Efficient enrichment of Rb during the magmatic-hydrothermal transition in a highly evolved granitic system: Implications from mica chemistry of the Tiantangshan Rb-Sn-W deposit

Rare metal mineralization is commonly associated with highly differentiated granitic systems, which evolve into pegmatite or hydrothermal veins. The newly-discovered large Tiantangshan Rb-Sn-W deposit provides an excellent opportunity to study Rb enrichment, as the Rb resources are mainly hosted in the hydrothermal micas (dominantly veins), rather than the K-feldspar or magmatic micas in the parent alkali feldspar granite and volcanic rocks, as is typical of other Rb-rich systems. Micas at Tiantangshan are found in the alkali feldspar granites, trachyandesites, layers with unidirectional solidification textures (USTs) and hydrothermal veins. The mica Rb2O concentrations in the alkali feldspar granites range from 0.19 to 0.81 wt% (mean 0.44 wt%), in the trachyandesites from 0.47 to 1.13 wt% (mean 0.65 wt%), in the USTs from 0.41 to 1.26 wt% (mean 0.81 wt%), and in the hydrothermal veins from 0.39 to 1.28 wt% (mean 1.02 wt%), and show a positive correlation with F and Li contents. The mica and K-feldspar in the alkali feldspar granites have low K/Rb values (13-29 and 25-103, respectively), indicating the highly evolved nature of the alkali feldspar granite. K/Rb and K/Cs values decrease gradually in the mica from the alkali feldspar granites to the USTs and then to the hydrothermal veins, whereas Li, F and Rb increase. Based on the textures and geochemistry of the micas, they are cogenetic with the alkali feldspar granites and represent the magmatic stage, whereas the USTs formed during the magmatic-hydrothermal transition stage and the hydrothermal veins correspond to the hydrothermal stage. The Tiantangshan micas record mineralogical and geochemical evidence of efficient Rb mineralization during magmatic-hydrothermal evolution in the alkali feldspar granitic system.