Geochemical indicators, such as Fe/Mn, B/Ga, B/Li ratios etc., play an important role in the palaeogeographic reconstructions and paleosalinity estimations of Precambrian basinal and terrestrial deposits which have any stenohaline fauna. Over the last years, together with other methods, a so-called "C/S method" is widely applied for these purposes. In Phanerozoic marine clayey sediments the ratio of organic carbon to authigenic (pyritic) sulphur is about 3. Clayey deposits of fresh water lakes have higher ratios because of deficiency of sulfate-ions and, accordingly, after chemical reduction of pyritic sulphur; under the euxinic conditions the C/S ratio is lowered to 0.5-1.5. This method was succesfully applied earlier to some Proterozoic deposits (Berner and Raiswell, 1983; Dannely and Jackson, 1988; etc.).
We investigated the contents of C and S in terrigenous deposits of Lower Riphean (1,4 Ga) Bakal and Middle Riphean Zigazino-Komarovo formations from the Russian Riphean stratotype (western slope of the South Urals). At present, genesis of these deposits are known rather well on the base of lithological methods. The deposits of Bakal formation were formed under open marine and near shore conditions; there are also lagunal facies. Zigazino-Komarovo sedimentary complexes were deposited in sublittoral and littoral zones of a shallow marine basin. 20 samples containing organic carbon and sulphur contents were investigated. Average C/S ratio for Bakal formation deposits is about 75. This ratio is typical for fresh water environments (limits of variations for C are from 0.65 to 1.4%; for S - from 0.02 to 0.16%). So, 300 - 400 m thick terrigenous members of Bakal formation with various littoral and sublittoral structures (Krupenin, 1987) were formed undoubtfully under low salinity conditions. Carbonate members of this level of Lower Riphean are of marine genesis. Wide variations of C and S contents are typical for clayey rocks of Zigazino-Komarovo formation (correspondingly, 1.1 - 4 and 0.28 - 2.8 %). The average C/S ratio for 6 samples is about 3.3 but in some samples this parameter is not more than 1. Thus, Middle Riphean Zigazino-Komarovo formation deposits were accumulated in marine basin with few euxinic events. Such a conclusion is supported by the presence among these of pyrite concretions and lens-like layers of diagenetic siderite.
This research was supported by RFFR, grant 95-05-14288.