Mineralogical evidence for magmatic and hydrothermal processes in the Qitianling oxidized tin-bearing granite (Hunan, South China): EMP and (MC)-LA-ICPMS investigations of three types of titanite

Abstract

The Qitianling granite, which is located in Hunan Province (southern China), is a metaluminous Sn granite that crystallized at relatively oxidized conditions, between the Ni–NiO (NNO) and Fe 2O 3–Fe 3O 4 (MH) buffers. Titanite (sphene, CaTiSiO 5) is a common tin-carrier mineral in the Qitianling granite. Based on electron-microprobe (EMP) and multi-collector (MC)-Laser-ablation (LA)-ICPMS investigations, we describe three types of titanite with various textures and SnO 2 contents, which probably formed during the magmatic and hydrothermal processes responsible for the granite crystallization and later evolution. The form of type I titanite ranges from subhedral to euhedral coarse-grained crystals and typically exhibits an envelope-like shape and sector-zoning. It is characterized by a high Fe 2O 3 content (on average 2.44 wt.%) and low SnO 2 (0.43 wt.%), which indicates crystallization during an early magmatic stage. Its formation temperature is estimated to have been 721 to 780 °C from EMP- and LA-ICPMS using the Zr-in-titanite geothermometer. Type II titanite occurs as both discrete grains and symplectitic intergrowths; and typically fills interstices between other minerals. It contains up to 26 wt.% SnO 2, which suggests a late magmatic origin. Chondrite-normalized REE patterns for type II titanite reveal a lanthanide tetrad effect that was linked to differentiation of the granite magma. Type III titanite is restricted to cleavage cracks in chlorite and is rich in Al 2O 3. It is obviously the secondary product of biotite alteration at a temperature that is estimated to be lower than 400 °C during the hydrothermal stage. This hydrothermal activity also impacted the primary titanites. Type I titanite experienced alteration to different degrees via coupled dissolution–reprecipitation, which gave rise to the formation of secondary minerals including cassiterite micro-grains. Some grains of the type II titanite reacted with fluids along the grain boundary, which led to the gradual leaching of Sn. Based on information from these types of titanite and their Sn behavior, we discuss the progressive enrichment of Sn during and after granite crystallization under a high oxidation state as well as the predominant role of fluids in the leaching, concentration and transportation of Sn during hydrothermal processes, which favors vein-type Sn mineralization.