Economic Geology Vol. 86, 1991, pp. 862-869 The Relationship between Carbon, Sulfur, and Pyritic Iron in the Amjhore Deposit, Bihar, India H. S. PANDALAI, Department of Earth Sciences, Indian Institute of Technology, Powai, Bombay 400 076, India A. CHANGKAKOTI, Geology Department, La Trobe University, Bundoora, Victoria, Australia H. R. KROUSE, Departmentof Physics, Facultyof Science, University of Calgary, Calgary,Alberta, Canada T2N 1N4 AND N. GUNALAN Department of Earth Sciences, Indian Institute of Technology, Powai,Bombay 400 076, India Abstract Low C/S ratioshave been generally observed in pyritiferous black shales of the lower Paleozoic andlate Proterozoic (Berner and Raiswell, 1983). In addition to factors such as high ratesof bacterialsulfate reduction which might haveexisted duringtheseearly periods due to the low oxygen content of ocean water, availability and qualityof organic matter,or lack ofbioturbation and availability ofreactive iron, lowC/Sratios may also reflect lowsedimentation rates.Three distinctsituations within a single vertical stratigraphic columnin the mid-Pro- terozoic Amjhore pyrite deposit reflectthe effect of anoxic levels andsedimentation rates on pyrite formation. In the first (lowershale with a mean C/S ratio of 2.03), pyrite formation was diagenetic, whereas in the second (massive pyrite ore with a negligible carbon content)and the third (uppershale with a low C/S ratioof 0.27), pyrite formation tookplace undereuxinic conditions. In environments whereanoxic levels are highandthe sedimentation rate low, low mean C/S ratios mayoccur. In such environments variations in total sulfate reduction per unit column of sediment deposited (resulting from variation in sedimentation rates or anoxic levels) are reflectedin the slope of the carbon-sulfur plot by a greaterslope. This may also resultin lower intercepts on the sulfuraxis. For upper shale samples the carbon-sulfur regression line hasa very high slope of 3.73 and a low intercepton the sulfuraxis of 0.21. Consistency of anoxic levels during deposition of upper shales is, however, indicated by the low standard deviation (0.051) of C/S dataanda dominant control of the sedimentation rate on the sulfur content of the sediments is inferred. Sulfurisotope data on pyrite samples indicatean envi- ronment closed to SO• 2 which was initially open to H2S(or HS-) and later during deposition of upper shale became closed to it due to a limited supply of reactive iron. This confirms that the pyrite bed anduppershales were laid down in a restricted environment andsubstantiates the interpretation of the carbon-sulfur relationships observed in them. Introduction OCCURRENCES of syngenetic pyrite in black shales have been widely studiedand different techniques have evolvedto interpret the origin of the sulfides. In most such occurrences, where the pyrite is formed either postdepositionally or "syngenetically" (in the sense of Raiswell andBerner, 1985), a systematic re- lationship between the organiccarboncontentand the sulfur content of the black shales has been ob- served(Berner, 1971; Leventhal, 1979, 1982; Rais- well and Berner, 1985, 1986; Davis et al., 1988; and several others). The relationships between organic carbon, pyritic sulfur,and iron in theserocks have been usedto distinguish between normal marine en- vironments and euxinic environments, and in addition, by using the concept of degree of pyritization, to dis- tinguish either a reactive iron-limited or an organic carbon-limited situation in the case of euxinic envi- ronments (Raiswell andBerner, 1985). In the present work theseconcepts havebeen appliedto the pyrite deposit at Amjhore where pyrite occurs both as a complete bed and as disseminations in black shales that enclose the pyritebed.The deposit isparticularly interesting because it occurs in a stratiform, strata- bound settingenclosed in a geologic formation of Precambrian age and contains pyrite ore rangingin wt percent of S from as low as 1 to as high as 47 percent.Earlier investigations of C-S relationships 862