HYDROTHERMAL VENTS IN LAKE TANGANYIKA, EAST-AFRICAN RIFT SYSTEM

Sublacustrine hydrothermal vents with associated massive sulfides were discovered during April 1987 at Pemba and Cape Banza on the Zaire sid e of the northern basin of Lake Tanganyika, East African Rift system. New investigations by a team of ten scuba divers during the multinatio nal (France, Zaire, Germany, and Burundi) TANGANYDRO expedition (Augus t-October 1991) found hydrothermal vents down to a depth of 46 m along north-trending active faults bounding the Tanganyika rift on the west ern side. Temperatures from 53 to 103-degrees-C were measured in hydro thermal fluids and sediments. Veins of massive sulfides 1-10 cm thick (pyrite and marcasite banding) were found associated with vents at the Pemba site. At Cape Banza, active vents are characterized by 1-70-cm- high aragonite chimneys, and there are microcrystalline pyrite coating s on the walls of hydrothermal pipes. Hydrothermal fluid end members s how distinctive compositions at the two sites. The Pemba end member is a NaHCO3-enriched fluid similar to the NaHCO, thermal fluids from lak es Magadi and Bogoria in the eastern branch of the rift. The Cape Banz a end member is a solution enriched in NaCl. Such brines may have a de ep-seated basement origin, as do the Uvinza NaCl brines on the eastern flank of the Tanganyika basin. Geothermometric calculations have yiel ded temperatures of fluid-rock interaction of 219 and 179-degrees-C in the Pemba and Cape Banza systems, respectively. Abundant white or red dish-brown microbial colonies resembling Beggiatoa mats were found sur rounding the active vents. Thermal fluid circulation is permitted by o pening of cracks related to 130-degrees-N normal-dextral faults that i ntersect the north-south major rift trend. The source of heat for such hydrothermal systems may relate to the existence of magmatic bodies u nder the rift, which is suggested by the isotopic composition of carbo n dioxide released at Pemba and Cape Banza.