40Ar-39Ar age constraints on timing of a late hydrothermal event in the Hutti greenstone belt, Karnataka, India

Abstract

Obtaining constraints on the timing of orogenic gold mineralization hosted in shear zones in greenstone belts is often difficult because of the lack of easily datable minerals and problems of overprinting of earlier events of mineralization by later hydrothermal events. In the orogenic Hutti gold deposit the U-Pb SHRIMP date obtained from a monazite grain within arsenopyrite that gave an age of 2547 ± 10 Ma, is generally accepted as the age of early hydrothermal mineralization. Nevertheless, other work has indicated that hydrothermal mineralization may have occurred in later episodes as well. This study reports new 40Ar-39Ar ages for hydrothermal biotite, amphibolite and hornblende in granites that occur along the northern and southeastern boundary of the greenstone belt. The 40Ar-39Ar ages of biotite samples are inferred to be the result of re-setting of the ages of alteration biotite by the last phase of hydrothermal activity in this area at ∼2332 Ma. The study indicates that the effects of the thermal event are more significant in areas of greater hydrothermal access such as within the Hutti shear zone. This late hydrothermal alteration event may be related to the D3 deformational event of Roy (1991). The results of the present work are important because it demonstrates a significant hydrothermal event much after the major 2.58–2.52 Ga accretionary event and accompanying TTG and felsic magmatism in the Eastern Dharwar Craton.