Summary:
The Turmalina Mining Complex (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 135°. 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.
Mineralization
The mineralization at the Turmalina deposit consists of a number of stratabound, tabular bodies that are spatially related to either a BIF/Iron Formation package or to a package of slightly silicified quartz-muscovite-biotite schists. These bodies are grouped together, according to the host stratigraphy, to the spatial configuration and to the gold content, into Orebodies “A”, “B”, and “C”. Gold mineralization 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.
The main past production of the mine has been from Orebody A, which is mostly comprised of slightly silicified and “veined” quartzmuscovite-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 350 m to 400 m (with an average thickness of 6 m) and to depths of approximately 1,150 m to 1,200 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 B is located in the 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 corresponds to two or three lower grade, tabular-shaped lenses that are 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 350 m to 400 m and to depths of 950 m to 1,000 m below surface.
Orebody C is a mineralized structure located to the southwest, in the structural footwall of Orebody A. At least three individual economic zones (orebodies “C SE”, “C Central”, and “C NW”) have been delineated in this zone along a strike length of a bit more than one kilometre, and to depths of 850 m to 900 m below the surface. The three individual stratabound economic orebodies are generally represented by 2 m to 10 m thick, pervasively altered/silicified/replaced lenses hosted by the unique Orebody C Iron Formation horizon. Its auriferous silicification is quite distinctive, being dark gray in colour and sulphide bearing (pyrite, pyrrhotite, and arsenopyrite constituting up to 5% to 12% in volume of the host rocks), and characteristically causes a marked obliteration of the original bedding lamination of the iron formations. The silicification zones are stratabound in relation to the host iron-formation layer. It is observed that the high-grade economic zones are generally confined to the silicification zones.
The quality of the average gold grades of the mineralized zones of orebodies C, A and B, is a direct function of the relative amount of arsenopyrite that is present in the total modal concentration of disseminated sulphides present in altered/silicified rock specimens.
Two recently discovered mineralized lenses are located between the Orebody A and the previously known lenses comprising Orebody C. These new lenses were discovered as a result of recent exploration drilling that was carried out from the underground drill bays to define and evaluate the lower portions of the Orebody C SE mineralized lenses. As these are newly discovered mineralized lenses, their full limits and economic potential are not fully understood at the moment. The presence of potentially economic mineralization therefore is, very likely, not restricted to only the previously defined mineralized horizons and orezones. The possibility of additional mineralized zones being located elsewhere in the mine stratigraphy must be considered and evaluated as exploration targets.
The auriferous gold mineralization at the Faina 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. Moreover, the mineralized quartz veinlets have locally been mesoscopically folded, in the same manner as the bedding/S0 surfaces and the Sn cleavage planes locally were folded. The mineralized quartz veinlets and the pervasive silicification hosted by amphibolitic packages at Faina are accompanied by disseminated sulphides (pyrite, arsenopyrite, pyrrhotite, and berthierite (FeSb2S4)), however, these accessory mineral phases rarely exceed 5% of the mineralized/economic rock volume.
The past open pit operation indicated that the individual folded economic zones and lenses of the Faina deposit have dimensions of 20 m to 140 m along the S0//Sn strike, 1 m to 15 m in thickness, and very reliable continuities down-plunge, noting the maximum attitude of -50° towards an azimuth of 73° for the intersection lineations inside the Faina open pit.
The economic mineralization at the Faina deposit is continuous down-plunge from surface for at least 850 m in length.
Gold mineralization at the São Sebastião deposit is contained within deposits hosted in three main strata-confined sulfidations zones within several stacked banded iron formation layers in the lower unit of the Pitangui greenstone belt. The mineralized zones are locally named Tomate, Biquinho, and Pimentão from top to bottom. The main mineralized zone is Biquinho. One of the smaller zoned, Pimentão, is located within a thrust-fold with the fold axis gently plunging to the north and gold mineralization concentrated in one of its limbs.
The main mineralized zones in the São Sebastião gold deposit are hosted in the two most continuous banded iron formation packages (Biquinho and Pimentão) 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.