Geochemistry of host rocks and role of organic matter in the precipitation of cassiterite-sulfide ores at the Dafulou deposit, Dachang tin field (south China)


Authors: Pašava J, Kříbek B

Published in: Bulletin of Geosciences, volume 76, issue 1; pages: 3 - 14;

Keywords: Dafulou cassiterite-sulfide deposit, South China, Geochemistry, Organic matter, Precipitation of ores,

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The Dafulou vein and stratabound cassiterite-sulfide deposit is located in the eastern part of the Dachang ore district. The deposit is hosted by a thick sequence of the Lower Devonian calcareous black shales. Geochemical study has shown an apparent increase of Sn, As, Bi, Sb, Cu and In from high-calcareous barren black shale through low-calcareous low-mineralized black shale to ore samples. Conversely, values of Sr, Ba, Pb, Zn, V and V/Cr ratio decrease in this order. Significant correlations of Bi and Cu with S and of V and Pb with organic carbon were encountered. Organic-geochemical study documented that the formation of pyrobitumen, which represents the 'distillation product' of the organic alteration at Dafulou, is associated with very strong pyrrhotitization of host rocks and with the formation of massive pyrrhotite ores of the main sulfide stage (Stage II). It was suggested that in-situ organic matter locally interacted with hydrothermal solutions which resulted in the decrease in fO2 of the ore solution and subsequent precipitation of ores. This interaction was evidenced by (1) an enrichment of organic matter (kerogen) in 13C isotope, (2) an increase of its structural organization and, (3) a decrease in the yield of pyrolysable organic matter.