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

C. Narasimha Murthy ¹, C. Naganna ¹, M. S. Sadashivaiah ¹

Affiliations
1 Department of Geology, Karnatak University, Dharwar, IN

Abstract

The quartz-dolerite dykes occurring in the Shirahatti area have been chemically analysed and a detailed petrological and mineralogical investigation has been made. The mineralogical investigation of the feldspars and pyroxenes has been done on the 4-axes Universal stage after the methods of Reinhard, Burri, Hess, Nemato, Turner, and others. The nature of the feldspars and the twin laws confirm the views of Gorai. The optical study of the pyroxene reveals the presence of enstatite, pigeonite and sub-calcic augite. The petrochemical. investigation indicates that the basic magma which has given rise to these dolerites had undergone fractional crystallization resulting in the enrichment of acidic residuum and volatiles. The deuteric and hydrothermal alterations seen in the dolerite dykes are due to the action of volatiles in the final stages of the consolidation of the basic magma.

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C. Naganna ¹

Affiliations
1 Department of Geology, Karnatak University, Dharwar, IN

Abstract

The rocks of St. Mary Islands consist of dacite, rhyodacite and granophyre, showing close similarity in texture, mineralogy and petrochemistry, to similar rocks described from the islands of Salsette and Bassein, near Bombay. Based on the petrological study, it is surmised that the rocks of these islands are genetically connected with the Deccan Trap igneous activity. The study suggests a wider Deccan Trap igneous activity extending to several hundred kilometres south of the now known outcrops of trap. It also hints the association of a rhyolite magma with Deccan Trap igneous activity.

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K. R. Subrahmanya ¹, C. Naganna ¹

Affiliations
1 Department of Geology, Bangalore University, Bangalore, IN

Abstract

Structural study of the Arsikere granite and the surrounding gneisses, has revealed that the former is intrusive into the latter. The planar and linear elements indicate that the granite was emplaced through injection of viscous magma in a north-easterly direction. The possibility that the Arsikere and Banavar granites form cupolas of a single batholith is also indicated.

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V. N. Vasudev ¹, C. Naganna ¹

Affiliations
1 Department of Geology, Central College, Bangalore, IN

Abstract

The gold deposits at Hutti situated on the north western fringe of Hutti-Maski Schist belt are of gold-quartz-sulphide reef type. A detailed study has shown three distinct stages of mineralization. Minerals formed during the first stage are rutile, pyrite (with traces of gold) arsenopyrite, specularite, quartz (reef and vein type) and carbonate. Minerals of the second stage are pyrrhotite. chalcopyrite, sphalerite. galena and gold. Minerals of the third stage are pyrite, calcite and quartz found along numerous minor faults (slips) and major faults. Based on the textural and associational criteria, three generations of gold are distinguished. They are: 1. Gold-pyrite association. 2. Gold-aresenopyrite-quarrz association. 3. Gold-sphalerite-quartz association. The study has further shown that only the gold of the second and third generations constitute the lode. Gold is in the native state and no tellurides are present.

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