PRECIOUS METALS IN BARITE-SILICA CHIMNEYS FROM FRANKLIN SEAMOUNT, WOODLARK BASIN, PAPUA-NEW-GUINEA

Barite-silica chimneys with disseminated sulphides and elevated conten ts of gold and silver occur at two sites on the caldera floor of the b asaltic andesite Franklin Seamount, situated near the western tip of t he 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 thro ugh Very late-stage sulphide outgrowths from barite into cavities; the se represent the fourth reported mineralogical occurrence of gold in s eafloor hydrothermal deposits. Silver, by contrast, was deposited as r eal or occult sulphosalt inclusions in pyrites apparently formed throu ghout the local paragenetic sequence in each growth zone of the chimne y walls. Correlations between silver, lead and antimony indicate that the main silver habitat is an unrecognized Ag-dominant Pb-Sb sulphosal t, but unresolved inclusions of a pyrargyrite-like sulphosalt in collo idal pyrite may also occur. Electron and proton probe microanalyses ha ve together established a balanced budget for many trace elements in m inerals and bulk contents, except it is necessary to appeal to variati ons throughout the chimney in distribution of As, Cd, Sb and Hg. The r esidence of Mo (which correlates with gold) remains unknown. Isotopic studies indicate that the local volcanic pile, or its parental magma c hamber, is the source of chalcophile metals in the chimneys, but neith er the source rocks nor the tectonic setting offer definitive explanat ions for the enrichment of precious metals at Franklin Seamount. Chemi cal aspects of the probably small-scale and short-lived hydrothermal s ystem 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 depositi on before venting. (C) 1997 Elsevier Science B.V.