The Otjihase massive sulfide deposit

Abstract

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The stratiform massive sulfide deposit of Otjihase, SWA/ Namibia is one of a number [of] cupriferous mineralisations of the Matchless Belt in the upper Proterozoic Damara Orogen. The Besshi-type mineralisation is associated with basic and minor ultrabasic, partly isoclinally folded intrusives. Three distinct MORB-type metabasica of the Matchless Belt differ macro- and microscopically as well as geochemically. Geochemical investigations of mineralized Kuiseb Schist proved a wall-rock alteration during ore forming processes with depletion of K, Rb, Na, Ti, V, Cr, Ni and P. Metallic elements as Cu, Fe, Zn and Pb as well as Ba and Ce have been enriched in altered zones. The discussion of structural problems focusses on aspects of the brittle deformation. The occurrence of an unmetamorphosed, tholeiitbasaltic sill close to faults is probably unique for the Matchless Belt. The intrusive rock with a geochemical pattern of a WPB (within plate basalt) crystallized with a temperature of 1130 ±. 30 °C and is thought to be related to the horst and graben tectonic of the Windhoek area. The basic magma is an assumed source of heat for the formation of tourmalonitic veins as well as the remobilisation and crystallisation of sulfides in kakirites.Textural evidences for remobilisation of the metamorphosed ore are visible at the micro-, meso- and macroscale of observation. Stronger anisotropic zones in pyrite, analysed by electron microprobe, are believed to be caused by exsolution of As. Pyrites from the Otjihase- and Matchless deposit differ in their lead isotope ratios. Residuum and leach solutions from Otjihase are more radiogenic. The common metamorphic age of both deposits is estimated on the basis of their Pb isotopic composition to have occurred 510 ±. 30 m. y. ago and are in agreement with Rb-Sr and 40Ar/39Ar age determinations. Otjihase- and Matchless mineralisations have low 208Pb/204Pb ratios compared with 206pb/204pb ratios. Different models for processes fractionating uranium and thorium are proposed. Sulfides from the Bambui group in Brazil have similar isotope ratios to samples from the southern Damara Orogen. A possible correlation between these Orogenic belts is indicated