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

Tapan Pal ¹, Biswajit Ghosh ², Sujit Chakraborty ³

Affiliations
1 Petrology Division, ER, Geological Survey of India, Kolkata, IN
2 Project: Andaman and Nicobar, Op: WSA, ER, Geological Survey of India, Kolkata, IN
3 Monitoring Division, CHQ, Geological Survey of India, Kolkata, IN

Abstract

The chromite bearing ultramafic rocks of Roro-Jojohatu areas in the Singhbhum Craton within the metasediments of the Iron Ore Group (IOG) is a part of Archaean greenstone terrain of Eastern India. In this area since all the primary silicates are altered, chromite has been used for host rock petrogenesis. Chromite occurs in different forms viz. (a) mainly cumulus phases in chromitite layers, (b) intercumulus phases forming banded/net textured chromite and (c) fine disseminations. Chemically all the chromites are aluminian chromite. The massive chromite and banded/net-textured chromites show high Cr-number (79-83) and intermediate Mg-number (51-60), but disseminated chromites have consistently low Cr-number (76-79) and low Mg-number (43-52). The plots of TiO₂ versus Cr-number, Al₂O₃ versus TiO₂, and Mg-number versus Cr-number reflect boninitic parental melt for the chromites. Al₂O₃ contents in liquid ranging from 9.67-11.03wt% and TiO₂ contents in liquid 0.42-0.75wt% also correspond for boninitic parentage. The FeO/MgO ratio in liquid ranging from 1.15 to 1.29 for massive chromites and 0.86-1.23 for banded/net-textured chromite, also suggest for boninitic source. Al₂O₃ versus TiO₂ plots indicate that chromites plot in the field of supra-subduction zone peridotites. Boninitic magma was produced from hydrous mantle melting in supra-subduction zone. At supra-subduction setting an oxidizing hydrous fluid derived from subducting slab facilitated the formation of chromitite layers at high fO₂ conditions.

Keywords

Chromites, Boninite, Supra-Subduction Zone, Roro-Jojohatu, Singhbhum Craton.

Full Text

     

M. N. Praveen ¹, Biswajit Ghosh ¹, H. S. Shrivastava ¹, M. L. Dora ², L. D. Gaikwad ¹

Affiliations
1 Geological Survey of India, E-5 Arera Colony, Bhopal - 462 016, IN
2 Geological Survey of India, Seminary Hills, Nagpur - 440 006, IN

Abstract

Sulphide mineralisation and syn-Volcanic hydrothermal alteration in the Betul Belt is associated with submarine bimodal volcanism. Felsic volcaniclastics form the favourable host rocks in most areas. Alteration zones have been metamorphosed to middle to upper-Amphibolite facies giving rise to various metamorphic mineral assemblages. Based on metal ratios, mineralisation can be classified into Zn-Cu and Zn-Pb-Cu types. The difference between the two types might be related to the composition of the footwall volcanics. Zn-Cu types are in close proximity to mafic volcanics towards their footwall side whereas, Zn-Pb-Cu types occur in areas dominated by felsic volcanics with little mafic volcanics towards the footwall. The sub-Parallel sheet like nature of the ore bodies and strata-Bound alteration might be related to the permeable nature of the host volcaniclastics.

Keywords

Sulphide Mineralisation, Classification, Hydrothermal Alteration, Subsea-Floor Replacement, Betul Belt, Central India.

Full Text

     

M. N. Praveen ¹, Biswajit Ghosh ¹

Affiliations
1 Geological Survey of India, E-5 Arera Colony, Bhopal-462 016, IN

Abstract

Multiple origin of gahnite of different generations and their relationship with other coexisting minerals like garnet, biotite, chlorite and sphalerite were studied in metamorphosed synvolcanic hydrothemally altered rock associated with base metal mineralisation from the Proterozoic Bhuyari prospect, Betul Belt, Central India. The present assemblages appear to be the result of overprinting of arnphibolite grade metamorphism on primary hydrothermal alteration products. Gahnite is found in a quartz + biotite + muscovite + pLagioclase + gahnite + garnet + chlorite assemblage. Petrographic evidence with supporting mineral composition data suggests the presence of two generations of gahnite. The first generation gahni te is found surrounding sphalerite grains and may be the product of desulphidation of primary sphalerite involving first generation garnet. Second generation gahnite show petrographic evidence of transformation from biotite and might be the result of overstepping of the zinc saturation limit of biotite during alteration to chlorite. The Bhuyari area was subjected to one prograde event followed by a retrograde metamorphic event and mineral paragenesis is in conformity with the metamorphic hlstory.

Keywords

Gahnite, Zn-Pb-Cu deposit, Hydrothermal alteration, Betul Belt, Madhya Pradesh.

Full Text

     

Biswajit Ghosh ¹, John Mahoney ², Jyotisankar Ray ³

Affiliations
1 Geological Survey of India, Op WB-SK-AN, ER, Kolkata - 462 016, IN
2 Department of Geology and Geophysics, University of Hawaii, SOEST, 1680 East-West Road, Honolulu, HI96822, US
3 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata - 700 019, IN

Abstract

The ophiolite assemblage around Mayodia, Dibang Valley, Arunachal Pradesh falls in the eastern extension of the Indus suture belt and is represented by peridotite tectonite hornblendite (dyke) and amphibolite. The entire succession is overlain by metabasalt carapace interlayered with metapelitic pelagic sedimentary rocks. The basal peridotite may be classified as wehrlite. The hornblendite typically occurs as intrusive within the peridotite tectonite and is represented by cumulus primary amphibole. The amphibolite is characterized by well developed gneissose banding. The pillow lava is represented by actinolite-Chlorite-Albite-Epidote schist. The evolutionary trend of the ophiolite suite has been assessed based on major, trace and rare earth element data which favours partial melting of a depleted mantle source. Tectonic discrimination diagrams for the amphibolite and metabasalt clearly indicate their MORB affinity. Such ophiolite assemblage has developed as a result of collision of India and Asia that started with the closing of Tethyan ocean during Mesozoic and Early Tertiary.

Keywords

Mayodia Ophiolite, Petrology, Geochemistry, Tectonic Setting, Northeastern Himalaya, Arunachal Pradesh.

Full Text

     

Biswajit Ghosh ¹, M. N. Praveen ², H. S. Shrivastava ²

Affiliations
1 Geological Survey of India, E-5 Arera Colony, Bhopal - 462 016
2 Geological Survey of India, E-5 Arera Colony, Bhopal - 462 016, IN

Abstract

Gahnite Chemistry from metamorphosed Zn-Pb-Cu sulphide occurrences of Betul Belt, Central India has been presented here. Gahnites of this Belt are suggested to be a product of desulphidation of sphalerite during metamorphism. Although the very presence of zincian spinels in rocks may appear to constitute an exploration guide for metamorphosed massive sulphide (MMS) deposits, spinels that are likely to be spatially associated with sulphides appear to have characteristic Zn-rich, Mg-poor compositions.

Keywords

Gahnite Chemistry, Zn-Pb-Cu-Deposits, Genetic Implications, Betul Belt, Central India.

Full Text

   

Sarmistha Mukhopadhyay ¹, Jyotisankar Ray ¹, Basab Chattopadhyay ², Shyamal Sengupta ², Biswajit Ghosh ¹, Subrata Mukhopadhyay ¹

Affiliations
1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata - 700 019, IN
2 Geological Survey of India, Central Petrological Laboratories, 15 A&B Kyd Street, Kolkata - 700 016, IN

Abstract

The Elagiri complex (12°31' N: 78°35' E) represents one of the important silica - oversaturated syenite plutons of the Southern Granulite Terrane of the Indian shield. This article for the first time reports the mineral chemistry of the Elagiri complex and brings out important petrogenetic significance. The litho-members of Elagiri complex are intrusive into high grade country rocks viz. granite gneiss, amphibolite and pyroxene granulite. The country rocks are foliated bearing evidences of multiple folding and deformation. On the other hand, the constituent litho members of the Elagiri complex (syenites, gabbro and later intrusives marked by lamprophyre and carbonatite) show preservation of igneous layering in terms of discernible parallelism of the constituent minerals. The Elagiri complex shows presence of sharp contacts among litho members and marked absence of chilled facies peripheral to the margin. Electron microprobe data have been critically used to systematize the constituent mineral-phases of the different lithomembers of the complex. Geothermobarometric data indicate a temperature of equilibration in the range of ∼700° to 500° C at ∼2.0 to 5.2 kb which corresponds to shallow level (cf. 18.2 km) equilibration-depth of the complex. Field observations and mineral chemistry data suggest that liquid immiscibility plays an important role during the evolution of the Elagiri complex.

Keywords

Mineral Chemistry, Pyroxene Thermometry, Equilibration Depth, Liquid Immiscibility, Elagiri Complex, Indian Shield.

Full Text

     

Biswajit Ghosh ¹, Sandip Some ¹, Animesh Thakur ¹

Affiliations
1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata – 700 019, IN

Abstract

Zincian spinels (gahnites) from the Mamandur Zn-Pb-Cu prospect of the Southern Granulite Terrain have been studied. Gahnites in the quartzofeldspathic gneiss occur either as (a) porphyroblastic grains closely in association with cordierite and sphalerite or as (b) inclusions in poikiloblastic quartz grains, restricted within quartz veinlets. Compositionally these gahnites belong to two different clusters corresponding to two modes of occurrences. The origin of the porphyroblastic gahnites is linked with the process of desulphidation of sphalerite whereas those occurring as inclusions within poikiloblastic quartz are direct crystallisation products from silica rich hydrothermal solution. A new compositional field for this latter group of gahnites is proposed here.

Keywords

Gahnite, Cordierite, Desulphidation, Mamandur, Base Metal.

Full Text