Mineralogical aspects of ores Sediments, ores and metamorphism: new aspects

Abstract

Recent years have witnessed the discovery of new large stratabound deposits of base metals, tungsten and uranium, as well as intensified research efforts in the investigation of known occurrences. Three problems are of particular interest: (1) The response of stratabound metal concentrations to varying degrees of metamorphism, and the limits of ‘survival’ of their geometrical and compositional parameters, (2) the distribution of manganese in the ore environment of sulphide deposits, and (3) the participation of elements not traditionally associated with the sedimentary cycle, such as tungsten, in the formation of stratabound deposits. The northwestern Cape Province, S. Africa, has been selected for an initial study of these aspects because of its wealth in large and economically significant base metal concentrations. These occur in volcano-sedimentary successions of probable Kheis age (2600 Ma) which have been metamorphosed during the 1200 Ma Kibaran orogeny. Metamorphism was of amphibolite facies grade (600-700 °C, 3- 4.5 kbar) in the east (deposits of Aggeneys and Gamsberg) and of granulite facies grade (800-1000 °C, 6—8 kbar) in the west (deposits of the O’okiep Copper District, including the Wolfram Schist). In amphibolite facies terrain, the stratabound nature of orebodies is generally well preserved, although complicated by several phases of folding. Electron probe analyses reveal high MnO-contents in silicates of the ore environment (garnets 20 %, pyroxene 5 %, stilpnomelane 11 %). These findings correlate with recent data from the Broken Hill, N.S.W. deposits (Stanton 1976) and from the Red Sea (Cronan et al. 1976). Their potential value lies both in the fields of ore genesis and of exploration geochemistry. Major concentrations of copper ore in transgressive ‘ noritoids ’ are limited to the granulite facies terrain of the O’okiep Copper District. Their isotopic, geochemical and petrological parameters favour crustal rather than mantle origin. A source bed model deriving the noritoids by partial melting of Cu-bearing members of the stratigraphic sequence, is thus proposed. Small tungsten deposits have in the past been mined in the granulite facies terrains of Namaqualand: concordant quartz-ferberite veins occur in the ' Wolfram Schist ’ and have previously been interpreted as hydrothermal and pegmatitic. Their distinct limitation to a specific stratigraphic horizon as well as new quantitative data on the age and grade of metamorphism in the area suggested a review of this genetic concept. The recent discovery of stratabound tungsten mineralization in Austria, Norway and Rhodesia, and the recognition of the sedimentary origin of the Sundong, S. Korea, deposit further stimulated these considerations. The Namaqualand tungsten deposits are thus interpreted as the product of granulite-facies metamorphism of sedimentary scheelite concentrations. These investigations necessitate the combination of many geoscientific disciplines, including field work, structural geology, geophysics, petrology, isotopic studies, geochemistry, ore microscopy and experimental mineralogy, here termed ‘the comprehensive geoscience approach’. This clearly requires team work; the author has had the privilege of closely cooperating with groups led by Professor T. N. Clifford (Witwatersrand University) and Mr J. Marais (O’okiep Copper Company Limited).