Summary:
The São Sebastião gold deposit is considered a hypozonal orogenic gold deposit type and, in general, is related to three different mineralization styles. Firstly, it is associated with replacement-style sulphidation over magnetite, characterized by the prevalence of massive to semi-massive pyrrhotite dissemination interlayered with chert bands. Secondly, sulphides are present in breccia zones, with varying thickness ranging from a few centimetres to a few metres, particularly evident in fold hinges within the deposit. Lastly, quartz-carbonate-sulphide veins play a role in gold mineralization, containing variable amounts of sulphides and showing spatial associations with breccia zones. Electron-microprobe analyses of arsenopyrite indicate a precipitation temperature range between 465°C and 560°C. This temperature interpretation is derived from the context of gold deposition with arsenopyrite occurring during compressional tectonic events. Gold is primarily found as inclusions in late- to post-kinematic pyrite and arsenopyrite, as fracture infills in arsenopyrite, and in contact with gangue minerals.
Gold mineralization at the São Sebastião deposit is contained within deposits hosted in three main strata-confined sulphidized zones within several stacked banded iron formation layers in the lower unit of the Pitangui greenstone belt. The main mineralized zones in the São Sebastião gold deposit are hosted in the two most continuous banded iron formation packages of the lower unit. The sulphide mineralization in these zones most commonly occurs as disseminations replacing magnetite, however, occasional massive sulphide mineralization in quartz-carbonate veins and breccias can occur. Pyrrhotite is the dominant sulphide, followed by arsenopyrite, pyrite, and chalcopyrite, which appear in smaller concentrations.
Lithostratigraphic Units
The lithostratigraphy of the Onças de Pitangui Project comprises three stratigraphic units: Lower, Middle, and Upper metavolcanic-sedimentary sequences, which are part of the Pitangui Greenstone Belt. These sequences have undergone metamorphism from upper greenschist facies (biotite-actinolite facies) to lower/middle amphibolite facies (grunerite-hornblende).
Lower Unit: This unit consists of five intervals from bottom to top:
I. Metavolcanic Basic and Ultrabasics Sequence: Includes metakomatiites, metabasalts, and metakomatiitic basalts.
II. Metasedimentary Sequence: Comprises interstratified metasandstones, BIFs, metacherts, and metapelites with carbonaceous rocks at the base.
III. Metavolcanic Ultrabasic Sequence: Predominantly metakomatiites.
IV. Metasedimentary Sequence: Features thicker zones of metasandstones, BIFs, and metapelites.
V. Metasedimentary and Metavolcanic Basic Interbedded Sequence: Contains metabasalts at the base, metasandstones with lenticular BIFs, and minor metacherts.
The BIF rock packages in intervals II and IV are significant as potential gold host rocks for the São Sebastião deposit and other satellite gold occurrences (Aparição, Caldas, Guardas, São Sebastião SE, and Vilaça targets).
Middle Unit: This unit is predominantly composed of clastic metasedimentary rocks such as metapelites and fine- to medium-grained metasandstones, interbedded with felsic metavolcanic rocks like metadacites and metaandesites. The contact with the Lower Unit is gradational, marked by an increase in the metasedimentary pile over the metavolcanic rocks. Minor gold occurrences in this unit include the Tabocal, Barreiro, and São Francisco targets.
Upper Unit: This unit consists of muscovite metasandstones and polymictic metaconglomerates containing clasts of BIF, metavolcanic-sedimentary rocks, metacherts, and quartzites. Few outcrops of this unit exist, and no gold mineralization has been reported.
Regionally, the Pitangui Greenstone Belt is a northwest-trending synclinorium structure surrounded by TTG and syn- to late-tectonic granitoids, such as the Florestal dome, Arruda and Casquilho granites, Jaguara granitoid, and the Bambuí group.
The São Sebastião gold deposit, part of the Onças de Pitangui Project, is located on the eastern limb of the Jaguara anticline, corresponding to an open fold structure (N300-N350/25-35º plunging structure, Dn+2). The main tectonic foliation (Sn) parallels the relic bedding structure (S0), represented by biotite and chlorite metamorphic minerals. Centimetric to metric isoclinal recumbent folds and intrafolial Dn folding structures are common. Locally, Dn+1 reverse fault structures, materialized by sulphide-quartz breccias, crosscut the Lower Unit and generate array folding as part of the faulting and folding process.
Mineralization
The São Sebastião gold mineralization is characterized by two main disseminated (rarely massive) sulphide-bearing mineralized zones, which are strataform with the BIF from the Lower unit, specifically in intervals II and IV, known as Biquinho and Pimentão. A third mineralized zone, named Tomate, has been found to contain less well developed mineralization to-date.
The gold is associated with a replacement-style sulphidation, where pyrrhotite replaces magnetite-rich bands along the BIF bedding lamination. The main mineral paragenesis of these mineralized zones includes pyrrhotite, arsenopyrite, pyrite, arsenian pyrite, and chalcopyrite, with subordinate sphalerite and galena. Accessory mineral assemblages include tourmaline, scheelite, Bi-sulphosalts, native Bi, and Bi-tellurides. These mineral assemblages indicate a diverse and complex evolution with multiple stages of sulphide association. Sulphide concentrations are commonly located on saddle reefs, boudin necks, and fractures parallel to the axial plane of deformation events Dn and Dn+1 folds. Gold occurs in various sulphide phases and gangue minerals, with sizes ranging from less than five microns to 200 µm. Generally, gold is included in arsenopyrite and pyrite, rarely in pyrrhotite, and most commonly along sulphide boundaries, filling crystal fractures and disseminated in the gangue as coarse grains (50 µm to 200 µm).
In addition to the sulphide zones, which are proximal zones of mineralization, there is a distal hydrothermal alteration present as a larger alteration halo of carbonate + quartz, mainly characterized by pervasive-disseminated ankerite and quartz, with minor calcite and siderite.
The gold mineralization model comprises an epigenetic hypozonal orogenic gold deposit, involving high- temperature fluids (greater than 600°C) with multi-stage mineralization, including syn-, late-, and post-kinematic mineral assemblages, likely linked to an intrusion-related model. The pathfinder elements of mineralization are associated with Au-Bi-Ag-Cu-As-Pb ± Co-Ni-TeW. Recent data of Re-Os dating from arsenopyrite and pyrite revealed ages at ca. 2.0 Ga.
Under these conditions, the São Sebastião gold deposit in the Pitangui Greenstone Belt may represent the deeper roots of the Iron Quadrangle gold system, differing from the gold deposits of the Rio das Velhas Greenstone Belt.