Chemical composition of tourmaline from the Yunlong tin deposit, Yunnan, China: implications for ore genesis and mineral exploration

摘要

The Yunlong tin deposit is located in the northern part of the Lancangjiang metamorphic zone of the Sanjiang Tethys orogen series in western Yunan province of China. It consists of vein-type cassiterite ores, which are mainly hosted in migmatites of Caledonian age. Abundant tourmaline is associated with the ores, quartz-tourmaline veins and barren migmatized gneiss and migmatites. A detailed electron microprobe study has been carried out to document the chemical compositions of tourmaline from this deposit. The results exhibit a systematic compositional change that might be used as tracer for ore genesis and in prospecting for tin mineralization. Tourmalines from the ore bodies are dravite with Fe/(Fe + Mg) ratios of 0.09 approx 0.31 and Ca/(Ca + Na) ratios of 0.03 approx 0.40. These tourmalines are also rich in chromium (up to 0.74 wt percent Cr_2O_3) and tin (up to 0.42 wt percent Sn). In contrast, tourmalines from the barren migmatites are mostly schorl with Fe/(Fe + Mg) ratios of 0.38 approx 0.94 and Ca/(Ca + Na) ratios of 0.00 approx 0.14. Tourmalines from quartz-tourmaline veins that occur between ore bodies and the migmatites show intermediate compositions, i.e., Fe/(Fe + Mg)=0.09 approx 0.59, Ca/(Ca + Na)=0.01 approx 0.22. It is suggested that the Mg-rich nature of the tourmaline can be used as an exploration tool in this region to target tin mineralization, because the tourmalines show increasing Mg contents and are more dravitic when approaching the ore bodies. It is likely that the formation of the Yunlong tin deposit was related to migmatitic-hydrothermal processes. The high Mg and Cr contents in tourmalines from the ore bodies were probably derived from the local meta-sedimentary and meta-volcanic rocks of the Precambian Chongshan Group rather than from the granites in the region.