A fluid inclusion and stable isotope (O, H, S and C) study of the Archean IOCG Cristalino deposit, Carajás mineral Province, Brazil: Implications to ore genesis

Abstract

The hydrothermal history of the Archean Cu-Au Cristalino deposit is recorded in moderately to intensely altered host rocks, which evolved from ~550 °C to ~150 °C. Distal sodic and proximal calcic-ferric, potassic and propylitic alterations developed successively with variable intensity. Two main mineralization types are also recognized: an earlier one characterized by a chalcopyrite + pyrite + magnetite + Au association that occurs as disseminations and breccias in calcic-ferric zones, and a later one represented by a chalcopyrite ± pyrite ± hematite ± Au association that occurs as breccias and veins mainly in potassic and propylitic zones. Estimated pressures (0.6 to 2.6 Kbar) are consonant with brittle and ductile-brittle deformational environments where both alteration and mineralization took place. The ore-bearing fluid was hot (T as high as ~550 °C), acidic and hypersaline (55.1% wt. NaCl equiv.) and chemically approached by the H2O-NaCl-CaCl2-CO2 ± MgCl2 ± FeCl2 system. Salinity might have exceeded 55 wt% NaCl equiv. in the early alteration stages but decreased progressively to 7.9 wt% NaCl equiv. from 250 °C onwards, due to incursion of surficial water into the deposit site. The fluid was initially 18O-enriched and D-depleted (δ18Ov-smow = +9.73 to +6.48‰; δDv-smow = −30.17 to −40.25‰) and most likely derived from magmatic sources. As result of dilution caused by mixing with meteoric water, the fluid became relatively 18O-depleted and D-enriched (δ18Ov-smow = +5.57 to −0.28‰; δDv-smow = −19.15 to −22.24‰). The δ13Cv-pdb values for vein and breccia calcite (−6.5 to −3.8‰) are consistent with a deep source for CO2, likely of mantellic origin. Most δ34Sv-cdt values for chalcopyrite show narrow variation (+1.6 to +3.5‰) and could indicate a homogeneous reservoir for sulfur. However, a few samples reveal significant influence of sedimentary rocks on their isotope composition (δ34Sv-cdt = −32.6‰), showing that sulfur may have been sourced from various reservoirs. It is inferred that most Cu and Au were transported as chloride complexes (>350 °C), and precipitated in response to decrease in temperature and Cl− activity, and increase in pH. The formation of the chalcopyrite ± pyrite ± hematite ± Au association was especially favored by an oxygen fugacity increase during the late stages of the hydrothermal alteration. Secondary fluid inclusions reveal that an aqueous, colder (200–150 °C) and relatively less saline (21–3.1 wt% NaCl equiv.) fluid of uncertain origin circulated in the Cristalino deposit area, which is likely related to a nearby Paleoproterozoic granitic intrusion. In comparison with other Archean Carajás IOCG deposits, particularly those that lie in the southern sector of the Carajás Domain, the Cristalino deposit shows similar structural styles, hydrothermal assemblages, and ore fluid composition and evolution. The data presented here support previous interpretations that consider Cristalino as of IOCG typology.