Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Roles of black shales in the origin of tin-polymetallic deposits in the Dachang ore district, south China
AbstractEconomic tin deposits of the Dachang tin field are hosted in a sedimentary sequence containing significant concentrations of organic-matter in the form of Lower Devonian calcareous black shales, that together with the younger granite intrusion actively participated in the formation of Sn-polymetallic deposits. Field observations coupled with the new petrologic, mineralogical, inorganic and organic geochemical, stable isotope and fluid inclusion data from the dafulou, Huile and Kangma stratabound and stockwork type cassiterite-sulfide deposits have confirmed that black shales have played several important roles in their origin. They became a source of S and partially of Sb, Co, Pb, V, and Fe during depositional period. Black shales have also played an important role controlling remobilized and epigenetic Sn-sulfidic vein- and replacement-type mineralization that occurred in connection with granitoid intrusions. Calcareous facies acted most likely as a geochemical barrier for metal-rich hydrothermal solutions that precipitated minerals in zones of lithologically and structurally controlled permeability. Bacteriogenic sulfides of black shales were also a dominant source of sulfur for epigenetic (vein and replacement) mineralization at the Huile and Kangma deposits. At Dafulou and Huile, oxidized organic matter have played a mjor function, in the origin of ore-bearing and post-ore carbonates. Black shales represented an important control of ore forming capacity of magmatic ore system associated with the emplacement of the Longxianggai granite through keeping sufficiently low fO2 in the exocontact. This resulted in the accumulation of Sn2+ in residual melt and in the formation of high-calcareous black shale/carbonate - replacement tin-sulfide ores in a longer distance from the intrusion.