34S values of samples from the three mineralizations range between −0.5 to +2.6‰ (CDT) with two exceptions at −2.8 and −2.9‰. The data support the existence of a zoned hydrothermal system largely controlled by a network of paleofractures and slightly reworked by a late muscovite-sulphide–quartz stockwork. The isotopic geothermometers record temperatures of ore deposition in the range of 200–300°C. The δD and δ18O values of the chlorites, tourmalines and muscovites would indicate a large input of seawater in the Dorlin hydrothermal fluids, whereas the δ18O and δ13C of the carbonates and δ34S of pyrite provide evidence of a magmatic component, resulting probably for a large part from leaching of the volcanic host rocks.