Abstract
This paper presents geological, petrographic and min- eralogical data about the granite that hosts the Tocantinzinho gold deposit and aimed at contributing to the understanding of the hydro- thermal processes related to the ore genesis. The host granite is a late to post-tectonic biotite monzogranite that fits in the oxidized sub-type of the ilmenite series. It was emplaced at depths of 6-9 km and reveals no deformation other than fracturing and brecciation. This intrusion has undergone mild to moderate hydrothermal alteration that generated two main rock varieties (salami and smoky), with no significant miner- alogical or chemical differences, though quite distinct macroscopically. Several types of hydrothermal alteration have been recognized in the granitic rocks, greatly represented by filling veins and/or replacement of primary minerals. The hydrothermal history started with microcliniza- tion, during which the granitic protolith was in part transformed into the salami variety. This process was followed by chloritization when the temperature dropped to ~330oC producing chamosite with X Fe in the 0.55–0.70 range. Then it evolved to sericitization, at the same time that ore-bearing fluids precipitated pyrite, chalcopyrite, sphalerite, ga- lena and gold. As alteration advanced, solutions saturated in silica and formed quartz veinlets. At the latest stage (carbonatization), aqueous and aqueous-carbonic fluids might have mixed, allowing Ca 2+ e CO 2 to react to form calcite. Most sulfides are present in veinlets that cross- cut the granite, some arranged as stockwork. Gold is normally very fine-grained and occurs mainly as submicroscopic inclusions or along microfractures in pyrite and quartz. The Tocantinzinho deposit is very similar to the Batalha, Palito and Sao Jorge deposits, and to those of the Cuiu-Cuiu goldfield. Topologically, it has been classed as an intru- sion-related gold deposit.
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