Dharwar stratigraphy and gold mineralisation in Kolar supracrustal belt Karnataka craton India.

Abstract

The Karnataka Craton is composed of four chief crustal components, viz., the Peninsular Gneiss Complex (PGC), the Sargur Supercrustals, the Dharwar Supercrustals, and the younger granites. The craton is roughly divided into a Western and an Eastern block by the Closepet Granite batholith. This study aims to correlate the stratigraphy of the two blocks and its relationship to gold mineralisation in the Craton. Extensive survey of the gold occurrences of both the blocks has been carried out to show their restriction to the Dharwar Supercrustals and to bring out the differences in the mode of mineralisation. A detailed study of the Kolar Belt has been made in an attempt to establish a genetic model for mineralisation. The PGC, of tonalite-trondhjemite-granodiorite composition, unconformably underlies the late-Archean, early-Proterozoic mafic platformal and geosynclinal succession of the Dharwar Supercrustals. The analysis and synthesis of geological and geochronological data has helped establish the chronostratigraphic relationships between the chief crustal components of the Western Block and this is presented in Table 1. Table 1: Chronostratigraphy of the Western Block Components • Dharwar Supergroup o Chitradurga Group: Deep-water, shelf and geosynclinal facies. o Bababudan Group: Shallow-water mafic platformal facies. o Age: 2345 Ma, 2620 Ma. • • Submarine Fe-rich tholeiitic volcanism took place around 2700 Ma, with resulting basalt–seawater reaction giving rise to the Oriental- and BIF-type stratiform and stratabound chemical sediments. • • The resulting hydrothermal solutions, typically rich in CO? and H?S, leached gold and sulphur from the host rock and transported them as thio-complexes at a temperature of around 350°C. • • Deposition of gold took place due to hydraulic fracturing, causing a sudden drop in pressure and resulting in the sulphidation of the wall rock, change in pH, and a drop in temperature accompanied by a loss of volatiles. This happened around 2650 Ma. • • Later deformation and metamorphism, between 2300 Ma and 1930 Ma, resulted in remobilisation and concentration of gold. • Peninsular Gneiss o Tonalite-trondhjemite-granodiorite components. o Age: 3000–3200 Ma. • Sargur Group o Ultramafic-mafic volcanics and minor anorthosites linked to orthoquartzites, metapelites, amphibolites, banded ironstones. o Age: 3095–3180 Ma. • Gorur-Hassan Gneiss o Tonalite-trondhjemite-granodiorite components. o Age: 3300 Ma. In the Eastern part, the PGC yields an Rb-Sr age close to 3000 Ma and the intrusive granites date at around 2600 Ma. The cordierite gneiss enclaves, dated by Rb-Sr method at 3010 Ma, are correlated with the Sargur Group. The Kolar supracrustal succession yields a Pb-Pb age of 2700 Ma and is considered a facies variation of the Bababudan Group. The metavolcanic succession of Hutti dates at about 2350 Ma, which is close to that of the Chitradurga Group. The study of the evolutionary history of the Dharwar belts suggests a number of broad controls on the distribution pattern of the gold deposits. The Sargur Group is almost barren of gold probably because of early silicification, absence of suitable fluid components and unfavorable metamorphic style. The mafic sequences of the Bababudan Group are subaerial and correspond to sulphide- and gold-depleted source rocks and do not contain any significant gold. The low-grade ore in the deposits of the western Dharwar belts can be attributed to the dominance of sediments and low-grade metamorphism. The volcanic dominant belts of the eastern Dharwar are shown to have the right combination of rock types and metamorphic history to offer excellent conditions for gold mineralisation. Based on the detailed study of the underground workings of the Kolar Gold Field, a viable genetic model for gold mineralisation is proposed. The lithostratigraphy of the Kolar supracrustal succession, evolved during the course of this investigation, is summarised in Table 2. Detailed petrographic studies of the metavolcanic succession reveal that they are fine- to coarse-grained. The pillow, variolitic and amygdular structures preserved in the metabasalts suggest a predominantly subaqueous environment of eruption. Additional evidences for this are the quench plagioclase textures and glomeroporphyritic basalts which have been identified. Mineralogical studies show that the basalts have been metamorphosed to amphibolite grade.