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Discrete Zr and REE mineralization of the Baerzhe rare-metal deposit, China

October 1, 2019

Although REE (lanthanides + Y) mineralization in alkaline silicate systems is commonly accompanied with Zr mineralization worldwide, our understanding of the relationship between Zr and REE mineralization is still incomplete (e.g. Škoda and Novák, 2007; Linnen et al., 2014; Petrella et al., 2014; Möller and Williams-Jones, 2016; Wu et al., 2018). The Baerzhe deposit in NE China is a source of Zr, REE, and Nb linked to the formation of an early Cretaceous, silica-saturated, alkaline intrusive complex. In-situ laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of zircon and monazite crystals constrain the relationship between Zr and REE mineralization at Baerzhe.
Three populations of zircon are identified and are differentiated based upon textural observations and compositional characteristics. Type I zircon display well-developed oscillatory zoning are are typically found as inclusions within sodic amphibole. Type II zircons are darker than Type I zircons in CL images, can overgrow Type I zircon, and have more irregular zoning and resorption features. Type III zircons contain irregular but translucent cores and rims with oscillatory zoning that are murky brown in color and grow in aggregates. Textural features and compositional data suggest that Types I and II zircon crystallized at the magmatic stage, with Type 1 being least altered and Type II being strongly altered, and Type III precipitated during the magmatic to magmatic-hydrothermal transition.
Whereas the magnitude of the Eu anomaly is moderate in the barren alkaline granite, all three populations of zircon exhibit pronounced negative anomalies. Such features are difficult to explain exclusively by feldspar fractionation and could indicate the presence of fluid modification of the rocks. Monazite crystals occur mostly through replacement of zircon and sodic amphibole; monazite clusters are also present. Textural and compositional evidence suggests that monazite at Baerzhe is hydrothermal.
Least and strongly altered magmatic zircon yield 207Pb-corrected 206Pb/238U ages of 127.3±1.2 Ma and 125.67±0.76 Ma, respectively. Deuteric zircon precipitated at 125.11±0.69 Ma. The chronological data suggest that the magmatic stage of the highly-evolved Baerzhe alkaline granite lasted less than two million years. Hydrothermal monazite records REE mineralization at 123.41±0.63 Ma, approximately 1 or 2 million years later than Zr mineralization. We therefore propose a model in which parental magmas underwent extensive magmatic differentiation while residual melts interacted with aqueous hydrothermal fluids. Deuteric zircon precipitated from a hydrosilicate liquid, and subsequent REE mineralization, exemplified by hydrothermal monazite, correlates with the metasomatic alteration from external fluid sources. Such interplay between magmatic and hydrothermal processes resulted in the formation of discrete Zr and REE mineralization at Baerzhe.