MINERAL DEPOSITS AND THEIR GENESIS 491 technology, where increasing use is being made of bacterially assisted leaching of refractory metallic ores Increased understanding of the role of bacteria in removing and precipitating metals from dilute solutions may have a profound influence upon theories of ore genesis The clearly important role of meteoric water in residual and depositional deposits in the present-day superficial environment reflects ‘the fundamental and powerful part played by fluids’ in nearly every kind of mineral deposit Subsurface fluids may be molten magmas, circulating meteoric waters, connate waters squeezed out of sedimentary basins, magmatic waters resulting from the solidification of igneous magmas, and waters generated by metamorphism All may contain dissolved solids and/or gases, and if heated they are known as hydrothermal solutions Because most processes of mineral concentration cannot be directly observed, various genetic theories have been advanced on the basis of available field observations and laboratory evidence Such theories seek to explain the origin of both the mineralizing fluid and its dissolved ore-forming constituents, and to relate the time of mineral concentration to the time of formation of the host rocks Mineralization considered to have been formed at the same time as the host rock is termed syngenetic, whereas epigenetic mineralization is considered to be introduced into the host rock at a later time Conceptual theories are vital to scientific understanding and advancement, but invite debate and controversy They tend also to trap dogmatic adherents into forcing good, but inconvenient new evidence into existing pigeonholes, or brushing it aside, unless a coherent new genetic mechanism is revealed There is no better example of this than the denial by many geologists of the evidence for continental drift, until the discovery of sea-floor spreading provided a credible mechanism for embracing that same evidence Development of the new theory of plate tectonics during the 1960s transformed the scientific framework of geology, including views on the nature and origin of mineral deposits During that same period some other discoveries of observable natural mineral concentration systems, currently active at or near the surface of the Earth, had a major impact on theories of ore genesis Modern sea-floor exploration revealed the vast extent of the manganese nodules described by the Challenger expedition of 1873 This demonstrated not only an enormous potential resource of Cu, Co, Ni and other associated metals, but also the potential of cold sea water as a dilute mineralizing fluid Also at the Earth–Ocean interface, the hot brine pools and underlying soft ferruginous muds rich in Zn, Cu and Ag discovered in 1965 in the Red Sea deeps demonstrated an exhalative deposit still forming at the present day in a continental rift system On the mid-Atlantic ridge the discovery of active ‘black smoker’ hydrothermal vents and massive sulphide deposits made a dramatic impact A 1600 m deep geothermal well in the Salton Sea in California (1962) tapped hot metalliferous brines that precipitated dark siliceous deposits containing about 20% Cu, 1% Ag and other metals in the discharge pipe at surface temperatures and pressures Contemporaneously with these revelations, fluid inclusion and chemical studies were highlighting the potency of brines and chloride complexes as solvents and transporters of metals Despite current knowledge of present-day processes, the interpretation of many features of mineral deposits in ancient sediments remains controversial, and will be discussed later Associations with Basic and Ultrabasic Rocks These deep-seated rock types with low silica, high FeMg content are the source of leading world supplies of minerals as diverse in use as chromite, platinum group minerals, vanadium, nickel, and diamonds In all these rocks, the problems of ore genesis are very much part of the problems of rock genesis The valuable elements or minerals are an integral part of the geochemistry of the parent magma, which is itself the ore forming fluid Layered basic intrusions display magmatic segregation during undisturbed cooling and differential crystallization, a process readily understood through clear field relationships supported by laboratory studies on ore textures Heavy oxide minerals such as chromite crystallize and settle into discrete layers as syngenetic magmatic sediments Sulphide minerals in contrast are deposited late (in many cases at the base of the intrusion or injecting wall rocks) from an immiscible sulphide melt that persists to the last stages of magma crystallization The oxide assemblage in strongly layered basic rocks is exemplified by the repetitive and regionally extensive chromite and vanadiferous magnetite layering of the giant early Proterozoic Bushveld Igneous Complex in South Africa The closely associated and famous platiniferous Merensky reef is coarse grained (pegmatoidal), with minor sulphides of Ni and Cu The sulphide assemblage is best known from the nickel mines of Sudbury, Canada, a differentiated, but not layered basic complex, now considered to have been triggered by meteoric impact, and the Norilsk deposit in flood basalts in Siberia Ultrabasic rocks of Archaean age (which include komatiite lavas) are also host to important nickel-copper-iron sulphide segregations in