Reply to Discussion on ?Dissolution channels in quartz and the role of pressure changes in gold and sulfide deposition in the Archean, greenstone-hosted, Hutti gold deposit, Karnataka, India? by Kolb J, Rogers A and Meyer FM

Abstract

The discussion on our paper (Pandalai et al. 2003) by Kolb et al. (2003, DOI 10.1007/s00126-002-0345-9) raises important issues related to the genesis of the Hutti gold deposit. The main points raised by Kolb et al. (2003) relate to the estimation of P and T of ore formation and the chronological (and genetic) order of vein formation in the shear zones at Hutti as identified by them. Rogers et al. (2002) and Kolb et al. (2003) recognize five stages of deformation (D1–D5), one prograde (M1) and two retrograde (M2, M3) stages of metamorphism, and four stages of quartz vein formation with attendant alteration. Rogers et al. (2002) and Kolb et al. (2001, 2003) do not refer to previously published literature on structural studies of the Hutti–Maski schist belt. Nevertheless, the deformational stages identified by Kolb et al. (2003) need to be placed in the framework of the regional tectonic history of the Hutti–Maski belt. Structural studies on this belt are reported in Roy (1979, 1991) and Biswas (1990). These studies identify three phases of deformation and metamorphism that are codified here as D1 , D2 , D3 * and M1 , M2 * , M3 * respectively to distinguish them from those of Kolb et al. (2003). A brief summary of the structural framework is given below before discussing the points raised in Kolb et al. (2003). The first deformational stage (D1 ) led to the development of tight isoclinal (F1 ) folds, penetrative schistosity (S1 ), synchronous prograde metamorphism (M1 ) and syntectonic emplacement of tonalite and tronjhemite around the Hutti–Maski belt. Additionally, mylonite and mylonitic foliation (Sm ) developed in narrow shear zones. On account of the parallelism of Sm * with S1 * it is inferred that they developed during the phase of D1 * deformation. The prograde M1 * phase of metamorphism is low grade (greenschist facies), but rises to medium grade (amphibolite facies) with development of garnet and cordierite in the north-eastern peripheral zone of the greenstone belt. The D2 * deformation stage gave rise to small and large-scale (F2 ) folds with subvertical axial planes (S2 ) and axes that plunge 50 to 60 NW. F2 folds bring F1 isoclinal folds into parallelism with F2 * limbs and are responsible for the map pattern of the exposed rocks. S2 *