Mineralogy and short wavelength infrared spectral analysis of white mica in the No. 782 REE–Nb–Zr deposit, NE China

Abstract

The No. 782 deposit is one of the rare earth elements (REEs), niobium and zirconium resources of NE China and contains ~17 million tons of ore @ 0.23 wt% total REEs and Y oxides, @ 0.12 wt% Nb2O5, and @ 1.00 wt% ZrO2. The REE–Nb–Zr minerals are hosted mainly in the late Ordovician porphyritic alkali-feldspar granite, which intrudes in a late Cambrian monzogranite. Zirconium is present mainly in zircon aggregates, niobium and heavy REEs mainly in fergusonite-(Y), and light REEs dominantly in bastnäsite-(Ce) and parisite-(Ce). Most of these economic minerals are hydrothermal in origin, occurred as replacement textures and fracture-filled veins. Six types of phyllosilicate mineral (muscovite, illite, phengite, montmorillonite, chlorite, and halloysite), which cannot be distinguished confidently by visual criteria, were detected using short wavelength infrared (SWIR) analysis. Given that white mica (i.e., muscovite, illite, and phengite) is a pervasive mineral throughout the pluton, SWIR spectroscopy on drill cores from two profiles (II and IV) of the deposit was carried out to decipher the distribution of alteration assemblages that are characterized by subtle variations in the alteration products. The illite Al–OH absorption position (Pos2200) and illite crystallinity (IC) display regular patterns, with Pos2200 values < 2,203 nm and IC > 1.26, coinciding with the high grades of REE–Nb–Zr mineralization. The phengite is limited to the mineralized zone of profile II. Such regular patterns of white mica might have been induced by the relatively high temperature fluids surrounding the center of alteration. Our results suggest that combinations of the SWIR spectral parameters of hydrothermal alteration products can be used as potential vectors to economic ores, tracing the pathways of hydrothermal fluids, and quantifying the ore grades of the No. 782 deposit. The super-enrichment of rare metals in granitic systems is controlled predominantly by extensive hydrothermal activity, and our results show that these REE deposits can be usefully assessed using the SWIR method, which has implications for future metal exploration.