Summary:
The gold metallogeny in the Iron Quadrangle district is complex. Mineralization within the district has been found in a number of different deposit types and/or styles. The five deposits described herein - the Turmalina Mine; Faina Zone; Pontal and Pontal South (collectively the Pontal Deposits); Zona Basal; and São Sebastião deposits - are examples of the deposit diversity in this area.
Geology and Mineralization
The MTL Complex is underlain by Archaean and Neoproterozoic age rocks. Archaean units include a granitic basement, overlain by the Pitangui Group, a sequence of ultramafic to intermediate volcanic flows and pyroclastics and associated sediments. The Turmalina deposit is hosted by chlorite-amphibole schist and biotite schist units within the Pitangui Group. A sequence of sheared, banded, sulphide iron formation and chert lie within the stratigraphic sequence. The stratigraphy locally strikes to azimuth 35°. The Turmalina deposits are believed to be typical examples of mesothermal, epigenetic deposits that are enclosed by host rocks that have undergone amphibolite grade metamorphism.
Turmalina Mine
At the Turmalina Mining Complex (MTL) Complex, Orebody A and Orebody B are understood to be typical examples of orogenic gold deposits influenced by WNW-ESE-trending shear zones. These deposits are associated with hydrothermal alteration and are surrounded by sedimentary host rocks that have experienced amphibolite-facies metamorphism, estimated to reach a hydrothermal temperature of approximately 650°C. Orebodies C and D share similar primary mineral assemblages but differ in host rocks (chemical metasediments in C and metabasalts in D).
The mineralization at the Turmalina deposit consists of a number of stratabound, tabular bodies that are spatially related to either a banded iron formation (BIF) package or to a package of slightly silicified quartz-muscovite-biotite schists. These tabular bodies are grouped together, according to the host stratigraphy, to the spatial configuration and to the gold content, into Orebodies A, B, and C (together the Orebodies). Gold can occur within the BIF package but can equally occur in the other host lithologies. Gold mineralization in the Turmalina deposits occurs in fine grains associated with sulphides in sheared schists and BIF sequences. Gold particles are mostly associated with arsenopyrite, quartz, and micas (sericite and biotite). Coarse grained gold, on a millimetre scale, is found locally with discrete quartz veins, but this type of occurrence is minor at the Turmalina deposit.
Faina Zone Deposit
At Faina Zone, the orebodies are classified as orogenic gold deposits with a folded geometry characterized by hinges plunging to the northeast and a faulted structure exhibiting an east-west striking orientation with near-vertical dip. These orebodies are predominantly hosted in metabasalts. Notably, the hydrothermal temperature at Faina Zone is intermediate, ranging between 450°C and 500°C.
The Faina Zone mineralization and mineralized zones hosted by the Mafic Metavolcanic Unit is inferred to be related, in a larger scale, to a regional, northwest-southeast oriented, transpressional faulting event that also generated a coeval smaller-scale system of east–northeast–west–southwest accommodation, transcurrent-movement faults. The auriferous gold mineralization at the Faina Zone deposit corresponds mainly to swarms of sulphide-bearing quartz veinlets (individual veinlets with millimetric to centimetric widths) which are hosted by amphibolitic packages of the Mafic Volcanic Unit. The mineralized swarms of quartz veinlets appear to occur within conformable horizons to the mine stratigraphic package, in at least several distinct “stratigraphic layers” of the Mafic Volcanic Unit.
Pontal Deposits
At Pontal, the orebodies are typically classified as orogenic gold deposits, however, certain veins display textural features associated with epithermal-type deposits. In Pontal, located at the northwest end of the shear zone, the mineralization shares a geometric resemblance with Faina Zone, but it is hosted in volcanic agglomerates. The primary mineral assemblage comprises pyrite + pyrrhotite + arsenopyrite + Au, linked to crustiform veins characterized by lower temperatures (approximately 300°C), potentially indicating an epithermal origin.
Three slightly different styles of gold mineralization have been recorded in the Pontal South target, despite the fact that all of them are gold-arsenic-antimony rich. The most common style consists of fine grained disseminations of sulphides composed mostly of arsenopyrite, pyrite, and pyrrhotite contained within the pyroclastic host rocks. The second style corresponds to massive concentrations of sulphides (mainly arsenopyrite and antimony sulphides) located around quartz veins and within highly (or pervasively) silicified domains of the same pyroclastic host rocks. The last style to be considered would be a result of the presence of multiple sulphidized clasts and coarse transported fragments, either primarily mineralized and redeposited, or eventually replaced by the same ore fluids that “sulphidized” the matrix of the pyroclastic host rocks.
Zona Basal Deposit
At Zona Basal, the orebodies have both mineralization representative of supergene gold deposits as well as hypogene mineralization that is most representative of structurally controlled sulphide replacement zones in stratabound deposits. The critical supergene reconcentration of gold and silver occurs in the leached horizons of Zona Basal. Hypogene mineralization at Zona Basal is primarily concentrated at the hinge zones of the major Basal Unit anticline. Additionally, it is found within thick chemical metasediment rock packages, including metacherts, BIFs, and carbonaceous schists, interspersed among metamafic rocks. Hydrothermal temperatures during both stages at Zona Basal are characterized as intermediate (approximately 450°C) and low (170°C).
The Zona Basal hypogenic economic mineralization can be understood as a system that is primarily controlled by a major northwest-southeast oriented transpressive structural movement zone, and which is spatially located approximately at the axial-plane setting of the Zona Basal overturned antiform. The Zona Basal “supergene” (or surficial) mineralization appears to concentrate economic gold grades as well as some silver and other base metals in the weathering halo. The more ubiquitous mineralization style recorded at Zona Basal corresponds to fine grained disseminations of sulphides (arsenopyrite, pyrite, and pyrrhotite) hosted by the favorably-replaced volcano-chemical horizons.
São Sebastião Deposit
The São Sebastião gold deposit is considered a hypozonal orogenic gold deposit type and, in general, is related to three different mineralization styles. 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.