496 MINERAL DEPOSITS AND THEIR GENESIS Metamorphic Associations Metamorphic processes may either generate new mineral deposits, or modify the characteristics of existing deposits A number of industrial minerals are formed by metamorphism, including kyanite for refractories, and garnet for abrasives Talc forms in metamorphosed ultrabasic rocks, which are also the major source of chrysotile asbestos When olivine rich rocks such as dunite are serpentinized, the fibrous form of serpentine, chrysotile, is formed in crossfibre veins that seem to develop under conditions of tensile stress and hydrofracturing A dense enough network of veins (a few per cent by volume) of goodquality fibre converts large masses and lenses of serpentinized rock into a mineral deposit of economic potential, usually suitable for mass-mining methods (Figure 12) Despite the health hazards associated with asbestos, world production is still about million tonnes per annum There are large deposits in the Urals, the Palaeozoic of the eastern Townships in Quebec, and in the Archaean at Zvishavani (Shabani) in Zimbabwe Tectonic deformation is also an important precursor to epigenetic mineralization (‘ground preparation’) as it forms shear zones or fractures that provide channelways for mineralizing fluids Dynamic metamorphism and the repetitive action of a process known as seismic pumping may drive mineralizing fluids from depth into higher level vein systems Activated by earthquake movements, fluids accumulating at depth in large fault systems are expelled upwards through channelways where dissolved substances may be precipitated as successive generations of vein fillings Large numbers of vein systems, commonly containing gold, are associated with the shear and fracture zones typical of greenstone schist belts in Archaean cratons and Precambrian Shield areas Geological and mining conditions may thus be very similar in such gold fields areas as far apart as Australia, Canada and West Africa A second and geologically important aspect of metamorphism relates to the fact that pre-existing mineral deposits, like all other rocks, must suffer the effects of regional metamorphism Deposits not only become faulted, folded and sheared into new geometric shapes and attitudes that affect the cost of mining, but may respond by recrystallization in ways that materially affect their amenability to mineral extraction and beneficiation Sulphide minerals are particularly prone to recrystallize and anneal, or become mobilized into new open spaces such as rock cleavage Veins of gangue and ore minerals may also be formed by lateral secretion driven by metamorphic temperatures and stresses The overall effect is to overprint and mask the original ore fabrics, especially at the micro and meso scales Such difficulties lead to additional debate about controls on ore formation and ore genesis, which in their turn affect decisions on how and where to explore for similar mineralization Some examples include the famous massive sulphide Pb-Zn ores of Broken Hill (Australia) in strongly folded sillimanite grade gneisses, the folded disseminated Cu deposits of the Zambian copperbelt, the great conglomerate hosted Au-U deposits of the Witwatersrand (South Africa) and the twice-folded and metamorphosed stratabound Cu-Co deposits at Kilembe (Uganda) Figure 12 A network of cross fibre chrysotile asbestos veins in serpentinite, Havelock mine, Swaziland This length of fibre and intensity of mineralization constitutes superior grade ore Photo: GR Davis