World‐class sediment‐hosted stratiform copper deposits: Characteristics, genetic concepts and metallotects

Abstract

Detailed studies of the host lithologies, mineral textures, metal zoning and configurations of numerous sediment‐hosted stratiform copper deposits have established a post‐sedimentary (diagenetic) timing and overprint concept for the genesis of this important mineral deposit type. Metals were probably deposited from low‐temperature oxidised saline brines which circulated across the colour‐distinctive redoxcline between host greybeds (commonly sulfidic ‘black shales’ or marginal‐marine microbial mats) 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, contrasting stratigraphic permeabilities, overpressured pore fluids and anomalous thermal gradients. Besides the identification of major greybed/redbed contacts and the longstanding recognition that stratiform copper deposits are commonly associated with extensive continental redbed and evaporitic units, exploration may be guided by the observation that stratiform copper deposits are restricted in geologic time to continental 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, successful exploration for stratiform copper deposits will probably depend more and more commonly on the detection and refined reconstruction of redbed/greybed sedimentary environments in zones of crustal extension at low palaeolatitudes post‐dating Archaean time and especially on hypothesised patterns of ore brine circulation in the redbed/greybed package following sedimentation.