Precious metals in barite-silica chimneys from Franklin Seamount, Woodlark Basin, Papua New Guinea

Barite-silica chimneys with disseminated sulphides and elevated contents of gold and silver occur at two sites on the caldera floor of the basaltic andesite Franklin Seamount, situated near the western tip of the Woodlark spreading axis where it propagates into continental crust. The mineralogical residences of silver and gold in these chimneys are very different. Gold occurs in tiny electrum particles dispersed through very late-stage sulphide outgrowths from barite into cavities; these represent the fourth reported mineralogical occurrence of gold in seafloor hydrothermal deposits. Silver, by contrast, was deposited as real or occult sulphosalt inclusions in pyrites apparently formed throughout the local paragenetic sequence in each growth zone of the chimney walls. Correlations between silver, lead and antimony indicate that the main silver habitat is an unrecognized Ag-dominant Pb-Sb sulphosalt, but unresolved inclusions of a pyrargyrite-like sulphosalt in colloidal pyrite may also occur. Electron and proton probe microanalyses have together established a balanced budget for many trace elements in minerals and bulk contents, except it is necessary to appeal to variations throughout the chimney in distribution of As, Cd, Sb and Hg. The residence of Mo (which correlates with gold) remains unknown. Isotopic studies indicate that the local volcanic pile, or its parental magma chamber, is the source of chalcophile metals in the chimneys, but neither the source rocks nor the tectonic setting offer definitive explanations for the enrichment of precious metals at Franklin Seamount. Chemical aspects of the probably small-scale and short-lived hydrothermal system appear the controlling factors, including low fluid/rock ratios leading to selective leaching of highly extractable constituents, and substantial subsurface mixing with seawater leading to pyrite deposition before venting.