REPLACEMENT PROCESSES IN VOLCANOGENIC MASSIVE SULFIDE DEPOSITS: THE KEY TO GIANT OREBODIES

We are studying sea floor massive sulfide formation in a variety of situations, including the Lucky Strike, Rainbow and Saldanha sites of the MOMAR area, the Pacmanus hydrothermal field of Papua New Guinea, hydrothermal exploration areas in the Bismarck archipelago (PNG) and in Vanuatu waters (N Fiji Basin). These sites are being compared in detail with ancient VHMS provinces, especially the exceptional Iberian Pyrite Belt, with over 1700 million tonnes of sulfide ores/sub-ores in a belt only 250 km long.

Lucky Strike and Rainbow are both candidates for drilling. There is striking evidence of sulfides (including sphalerite and massive chalcopyrite in places) replacing basalt and serpentinite, respectively. At Rainbow progressive replacement of the rock starts with beautiful stockwork veins and grades into massive sulfide. Saldanha shows large-scale diffuse discharge of methane with observed discrete venting of warm hydrothermal fluids through sediments. These contain conduits with hydrothermal minerals, including pyrite, sphalerite and chalcopyrite. At Pacmanus, ODP drilling seems to have intersected an actively forming sub sea-floor deposit, over a thickness of about 80 meters, under a 30-meter thick cover of fresh dacite. In Vanuatu and other exploration areas we are trying to identify hydrothermal components in sediments that may overlie sub-sea floor sites of ore precipitation with no obvious expression on the sea floor.

Similar models of sub-sea floor ore precipitation were proposed for the Iberian Pyrite Belt ores as early as 1983 and have since gained wide acceptance. The new data reported here show direct evidence in favor of the model and suggest its general applicability.

In conclusion, most ore in massive sulfide deposits, in a vast array of geologic settings, results from sub-sea floor replacive deposition. The key to giant orebodies, given appropriate hydrothermal conditions, may be this efficient form of sulfide precipitation, on a large scale. Sediments seem to play a key role in providing cover rocks and perhaps appropriate material to host ores via replacement. This may lead to hybrid, volcanic-sediment hosted giant deposits, less obvious on the sea floor than most of the hydrothermal fields discovered so far. Submarine mineral exploration may still be in its infancy.