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

A. G. Ugarkar ¹, S. G. Tenginkai ¹

Affiliations
1 Department of Studies in Geology. Karnatak University, Dharwad 580003, IN

Abstract

Gold mineralization of gold-quartz vein-type is confined to narrow shear zones in the metabasic volcanic suite of the Mangalur Greenstone Belt. The dominant sulphide phase associated with the gold mineralization is pyrrhotite-pyrite assemblage followed by arsenopyrite, chalcopyrite and sphalerite in the decreasing order of abundance. Gold occurs in native state and within quartz.

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A. G. Ugarkar ¹, T. C. Devaraju ¹

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580003, Karnataka, IN

Abstract

Gadag greenstone belt, which has a history of ancient gold mining, hosts three distinct auriferous zones viz., Western Zone, Middle Zone and Eastern Zone distributed both in metavolcanic and metasedimentary suites. The paper presents ore mineralogy of the Western Auriferous Zone which is hosted by pillowed and sheared metavolcanic rocks. Based on the relative abundance of quartz veins and sulphide minerals this zone could be classified into Type-A ore and Type-B ore.

Type-A ore comprises white quartz veins (30-90 cms thick) displaying evidence of early deformations. They contain chloritic/micaceous leafy aggregates and traces of arsenopyrite, pyrite, chalcopyrite and sphalerite ± scheelite; gold occurs in native form in vein quartz.

Type-B ore comprises mainly chlorite-muscovite/sericite-ankerite-monazite sulphide schist with subordinate vein quartz. Associated sulphides are arsenopyrite, pyrite, pyrrhotite, chalcopyrite, sphalerite and galena in the orderof decreasing abundance. Scheelite is seen occasionally. Gold occurs as: (i) native gold in silicate and quartz gangue, (ii) native gold inclusions and microfracture fillings in arsenopyrite and (iii) in combined state in arsenopyrite. Arsenopyrite could serve as a main pathfinder mineral for exploration, mining as well as processing of Type-B ore.

Mineralogical characteristics, wall-rock alterations and structural features related to Type-A ore, Type-B ore and host rocks characterise the gold mineralisation in the Western Auriferous Zone as structure controlled hydrothermal epigenetic lode.

Keywords

Gold, Sulphides, Ore Mineralogy, Greenstone Belt, Gadag, Karnataka.

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T. C. Devaraju ¹, Ali Khoshroo ², A. G. UGARKAR ¹

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580003, IN
2 Aerodynamic Research Center, P.O. Box 19584/449, Tehran, IR

Abstract

A dozen clay deposits occurring in the districts of Belgaum, Dharwad and Chitradurga of Karnataka State have been studied. ln Dharwad district, while the clay deposits derived from greywackes/phyllites are dominated by kaolinite, those derived from the acid metavolcanics contain illite as the sole clay mineral. In Belgaum and Chitradurgadistricts the deposits are kaolinitic, characterised by poor crystallinity and near absence of illite.

Chemically, the clay deposits studied have relatively high silica and low titania. The transformation of precursor rocks to clay deposits which was brought about by insitu chemical weathering and subsequent reworking of the weathered products under tropical climatic conditions was accompanied by enrichment of Al, Zr and Cr and removal of Sit Fe. Ni, Cu and Zn. Presence of relicts of nearly all the major and accessory minerals native to the precursor rocks, in all the litho-units of clay profiles is in agreement with such an interpretation.

Keywords

Economic Geology, Clay, Geochemistry, Mineralogy Chitradurga, Dharwad, Belgaum, Karnataka.

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M. V. Koti ¹, S . G. Tenginkai ¹, A. G. Ugarkar ²

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad-580003, IN
2 Department of Environmental Science, Bangalore University, Bangalore- 560056, IN

Abstract

Copper mineralization in Machanur is confined to the fracture and breccia zone (shear zone) traversing mainly pink porphyritic granite of the Closepet granite suite (2,400-2,000 my.) including minor diabase dykes and pegmatites, The mineralization is fractule-controlled cavity filling epiigenetic hydrothermal type. The associated wall-rock alterations are chloritization, saussuritizalion, sericitization and silicification of the host-rocks. The  mineralization consists essentially of chalcopyrite and pyrile with subordinate bornite, chalcocite, covellite, digenite and linnaeitc. Native copper occurs but rarely. Hematite(specularite) and magnetite are commonly associated with the mineralization. Three stages of mineralization have been inferred. It is suggested that the second stage has been dominated by precipitation of chalcopyrite. The paragenetic scquence is suggestive of high fO₂ condition in fluid at the beginning and progressive increase of Cu/Fe ratio and high fS₂ condition in fluid during the main Cu deposition stage.

Keywords

Copper, Ore Mineralogy, Paragenesis, Economic Geology, Machanur, Karnataka.

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