HYDROTHERMAL OXIDE AND GOLD-RICH SULFATE DEPOSITS OF FRANKLIN SEAMOUNT, WESTERN WOODLARK BASIN, PAPUA-NEW-GUINEA

Complexly shaped hydrothermal spires and mounds of Fe-Mn-Si oxide up t o several meters thick and 100 to 200 m in extent are widely distribut ed at a 2,143- to 2,366-m depth on and near Franklin Seamount. Frankli n is a young submarine basaltic andesite volcano with rare occurrences of sodic rhyolite located near the western propagating tip of a sea-f loor spreading axis in the Woodlark basin, Papua New Guinea. Some chim neys are venting a 20-degrees to 30-degrees-C, mildly acid, base metal -containing clear fluid which is a mixture of seawater and an estimate d 270-degrees to 350-degrees-C hydrothermal end member. The compositio n of the end-member fluid is similar to those calculated for midocean black smokers but lacks reduced sulfur. Some of the oxide material ran ges to about 200 years old, and the deposits formed during the closing stages of volcanic activity, mainly after a collapse caldera develope d at the summit of the seamount. Inactive gold-rich barite-silica chim neys with sparsely disseminated sulfides occur in the summit caldera. Similar material may underlie Fe-Mn-Si deposits. The yellow-orange to red-brown Fe-Mn-Si deposits consist mainly of an Si-bearing Fe oxyhydr oxide phase. Although there are no present faunal concentrations near active vents, microstructures of these deposits suggest that biogenic processes may have been important in the earlier stages of their growt h. Silica filaments of probable microbial origin forming the original constructions became overgrown or were replaced by hydrothermal Fe oxy hydroxide. Bright green nontronite crystallized in internal patches an d veinlets, and locally formed where relatively reduced fluids reached the outer surfaces of deposits. Manganese oxides developed as replace ments and cavity fillings at oxidation fronts in the outer portions of the hydrothermal constructions. Manganese also deposited with Fe in b lack external crusts on older constructions. Similar crusts coat the b aritic chimneys and occur on lava surfaces up to 100 m from the main h ydrothermal deposits. Trace element geochemistry of the Fe-Mn-Si depos its partly reflects the presence of basaltic glass contaminants: most Al, Ti, Cr, and Zr are from this source, for instance. Among hydrother mal components, only Co, Ni, and Mo are prominently correlated with Mn . The deposits contain anomalous As, Sb, and Hg. Rare earth element pa tterns and Sr isotope ratios confirm the importance of seawater compon ents in their formation. The baritic deposits are particularly rich in Ag (to 545 ppm) and Au (to 21 ppm), and contain significant Zn, Cu, a nd also Pb both as galena and rare cerussite. The silver occurs within disseminated pyrite spheroids and anhedra of colloidal origin, partly as submicron-sized Sb sulfosalt inclusions. The site of Au has not be en determined. Fluid inclusions suggest formation from a fluid of mode rate temperature (184-degrees-244-degrees-C) slightly more saline than sea water (3.4-5.8 wt % NaCl equiv). No massive sulfide deposits have been found, but vent fluid chemistry suggests that sulfide stockworks probably occur within the underlying volcanic pile. The Franklin Seam ount occurrences confirm that sea-floor hydrothermal activity may be a ssociated with submarine volcanism where accretional spreading propaga tes into a continental margin environment, a setting that may have num erous ancient analogues. The Fe-Mn-Si deposits support an exhalative o rigin for iron-formations associated with ancient volcanic sequences, the chemistry of which may constitute a useful pathfinder to base meta l and precious metal ores. The discovery of ore-grade Au and Ag in bar itic chimneys at Franklin Seamount suggests targets for land-based exp loration.