Tourmaline as a recorder of contrasting boron source and potential tin mineralization in the Mopanshan pluton from Inner Mongolia, northeastern China

Abstract

Abundant tourmaline occurs in the Mopanshan pluton in Inner Mongolia of NE China, and can be classified by its occurrence into four types: nodular tourmalines (ND type), pegmatite-pocket tourmalines (PP type), stockwork tourmalines (SW type) and vein tourmalines (VT type). All these tourmalines belong to the alkali group and are dravite-schorl. Petrography and major elements data suggest that the ND- and PP-type tourmalines crystallized during the transition from late-magmatic to early-hydrothermal evolution of granite with all the components derived from the magma. Petrography and trace elements (especially Sr、V、Cr、Ni、Ti) data suggest that the SW- and VT-type tourmalines may form by the mixing of magmatic and country rocks components in the post-magmatic hydrothermal environment. Due to the similar δ11B of the SW-type with the ND- and PP-type tourmalines (variation <2‰, and all averaged at −15.8‰), the boron source of the SW-type tourmalines is likely the same as the ND- and PP-type ones, i.e., from the granitic magma. The VT-type tourmalines have extremely negative δ11B values from −27.0‰ to −29.8‰, indicating a distinct and isotopically lighter boron source. The potential presence of non-marine evaporites (with very light boron isotopes) in the meta-sedimentary strata deep in the region may contribute boron to the VT-type tourmalines. The high Sn contents in the tourmalines, in particular the SW-type (918–2172 ppm) may have prospecting significance to target tin mineralization related to granite in the region.