Geochemical Contrasts between Late Triassic Rb-Rich and Barren Pegmatites from Ningshan Pegmatite District, South Qinling Orogen, China: Implications for Petrogenesis and Rare Metal Exploration

Xiao Nie,Zongqi Wang,Gang Wang,Lei Chen,Zengda Li

doi:10.3390/min10070582

Abstract

The Ningshan pegmatite district in the South Qinling Orogen hosts numerous Rb-(Be) mineralized pegmatites. In this study, whole-rock geochemistry, mineral geochemistry, and zircon U–Pb isotopes of the Rb-rich and barren pegmatites were determined. The barren pegmatites consist mainly of muscovite, microcline, albite, quartz, and garnet, whereas the Rb-rich pegmatites are mainly composed of muscovite, albite, quartz, and beryl, with minor chrysoberyl, cassiterite and columbite-group mineral. The muscovite and albite are the main Rb-bearing minerals. The U–Pb zircon dating of the Rb-rich and barren pegmatites yielding an age of 212–203 Ma, which is similar to that of the neighboring two-mica monzogranites distributed in the Ningshan area. Compared with the two-mica monzogranites, geochemical features, such as the Zr/Hf, Rb/Sr and Nb/Ta ratios and trace element contents indicated that the Rb-rich and barren pegmatites derived from fractionation of the two-mica monzogranites. In combination, the current and previous results suggest that the fractionation of the two-mica monzogranites caused the generation of the affinitive residual melts that, finally, crystallized to form the pegmatites. Compared to those from the barren pegmatites, the apatite from the Rb-rich pegmatites have higher MnO (14.51–19.12 wt.%) and Cl (0.12–0.16 wt.%) contents and lower F/Cl rartios (20–29). We conclude that these differences reflect unique geochemical signatures, and the geochemical composition of the apatite can be used as exploration guidance for rare metal-rich pegmatites.

Highlights

Rare metals, such as lithium, beryllium, cesium, rubidium, tin, niobium, tantalum, and uranium, are important to the economy and defense of the country
Zircon U–Pb dating for granitic pegmatites has a few problems, including huge amounts of inherited zircons, or strongly metamicted zircons, the U–Pb zircon geochronometer is one of the most important dating methods for rare metal-rich pegmatite, as the closure temperature of the zircon
U–Pb system is close to the temperature of the magmatic stage of pegmatites [24,25,55,56,57,58,59,60]

Summary

Introduction

Rare metals, such as lithium, beryllium, cesium, rubidium, tin, niobium, tantalum, and uranium, are important to the economy and defense of the country. The NYF family is suggested to be associated with A-type granite derived from a mixed source of lower crust. The most accepted model assumes an igneous origin, with most pegmatites originating from volatile-rich residual melts derived from the crystallization of parental granitic magmas. In this model, pegmatite is assumed to be linked genetically with fertile granite(s) exposed at the surface or buried at depth [1,3,5,6,7,8,9]. The other model assumes an anatectic origin (e.g., in migmatitic terranes), whereby the pegmatites form by direct partial melting of metasedimentary rocks, and have no petrogenetic relation to the neighboring granite(s) [10,11,12,13,14,15,16,17,18,19,20]

Methods

Results

Conclusion