Isotopic Study of Siderite Concretion, Tuscarawas County, Ohio

52 M. A. TORAASON AND G. L. WRIGHT Vol. 82

ISOTOPIC STUDY OF A SIDERITE CONCRETION, TUSCARAWAS COUNTY, OHIO1

S. R. ORR and G. FAURE, Department of Geology and Mineralogy, Ohio State University, Columbus, Ohio 43210 G. BOTOMAN, Division of Geological Survey, Ohio Department of Natural Resources, Fountain Square, Columbus, Ohio 43216

ABSTRACT. A carbonate concretion from the shale bed overlying the No. 5 coal of Pennsyl- vanian age in Tuscarawas County is composed of siderite and contains pyrite, barite, sphalerite and traces of calcite in its interior and in fractures. The siderite has 8I3C = + 0.65%o, Sr=289 ppm and 87Sr/86Sr=0.7110±0.0009- These results indicate that the siderite was deposited from brackish water in the pore spaces of the sediment and discredit the hypothesis that the carbonate ions originated by oxidation of peat or organic material. The barite has 8 S=— 4.6%o, indicating that it was not precipitated from sea water. Pyrite and sphalerite have 83 S= — 10.8%o and — 4.6%o, respectively, and were not deposited in isotopic equilibrium with each other. The sulfur in all three minerals is enriched in 3 l6S as a result of bacterial reduction of sulfate to sulfide.

OHIO J. SCI. 82(1): 52, 1982

INTRODUCTION utions after deposition of the sediment. Carbonate concretions exist in many They are composed not only of carbonate shale formations in the Paleozoic bedrock of minerals but also may contain layers of py- Ohio. Clifton (1957) summarized their oc- rite or marcasite. Therefore, the geochemi- currence in the Ohio Shale of Upper Devo- cal process by which they formed must be nian age. Olson (1975) reported average capable of producing both carbonate and concentrations for FeO = 2.39±0.11% and sulfide minerals. Barth (1975) proposed Sr=75.3± 1.6% ppm in a profile across a that concretions in the lower section of the ferroan dolomite concretion from the Olen- Ohio Shale north of Columbus were formed tangy Shale in Delaware County. A com- by the action of bacteria reducing sulfate plete chemical analysis of a concretion from ions to H2S and oxidizing organic com- the Olentangy Shale was published by pounds to CO2. These gases dissolved in Westgate(1926). the pore water of the sediment and caused precipitation of iron sulfide and carbonate The concretions apparently formed by minerals, respectively. precipitation of minerals from aqueous so- Concretions also are found in the shale units between the coal seams of Pennsylva- ^anuscript received 28 June 1980 (#80-37). nian age in Tuscarawas County. In this OhioJ. Sci. SIDERITE COMCRETION case, the presence of peat and organic mate- viewed by Faure (1977), have revealed that l2 rial in the shale may have provided an abun- plants are enriched in 6C as a result of dant food source for bacteria. The CO2 re- photosynthesis. Since coal, petroleum, and leased by their metabolic activity may have natural gas formed from plant tissue, they contributed to the growth of carbonate con- are enriched in ^C and have 8 C values cretions in the overlying shale units. We that are commonly in excess of — 25%o rela- have attempted to test this hypothesis by a tive to the PDB standard. By contrast, cal- study of one concretion collected from a cite in marine limestones has 8 C values shale unit just above the No. 5 coal seam close to zero. The use of carbon isotopic exposed at the strip mine of the Eberhart compositions to determine the environ- Coal Co. in Tuscarawas County, Ohio. ment of deposition of siderite concretions and to identify the source of the carbon is MINERAL COMPOSITION described in detail by Weber et al (1964). The concretion was about 30 x 18 x 13 The siderite of the concretion from Tus- cm in size. The bulk of it was composed of carawas County has a 8 C value of siderite (FeCO3) identified by x-ray diffrac- + 0.65%c. According to the criteria of tion and we detected small amounts of Weber etal (1964), this value indicates a re- quartz and kaolinite or chlorite in the insol- stricted marine (brackish) environment and uble residue of the siderite. The interior thereby discredits the hypothesis that this contained a veinlet of pyrite (FeS ) covered 2 concretion formed from CO2 derived from by a layer of barite (BaSO4). A few tabular bacterial oxidation of peat or coal. crystals of dark brown sphalerite (Zn, Fe)S, up to 2 cm in diameter, were embedded in The siderite has a Sr concentration of 289 ppm, determined by x-ray fluorescence. Its the barite. Both pyrite and barite occurred 87 86 in fractures that extended from the outside Sr/ Sr ratio is 0.7110±0.0009- The var- of the concretion into the interior. Pyrite iation of the isotopic composition of Sr in appears to have formed first followed by ba- the oceans during the past 600 million rite and sphalerite. The presence of a small years has been reviewed by Faure and amount of calcite (CaCO3) in the interior of Powell (1972) and by Faure (1977). A the concretion was detected by the reaction study by Mukhopadhyay and Brookins with HC1. (1976) indicates that the 87Sr/86Sr ratio of limestones in the Madera Formation CARBON AND STRONTIUM (Pennsylvanian) of New Mexico varied from The isotopic composition of carbon in 0.7066 to 0.7118. The value obtained for the siderite (table 1) indicates the source of the concretion from Tuscarawas County is carbon in this mineral. Studies of the near the high end of this spectrum and isotope composition of carbon in nature, re- confirms a brackish near-shore environment of deposition. TABLE 1 Analytical Data for Concretion from Eberhart Coal Co., SULFUR Tuscarawas County, Ohio.* The isotopic composition of sulfur in 834S 813C Sr marine sulfate deposits varies widely de- 87 86 (%o) (%o) (ppm) Sr/ Sr pending on their geologic age. The varia- Siderite — +0.65 289 0.7110±0.0009 tion is the result of isotope fractionation Barite — 4.6 — — — during the reduction of sulfate to sulfide ion Sphalerite —4.6 — — — by the bacterium Desulfovibrio desulfuricans. Pyrite -10.8 — — — This process enriches the sulfide in \^S # The isotopic compositions of sulfur and carbon were and concentrates \ S in the residual sul- determined by Geochron Laboratories. The Sr/ 6 Sr ratio was measured by the authors and was cor- fate. The systematics of sulfur isotope frac- rected for isotope fractionation to a value of 0.1194 tionation have been reviewed by Faure for the 86Sr/88Sr ratio. (1977). According to ClaypoolShales along the Olentangy barite. River. OhioJ. Sci. 75: 162-163- Botoman, G. and G. Faure 1976 Sulfur isotope com- The 8S3 S values of the pyrite and sphal- position of some sulfide and sulfate minerals in erite in the concretion are — 10.8%o and Ohio. OhioJ. Sci. 76: 66-71. — A.6%o, respectively (table 1). The enrich- Claypool, G. E., W. T. Holser, I. R. Kaplan, H. 3 Sakai and I. Zak 1980 The age curves of sulfur and ment in gS of both minerals is compatible oxygen iostopes in marine sulfate and their mutual with bacterial fractionation of marine sul- interpretation. Chem. Geol. 28: 199-260. fate discussed above. The difference be- Clifton, H. E. 1957 The carbonate concretions of the tween the 83 S values corresponds to an Ohio Shale. OhioJ. Sci. 57: 114-124. Faure, G. and J. L. Powell 1972 Strontium isotope isotope equilibration temperature of geology. Springer Verlag, New York. 166 pp. -33.4°C (Orr 1980). This result is not sen- 1977 Principles of isotope geology. J. Wiley sible and indicates that pyrite and sphaler- and Sons, New York, 466 pp. ite in the concretion did not form in Mukhopadhyay, B. and D. B. Brookins 1976 Stron- isotopic equilibrium with a common reser- tium isotopic composition of the Madera Forma- 3 tion (Pennsylvanian) near Albuquerque, New voir. The large difference in the 8 S values Mexico. Geochim. Cosmochim. Acta 40: 611- probably indicates that pyrite and sphaler- 616. ite formed at different times from solutions Olson, G. A. 1975 A geochemical investigation of a containing sulfide of different isotopic com- carbonate concretion from the Devonian Olen- position. This conclusion is supported by tangy Shale. Unpubl. B.Sc. thesis, Dept. Geol. and Mineral., Ohio State Univ. Columbus, 25 pp. the evidence that pyrite was deposited first, Orr, S. R. 1980 Isotopic study of an eastern Ohio car- followed by barite and sphalerite. A more bonate concretion. Unpubl. B.Sc. thesis, Dept. extensive discussion of sulfur-isotope Geol. and Mineral., Ohio State University, Col- equilibration temperatures for pyrite, umbus, 26 pp. Weber, J. N., E. G. Williams and M. L. Keith 1964 sphalerite and galena in Ohio is given by Paleo-environmental significance of carbon Botoman and Faure (1976). isotopic composition of siderite nodules in some The results of this study indicate that the shales of Pennsylvanian age. J. Sed. Petrol. 34: 814-818. concretion formed by precipitation of sider- Westgate, L. G. 1926 Geology of Delaware County. ite from brackish water and that very little, Ohio Geol. Surv., 4th ser., Bull. 30: 31-35, 50- if any, carbonate ion originated by oxida- 54.