Rare earth element evidence for syngenetic origin of an Archean stratiform gold sulfide deposit, Kolar schist belt, South India

Abstract

Both vein-type gold-bearing quartz deposits and stratiform-type gold sulfide deposits occur in the Archean Kolar schist belt. We present here the results of rare earth element (REE) studies on the banded iron-formation-hosted Mallappakonda gold sulfide deposit from the south Kolar schist belt. Sulfides in this deposit constitute about 25 vol percent of the ore and consist of approximately equal amounts of pyrrhotite and arsenopyrite with minor 1oellingite. The deposit has on average 3 ppm gold which occurs as inclusions within arsenopyrite. Sulfides, silicates, and oxides exhibit metamorphic intergrowth textures. Arsenopyrite thermometry yields temperatures of 540 degrees + or - 10 degrees C, and is compatible with regional lower to middle amphibolite facies metamorphism.The major and trace element geochemistry of the Mallappakonda gold sulfide deposit is similar to that of many sulfide facies banded iron-formations associated with Archean volcanic-dominated greenstone belts. The chondrite-normalized REE patterns for the Mallappakonda ore show both light REE and heavy REE enrichment with a strong positive Eu anomaly. The REE abundances vary by a factor of 10 and correlate positively with iron contents, but the REE patterns do not depend on mineralogy or mineral proportions. The REE patterns for whole ore and sulfide separates are similar. The sulfide ore and the host banded iron-formation have epsilon_Nd values at 2700 Ma (4.01 and 2.54) within the range of values (1.5-8.5) observed for the amphibolites in the belt.The REE patterns of the Mallappakonda gold sulfide deposit are distinctly different from those of the epigenetic vein-type gold quartz deposits in the Kolar schist belt, but they are similar to those of the host banded iron-formation and to those of banded iron-formation in other Archean greenstone belts. In many geochemical features, more so in REE and gold, the deposit is comparable to hydrothermal vent fluids and proximal hydrothermal metalliferous sediments at modern ocean ridges. Several lines of evidence including geologic, geochemical, and limited isotopic data suggest that the Mallappakonda gold sulfide deposit could have had a syngenetic volcanic exhalative origin.