Deformation history and P-T condition of metamorphism of Gadag greenstone belt of western Dharwar craton, southern India: Implications on the genesis of orogenic gold mineralisation

The Late Archean Gadag greenstone belt (GGB) in the northern continuity of extensive Chitradurga greenstone belt in the western Dharwar craton has extensive meta volcano-sedimentary cover apart from older TTGs (Tonalite-trondhjemite-granodiorite), pink granite and other intrusives. The GGB encompasses a very unique and classical orogenic type gold deposit known as Gadag gold field (GGF) where auriferous lodes are hosted by metabasalt, greywacke and BIFs in western, central and eastern part of GGF respectively. In this study, we focus on the deformation history and P-T conditions of metamorphism of greenstone rocks which could have influenced in the generation of auriferous fluid in GGB. Two events of deformation were suggested in GGB so far where the first event (D1) is represented by NW-SE trending pervasive fabrics (S1) and schistocity, whereas the second event (D2) marking the regional disposition of NNW-SSE tectonic trend. This is further corroborated by microscopic evidences of Q-M microlithons development and garnet porphyroblast growth that is post tectonic to D1 deformation, syn-tectonic to early D2 and stage 2 porphyroblast development of non-rotating porphyroblast model in garnetiferous amphibolite in the north-eastern part of GGB. However, the matrix continued to get deformed and reached stage 6 in garnetiferous schist with dominant sinistral sense of shear in the southern part of GGB. The estimated peak metamorphic temperature of garnetiferous amphibolite using hornblende-plagioclase thermobarometry is 578 ± 21°C and the pressures are 7.1 ± 3kb and 7.4 ± 2.3kb. Garnet-biotite thermometric estimation for the same garnetiferous amphibolite from north eastern part of GGB yielded 525 ± 25°C indicating a lower amphibolite facies of metamorphism. The auriferous quartz veins were associated with both the D1 and D2 phases of deformation, and the genesis of this auriferous fluid may be related to lower amphibolite facies of metamorphism of these host rocks as evidenced from the fluid inclusion study.

Keywords: craton, P-T conditions, thermobarometry