Summary:
Deposit Types
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.
In a broad context, the primary deposit types in this region include the following:
1. Structurally controlled sulphide replacement zones in stratabound deposits such as Algoma-type BIF;
2. Orogenic gold deposits;
3. Supergene gold concentration of gold through weathering processes near the surface.
Where grades and thicknesses permit, gold deposits of these types and styles are amenable to both bulk mining and more selective high-grade underground operations.
The MTL Complex region, with its poly-orogenic geological evolution (Archean, Transamazonian, and Brasiliano orogenies), is characterized by the amalgamation of Archean blocks where the rocks of the Pitangui Group outcrop. This stratigraphic group is surrounded by the Mesoarchean basement of the Divinópolis Complex and intruded by high-K calc-alkaline magmatism (e.g., the Casquilho granitoid, adjacent to some of the Jaguar MTL ore bodies).
At the 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.
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.
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 (folded/faulted), 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.
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. Despite not being situated at the MTL major shear zone, Zona Basal is positioned approximately two kilometres southwest of it, adjacent to the normal limb of the Pitangui Group synclinorium. 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 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.
Mineralization
The mineralization at the Turmalina Mine consists of a number of stratiform, tabular bodies that are spatially related to either a 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. The down-plunge continuity of mineralization within the Orebodies follows the intersection between bedding planes/S0 and the main penetrative tectonic cleavage/Sn and the attitude of this intersection lineation has been identified and statistically measured underground.
Much of the past gold production of the mine has been derived from Orebody A, which is mostly comprised of slightly silicified and “veined” quartz-muscovite-biotite schist host rocks (swarms of small, prevalent, quartz veinlets that are centimetres in width). The economic mineralization in this zone has been outlined along a strike length of approximately 550 m to 600 m (with an average thickness of six metres) and to depths of approximately 1,300 m to 1,400 m below surface. The southeastern portion of Orebody A is composed of two parallel narrow veins. The northwestern portion of Orebody A is much the same as the southeastern, however, the two parallel zones nearly or completely merge, and therefore the economic zone is much wider overall to the northwest direction (locally up to 10 m to 15 m in thickness). Orebody A is mostly comprised of slightly silicified (with swarms of thin quartz veinlets) biotite-muscovite-quartz schist host rocks (a metasedimentary package). Mining activities on Orebody A were put on hold in late 2022.
Orebody B is located in the structural hanging wall of the Orebody A, and is geologically somewhat similar to Orebody A, both in terms of the type of the host package and of the visual style of the gold mineralization. The Orebody B comprises to two or three lower-grade, tabular-shaped lenses that are oriented generally parallel to Orebody A. These lenses are located approximately 50 m to 75 m in the structural hanging wall and are accessed by a series of crosscuts that are driven from Orebody A in the upper levels of the mine. The mineralization in this zone has been outlined along a strike length of approximately 425 m to 475 m and to depths of 950 m to 1000 m below the surface.
Orebody C is the current primary source of gold production at the Turmalina Mine. It is a mineralized structure located to the southwest, in the structural footwall of Orebody A. At least three individual economic zones (Orebodies CSE, C Central, and CNW) have been delineated in this zone along a strike length of approximately 1,000 m to 1,100 m and to depths of 850 m to 950 m below the surface. The three individual stratiform economic orebodies are generally represented by two metre to 10 m thick, pervasively altered/silicified/replaced lenses hosted by the unique Orebody C Iron Formation horizon.