Gold mineralization in the Hutti mining area, Karnataka, India

Abstract

Economic deposits of gold associated with sulfides occur in the Precambrian (Dharwar) rocks of the Hutti-Muski schist near Hutti village (16 degrees 12' N, 70 degrees 39' E) in the northern part of Karnataka State and have been exploited since 1904. The principal mine has produced about 22,500 kg of gold. The average yield, which was 19.49 g per ton of ore in 1920 has now decreased to an average of about 6.3 g per ton. The auriferous lodes, confined to the schist belt composed of metabasalts and massive chert bands, comprise vein quartz with parallel to subparallel, narrow, linear layers of chlorite-biotite schist. The entire schist belt has been intruded by granite and aplite in its western and northern parts. Of nine essentially parallel gold-quartz-sulfide lodes only six are being mined. The lodes are localized along narrow zones of highly sheared chlorite-biotite schist. Structurally the schist belt appears to represent an original isoclinal fold, with its axis striking north-northwest-south-southwest, which has suffered later cross folding. There is evidence of at least three periods of folding. The area is also traversed by a major postmineralization fault trending N 70 degrees W-S 70 degrees E. On a regional scale ore mineralization exhibits stratigraphic, lithologic, and predominantly structural controls. Mineralogically, the ores are composed of gold, sulfides, and oxides. The gangue minerals are quartz, chlorite, biotite, sericite, actinolite, hornblende, tourmaline, calcite, siderite, and ankerite. Textural studies indicate that in the first stage, rutile, pyrite containing inclusions of gold, arsenopyrite, specularite, quartz, and carbonates were formed. During the second stage, pyrrhotite, chalcopyrite, sphalerite, galena, and gold were deposited. During the last stage, pyrite, calcite, and quartz were formed. Further, the studies also suggest that gold deposition occurred mainly during the second stage. The gold occurs in the native state and contains silver up to a maximum of 9 percent, copper up to 0.05 percent, and traces of iron, nickel, and cobalt. Genetically, the gold lodes appear to have been formed by remobilization of the metals present in the mafic volcanic rocks during late periods of tectonic disturbance.