Geochemical constraints of bulk rocks and in-situ zircon from the granitoids of the Hungund Schist Belt: An insight for hydrothermal activity

Orogenic, gold hosting greenstone belts in Eastern Dharwar Craton (EDC), India, are surrounded by granitic bodies and affected by hydrothermal activities. In this paper, field, petrographic and geochemical study of granites coupled with LA-ICP-MS analyses of the trace element abundances of zircons from the granites of the Hungund Schist Belt, the northwest continuation of Kolar and Ramgiri schist belts in EDC is discussed to report the effects of hydrothermal alteration. The geochemical characteristics such as Nb/Ta ratio (~5), low Zr/Hf ratio, depleted ∑REE from the granites close to the shear zone provide clue towards magmatic–hydrothermal interactions in the source rock, while granites away from the shear zone show feeble alteration signature. Zircons from the altered granite contain higher concentration of trace elements, LREE enrichment and high ∑REE with weak positive Ce anomalies and strong negative Eu anomalies indicating hydrothermal alteration. While the zircons from unaltered granites have low to moderate trace element concentration and REE patterns similar to unaltered magmatic zircons. Contrarily, in the ternary plot, the zircons from unaltered granites show minor affinity towards metasomatised hydrothermal field instead of clustering under purely magmatic field. Hence it is considered that these zircons from unaltered granite underwent some minor alteration effect, while the zircons from altered granite are affected completely due to hydrothermal activity. Therefore, it is concluded that movement of hydrothermal solution has taken place along the shear zone in the study area, which has affected more on granites close to the shear zone and minor effects are seen in granites away from the shear zone. Hydrothermal alterations often result in concentration of specific economic minerals and hence, can be used as an indicator for mineral exploration. Research Highlights Petrography and geochemistry of granites and trace element analysis of zircons from these granites of Hungund schist belt are presented. The highlights are: Sericitization, chloritization and epidotization are principle signatures observed in hydrothermally altered granites. Nb/Ta ratio (~5), low Zr/Hf ratio, depleted ∑REE from the granites close to the shear zone suggests magmatic-hydrothermal interactions in the source rock, while the granites away from the shear zone do not reflect any significant alteration signature. Zircon geochemistry from the altered granite reveals that it contains higher concentration of trace elements, shows LREE enrichment and high ∑REE with weak positive Ce anomalies and strong negative Eu anomalies indicating hydrothermal alteration, whereas the zircons from unaltered granites have low to moderate trace element concentration and also REE patterns similar to unaltered magmatic zircons. The zircons from unaltered granites are not purely magmatic zircons and have undergone some minor alteration effect which is also evident from the LREE overabundances. Therefore, we conclude that movement of hydrothermal solution has taken place in the study area along the shear zone, showing prominent effects on the granites close to the shear zone and minor effects to those away from the shear zone. Such hydrothermal alteration signatures can be used as an indicator for mineral exploration.