Sedimentary environment of Zambian copper deposition

Abstract

Superseding of the hydrothermal origin by syngenesis subsequent to 1928 is recorded historically. Granites claimed to be intrusive were in 1940 found to be overlain unconformably by Katanga sediments. The bulging intrusive granites became bulging paleo-hills, avoided by copper mineralization. Discovery that ore grade in the deformed and shattered orebeds of a dragfold was no richer than in the unfolded sections, proved that the mineralization was prefolding. Two decades later dolomitic facies over some granite bulges proved to be algal bioherms. By the syngenetic theory the zonal sequence, from copper-rich to iron-rich sulphide, was interpreted as representing depth zones parallel to shoreline controlled by the activity of anaerobic bacteria. Recognition of anhydrite explained mysterious cavities in veins, conditions of sedimentation, and the source of the sulphur to form the sulphides and, by its solubility, explained the high porosity of the Katanga sediments to depths of 600 m. Features of the ore shale deposits such as dissemination instead of selective replacement, mineral zoning, association with basins flanking paleo-hills, barren gaps over bioherms, apply also to mineralization in arenites. Pyritic disseminations in sediments over granite hills and sulphide concretions are confined to orebodies in arenite, explainable by its greater original permeability. Evidence for syngenesis derives from interpretation of the depositional environment, although many of the processes are still being elucidated. Prior to formation of the major ore deposits, in marine waters, evaporation of playa lakes gave high concentrations of sulphates, borates, and, under anoxic conditions, sulphide precipitation. Drying out of such lakes exposed mud flats on which efflorescences of gypsum, borates, and iron and cobalt sulphates presumably formed. Wind erosion at Mufulira produced dunes containing high proportions of gypsum later converted to anhydrite. At Chibuluma floods dissolved iron and cobalt sulphate from desiccated mud flats and on entering the saline lake precipitated hydroxides of these two metals as a heavy red mud with suspended sand grains and pebbles. The hydroxides were then converted to sulphide, whilst copper and iron were precipitated as sulphide in the overlying waters. Borates diffused into the underlying sand and combined with muddy coatings and matrix of the sand grains, to form tourmaline. Copper and cobalt were precipitated in greatest quantity at convergence of currents in the playa. Sulphide in sediment became sufficiently indurated to be eroded and redeposited as detrital grains, concentrated on foresets and truncation planes with tourmaline, sphene, epidote, and zircon grains. Compaction folding over Basement hills and even over biohermal and aeolian accumulations were controls for ore deposition.