Summary:
The Pitangui area, where the Turmalina Complex is located, is underlain by rocks of Archaean and Proterozoic ages. Archaean units include a granitic basement, which is 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 schists and silicified biotite schist units within the Pitangui Group. A sequence of sheared, banded, sulphide-facies and silicate-facies iron formations and cherts lies within the stratigraphic sequence. The stratigraphy broadly strikes towards an azimuth of 135°.
At the Complex, Orebody A and Orebody B are understood to be typical examples of mesothermal, epigenetic, and veined deposits that are enclosed by sedimentary host rocks that have undergone amphibolite-facies metamorphism. The origin of the three individual economic zones of Orebody C is a bit more controversial, as they are hosted by a stratigraphic horizon composed essentially of unique volcano-chemical BIFs and black carbonaceous schists. A summary of some of the main and common geological characteristics of these two potentially distinct groups of Archean gold deposits present in the Iron Quadrangle district follows.
The dimensions of the economic orebodies possess strike-lengths ranging between 50 m to 350 m for individual mined zones. Average thicknesses of the orebodies may range from 2 m to 20 m.
PROPERTY GEOLOGY
The mine sequence (part of the Pitangui Group) consists of bedded metasediments including quartz- sericite schists and sericite-chlorite-biotite schists, mafic volcanic rocks including amphibole- plagioclase-quartz-chlorite schists, and horizons of meta-cherts, banded ironstones (BIFs), and black carbonaceous schists. All lithologic types of the stratigraphic units of the Turmalina deposit have been metamorphosed to the amphibolite facies.
The bedding (S0) surfaces of the mine package consistently strike at 315° and dip 50° to 60° towards an azimuth of 45°. The consistent average strike orientation of the Sn cleavage is 290° to 300°, with a dip of approximately 60° to 65° towards an azimuth of 20° to 30°. The highly important intersection lineation plunges approximately 45° to 50° towards an azimuth of 65° to 75°. As a direct result of the abovementioned recent mapping activities underground, it is now postulated that the down-plunge continuity of the stratabound, tabular economic bodies of the Turmalina deposit mimics, in terms of geographic orientation, the attitude of the intersection lineation, which has been identified and statistically measured underground (intersection lines between bedding planes/S0 and the main penetrative tectonic cleavage/Sn). This interpreted spatial “linear control” would also correspond to the orientation of axes of hypothetical (plunging and overturned) major amplitude Dn/Fn overturned folds that deformed regional stratigraphic packages of the Pitangui greenstone belt.
MINERALIZATION
The mineralization at the Turmalina deposit consists of a number of stratabound, 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.
The main past production of the mine has been 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 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 A is mostly comprised of slightly silicified (with swarms of thin quartz veinlets) biotite-muscovite-quartz schist host rocks (a metasedimentary package).
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 1000 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 CSE, C Central, and CNW) 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 CSE 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.
Gold mineralization in the Turmalina orebodies occurs as very fine gold grains associated with sulphides in sheared/foliated and silicified schists and BIF sequences. Gold particles average 10 µm to 20 µm in size, and 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.