494 MINERAL DEPOSITS AND THEIR GENESIS controversy and lack of understanding surrounds many deposits found in sediments, especially metalliferous ores Dispute tends to centre on the nature and origin of the ore forming fluid, its movement through the crust, the origin of the metals, and the mechanism of their precipitation The controversy is well illustrated by the continuing debate concerning the origin of the giant Witwatersrand gold deposits of South Africa, the world’s greatest gold producer for over a century A wellpreserved Late Archaean sedimentary basin, about km thick and 280 km across, contains pyriteuraninite-gold mineralization in conglomerates developed at several unconformities from the bottom to the top of the sequence (Figure 9) Economic interest is focused on a series of conglomerates towards the top of the basin and situated near its edge Despite compelling evidence for a paleoplacer origin, undisputed signs of some hydrothermal activity in the sediments are still being used to argue the minority view that epigenetic hydrothermal processes played a major role in the introduction of the gold Another syn-sedimentary hypothesis holds that gold was precipitated from solution, on the evidence of carbon (‘thucholite’) that may be derived from primitive organisms The uncertainties continue, as some others hold that the carbonaceous matter was introduced from an external source long after consolidation of the sediments Ores in sediments may be discordant irregular masses of clearly epigenetic origin; or concordant and possibly, therefore, either syngenetic with the enclosing sediment, or diagenetic (that is, due to processes of diagenesis) The term ‘stratabound’ is used to indicate confinement of the mineral deposit between or within sedimentary strata, and ‘stratiform’ to indicate that the deposit shows internal layering or stratification Most Phanerozoic ironstones are sedimentary, and the intriguing Banded Iron Formation, widespread in Proterozoic and Archaean basins, is well documented as the major sedimentary protore of high-grade iron ore production Many stratiform massive base-metal sulphide deposits, once considered by many to be epigenetic replacements due to hydrothermal fluids from hidden igneous bodies, have been shown to be the products of hydrothermal exhalations and a volcano-sedimentary environment, as for the Kuroko and Iberian deposits mentioned above At the other pole are the Late Proterozoic stratabound Cu-Co disseminated sulphide ores of the Copperbelt in northern Zambia and the adjacent Congo Mineralization of just a few percent sulphides is confined to strata only tens of metres thick, but extending (in large regional folds) over thousands of square kilometres The only igneous rocks known nearby are basement granites that form the hilly topography upon which the cupriferous sediments were deposited, and much later basic dykes The detailed evidence, including a marked facies zoning upwards and shoreward of pyrite-chalcopyrite-bornite-chalcocite (Figure 10), convinced most geologists that the ores are not simple hydrothermal replacements as favoured after mine development in the 1920s An origin due to syngenetic precipitation of pyrite and copper sulphides by bacterial action has long been advocated, together with effects due to diagenesis and metamorphism, but not convincingly demonstrated Current research based on increased regional knowledge (over 1000 million Figure A typical specimen of Witwatersrand gold ore from South Africa Pyrite and minor gold in the groundmass supporting well rounded, unmineralized quartz pebbles, 25 cm across Photo: GR Davis