Oligocene Nb-Ta-W-mineralization related to the Xiaruleucogranite in the Himalayan Orogen

Abstract

<p indent=0mm>Himalayan leucogranites are abundant within a ~2000-km-long, E-W trending belt on the Tibetan Plateau. Recently, it has been shown that several leucogranites and pegmatites (including aplites) of the Himalayan leucogranitic belt are closely related to rare-metal mineralization. The leucogranites and pegmatites are commonly peraluminous to strongly peraluminous, classifying as highly-fractionated granites in terms of whole-rock geochemistry and characteristic mineral assemblage with phases such as garnet, mica, zircon and oxides. The leucogranites contain typical rare-metal-bearing minerals formed during the magmatic-hydrothermal stage, including spodumene, beryl, columbite-group minerals (CGMs), cassiterite, and wolframite, associated with Li-, Be-, Nb-Ta-, Sn- and W-mineralization. Multiple episodes of magmatism and mineralization (~25−12 Ma) have been identified for the leucogranites. Nevertheless, our understanding of the rare-metal mineralization in the Himalayan orogeny and its age range remains limited. The Xiaru Dome, which crops out in the central part of the northern Himalaya, is mainly composed of middle-high grade metasedimentary rocks and granitic gneiss. Intrusions of tourmaline-garnet-bearing leucogranite and Be-Nb-Ta pegmatite are relatively common. Beryl and CGMs have been previously observed within pegmatite with the total Nb₂O₅ and Ta₂O₅ contents of up to ~0.014wt%. During recent field work, Nb-Ta-W-bearing tourmaline granite/pegmatite was found for the first time. To further constrain the leucogranite evolution and related Nb-Ta-W mineralization, we have studied the rock-forming and ore-forming phases of the Nb-Ta-W-bearing leucogranite/pegmatite and country rock granitic gneiss. We have characterized whole-rock geochemical compositions, the textures and the chemical compositions of the Nb-Ta-W oxide minerals, and complemented these by U-(Th-)Pb dating of CGMs, zircon and monazite. Our work shows that the granitic samples are peraluminous (ACNK =1.06−1.26), with high silica (SiO₂ = 72.2wt%−75.5wt%) and alkali contents (Na₂O+ K₂O = 8.4wt%−9.4wt%). They contain high concentrations of B, Nb, Ta and W (780×10⁻⁶, 58×10⁻⁶, 17×10⁻⁶ and 67×10⁻⁶, respectively), which are hosted in schorl, CGMs, wolframoixiolite, qitianlingite, and wolframite. Zircon and monazite U-(Th-)Pb dating shows that the leucogranites formed at ~530−387, ~44, and ~36−35 Ma, indicating multiple episodes of magmatism for the Xiaru intrusions, while U-Pb dating of CGMs reveals that the Nb-Ta mineralization related to the Xiaru leucogranites occurred exclusively at ~34−33 Ma. The Oligocene Nb-Ta mineralization of the Xiaru district is thus the oldest (earliest) rare-metal mineralization related to Himalayan leucogranites ever discovered. We highlight that the ore-forming age is best determined by dating ore-minerals directly instead of associated zircon or monazite. We conclude that a high B concentration in the parental magma possibly permitted high fractionation of the magma and concentration of Nb and Ta. Magmatic columbite formed early, which was later metasomatically altered by Ca-W-F-rich fluids. At the hydrothermal stage, a characteristic mineral assemblage formed, including qitianlingite, wolframite, scheelite, and microlite. The enrichment of W in the local granitic gneiss, located adjacent to the Nb-Ta-W-bearing granite, is possibly related to the ingression of W-bearing fluids from the leucogranite. Compared with skarn-type W mineralization related to the Cuonadong pluton in the eastern Himalayan orogen, there are at least two types of W mineralization related to the Himalayan leucogranites: Skarn-type and granite-type. The discovery of Nb-Ta-W mineralization related to the leucogranite/pegmatite in the Xiaru gneiss dome deepens our understanding of Himalayan rare-metal mineralization and provides the basis for exploration of similar mineralization types elsewhere in the Himalayan Orogen.