GENESIS OF GOLDMINERALIZATION IN HIRA-BUDDINI, HUTTI SCHIST BELT, KARNATAKA, INDIA

Abstract

Hira-Buddini gold deposit is located in the eastern margin of the Hutti Schist Belt (also known as Hutti-Maki Schist Belt). Amphibolite is the major rock type in the area with a linear patch of felsic volcanic rock. Gold mineralization is associated with an auriferous lode which is almost parallel to the contact of amphibolite and felsic volcanic rock. The lode strikes along N75°-N255° and dips due north. The lode is about 600 m in length and thickness varies from 1 to 3.5m. Currently there are five underground levels in the mine and the approximate depth of mining is 150 m. There are four types of quartz veins associated with gold mineralization in the auriferous lode. The veins contain slivers of wall rock which occur either as continuous bands (of mm to cm scale) or as brecciated fragments alternating with layers of quartz-rich vein material. Apart from quartz, the other vein minerals include plagioclase (albite), muscovite, calcite, chlorite, tourmaline and sulphide minerals. Quartz and plagioclase grains of the veins exhibit microstructures of dynamic recrystallization/ intracrystalline deformation. The hydrothermally altered amphibolite proximal to vein is rich in biotite with an alteration assemblage of biotite-tourmaline-chlorite-calcite-sulphide minerals. The alignment of biotite in the altered amphibolite defines a strong foliation which is almost parallel to sub-vertical veins. Two types of alteration zones have been found in the altered felsic volcanic rock (i) muscovite – calcite rich zone represented by muscovite-sericite-calcite-chlorite assemblage and (ii) albite-biotite rich zone represented by albite -biotite-calcite (±muscovite) assemblage. Altered amphibolite and altered felsic volcanic rocks are associated with abundant sulphide minerals. Major sulphide minerals include pyrite, pyrrhotite and chalcopyrite. Gold is present as native grains and also in association with sulphide minerals in altered wall rocks. Veinlets of calcite that crosscut several quartz grains of vein are quite common. Such veinlets have brecciated fragments of vein quartz and are discordant to the foliation developed in altered wall rock. Quartz grains of the veins contain four types of primary fluid inclusions: i) carbonic (Type-Ia), ii) aqueous (Type-Ib), iii) aqueous-carbonic (Type-Ic) and iv) salt crystal bearing aqueous ployphase (Type-Id) fluid inclusions. Carbonic inclusions are the most abundant type of fluid inclusions. Fluid inclusions along the intragranular trails are mostly carbonic (Type-II). Presence of aqueous (Type-IIIa) and salt crystal bearing aqueous ployphase (Type-IIIb) fluid inclusions along the transgranular trails are quite common in the samples. Significant areas of quartz grains are devoid of fluid inclusions and even some quartz grains are entirely devoid of i fluid inclusions. Some fluid inclusions show textural evidence of re-equilibration and certain portions of quartz grains contain highly re-equilibrated fluid inclusions. Very small and stretched fluid inclusions are present along the grain boundaries of quartz. The temperature of homogenization of CO2 in primary carbonic fluid inclusions exhibit wide range in histograms. The salinity of aqueous biphase primary fluid inclusions varies from 0.35 to 20.15 wt. % NaCl equivalent. These inclusions homogenized into liquid state within a temperature range of 117-340°C. The aqueous-carbonic primary fluid inclusions have salinity between 0.02 and 11.61 wt. % NaCl equivalent. Total homogenization of these inclusions occurred into vapour state and the temperature varies from 212 to 327 °C. Most of the aqueous polyphase primary fluid inclusions homogenised by dissolution of salt crystal between 205 to 390, and the calculated salinity varies from 30 to 46 wt. % NaCl equivalent. Isocon analysis and mineralogical studies of unaltered amphibolite and altered amphibolite indicated that the auriferous lode is enriched in SiO2, K, Pb, LOI and S due to rock-fluid interaction during gold mineralization. The gold mineralization took place during a brittle-ductile deformation episode. The temperature is around 500 °C and pressure values vary from 2.05 to 4.36 kbar. Low-saline aqueous carbonic metamorphic and high saline aqueous magmatic fluids were involved during gold mineralization. Preferential loss of H2O from fluid inclusions due to intracrystalline deformation of quartz was responsible for abundance of primary carbonic fluid inclusions. Significant amount of gold deposition took place from gold sulphide complexes due to wall rock sulphidation. The gold mineralization may be related in time and space to Late-Achaean metamorphism, deformation and intrusion of granites in the Hutti Schist Belt.