Die oberproterozoische Duruchaus-Formation in Namibia und ihre Kupfervererzungen

Abstract

Abstract by author:

Higher heat flow characterized the depositional environment of the sediments of the Nosib Group during the synrift phase, and enabled, supported by the arid climate, the formation of saline, chloride rich diagenetic solutions from the characterized rift sediments (including evaporitic intercalations) of the Duruchaus Formation. These solutions were able to leach and mobilize metal ions from sedimentary and magmatic rocks. During the regional metamorphism of the Damara Orogeny those solution and mobilization processes continued and were intensely reactivated. Several orthoamphibolites (2-4 meters thick) provide evidence of volcanic activity of an early Damara Orogenic magmatism during the deposition of the Duruchaus sediments. The tholeiitic chemistry of these basic volcanic rocks may be due to the formation of a basaltic magma within the Upper Proterozoic continental crust between the Kongo and the Kalahari Cratons. Additional basic volcanic rocks which show a great distribution along the Southern Margin of the Damara Orogen Belt are the metabasalts of the Dordabis Formation (Sinclair equivalents ?) and the amphibole-schists of the Chuos Formation. Owing to the close similarity of their chemistry and their geological and tectonical framework, compared with the Duruchaus amphibolites, it seems that they were also the result of the rift-related basic magmatism of the Damara Orogeny

Regional metamorphism of the Damara Orogeny affected the sediments of the Nosib Group 530-510 Ma ago. Within the most southerly part of the Southern Orogen Margin, the upper greenschist to epidote-amphibolite facies was reached. The illite-rich pelites of the Duruchaus Formation became mainly quartz and albite bearing biotite schists. In contrast to the Central Zone of the Damara Orogen, where domal synforms and antiforms predominate, intensive south vergent upfaulting and over-thrusting characterize late Orogenic tectonics of the Southern Margin. Due to the metamorphic alteration of the Duruchaus sediments, silica- and carbonate-rich fluids of high salinity originated at temperatures up to 500-550°C. These metamorphic solutions increased rock creep and acted as sliding masses during the above tectonic activity. The fluids moved upward along zones of weakness and faults into higher horizons where they formed discordant quartz-carbonate-breccias and intrusive "pegmatitic" quartz bodies. Calcite rhom-bohedrons, up to 0. 8 m long, and quartz crystals, up to several meters long, are common in these deposits

Along the Southern Margin of the Damara Orogen, copper mineralizations occur frequently within zones of intensively sheared and foliated mica and chlorite schists of the Duruchaus Formation. Mean copper ore grade reaches 2 wt. - percent containing up to some tens g/t silver. Normally these mineralized zones are not thicker than a few decimeters, reaching a maximum of 1-2 meters. Their horizontal extent ranges from several hundred meters to 1-2 km. The primary sulfides (chalcocite, chalcopyrite, pyrite) have been mostly completely oxidized to malachite, copper and iron oxides near the outcrops. Even in the intrusive quartz-carbonate-breccias and "pegmatitic" quartz bodies strongly limonitized iron oxides (up to 2 cm diameter) and copper bearing sulfides, very often altered to malachite, have repeatedly been observed. These epigenetic copper bearing mineralizations were formed by hydrothermal events during the Damaran regional metamorphism and are closely associated with fault and thrust zones within the Duruchaus Formation. Their copper has been transported varying distances from the source rock. Basic volcanic rocks of the early Orogenic magmatism have an average copper content of 110 ppm and are considered to be, besides sedimentary rocks, the source rocks for the copper in these saline solutions. In the metasediments of the Duruchaus Formation copper contents are less than 20 ppm

Depending on the thickness of overlying rock sequences during diagenesis and continuously intensifying regional metamorphism, excess hydrostatic and tectonic pressure was built up in the increasingly metalliferous formation waters. On zones of weakness and fault zones which may have been already intended or active during the synrift phase, tectonic movements took place at least due to expanding internal and external pressure. These movements culminated with the intensive folding and overthrust tectonics which characterize the areas where the Duruchaus metasediments are exposed along the Southern Margin of the Damara Orogen. The fluid phases easily left the overstressed system along the dislocation planes and reduced considerably the internal static and dynamic friction of the sliding planes

Copper rich sulfides precipitated where Cu-Fe-chloride bearing fluids met sulfide sulphur. Changes of P-T-conditions and the redox potential, due to lithological variations of the host rock, drove the crystallization process. At the same time, structural traps like up- and overthrusts, hinge folds, and karst related cavities close to surface were of great genetic importance. In the copper mineralized zones of the intensively sheared and/or foliated Duruchaus metapelites, low organic carbon contents were sufficient to reduce sulfate sulphur from the intercalated sulfate bearing carbonate rocks. This sulfide sulphur was available to form copper sulfides from the copper bearing fluids. Thereby traces of the precious metals gold and silver were incorporated in the primary sulfides. Under later near surface conditions, after uplifting and cutting the mineralized metasediment zones and the overlying rocks by erosion, primary copper and iron sulfides were altered to copper oxides and carbonates, and iron oxides and hydroxides. Related to these weathering processes, partial remobilization and reconcentration of the valuable metals took place and may have continued until recently. -