WORLD-CLASS SEDIMENT-HOSTED STRATIFORM COPPER-DEPOSITS - CHARACTERISTICS, GENETIC CONCEPTS AND METALLOTECTS

Detailed studies of the host lithologies, mineral textures, metal zoni ng and configurations of numerous sediment-hosted stratiform copper de posits have established a post-sedimentary (diagenetic) timing and ove rprint concept for the genesis of this important mineral deposit type. Metals were probably deposited from low-temperature oxidised saline b rines which circulated across the colour-distinctive redoxcline betwee n host greybeds (commonly sulfidic 'black shales' or marginal-marine m icrobial mars) and immature footwall redbeds filling continental rift basins. Adequate sources of ore metals (principally copper+/-silver or cobalt) and associated metals (e.g. lead, zinc, cadmium) can be found in the redbeds (or deeper rocks), and the circulation of saline pore fluids to and across the redoxcline may have been induced and focused by normal features of rift geology such as marginal basin faults, cont rasting stratigraphic permeabilities, overpressured pore fluids and an omalous thermal gradients. Besides the identification of major greybed /redbed contacts and the longstanding recognition that stratiform copp er deposits are commonly associated with extensive continental redbed and evaporitic units, exploration may be guided by the observation tha t stratiform copper deposits are restricted in geologic time to contin ental rift zones, to low palaeolatitudes where evaporites tend to form , and to sediments younger than the apparent oxidation of the Earth's atmosphere at about 2.3 Ga (essentially post-Archaean time). Thus, suc cessful exploration for stratiform copper deposits will probably depen d more and more commonly on the detection and refined reconstruction o f redbed/greybed sedimentary environments in zones of crustal extensio n at low palaeolatitudes post-dating Archaean time and especially on h ypothesised patterns of ore brine circulation in the redbed/greybed pa ckage following sedimentation.