SHALLOW SUBMARINE VENTING IN THE TABAR - FENI ARC; MINERALOGY AND GEOCHEMISTRY OF HYDROTHERMAL PRECIPITATES

In April, 1994 fieldwork was carried out on and off-shore Lihir island in the Tabar-Feni chain using basic mapping and sampling tools, a handhold Global Positioning System (GPS) and scuba diving equipment for off-shore work. The Tabar-Feni chain is host to several large porphyry stocks and epithermal gold deposits, including the giant Ladolam gold deposit on the island of Lihir. The island of Lihir is the perfect site to investigate the transition from terrestrial epithermal to deep-sea hydrothermal systems.
Deep-sea hydrothermal deposits closely resemble ancient massive sulfides mined on land today, whereas hydrothermal systems at much shallower water depths may resemble terrestrial epithermal systems. The presence of heated seawater allows for significant transport of base metals as chloride complexes and the resulting mineralization may be closer to that of gold rich deep-sea polymetallic sulfides than to terrestrial epithermal systems. In terrestrial hot springs Cu, Fe, Zn and Pb concentrations are low because dilute meteoric waters are unable to transport significant amounts of base metals as chloride complexes.
Areas of active submarine gas and fluid venting were discovered off-shore Lihir. Vent sites were approximately 100 m from the beach and occurred at a water depth of 6 to 10 m. Measured fluid temperatures ranged form 60 to 96 deg C. Sites of gas and fluid exhalation did not show vent edifices similar to 'black or white smokers' on mid-ocean ridges. Rather, they are small, funnel like indentations in the sandy bottom where gas and water percolate through the sand before exiting into seawater. Hydrothermal precipitates were collected from just below the sand-seawater interface adjacent to the vent sites.
The samples from Lihir mainly consist of dolomite, pyrite and other sulfides. The variety of base-metal sulfides, e.g., galena, sphalerite, cinnabar and chalcopyrite, present in the Lihir specimens may already indicate a seawater contribution to the hydrothermal fluid.
In addition to the off-shore hot springs several on-shore hot springs were mapped and sampled for comparison of the two environments. On-shore hot springs are characterized by the occurrence of hot pools, hot mud, boiling springs and geysers and may be compared to other geothermal areas (e.g., Yellowstone National Park and Steamboat Springs, Nevada).