Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

J. P. Shrivastava ¹, M. S. Salil ¹, S. K. Pattanayak ¹

Affiliations
1 Department of Geology, University of Delhi, Delhi - 110 007, IN

Abstract

Clay mineral studies on the K/T boundary succession at Anjar, Kutch show smectite, sepiolite and palygorskite as its major clay mineral assemblage. There is a distinct variation in the clay mineralogy across the section with sepiolite and palygorskite dominating the lower and upper lithounits, respectively, in association with smectite. The KTB layer, marked by the Ir (1271 pg/g) and Os (1414 pg/g) anomaly contain sepiolite and smectite in subequal proportions. The clay stratigraphy of the succession reflects progressive increase in aridity and basicity in the depositional environment during the deposilion of Anjar KTB sediments. It appears that the sepiolite-palygorskite-smectite complexes in the sediments were developed in a lacustrine or peri-marine environment of high basicity under arid conditions induced and influenced by Deccan volcanism. In this environment, the Si, Mg and Al ions necessary for the formation of sepiolite and palygorskite could be supplied by the hydrothermal fluids associated with volcanism by the dissociation of silicates already available in the depositional environment or simultaneously by both the processes. Clay mineral associations in the Ir-bearing Anjar intertrappean sediments across its lithounits are more compatible with the depositionai environment influenced by the volcanism, than an asteroidal impact. In the upper part of the succession, the predominance of palygorskite and smectite suggests that the latter contributed the necessary Al, Si and Mg ions to result the former. The REE signatures indicate that sepiolite-smectite is most likely the carrier phase of Ir in Anjar intertrappeans.

Keywords

Clay Mineralogy, Iridium, Intertrappeans, Deccan Trap, KTB, Kutch, Gujarat.

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M. S. Salil ¹, S. K. Pattanayak ¹, J. P. Shrivastava ¹

Affiliations
1 Department of Geology, University of Delhi, Delhi-110 007, IN

Abstract

The studies on the structural formulae of smectites in the Larneta sediments indicate that they are rich in octahedral Fe, Mg and tetrahedral Al. Identical patterns of Mg, Fe and Al abundances in the octahedral and tetrahedral layers respectively, were also observed in the smectites of intertrappean sediments and weathered Deccan Basalts. The thermodynamic components and charge configurations of smectites in the Lameta, intertrappean and Deccan Basalt indicate that they fall within the compositional plane of smectite solid solution. The compositional commonality of smectite in the Lameta sediments with those in the intertrappeans and weathered Deccan Basalt suggests, Smectites in the Lameta sediments are derivatives of Deccan volcanic products. This implies the commencement of Deccan Volcanism during the Maastrichtian Lameta sedimentation or prior to it.

Keywords

Smectite, Larneta Sediments, Deccan Basalt, Central India.

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J. P. Shrivastava ¹, S. K. Pattanayak ¹, Chatar Singh ²

Affiliations
1 Department of Geology, University of Delhi, Delhi- 110 007, IN
2 ITMMEC, IIT, Delhi - 110 016, IN

Abstract

Disseminated microscopic gold grains, measuring 8-12 microns across and possessing a fineness of 950 to 960 have been observed in the quartz normative tholeiitic lava flows from the Amarkantak region of the Eastern Deccan Volcanic Province. High temperature of equilibration (1060-1470°C) and very low oxygen fugacity (<5) as determined from the co-existing ulvospinel-magnetite and ilmenite-haematite pairs are proposed to be the controlling factors for the occurrence of gold in these volcanics.

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