Summary:
The RGM is located within the Mutare Greenstone Belt (MGB), which extends from the Mozambique Belt in the east, into the granites and gneisses of the Zimbabwe Craton in the west. The geology is dominated by an east-west trending series of metavolcanics consisting of olivine cumulates, komatiites, komatiitic basalts and high iron tholeiites. The rocks are extrusive, indicated by the presence of spinifex texture and pillow lavas as well as lenses of intercalated clastic sediments and discontinuous strings of Banded Iron Formation (BIF). The MGB is bounded by the Penhalonga Diorite to the north and south.
Local and property geology
Felsite
The Felsite consists of a tabular micro-granitic body, which intruded the Penhalonga diorite. It is a robust orebody, constituting the main mineralisation style at the RGM. It strikes east-west and dips at a locally variable but shallow angle of approximately 35° to the south. At its widest, the orebody attains a thickness of approximately 7 m. It has been worked on a strike length of more than 2,500 m. The Felsite lies in the core of the Penhalonga synform and it is the folding of this micro-granitic sheet within the diorite that is believed to be responsible for the localisation of gold mineralisation (KDM 2018).
Two phases of deformation are recognisable in the Felsite, and these are believed to be responsible for the development of three cleavage plane generations within the orebody. These cleavage plane generations consist of the following (KDM 2018):
- Shallow dipping, northeast trending, un-mineralised cleavage planes, which lack quartz infill.
- East-southeast dipping, quartz-filled, mineralised planes with variable inclinations. The shallow dipping (approximately 25°) cleavage planes are mineralised but lack quartz infill. These generally displace the steeply dipping cleavage planes. The steep dipping (+25°) planes are predominantly quartz filled.
- The contact between the diorite and the Felsite is locally sheared and mineralised. This shearing, either from the contact or in most cases from the fold hinges, continues into the diorite as a single dilational quartz filled plane, shallowly dipping, predominantly towards the east-northeast.
Superimposed on the first folding event of the Felsite, is a second folding event with an axis appearing to plunge steeply towards the north. This has resulted in a fold interference believed to have produced the apparent dome and basin configuration that is evident, particularly on the upper levels of the RGM (KDM 2018).
MSZ
There are several sub-parallel shears that cut across both the Penhalonga diorite and Felsite. These shears are southeast striking deformation zones, which have a variable but steep dip, ranging between 45° and 75° to the north-northeast. These shears include the Rezende, Village North and Kent reefs. Elongated lineations in the Rezende mineralised shear zone are oriented west-northwest and east-southeast, suggesting both dextral and sinistral senses of movement (KDM 2018).
The MSZ host grey to white, fine-grained quartz veins and veinlets. These are characterised by a distinct crack-seal, banded or ribboned appearance, with anastomosing slivers of altered country rock. Associated sulphide mineralisation includes pyrite, galena, chalcopyrite, accessory arsenopyrite, and occasional native gold. The Rezende mineralised shear zone averages 2.5 m in thickness, with a strike extent of approximately 1,400 m and 800 m up and down-dip. The Village North and Kent MSZ have widely variable thickness, ranging from several cm to more than 6 m in places (KDM 2018).
Deposit type and geology
Mineralisation at RGM may be described as a lithological and structurally controlled/shear hosted gold deposit, hosted predominantly within the Felsite and MSZ.
Mineralisation
There is strong structural and lithological control on gold mineralisation at the RGM. The sulphide and gold mineralisation in the Felsite orebody is bound within the structurally deformed micro-granitic body, with few transgressive shear or vein extensions into the host diorite. To date, no significant mineralisation has been delineated within the diorite. While no specific metallogenetic studies have been completed, the prevalence of galena in the Felsite orebody reduce with depth. This also appears to be the case with gold (KDM 2018).
Mineralisation within the MSZ (narrow reefs) is distinctly limited to physical deformation that has allowed the migration of hydrothermal fluids resulting in the deposition of quartz, sulphides and gold. While comparatively high gold values are associated with grey quartz and pyrite impregnation, zones of sheared and propylitised diorite commonly form part of contiguous reef structure (KDM 2018).