Gold and native copper in supergene sulphides from the Mid-Atlantic Ridge

Abstract

In 1985 black smokers and massive sulphide deposits were discovered in the TAG hydrothermal field in the rift valley of the Mid-Atlantic Ridge near latitude 26°08′ N (ref. 1). Sulphide samples from chimneys and mounds were recovered by dredging at depths between 3,620 and 3,670 m. Mineralogical and chemical analyses of the samples reveal (1) primary, unaltered Zn-Fe-Cu-sulphides containing 0.8–4.0 p.p.m. Au (0.02–0.12 oz per ton) and (2) secondary Cu-rich sulphides with native copper, formed by supergene alteration of primary Cu-Fe-sulphides, containing up to 16.4 p.p.m. Au (0.48 oz per ton) as free native gold. This is the first reported occurrence of secondary enrichment of gold and copper in recent submarine sulphides. The high gold grades and native copper associated with secondary Cu-sulphides resemble occurrences in some supergene gossans overlying ancient massive sulphide deposits on land2.