International Geologiical Congress - Oslo 2008


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MRD-13 Ore deposits associated with black shales: from their origin to their environmental impacts


Carbonaceous tuff-stratiform barite association in Mangampet deposit, Andhra Pradesh, India: Implications on depositional environment and origin


Mihir Deb, University of Delhi (India)
Konka Bheemalingeswara, University of Delhi (India)


The Mangampet barite deposit in Cuddapah district, Andhra Pradesh, comprising two barite lenses, is one of the largest volcano-sedimentary stratiform deposits in the world with a resource of 37 Mt. The barite in the deposit is closely associated with carbonaceous tuff overlying dolostones of the Pullampet Formation in the Mesoproterozoic Cuddapah Supergroup. Barite occurs intimately associated with minor quantities of pyrite, mainly as bedded, grey, granular masses or in lapilli or rosette form; veins, fracture filling and replacement patches also occur sporadically.
Petrographic studies of 28 samples along a complete profile in the deposit revealed layers of medium to coarse grained barite alternating with dark and stringery carbonaceous layers, representing algal mats. The association is speckled with fine cubes of pyrite. An incipient schistosity transecting the primary stratification of the tuff suggests a very low grade of metamorphism. Absence of graphite in XRD studies of separated particulate organic matter and peaks in FT-IR analysis representing C-O, OH and C-H bonds in them suggest immature nature of the organic matter associated with the tuff. Atomic ratios of H/C and O/C, varying from 0.57 to 1.15 and from 0.20 to 0.36 respectively, point towards both marine (type II kerogen) and terrestrial (type III kerogen) source for the organic matter, which due to low grade thermal effects, exhibits only low to medium level of aromaticity.
Total carbon values in the tuff samples range between 0.01 and 5.49 wt. % and sulfur between 0 and 2.70 wt. %. δ13Corg values, ranging between -28.0‰ to - 32.4‰ and δ34SSO4 between +43.4‰ to +44.1‰ indicate biogenic source for carbon and a modified seawater for the barite sulfur respectively.
Regression line in Corg-S plot is parallel to the normal marine regression line but has an intercept at 1.2% S. The degree of pyritisation (DOP) remain uniform around 0.8 for differing values of Corg suggesting an euxinic (H2S-containing) environment of deposition and a syngenetic origin of the pyrite. The C-S pattern appears to be due to C-Fe coupling and the uniform DOP values appear to signify that pyrite formation was Fe-limited, a condition brought about probably by the low rate of organic matter deposition with higher levels of pyroclastic influx. Formation of the barite with minor secondary pyrite seems to be the result of fluctuating redox condition of the depositional environment.
A preferred model of startiform barite mineralisation envisages mixing of seawater with hydrothermal solutions carrying barium in a restricted basin with prolific biologic activity, receiving argillaceous sediments from continental sources and pyroclastic matter from explosive volcanism. The restricted nature of the basin resulted in a gradual decrease of oxygen content in the bottom waters and low recycling rates for the organic matter. Change from normal marine to euxinic conditions helped the formation of syngenetic pyrite.


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