SUBLACUSTRINE HYDROTHERMAL SEEPS IN NORTHERN LAKE TANGANYIKA, EAST-AFRICAN RIFT - 1991 TANGANYDRO EXPEDITION

Sublacustrine hydrothermal seeps were recently discovered (April 1987) at Pemba and Cape Banza along the Zaire side of the North Tanganyika Trough, in the Western Branch of the East African Rift System. This hydrothermal activity as well as two newly discovered sites, Luhanga and Mbemvi, have been investigated by scuba divers during the multinational (France, Germany, Zaire, Burundi) and multidisciplinary (structural geology, mineralogy, geochemistry, microbiology) TANGANYDRO Expedition (28 August-12 October, 1991). This paper presents the main field results of the TANGANYDRO Project on the North Tanganyika hydrothermal sites, and comparisons with data obtained during the 1987 field work. The Pemba and Luhanga sites are located along the seismically active Uvira Border Fault. Two wide bubble fields and eleven active vents of varying morphologies have been observed at Pemba down to a water depth of 46 m, with fluids pouring out at a temperature of 53-88-degrees-C. These sites are located at the intersection of N120-140-degrees Precambrian cleavage and N45-50-degrees open fractures. N120-degrees-trending veins of massive sulphides (pyrite and marcasite) up to 0.70 m wide associated with these vents have been found locally. Methane and heavier hydrocarbon gases have been identified in the Pemba fluids. Fossil hydrothermal activity was also recognized on land. Fifteen km north of Pemba, the new discovered site of Luhanga has shown 10 active vents and some small bubble fields between 3 and 6 m water depth. Fluid temperature is 50-54-degrees-C. There, vents are located at the intersection of N20-30-degrees sinistral strike-slip faults and an E-W twisted Precambrian cleavage. At Cape Banza, seventy-two vents or groups of vents have been identified down to a water depth of 6 m, along a N-S trending, 200 m long belt. These vents are characterized by aragonite chimneys with varying sizes and morphologies. Sulphide mineralization (pyrite) has been found on the walls of fractures and pipes through which the hydrothermal fluid passes. Temperatures between 66 and 103-degrees-C have been measured. Methane and heavier hydrocarbon gases have also been identified in the Cape Banza fluids. The Mbemvi site, located in Burton's Bay, has shown five active vents between 6 and 10 m water depth. Fluids pour out through sediments at a temperature of 60-degrees-C. Attention has also been devoted to some subaerial sites belonging to the northern Tanganyika structural system in Zaire (Mashuzi) and Burundi (Kabezi and Ruhwa). Possible relationships between heat flow anomalies, regional volcanism, neotectonics and hydrothermal activity are discussed.