Summary:
The gold metallogeny in the Iron Quadrangle is complex, with gold mineralization predominately observed within three general types of deposits:
- Archean-age, invariably stratabound-like, Algoma BIF hosted deposits.
- “Quartz vein swarms-style”, clearly mesothermal deposits.
- Early-Proterozoic, Witwatersrand type paleo-placer deposits.
While 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.
- At Córrego Brandão the sulphide mineralization extends approximately 1,000 m along strike and 250 m down dip. The thickness of the mineralized zones may range from two metres to 20 m.
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).