RARE-EARTH ELEMENT EVIDENCE FOR SYNGENETIC ORIGIN OF AN ARCHEAN STRATIFORM GOLD SULFIDE DEPOSIT, KOLAR SCHIST BELT, SOUTH-INDIA

Both vein-type gold-bearing quartz deposits and stratiform-type gold s ulfide deposits occur in the Archean Kolar schist belt. We present her e the results of rare earth element (REE) studies on the banded iron-f ormation-hosted Mallappakonda gold sulfide deposit from the south Kola r schist belt. Sulfides in this deposit constitute about 25 vol percen t of the ore and consist of approximately equal amounts of pyrrhotite and arsenopyrite with minor loellingite. The deposit has on average 3 ppm gold which occurs as inclusions within arsenopyrite. Sulfides, sil icates, and oxides exhibit metamorphic intergrowth textures. Arsenopyr ite thermometry yields temperatures of 540-degrees +/- 10-degrees-C, a nd is compatible with regional lower to middle amphibolite facies meta morphism. The major and trace element geochemistry of the Mallappakond a gold sulfide deposit is similar to that of many sulfide facies bande d iron-formations associated with Archean volcanic-dominated greenston e belts. The chondrite-normalized REE patterns for the Mallappakonda o re 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 p ositively with iron contents, but the REE patterns do not depend on mi neralogy or mineral proportions. The REE patterns for whole ore and su lfide 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 th e range of values (1.5-8.5) observed for the amphibolites in the belt. The REE patterns of the Mallappakonda gold sulfide deposit are distin ctly different from those of the epigenetic vein-type gold quartz depo sits in the Kolar schist belt, but they are similar to those of the ho st banded iron-formation and to those of banded iron-formation in othe r Archean greenstone belts. In many geochemical features, more so in R EE 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 limite d isotopic data suggest that the Mallappakonda gold sulfide deposit co uld have had a syngenetic volcanic exhalative origin.