Hydrothermal activity in the Northern Tanganyika Rift, East Africa

Abstract

Hydrothermal fluids from sub-lacustrine and sub-aerial sites were sampled in the North Tanganyika Rift during the 1991 TANGANYDRO expedition. Two types of hydrothermal end-member fluids, a NaHCO 3 type and a NaCl type, have been identified within the sample set. The NaHCO 3 hydrothermal fluids are emitted at the level of the sub-lacustrine Pemba and sub-aerial Kabezi and Mashuzi sites, which are located in the northernmost Bujumbura sub-basin. The NaCl hydrothermal fluid vents at the level of the sub-lacustrine Cape Banza field, on the southeastern edge of the Ubwari ridge. The two types of hydrothermal fluids display isotopic and chemical evidence of interaction with magmatic volatiles. The magmatic effects are strongest in the fluid discharging on the Pemba hydrothermal field, where abundant CO 2 bubbling accompanies fluid emission. The δ 13C-values of the CO 2 gas sampled at Pemba and at Cape Banza indicate a magmatic origin. The Pemba fluid shows the highest alkalinity and boron content and the lowest pH of the hydrothermal end-member fluids, and it has equilibrated at depth at the highest temperature (214 ± 7°C). In addition, the sulfide mineralizations associated with the Pemba hydrothermal vents are enriched in As and Tl which, similarly to B, can be transported as volatiles in magmatic fluids. The northern Tanganyika NaHCO 3 fluids are similar to the NaHCO 3 fluid of the geothermal system of Lake Bogoria in the eastern branch of the East African Rift and they can be compared with the NaHCO 3 hydrothermal water that is inferred to flow into Lake Kivu. The Cape Banza NaCl fluids may derive from the mixing between meteoric water and a deep-seated basement brine similar to the ones found in old continental shields such as the Canadian Shield. The high equilibrium temperatures and the geochemistry of the North Tanganyika Rift hydrothermal fluids suggest that degassing magmatic bodies are present at depth in the area. These magmatic bodies may be either the southern expression of the South Kivu magmatism or a more recent activity not yet expressed as surface volcanism. The hydrothermal sites are located along major active rift bounding faults, which can be associated with deep and strong seismicity as in the case of the Pemba field. Such deep fractures may have favoured the uprise of the magmatic bodies that are the source of heat and volatiles to the hydrothermal systems. The circulation of hydrothermal fluids within the Tanganyika lacustrine sedimentary pile could play an important role in the maturation of the organic matter enclosed in the lake sediments and the subsequent oil generation and migration.