Seiffert, Gerhard (1995): Geochemical analysis of sulfides from Lake Tanganyika [dataset publication series]. PANGAEA, https://doi.org/10.1594/PANGAEA.881644, Supplement to: Seiffert, G (1995): Hydrothermalismus im Ostafrikanischen Riftsystem: Mineralogische und geochemische Charakterisierung und Genese von Massivsulfiden sublakustriner Geothermalfelder im nördlichen Tanganyika-See. Berichte-Reports, Geologisch-Paläontologisches Institut der Universität Kiel, 72, 116 pp, https://doi.org/10.2312/reports-gpi.1995.72
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Abstract:
Geothermal waters with maximum temperatures of 103°C emanate from two sublacustrine hot spring areas which are located in the northem part of Lake Tanganyika (East African Riftsystem). The hydrothermalism leads to the formation of crust- and stockwork-like massive sulfide bodies on the lake bottom to a maximum water depth of 46 m. These geothermal vent areas were investigated and sampled during the German-French TANGANYDRO-campaign in 1991.
The aim of this work is to characterize the mineralogy and geochemistry of these sulfides and to reconstruct their genesis. Mineralogical methods that have been used inc1ude scatter electron microscopy (SEM), polarization microscopy, and X-ray diffraction analysis. The geochemical methods inc1ude electron microprobe analysis (EMP) and inductively coupled plasma mass spectrometry (ICP-MS) for the major and trace elements and sulfur isotope measurements in order to determine the d34S-values of the samples.
The samples consist exc1usively of iron sulfides. The dominant minerals are marcasite and melnicovite with subordinate pyrite. All samples show collophorm textures indicating their origin from gel-like precursors. They are characterized by high contents of As (up to 3.4 wt%), Sb (up to 0.6 wt%) and Tl (up to 2.6 wt%). Low d34S-values in the range of -11.6 %0 to +2.4 %0 (rel. PDB) indicate bacterial sulfur fractionation.
The crystallization of the gel-like precursor leads to the formation of marcasite and pyrite with melnicovite as a transitional phase. Pyrite is formed by the replacement of either melnicovite or marcasite. This mechanism accords to previously postulated models for the formation of iron sulfides in low temperature (<100°C) hydrothermal systems. A significant sulfur isotope fractionation (increase of d34S) has been observed during the replace- ment of melnicovite by the mature phases marcasite and pyrite.
Biogenie impact on the sulfide formation is indicated by low Co/Ni-ratios (<1), the negative d34S-values and the occurrence of framboidal pyrite. The metabolism of sulfur oxidizing and sulphate reducing bacteria at the wall rock of the vents and in the spring waters is suggested to actively influence the setting of specific pH-values required for the formation of either marcasite or pyrite. The altemating pyrite-marcasite layers are the result of fluctuations in the productivity of those bacteria, which may depend on seasonal variations or changing nutrient support.
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Median Latitude: -3.715833 * Median Longitude: 29.065833 * South-bound Latitude: -4.270000 * West-bound Longitude: 29.015000 * North-bound Latitude: -3.605000 * East-bound Longitude: 29.320000
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Datasets listed in this publication series
- Seiffert, G (1995): Table A.1 Trace element concentrations of sulfides from Pemba. https://doi.org/10.1594/PANGAEA.881549
- Seiffert, G (1995): Table A.5 Geochemical results from microprobe analysis. https://doi.org/10.1594/PANGAEA.881631
- Seiffert, G (1995): Table A.8 Geochemical composition of sediments and rocks from Pempa. https://doi.org/10.1594/PANGAEA.881632