Geology and vein tin mineralization in the Dadoushan deposit, Gejiu district, SW China

Abstract

Vein-type tin mineralization in the Dadoushan deposit, Laochang ore field, Gejiu district, SW China, is predominantly hosted in Triassic carbonate rocks (Gejiu Formation) over cupolas of the unexposed Laochang equi- granular granite intrusion. The most common vein mineral is tourmaline, accompanied by skarn minerals (garnet, di- opside, epidote, phlogopite) and beryl. The main ore mineral is cassiterite, accompanied by minor chalcopyrite, pyrrho- tite, and pyrite, as well as scheelite. The tin ore grade varies with depth, with the highest grades (~1.2 % Sn) prevalent in the lower part of the vein zone. Muscovite 40 Ar- 39 Ar dating yielded a plateau age of 82.7±0.7 Ma which defines the age of the vein-type mineralization. Measured sulfur isotope compositions (δ 34 S0�4.1 to 3.9 ‰) of the sulfides (arseno- pyrite, chalcopyrite, pyrite, and pyrrhotite) indicate that the sulfur in veins is mainly derived from a magmatic source. The sulfur isotope values of the ores are consistent with those from the underlying granite (Laochang equigranular granite, �3.7 to 0.1 ‰) but are different from the carbonate wall rocks of the Gejiu Formation (7.1 to 11.1 ‰). The calculated and measured oxygen and hydrogen isotope com- positions of the ore-forming fluids (δ 18 OH2O0�2.4 to 5.5 ‰, δD0�86 to �77 ‰) suggest an initially magmatic fluid which gradually evolved towards meteoric water dur- ing tin mineralization.