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

J. Mallikharjuna Rao ¹, S. Nirmal Charan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

The diamond-rich Pipe-7 kimberlite found about l0km east of Lattavaram in Anantapur district, Andhra Pradesh in south India is an elongated dyke - like body, that extends over a length of 450 m and has a maximum width of 25 m. It is porphyritic in texture and contains sub-angular to rounded xenoliths of peridotite, garnet-clinopyroxene rocks, websterite, amphibolite, metabasic rocks and granite. Petrographic studies reveal that it is porphyritic in texture and contains euhedral to sub-hedral altered olivine. phlogopite, garnet, clinopyroxene (chrome diopside), chromite, perovskite and ilmenite which are the uneqvivocal primary liquitlus phases whereas serpentine, chlorite, phlogopite, calcite, amphibole, magetite, monticellite, clay minerals and others represent secondary alteration assemblages. Chrome-diopsides and garnets represent an important megacryst suite.

The Pipe-7 kimbeerlite has higher CaO and lower MgO, Al₂O₃, K₂O and iron contents compared to rhe other six known kimberlites in the vicinity. Fe/Mg ratio in coexisting garnets and clinopyroxenes, indicates the equilibration temperature for (a) the nodules to be 1040 - 1070°C and for (b) the megacrysts of garnet and clinapyroxene to be 1350°C respectively.

Pyrope garnets from titis kimberlite show a positive correlation between CaO and Cr₂O₃ and are similar in nature to those of the low calcium garnets reported from the diamondiferousSouth African kimberlites.

Keywords

Kimberlite, Petrography, Geochemistry, Anantapur Dist, Andhra Pradcsh.

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D. V. Subba Rao ¹, S. Nirmal Charan ¹, B. L. Narayana ¹, S. M. Naqvi ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500 007, IN

Abstract

Occurrence of fine-grained, fingerprint-like symplectic intergrowths of cordierite, K-feldspar and quartz formed due to the retrogressive, garnet breakdown reactions following decompression during uplift, is reported here from metapelites of Usilampatti area. These metapelites consisting of cordierite, garnet, hypersthene, biotite, hercynitic spinel, sillimanite, K-feldspar assemblage were metamorphosed at 6.5-7.5 Kb and 750-800°C. The symplectites were formed at 600-670°C and 5 Kb pressure. The textural and mineralogical evolutions in these metapelites indicate a rapid decompressional P-T evolution. Petrological and geochemica1 data suggest that these metasediments were derived mostly from an evolved basement source of granodiorite-granite composition with a minor basic component.

Keywords

Metamorphic Petrology, Gneiss, Tamil Nadu.

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R. M. K. Khan ¹, S. Nirmal Charan ¹, M. Arora ¹, S. M. Naqvi ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500007, IN

Abstract

The iron-formations ofKudremukh mainly contain magnetite, recrystallized chert, gruneritecu mmingtonite (X_Mg 0.21 to 0.33), hornblende, bioti te along with minor amounts of ankerite (X_Mg O. 30 to 0.52), calcite (X_Mg 0.06 to 0.22), chamosite and pyri teo Chamosite, a rare relict mineral, was probably formed by reaction between high Al-clay and Fe. Magnetite was formed during metamorphism. Along with the precipitation of alternating layers rich in silica and iron, clayey material in the form of montmorillonite, illite and/or glauconite along with Fe-Mg carbonates were also deposited, which after metamorphism (430-520°C) gave rise to the present mineralogy.

Keywords

BIF, Mineral Chemistry, Kudremukh Schist Belt, Karnataka.

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D. V. Subba Rao ¹, S. Nirmal Charan ¹, S. Moeen ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500007, IN

Abstract

Sapphirine is found in the pelitic gneisses of Kosavankovilpatti near Usilampatti in Tamil Nadu. It is intimately associated with melt products and the associated minerals are garnet, biotite, orthopyroxene, plagioclase, mesoperthite and K-feldspars. The sapphirine-bearing rock is characterised by (i) enrichment of MgO, depletion of SiO₂ (ii) absence of coronas and symplectites, (iii) presence ofmesoperthites (iv) high X_Mg values of the associated minerals and (v) high P-T conditions (855±20°C and 8.0±O.3 Kb) in contrast to the adjacent sapphiri ne absent rocks (750-800° and P of 6.5-7.5 Kb). This occurrence of prismatic sapphirines is in contrast with the reported sapphirine occurrences where it occurs as symplectites and coronas defining a decompression path (T= 700°C and p 5-7 Kb) in the adjacent Kodaikanal massif. It is suggested that the sapphirine being reported is of prograde origin.

Keywords

Petrology, Precambrian, Sapphirine, Metamorphism and Geothermobarometry.

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R. H. Sawkar ¹, Rahul Baldota ², S. R. Suresh ², P. T. Sastri ², A. S. Kulageri ³, B. R. Nagabhushan ³, D. S. Sarma ⁴, D. V. Subbarao ⁴, S. Nirmal Charan ⁴, S. M. Naqvi ⁴

Affiliations
1 Bangalore, IN
2 MSPL Ltd., Hospet, IN
3 Hutti Gold Mines Co. Ltd., Chitradurga, IN
4 National Geophysical Research Institute, Hyderabad, IN

Abstract

No Abstract.

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B. Kumar ¹, S. Nirmal Charan ¹, Malti Goel ²

Affiliations
1 National Geophysical Research Institute, Hyderabad-500007, IN
2 Department of Science & Technology, New Delhi-110016, IN

Abstract

No Abstract.

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P. V. Sunder Raju ¹, S. Nirmal Charan ¹, D. V. Subba Rao ¹, B. Uday Raj ¹, S. M. Naqvi ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

Gold mineralisation occurs at Chikkasiddavanahalli (C S Hall!) in Archaean volcano-Sedimentary sequence of the Ingaldhal Formation, in Chiradurga Group, Dharwar craton. The C S Hal1i ridges and low mounds are about 1 2 km long and 100 in wide with N-S strike and vertical to steep easterly dips comprising mixed sulphide/oxide facies banded iron formations (MSOBIF) and associated ferruginous phyllites, metavolcanics and minor carbonates. Several horizons of BIF's occur as parallel to subparallel linear brands and extend over a strike length of 20-40 km with widths ranging from 2-25 m. These BIF s are highly sheared, deformed and metamorphosed to upper greenschist facies. The shears in BIF's are occupied by several syn-Tectonic quartz carbonate veins of various dimensions and found to be favourable loci for Gold-Sulphide mineralisation. Petrological studies show major mineral constituents with pyrite, arsenopyrite and subordinate grunerite, hornblende, chlorite, muscovite, ankerite, calcite, magnesian siderite and quartz. Most gold deposits world over are hosted in second order and subsidiary shear zones. The primary gold in BIFs is enriched with intense fracturing controlled by major shear systems extending from Gadag in the north and Doduguni in the south Active convergent margin settings related to accretionary processes and fluid activity are suggested to have led to Epigenetic gold mineralisation in BIFs at C S Halli area with an average gold grade of 2 08 g/t.

Keywords

Au Mineralisation, Structural Controls, Shear Zones, C S Halli, Chitradurga Schist Belt, Karnataka.

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