Native copper and alpha-copper-zinc in sediments from the TAG hydrothermalfield (Mid-Atlantic Ridge, 26 degrees N): nature and origin

Native copper and alpha-copper-zinc occur as strands and elongated grains ( up to 300 mu m in length) within the sediments from the Trans-Atlantic Geot raverse (TAG) hydrothermal field (Mid-Atlantic Ridge, 26 degrees N). They a re remarkably similar in composition and crystal structure to copper and co pper-zinc occurrences found in other natural environments. The results of m ineralogical studies are discussed in terms of the possible mechanisms of n ative metal formation in the complex TAG field: with an asymmetric and high ly-fractured rift valley, and mature active and relict sulfide mounds. Nati ve copper and alpha-copper-zinc grains disseminated in the TAG sediments ar e either inherited from: (1) primary magmatic or metamorphic crustal source ; (2) hydrothermal deposits; and (3) the alteration of primary deposits, or formed (1) authigenically, or (2) biogenically within/on the sediment cove r. Native metallic particles could have been formed as accessory minerals d isseminated in the ridge crest basic rocks and/or massive sulfide mounds. D egradation of these rocks and mass wasting of the mounds have liberated the metallic grains which have, in turn, been deposited into adjacent sediment s. (C) 1999 Elsevier Science B.V. All rights reserved.