| Hydrothermal circulation that produces sulfide deposits in the marine environment can also produce sulfide deposits in lakes. Sulfides that are presently forming decimeter-tall chimneys in relatively shallow waters of Lake Tanganyika in east Africa have been examined.; Ore microscopy and X-ray diffraction study of samples from the Pemba hydrothermal site revealed marcasite and pyrite as the most common sulfides. These commonly display growth zoning and colloform banding. Some pyrite is framboidal. Minor minerals include barite, kaolinite, hematite and quartz.; Bulk analyses by XRF and INAA of the sulfides revealed high abundances of As, Tl and Ba and very low to non-measurable concentrations of Cu, Zn, Pb, Au and Ag. Chondrite normalized REE diagrams show a typical continental sedimentary pattern with added hydrothermal components.; The continental rift environment may be responsible for high abundances of As and Tl. These two elements probably occur as nonstoichiometric substitutions in the pyrite lattice.; Evidence suggests that large lakes could be sites for large sulfide deposits formation today and in the past. |
