Summary:
Deposit Types
The Borborema deposit, Borborema Project, is considered to be a classic orogenic-gold deposit type in a sheared and deformed Archaean to Proterozoic age greenstone belt sequence, that is comprised of metamorphosed volcanic-sedimentary rocks units intruded by slightly younger post-tectonic igneous bodies.
Studies carried out in the Borborema Project area (Phillips, 2011) concluded that the gold deposit is classified as a mesothermal orogenic-gold type in view of its key characteristics.
Gold mineralisation at the Borborema Project, Borborema Province, occurs in a succession of (meta) pelitic and psammopelitic schists intruded by minor occurrences of pegmatite and granitic orthogneiss (Stewart: 2011). Rocks are metamorphosed in upper greenschist facies to amphibolite conditions (Araújo et al., 2002; Stewart, 2011).
Mineralization And Alteration
The Borborema deposit is located within a northeast–southwest trending structure which forms part of the northern segment of the Santa Mônica dextral shear zone (Araujo et. al, 2002). The shear zone displays a penetrative north-northeast-trending fabric, dipping southeast at around 40 degrees. In the Project area the principal mineralised shear zone, termed the Morro Pelado Shear, is around 30 metres thick.
The mineralization is strongly controlled by regional structure with secondary structuring providing the preferred host for gold (Figure 29). In addition to the main mineralized zone, several thinner sub-parallel zones of increased gold mineralization (> 0.1 g/t Au) can be seen in drill core.
Genesis of the gold mineralizing event or events is poorly understood. Two distinct gold mineralization types are identified both by SRK and Aura Geologists in drill cores: 1) disseminated free gold, and 2) gold in association with sulphide mineralization represented by pyrrhotite, chalcopyrite, pyrite, sphalerite, and galena. Additionally, the sulphide mineralization was observed in the outer contact between chert boudins and schist along with or within schist foliation.
The continuity of mineralization observed in select diamond drill core shows a highly discontinuous nature to both types of observed gold mineralization. Sulphide-hosted gold appears primarily along psammitic schist foliations and around the perimeter of quartz veins and boudins. The visual inspection of sulphide mineralization in core with correlated analytical results appears to indicate a relatively high concentration of Au in pyrrhotite such that a sub-cm scale zone of sulphide mineralization resulted in grades commonly exceeding 1 g/t Au (SRK, 2022). Sulphide mineralization throughout the main mineralized zone is sporadic in nature. For example, a 10 cm zone of sulphides hosted the entirety of the metal for the 1 m sampling interval while the remaining core appeared barren was noted (SRK, 2022).
The mineralised sequence has been subjected to a complex, multi-stage deformational history, with folded, sheared, dismembered and boudinage quartz and quartz-carbonate veins and veinlets commonly associated with the gold mineralisation. Recrystallised sulphides, both finely disseminated and locally forming centimetre-scale patches, dominated by pyrrhotite with lesser pyrite, chalcopyrite, sphalerite, and galena are common within the mineralised zones. Microscopic examination, however, does not appear to indicate a direct relationship between gold mineralisation and sulphide abundance. Magnetite closely associated with gold, post-dates the sulphides (Baars, 2011).
Baars (2011) believes that the deformational event which accompanied gold mineralisation was an extensional event forming a linear dilatational feature. Limited analytical data for silver indicate overall a silver/gold ratio of approximately 2:1, although on an individual sample basis there appears to be little or no correlation between gold and silver values. Phillips (personal comm.) suggests that the base metal sulphide mineralisation event may be independent of the gold event; the lack of direct correlation between gold and silver also suggests deposition in separate events or pulses.
Holcombe (2012) concluded that the main host of mineralization developed along steeply dipping, retrograde reverse-sense shear that occurs within the Sao Francisco Pit. He concluded also that a second shallow-dipping structure was associated with mineralization that was separate and oblique to the main shear zone. The shallowly dipping mineralized system lies in a strongly attenuated, axial plane-parallel zone within the overturned limb of a large, inclined fold. The mineralization is locally sheared but the displacement between its hanging wall rocks, and its footwall rocks is not significant from a crustal point of view.
Mineralisation within the wide retrograde shear within the pit is dominantly within deformed veins. The mineralisation extracted from the eastern part of the main pit may have been from dissection of the pre-existing shallowly dipping mineralized zone that now forms the main mineralized orebody. Mineralisation within the remainder of the shear, within the pit, is likely from a separate source at depth.
Evidence of alteration of the host rock in association with gold mineralization is not well understood. The retrograde metamorphism in the Sao Francisco Pit has not examined for altered minerals and low-temperature mineral assemblage. There is no or little geochemical analysis was done to identify the mineralogical and chemical signatures of gold bearings host rocks. Further analysis on alteration chemistry and mineralogy needs to be done to better clarify the genesis of gold in the Borborema deposit.