ORE MINERALOGY OF WESTERN AURIFEROUS ZONE OF GADAG GREENSTONE-BELT, KARNATAKA

Gadag greenstone belt, which has a history of ancient gold mining, hos ts three distinct auriferous zones viz., Western Zone, Middle Zone and Eastern Zone distributed both in metavolcanic and metasedimentary sui tes. The paper presents ore mineralogy of the Western Auriferous Zone which is hosted by pillowed and sheared metavolcanic rocks. Based on t he relative abundance of quartz veins and sulphide minerals this zone could be classified into Type-A ore and Type-B ore. Type-A ore compris es white quartz veins (30-90 cms thick) displaying evidence of early d eformations. They contain chloritic/micaceous leafy aggregates and tra ces of arsenopyrite, pyrite, chalcopyrite and sphalerite +/- scheelite ; gold occurs in native form in vein quartz. Type-B ore comprises main ly chlorite-muscovite/sericite-ankerite- monazite sulphide schist with subordinate vein quartz. Associated sulphides are arsenopyrite, pyrit e, pyrrhotite, chalcopyrite, sphalerite and galena in the order of dec reasing abundance. Scheelite is seen occasionally. Gold occurs as: (i) native gold in silicate and quartz gangue, (ii) native gold inclusion s and microfracture fillings in arsenopyrite and (iii) in combined sta te in arsenopyrite. Arsenopyrite could serve as a main pathfinder mine ral for exploration, mining as well as processing of Type-B ore. Miner alogical characteristics, wall-rock alterations and structural feature s related to Type-A ore, Type-B ore and host rocks characterise the go ld mineralisation in the Western Auriferous Zone as structure controll ed hydrothermal epigenetic lode.