Magma differentiation and rare metal mineralization of the Qongjiagang area, Himalayan orogen: evidence from trace element and boron isotope compositions of tourmaline

Abstract

Strongly fractionated granitic-pegmatitic rocks of the Qongjiagang area, Himalayan orogen are associated with economically important rare metal (e.g., Li, Nb, Be, and Ta) mineralization. Tourmaline is a frequent mineral in both the granites and pegmatites, which makes it an ideal candidate for investigating magmatic to hydrothermal processes that led to the rare metal enrichment. However, a systematic investigation of the response patterns of tourmaline compositions to differentiation processes is still lacking. In this study, we present petrographic and characteristics of the granitic-pegmatitic rock suites, as well as the chemical and boron isotopic compositions of tourmaline. Tourmalines in all samples exhibit schorl characteristics. Those from part of the muscovite granites exhibit pronounced zoning with highest MgO contents and δ¹¹B ratios being recorded by their cores (δ¹¹B: −10.0 ∼ −7.74 ‰) whereas the rims trend to lower δ¹¹B (−11.8 ∼ −10.5 ‰). The δ¹¹B ratios of the tourmaline rims are close to those of unzoned tourmalines of the other muscovite granite samples (−12.5 ∼ −12.1 ‰). Tourmalines of tourmaline granite (−14.5 ∼ −13.9 ‰) and barren pegmatite (−14.0 ∼ −12.9 ‰) exhibit comparably low δ¹¹B ratios. The ones in beryl pegmatites are characterized by the lowest δ¹¹B ratios (−15.7 ∼ −14.3 ‰) but at the same time high concentrations of B₂O₅ and enrichment in Pb, Nb, Ta, and the light rare earth elements. Notably, tourmalines of spodumene pegmatites show variable δ¹¹B ratios (−14.6 ∼ −10.3 ‰) and at the same time high values of Al₂O₃, B₂O₅, Li, Be, Sn, Cr, La, Ce, Pb, and Zn, yet low SiO₂. The textural position, as well as element and B isotope composition of tourmaline suggest a magmatic origin of all tourmalines, and record the magma differentiation from early muscovite granite to the subsequently formed pegmatites, with tourmaline of the spodumene pegmatite being subsequently altered by hydrothermal fluids. The variable composition of the tourmalines records independent enrichment of different rare metals during differentiation. Following this, tourmaline is a useful tool to reconstruct rare metal enrichment and mineralization processes controlled by granite magma differentiation and during interaction with externally derived fluids at the magmatic–hydrothermal transition.