The geologic setting, mineralogy, geochemistry, and genesis of gold deposits of the Archean Kolar schist belt, India

Abstract

The Kolar schist belt, one of the auriferous Archean schist belts in the Dharwar craton, includes two types of gold deposits: (1) a stratiform sulfide type, associated with amphibolites and banded ferruginous quartzite, and (2) a gold quartz-carbonate vein type associated with light rare earth element (LREE)-enriched komatiitic and tholeiitic amphibolites on the eastern part of the belt. Many lodes of the former are banded and show deformation similar to that of their host rocks. The latter type consists of veins localized within shear zones. Both types have only a few inches of altered border rock characterized dominantly by biotite with minor diopside and garnet. The sulfide lodes contain 5 to 30 vol percent sulfides which include dominantly pyrrhotite and arsenopyrite. The lodes also contain variable amounts of magnetite and ilmenite. No systematic variation in ore minerals is discernible among the lodes of the entire belt. However, in the Kolar gold fields area, a set of four parallel sulfide lodes exhibit certain textural and mineralogical zoning. Ore minerals show metamorphic equilibrium textures and the temperature of final equilibration of sulfides is 500 degrees C or higher. Sulfide lodes have low but variable abundances of base metals. No correlation among total sulfide contents, sulfide minerals, and abundances of base metals and gold is observed. Nor is there any geochemical coherence between gold and arsenopyrite in the several lodes studied. Major oxides, particularly Al₂O₃, TiO₂, and K₂O also show a large variation. The REE abundances of the sulfide lodes vary widely, but their patterns are similar, with enrichment in both light REE and heavy REE. Normalized Eu shows strong to negligible enrichment. The REE patterns are similar to those of the banded ferruginous quartzite or iron-formation. Samples with high REE abundances also have high base metals, Al₂O₃, TiO₂, and K₂O abundances and low arsenopyrite contents. Based on these observations and limited Nd and O isotope data it is suggested that the sulfide lodes were originally auriferous, complex interflow sediments formed from submarine hydrothermal exhalations, seawater, and detrital sediments.The vein-type mineralization has only trace amounts of sulfides with galena as the dominant phase (Narayanaswami et al., 1960). The veins consist dominantly of quartz with minor amounts of calcite and alkali feldspar and trace amounts of scheelite and tourmaline. The gold content of the veins is generally >10 ppm. The veins are also unusually enriched in Cr and Ni. These observations and the fluid inclusion studies on vein quartz (Santosh, 1986) suggest the presence of CO₂ -bearing ore fluids. One sample of the ore vein has a strongly fractionated, light REE-enriched pattern with an epsilon_Nd value of +2.8 at 2,600 Ma. Galena from the ore samples have Pb isotope ratios, reported by Venkatasubramanian et al. (1977) and Chernyshev et al. (1980), that are similar to those of the 2,600-Ma intrusive granodioritic gneisses present on the west side of the schist belt (Krogstad et al., 1989). The delta¹⁸O of vein quartz is 11.6 + or - 0.7 per mil (Golding, 1982). These geochemical data suggest that the gold quartz veins were formed dominantly from magmatic fluids derived from the crystallization of granitic rocks. A contribution from metamorphic fluids is also conceivable as indicated by the abundance of Ni and Cr in the veins.