Summary:
The Pilar and Roça Grande deposits are examples of the Algoma BIF hosted type, Córrego Brandão is a folded and sheared deposit hosted in carbonate-quartz-chlorite schists with iron occurrences. The main geological characteristics of this group are summarized as follows:
- Main host/fertile “Algoma type” BIF Units: These units host the mineralization and are stratigraphically located at the waning stages of major volcanic cycles of the Rio da Velhas greenstone belt. They are overlain by later sedimentary rocks composed of greywackes and turbidites.
- Mineralization style:
- The mineralization consists of predominately “lateral” replacements/sulphidations of the iron carbonate-rich bands of the host Algoma type BIF units. The BIF hosted gold mineralization at Pilar, however, is not syngenetic in nature (in relation to the deposition of the host rock packages), rather it is clearly an epigenetic event that has occurred after the formation of the host rock units. There is a clear temporal-spatial-genetic relationship between the replacement/sulphidation of the host BIFs and the onset of a structurally controlled, district scale silicification event at Pilar. Textural observations suggest that at least two episodes of quartz veining has occurred. The early stage veining is deformed by the regional strain and clearly pre-dates the deformation event. The later stage veins are observed to crosscut the strain-related fabrics and so are interpreted to represent a younger mineralizing event that has taken place some time after the regional deformation event.
- Dimensions of the economic orebodies: Economic strike lengths of only 50 m to 350 m for individual mined zones. The average thicknesses of the BIF hosted orebodies may range from two metres to 20 m.
PROPERTY GEOLOGY
The Pilar deposit is hosted by the basal units of the Nova Lima Group, and by sequences of the QuebraOssos Group. The rock packages in the immediate Pilar area are comprised of tholeiitic metabasalts, mica-quartz schists, chlorite-quartz schists, quartz-chlorite-sericite schists, and volcano-chemical and clastic meta-sedimentary rocks of the Santa Quitéria Unit (Nova Lima Group), and further to the east, of meta-komatiite flows (along with their intrusive equivalents) of the Quebra- Ossos Group. The volcanochemical meta-sedimentary rock packages include cherts, BIFs, and carbonaceous phyllites. Along the eastern edge of the Pilar property, the supracrustal units of the Rio das Velhas Supergroup are in fault contact with migmatites and granitic gneisses of the Santa Bárbara Complex, the unit that locally represents the basement sequence.
Pilar occurs at the northernmost end of the northeasterly oriented Brumal-Pilar BIF trend, which extends for many kilometresto the southwest from the Pilar deposit. In regional terms, the Brumal-Pilar BIF linear trend corresponds to a package of “Algoma type” BIFs (oxide-facies, silicate-facies, and carbonate-facies lithotypes) that represent the main economic target as hosts of the Pilar deposit.
Past regional mapping demonstrated that the Brumal BIF trend within the Pilar site is folded into a considerably tight, overturned synform-antiform fold of approximately one kilometre in amplitude, with axes statistically plunging steeply to the southeast and with an axial-planar tectonic cleavage dipping steeply in the east-southeast direction.
While the “Algoma type” BIFs typically range between five metres and 15 m to 20 m in thickness, at Pilar, they have been severely and tightly folded and thickened as a result of a west-verging compressional regional deformation event that affected the entire eastern border of the Rio da Velhas Supergroup exposures in the Iron Quadrangle Terrain. Structural geometries recorded at Pilar indicate that the mine stratigraphic package may have been folded and refolded during this event. Moreover, some major reverse faults and/or accommodation faults (such as faulted synform closures) formed during this regional compressional event locally exhibit evidence of the presence of later superimposed events (mainly tilting and/or rotation of the previously faulted blocks).
Stratigraphically, the Pilar BIF Unit is overlain by a two to five metre thick layer of carbonaceous phyllites, which in turn is overlain by a thick package of greenschists (meta-basic volcanic rocks - “Upper Basic Volcanic Unit”). The Pilar BIF Unit is underlain by a thick package of greenschists (“Lower Basic Volcanic Unit”). The Lower and Upper basic volcanic units are very similar in nature, if not identical, considering their lithologies, lithostratigraphic record, and penetrative structural petrofabrics mapped.
MINERALIZATION
Economic gold mineralization at Pilar is hosted by the folded, and locally refolded, Pilar BIF Unit including the SW Limb, São Jorge, BF III, BF II, BF, LPA, and BA mineralized zones, and by the conformably folded Torre meta-volcanic sequence.
The main zones of mineralization occur as scattered, stratabound lenses (or “pods”) of sulphide-facies
BIFs within the “carbonatic-oxide-facies” deposit scale Pilar BIF Unit. Economic mineralized bodies consist of stratabound, but not stratiform, concentrations of gold bearing sulphides that occur in scattered grains, seams, and irregular shaped granular aggregates located along and replacing iron carbonates-rich bands of the BIFs. Arsenopyrite and pyrrhotite are the most important sulphide minerals in mineralized bodies, with pyrite, chalcopyrite, galena, and sphalerite commonly present as accessory minerals. A direct relationship can generally be established between the amounts of arsenopyrite (percentage per volume) and the gold concentrations in mineralized BIF samples of the Pilar deposit.
There is a clear temporal-spatial-genetic relationship between the epigenetic replacement/sulphidation of the host BIFs and the onset of a structurally controlled, district scale silicification event. Carbonate-rich bands of mineralized BIFs commonly exhibit sulphide enriched alteration/replacement halos that are symmetrically distributed around swarms of quartz veins and veinlets. The sulphide minerals occur predominately as disseminations in the host rocks, but can achieve semi-massive to massive concentrations locally, over a few metres.
Individual quartz veins are typically less than one metre in width and can be observed to be of three generations. Quartz veins of the first generation are typically associated with the gold mineralization and are folded by the main tectonic event which affected the Pilar package as a whole. Quartz veins of the second generation are typically lower grade or barren and are not affected by folding. Lastly, quartz veining of the third generation is associated with the aforementioned extensional tectonic cleavage and may also be related to halos of mineralization/sulphidation where related to hinge structural domains.
At the Pilar deposit, increased average gold grades and higher sulphide/arsenopyrite concentrations (within the economic mineralized zones and orebodies) are almost everywhere (or at least very frequently) mapped in association with the deposit scale, larger, higher-amplitude fold hinge geometries. Increased average gold grades and higher sulphide/arsenopyrite concentrations have also been observed in association with a second folding event (and/or with a subsequent third folding event) in the Pilar BIF package (Tiago Souza, 2020). The second folding event would have brought mineralized quartz veins into the BIF Pilar package, however, according to Tiago Souza (2020), the third folding event was responsible for the generation of the economic mineralization hosted by the large fold hinge geometries of the Pilar package (São Jorge synform, BF II antiform, BF antiform, BF II-BF Junction synform, BA antiform, and the smaller-amplitude Torre antiforms).