Summary:
The Twin Hills Project is located in an area of known gold deposits hosted within the inland arm of the Pan African Neoproterozoic Damara Belt, a northeast-striking Neoproterozoic fold, thrust, and metamorphic belt. It reflects a Neoproterozoic rifting-accretionary event between the Congo Craton to the north and the Kalahari Craton to the south. Peak deformation within the Damara Belt occurred between 500 Ma and 530 Ma. The Damara Belt comprises a gently folded Northern Zone, a Central Zone and a Southern Zone.
Mineralization
Styles Orogenic gold deposits occur in Archaean to Tertiary aged metamorphic belts where accretion or collision has added to or thickened continental crust. The deposits are generally hosted by volcanic and turbiditic sequences that have been metamorphosed to greenschist or less commonly amphibolite facies. The deposits are generally interpreted to form late in the orogenic cycle from mid to lower-crustal metamorphic fluids. The gold ores develop syn-kinematically with at least one stage of the major deformation of the country rocks. They inevitably have a strong structural control involving faults or shear zones, folds, and other areas of competency contrasts. The deposits show vertical dimensions up to 1–2 km with strong lateral zonation of wall-rock alteration, normally potassium, arsenic, antimony, large-ion lithophile elements, CO2, and sulphur, with additions of sodium or calcium particularly in rocks of amphibolite facies. Proximal wall-rock alteration varies from sericite-carbonate-pyrite at high crustal levels through biotite-carbonate-pyrite, to biotite-amphibolite-pyrrhotite and biotite / phlogopitediopside-pyrrhotite at deeper crustal levels. Quartz ± carbonate veins are ubiquitous and commonly gold-bearing, although in many systems it is the sulphidized, high iron / iron + magnesium +calcium wall-rocks adjacent to the veins which contain most ore (Groves et al., 2003).
Twin Hills
The gold mineralization style at the Twin Hills Gold Project can be broadly categorized as orogenic, related to the Damaran Orogeny 550–500 Ma. The Twin Hills deposit and associated targets are located along a crustal-scale lineament (KFZ), on the southern margin of a turbidite basin folded into a tight syncline during the Damara Orogen. The mineralization in the Twin Hills area has a defined strike extent of 11 km long, part of more than 20 km strike length of anomalous geochemistry along the KFZ and associated with splays and second and third-order structures to the south of the KFZ. The lithologies at Kudu and Oryx are tightly to isoclinally folded and overturned towards the north. The lithologies at Bulge, Twin Hills Central and Clouds are tightly folded and overturned towards the south. In general. the folding becomes more open towards the east.
The priority portion of the Twin Hills area is a structural jog along the KFZ on the margin of the Dobbelsberg anticline. Three splay structures off the KFZ are visible in the magnetic data and are coincident with anomalous gold assays in bedrock and the calcrete cover above the bedrock.
The gold mineralization at Twin Hills is hosted by meta-greywackes which can be divided into interbedded and massive units. At a deposit scale, the mineralization is associated with shearing within the greywackes parallel to the axial plane of the tightly folded package. The gold mineralization is closely associated with arsenopyrite mineralization in millimetre-scale veinlets as well as fine-grained disseminate. Gold is associated to a lesser extent with pyrrhotite which has a much larger footprint and also occurs in units not well mineralized with gold. Selvages to the sulphide-quartz veinlets are characterized by potassic alteration (biotite) and higher-grade zones have often been silicified.
Goldkuppe
Goldkuppe is a 3 km long mineralized system with gold occurring in several discontinuous, plunging shoots hosted in (or on the contact of) a brown dolomitic marble unit. The brown colour on the surface of the marble is due to the weathering and oxidation of common disseminated sulphides. The sedimentary package, including this brown dolomitic marble, is deformed into a large anticlinal structure which was mapped in detail by Kasch in 2017. The dolomitic marble is overlain by grey marble (hangingwall) and underlain by coarse and banded white marble (footwall). These carbonate rocks belong to the Karibib Formation.
Mineralization is associated with secondary fold noses as small-scale saddle reefs and limb faults. Massive or semi-massive sulphide concentrations with annealed textures, often with strong copper and gold grades, comprise coarse-grained aggregates of chalcopyrite-pyrite and lesser pyrrhotite. In addition, there is low-grade mineralization associated with pervasive skarn alteration adjacent to the intrusive bodies around Goldkuppe. The skarn altered areas contain diopside, biotite-phlogopite, and tremolite ± actinolite. Less abundant, reddish-brown garnet is also present. Some of these skarn zones may have a distal weak iron-magnesium carbonate envelope, which is particularly obvious in oxidized older drill core.
At Oasis to the south of Goldkuppe, gold mineralization is hosted by banded marbles where the banding is created by thin calc-silicate layers. Gold is associated with north-south striking quartz veins with pyrrhotite and arsenopyrite mineralization.