Formation of the Tocantinzinho 1989 Ma gold-only deposit (Pará State, northern Brazil), based on sulfur, oxygen, hydrogen, and carbon isotope data

Abstract

The Tocantinzinho gold deposit in the Brazilian Amazon region, approximately 1989 Ma in age, contains 16.7 Mt of ore in syenogranite and monzogranite breccias and microbreccias, most with gold in pyrite, silicates and quartz, and a small part in quartz vein pyrites. Three hydrothermal phases are recognised, H1, H2, and H3, where H1, the first hydrothermal phase, is the oldest. The H1 transitional, late-magmatic hydrothermal phase altered feldspar to sericite and mafic minerals to chlorite. During the H2 hydrothermal phase, pyrite, gold, and minor galena were precipitated, followed by intense sericite alteration. The last hydrothermal phase, H3, was coeval with a large number of andesite intrusions that generated high-pressure fluid-driven cataclasis. This event opened veins, tension gashes, and pull aparts that were filled with carbonate, worm-like chlorite, prismatic quartz, galena, pyrite, and rutile, which likely displaced and concentrated the gold near the carbonate-filled structures.The minerals sulfide δ34S values vary between −4.4 and + 5.6‰, which is compatible with a sulfur magmatic source, and indicate that the sulfur and likely the gold are both magmatic. The δ18OSMOW values of carbonates vary between +7.63 and + 21.23‰ in a narrow range of δ13CPDB values between −3.94 and −1.69‰. Two explanations are possible for these values. If that the carbonate fluids were genetically related to the andesite magmatism, this fluid mixed with meteoric water, and degassing of this mixture occurred in the near-surface to produce dissolved inorganic CO2 with high δ18O value. Alternatively, when fluid mixing occurred, the oxidation state of the environment increased, and the δ13C values of the carbonates remained relatively uniform, whereas the δ34S values of the sulfides varied. The δ18O values of the quartz, sericite, and chlorite of the mineralized and barren rocks are consistent with δ18O values > +8 for the fluids that crystallized the chlorites and δ18O > + 4‰ for those that crystallized the muscovite. High δD values of these fluids, estimated from mineral δD values range between −2 and −25‰, and differentiate them from the fluids that form porphyry-type Cu–Au and orogenic deposits. The isotope data for Tocantinzinho present characteristics of an intrusion-related gold deposit that combines the features of equigranular, intrusion-hosted, mesozonal deposits, such as those hosted in reduced granite, with Au-rich porphyry deposits hosted in oxidized granites.