Summary:
The Kombat Mine is located in the Otavi Mountainland, just north of the boundary between the Northern or Outjo Tectonic Zone and the Northern Platform Margin of the Damara Orogenic Belt.
The orebodies within the Kombat license area are situated on the northern limb of the double plunging, canoe shaped Otavi Valley Syncline with its northern limb dipping south at 20° to 75° to the south. Several northeast and east trending normal and strike-slip faults crosscut the syncline. The northeast trending normal faults postdate mineralisation.
Seven distinct zones of mineralisation separated by barren dolostone are strung out over a distance of 6 km along the so-called Kombat monoclinal lineament. All zones have surface expression except for Asis West where the orebody is downfaulted along the Kombat West Fault.
Hosted by the dolostone of the Hüttenberg Formation, the ore occurs below monoclinal flexures on the contact between the Kombat and Hüttenberg Formations. This affinity for the contact is not obvious at Asis Ost and E900 as the orebodies are truncated here by erosion. The amplitude of the flexures varies from 75 m to 100 m and the wavelength ranges from 150 m to 250 m. In general, the ore loci are defined by breccia bodies in dolostone and a variety of structural controls (e.g., steeply dipping zones of shearing, net-vein fractures, joints, and fracture cleavages). These planar structures are sub-parallel within the orebodies and diverge from the contact, hence imparting en-échelon pattern to the orebodies and a crosscutting relationship with the contact (Innes and Chaplin, 1986; Dean, 1995). They are interpreted as D2 structures into which the Pb- and Cu-sulphides were remobilised.
The country rock above the orebodies is sheared and fractured into what is described by the term “roll structures”. A relation between the orebodies and the feldspathic sandstone of the Kombat Formation is also indicated. The ore lenses abut against the contact and hang like pendants beneath the flexures.
The orebodies are steep in orientation and transgressive to stratigraphy. With depth the massive sulphides horsetail and merge into thready, stringer type until they become disseminated in calcitised zones of net-vein fractures. The Kombat orebodies are interpreted to have formed as a result of the release of both CO2 and CH4 from the Mulden shales. This converted the anhydrite (in the dolostones) to calcite releasing SO4 into the brines. The brines also migrated along the basin margin faults and thrusts, picking up base metals along the way.
The CO2 and S reacted with downward-migrating, oxidising groundwater producing sulphuric acid that ate its way up through the last 400 m of the rotated fold-thrust fracture systems in the carbonates, forming a hypogene karst system. Unconsolidated sand was subsequently forced through the fracture system forming sandstones. The overlying Mulden phyllite acted as a barrier preventing the upward migration of base metal bearing brines, subsequently precipitating sulphides by reduction in structurally controlled roll structures.
The Kombat mineralised zones are carbonate-hosted base metal sulphide deposits associated with hypogene filled karst cavities and only occur along parallel “roll structures”, which are thrust-related folds. One “roll” parallel to the main Kombat Mine “roll” is present at surface at Kombat Station approximately 1,500 m to the north. The mineralised karst is thought to be caused by the upward migration of corrosive, evaporitederived brines through the Huttenberg carbonates. These brines were expelled from the basin during compression, migrated up the thrusts into folds and encountered oxidized meteoric groundwater and formed corrosive suphuric and carbonic acids. These acids were blocked by the impermeable and reducing Mulden shales resulting in the precipitation of base metal sulphides.
Orebody Dimensions and Mineralisation Zonation
Sulphide and carbonate minerals occur in zones around and running parallel to the major northeast striking cross-cutting faults. The malachite-azurite zone averages 50 m in width and is closest to the faults. The covellite-chalcocite zone is approximately 50 m wide and further away from the fault it widens to up to 100 m wide, surrounded by the chalcopyrite zone. The zonation marks the alteration of the basic chalcopyrite mineralisation by oxidizing groundwater.
Broad zones of calcitisation flank sulphide lenses; at depth, these can form 200-300 m widths of sugary limestone. Calcitisation is the dominant alteration associated with mineralisation. Steeply dipping lenses of compositionally and texturally layered Fe-Mn oxide-silicate mineralisation are generally found near feldspathic sandstone lenses and are commonly associated with the peripheries of the Cu-Pb mineralised zones. These Fe-Mn bodies are layered, lenticular and typically 100 m long by 50 m wide and may reach sizes up to 300 m long by 100 m wide.