Structural, mineralogical, geochemical and geochronological constraints on ore genesis of the gold-only Tocantinzinho deposit (Para State, Brazil)
Résumé
The Tocantinzinho gold deposit is located in the north of Brazil, in the Tapajós Gold Province of the Amazon Craton. Current total measured and indicated resources are 70.2 Mt @ 1.06 g/t Au (2.4 Moz). The mineralized bodies are hosted in equigranular hornblende-biotite syeno and monzogranites plutons, with many aplite and pegmatite pockets. The magmas have I type signatures and were oxidized. The ages of the mineralized granites vary between 1996.1 ± 2.2 Ma and 1989.1 ± 1.1 Ma, and they are situated in the middle of a tectonic corridor formed by shear zones oriented parallel NW-SE that cut regional biotite-hornblende granodiorites with ages between 2007 ± 8 Ma and 1997 ± 10 Ma. Three hydrothermal phases, simultaneous to tectonic deformations, affected the Tocantinzinho Au deposit: the first, H1, occurred approximately at 1996.1 ± 2.2 Ma, during the magmatic-hydrothermal transition, which altered the igneous feldspars, biotite and hornblende. Then, the granites were brecciated in a transpressional, brittle and hydraulic fracturing regime, forming B1 breccias simultaneous to hydrothermal alteration H2, developed between 1996.1 ± 2.2 Ma and 1989.1 ± 1.1 Ma. The mineralization occurred during this phase, which simultaneously disseminated 1.0– 1.5 g/t of gold, together with pyrite and minor galena, and formed few quartz + pyrite veins with high gold contents, between 1.5 and 70.0 g/t. In an undated tectonic event occurred after and probably near 1989.1 ± 1.1 Ma, the entire mineralized region was cut by andesite dikes that reached the Proterozoic surface, and generated a new hydrothermal, degassing episode, H3. This last hydrothermal stage generated micro-fractures filled with B2 micro-breccia formed by high-pressure water-driven cataclasis, and probably remobilized previously precipitated gold but did not bring new gold to the deposit. Muscovite 40Ar/399Ar ages of the altered zone suggest that the isotopic system of this mineral was reseted at approximately 1860 Ma, about 130 Ma after the end of H3. The Tocantinzinho seems to be a single deposit, which combines characteristics of mesozonal, equigranular type, intrusion-related gold deposit, with those of porphyry-type gold deposits, although in both cases the differences are more frequent than the similarities. The available data does not allow discard that the Tocantinzinho is an orogenetic deposit.