Sulfur evaporites

Evaporite Basin Sulfur Deposits. Elemental sulfur occurs in another type of subsurface deposit similar to the salt-dome stmctures in that the sulfur is associated with anhydrite or gypsum. The deposits are sedimentary, however, and occur in huge evaporite basins. It is befleved that the sulfur in these deposits, like that in the Gulf Coast salt domes, was derived by hydrocarbon reduction of the sulfate material and assisted by anaerobic bacteria. The sulfur deposits in Italy (Sicily), Poland, Iraq, the CIS, and the United States (western Texas) are included in this category. 

The great evaporite basin deposits of elemental sulfur in Poland were discovered only in 1953 but have since had a dramatic impact on the economy of that country which, by 1985, was one of the world s leading producers (p. 649). The sulfur occurs in association with secondary limestone, gypsum and anhydrite, and is believed Ui be derived from hydrocarbon reduction of sulfates assisted 1 bacterial action. The H2S so formed is consumed by other bacteria to produce sulfur as waste — this accumulates in the bodies of the bacteria until death, when the sulfur remains. 

What are the relative contributions of these two sources Two approaches have been taken. One is to establish the geology and hydrology of a basin in great detail. This has been carried out for the Amazon (Stallard and Edmond, 1981) with the result that evaporites contribute about twice as much sulfate as sulfide oxidation. The other approach is to apply sulfur isotope geochemistry. As mentioned earlier, there are two relatively abundant stable isotopes of S, and The mean 34/32 ratio is 0.0442. However, different source rocks have different ratios, which arise from slight differences in the reactivities of the isotopes. These deviations are expressed as a difference from a standard, in the case of sulfur the standard being a meteorite found at Canyon Diablo, Arizona. 

Evaporitic sulfur has a range of sulfur isotopic composition from +10%o to +30%o, while sedimentary sulfur is depleted in the heavy isotope and has a range of isotopic composition of about —40%o to +10%o. Most of this variation reflects systematic changes with geological age. The source fractions of a river water can be estimated from an isotopic mass balance ... 

Accepting these relative proportions from evaporites (2/3) and sulfides (1/3), the characteristic times, T of cycling of the evaporite sulfur and sulfide sulfur reservoirs can be estimated from the reservoir sizes (R,) in Table 13-3, and the river flux of sulfur. For evaporites ... 

Sulfur for commercial purposes is derived mainly from native elemental sulfur mined by the Frasch process. Large quantities of sulfur are also recovered from the roasting of metal sulfides and the refining of crude oil, i.e., from the sulfur by-products of purified sour natural gas and petroleum (the designation sour is generally associated with high-sulfur petroleum products). Reserves of elemental sulfur in evaporite and volcanic deposits and of sulfur associated with natural gas,... 

Chlorine is released as HCl, which dissociates upon dissolution in water to generate Cl (aq). Sulfur is released as either H2S or SO2. Both are transformed into S04(aq) through chemical reactions involving oxidation by O2 and dissociation/dissolution in water. The amounts of primary magmatic volatiles that have been degassed thus far are given in Table 21.5. About half of the chlorine has been retained in the ocean and the other half has been converted into evaporite minerals. In comparison, virtually... 

Thiemens MH, Heidenreich JE (1983) The mass independent fractionation of oxygen - A novel isotope effect and its cosmochemical implications. Science 219 1073-1075 Thiemens MH, Jackson T, Zipf EC, Erdman PW, van Egmond C (1995) Carbon dioxide and oxygen isotope anomalies in the mesophere and stratosphere. Science 270 969-972 Thode HG, Monster J (1964) The sulfur isotope abundances in evaporites and in ancient oceans. In Vinogradov AP (ed) Proc Geochem Conf Commemorating the Centenary of V I Vernadsku s Birth, vol 2, 630 p... 

Sulfur content in crude oils and natural bitumens varies from less than 0.05 to more than 14 weight percent, but few commercially produced crude oils exceed 4% sulfur. Tissot and Welte (25) show a frequency distribution of crude oils based on over 9,000 samples and report the average sulfur content as 0.65%. The distribution is clearly bimodal with a minimum at about 1% sulfur. Oils with less than 1% sulfur are classified as low-sulfur, and those above 1% as high-sulfur. In general, high-sulfur oils are derived from carbonate or carbonate-evaporite rock sequences whereas low-sulfur oils are derived largely from clay-rich clastic sequences f25-26.28-29). 

High-sulfur coals of Pennsylvanian age in the Canadian Maritimes Basin contain 5-8% sulfur. However, this sulfur is associated predominantly with freshwater geologic settings. Recent sulfur isotopic results support the hypothesis that the sulfur in these coals is derived from a bedrock evaporite source (122). 

Thus, in an analogous fashion to carbon, if total exogenic sulfur has remained constant, then the 834S values of sulfate in seawater, and consequently in evaporite minerals, are functions of the ratio of oxidized sulfur to reduced sulfur in the sedimentary reservoirs of this element. If the size of the sulfate reservoir increases, then the sulfide reservoir shrinks, and the mean 34s/32s ratios of each reservoir changes. 

Claypool G.E., Holser W.T., Kaplan I.R., Sakai H. and Zak I. (1972) Sulfur and oxygen isotope geochemistry of evaporite sulfates. Abstr. Geol. Soc. Amer. 4, 473. 

The connection between weathering, evaporation, bacterial activity in marine sediments in shallow water areas and stable sulfur isotope distribution was studied by Holser and Kaplan (1966), Nielsen and Ricke (1969), Nielsen (1968), and Eremenko and Pankina (1972). The sulfur isotope ratios of evaporite sulfates show enrichment of 34S in the Devonian deposits (5 +23 °/oo), a pronounced dip in the Permian ( 5 +12 °/oo), values of +20 °/oo in the Triassic, a fall (+17 to +15 °/oo) in the Jurassic and Cretaceous, stabilizing in the Tertiary to the Recent value of +20 °/oo. 

Geologic sources of sulfur include primary and secondary sulfide minerals as well as chemically precipitated sulfate minerals. In sedimentary rocks, sulfur is often concentrated. Examples include pyritic shales, evaporites, and limestones containing pyrite and gypsum in vugs and lining fractures. In many cases, these rocks can be both the source of organic matter as well as sulfate for the biogeochemical reduction of sulfate to occur. 

Sulfur in groundwater is primarily in the oxidized sulfate species, SO . Sulfate in ground-water can have several sources. These include (i) dissolution of evaporite sulfate minerals such as gypsum and anhydrite, (u) oxidation of sulfide minerals, (rii) atmospheric deposition, and (iv) mineralization of organic matter. 

The principal sources of sulfate in formation waters are dissolved marine sulfate, sulfate derived from the dissolution of evaporites, and sulfate formed by the oxidation of sulfides. Sulfate is destroyed by reduction to hydrogen sulfide. The value of 5 " S in gypsum is only — 1.6%o heavier than sulfate in solutions from which it precipitates. The isotopic composition of sulfur in gypsum in Phanerozoic deposits precipitated from seawater during evaporation thus tracks the secular changes in the isotopic composition of sulfur in seawater ( 10% to 30%o). 

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