Complexly shaped hydrothermal spires and mounds of Fe-Mn-Si oxide up to several meters thick and 100 to 200 m in extent are widely distributed at a 2,143- to 2,366-m depth on and near Franklin Seamount. Franklin is a young submarine basaltic andesite volcano with rare occurrences of sodic rhyolite located near the western propagating tip of a sea-floor spreading axis in the Woodlark basin, Papua New Guinea. Some chimneys are venting a 20-degrees to 30-degrees-C, mildly acid, base metal-containing clear fluid which is a mixture of seawater and an estimated 270-degrees to 350-degrees-C hydrothermal end member. The composition of the end-member fluid is similar to those calculated for midocean black smokers but lacks reduced sulfur. Some of the oxide material ranges to about 200 years old, and the deposits formed during the closing stages of volcanic activity, mainly after a collapse caldera developed at the summit of the seamount. Inactive gold-rich barite-silica chimneys 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 oxyhydroxide 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 growth. Silica filaments of probable microbial origin forming the original constructions became overgrown or were replaced by hydrothermal Fe oxyhydroxide. Bright green nontronite crystallized in internal patches and veinlets, and locally formed where relatively reduced fluids reached the outer surfaces of deposits. Manganese oxides developed as replacements and cavity fillings at oxidation fronts in the outer portions of the hydrothermal constructions. Manganese also deposited with Fe in black external crusts on older constructions. Similar crusts coat the baritic chimneys and occur on lava surfaces up to 100 m from the main hydrothermal deposits. Trace element geochemistry of the Fe-Mn-Si deposits partly reflects the presence of basaltic glass contaminants: most Al, Ti, Cr, and Zr are from this source, for instance. Among hydrothermal components, only Co, Ni, and Mo are prominently correlated with Mn. The deposits contain anomalous As, Sb, and Hg. Rare earth element patterns and Sr isotope ratios confirm the importance of seawater components in their formation. The baritic deposits are particularly rich in Ag (to 545 ppm) and Au (to 21 ppm), and contain significant Zn, Cu, and 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 been determined. Fluid inclusions suggest formation from a fluid of moderate 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 Seamount occurrences confirm that sea-floor hydrothermal activity may be associated with submarine volcanism where accretional spreading propagates into a continental margin environment, a setting that may have numerous ancient analogues. The Fe-Mn-Si deposits support an exhalative origin for iron-formations associated with ancient volcanic sequences, the chemistry of which may constitute a useful pathfinder to base metal and precious metal ores. The discovery of ore-grade Au and Ag in baritic chimneys at Franklin Seamount suggests targets for land-based exploration.
