Zn-Pb-Cu VHMS deposits ? extending the brine-pool pool

Abstract

The massive sulphide deposits of the Devonian-Carboniferous Iberian pyrite belt (IPB) and the Ordovician Bathurst mining camp (BMC) have much in common: relative to others of this group like those of the Hokuroku basin they are large; have high aspect ratios; low Zn, Pb and Cu grades; high Fe/Cu, Zn/Cu and Pb/Zn ratios; high Sn; low fO2 mineral assemblages; evidence of Ba but scarcity of barite; ill-defined or no zone refining; and abundance of framboidal pyrite, mineral banding (bedding) and syn-depositional breccias. Differences include the vent complexes above stockwork pipes in the BMC, and biogenic S (negative d34S) in the IPB. There is evidence of sea-floor deposition, and fluid inclusion data from stockwork quartz in eight IPB deposits predict that the ore-forming fluids reversed buoyancy on mixing with seawater and ponded, sulphides precipitating immediately in the quenched ore-forming fluids. This manner of deposition explains many of the above ore characteristics while the biogenic S may be due to biostatic evolution. There is no reliable fluid inclusion evidence for the BMC but we suggest that the high degree of commonality between the camps points to brine-pool deposition for the BMC ore deposits. A previously proposed model involving buoyant fluids emerging into anoxic ocean water is vitiated by the evidence that host shales were deposited in oxic ocean water during mineralisation, and the lack of evidence for chimneys and rubble mounds that would be predicted by this hypothesis.