Timing and genesis of the Tudiling trachyte Nb-Ta-Zr-REE deposit in the South Qinling (Central China): Implications for rare metal enrichment in extrusive peralkaline magmatic systems

Abstract

Relative contribution of magmatic and hydrothermal processes to rare metal enrichment has been a key issue for the alkaline-associated rare metal deposits. Till now, our understanding on the rare metal concentration of extrusive peralkaline magmatic systems is rather limited. We try to address this issue by studying the Tudiling trachyte Nb-Ta-Zr-REE deposit in the South Qinling (SQ), which is well endowed with abundant alkaline-associated rare metal deposits. Four lithofacies (i.e., trachyte, trachytic tuff, trachytic phyllite and trachytic ignimbrite) with rare metal mineralization were thoroughly investigated on their emplacement ages, whole-rock geochemistry, and Nb-REE mineralogy to decipher the evolution of the system and the concentration mechanisms for the rare metals. Zircon U-Pb dating indicates that the former three lithofacies developed during the Early Silurian (445–442 Ma) and are coeval with their intrusive counterparts, i.e., the Miaoya and Shaxiongdong syenites in the SQ. They share similar element distribution patterns (Nb-Ta, Zr-Hf, LREE enrichments, and Sr, P, Ti depletions) and moderately depleted Nd isotopes (εNd(t) values: 2.3–3.1), indicating a common mantle source. In contrast, the trachytic ignimbrite erupted during the late Jurassic (157 ± 1.9 Ma) and has enriched Nd isotope compositions (εNd(t) values: −2.9–−2.8), implying an enriched mantle source, which had probably been modified by the residuals of the Mianlue oceanic sediments. Their bulk-rock rare metal concentrations indicate that the former three lithofacies are characterized by Nb-Ta-Zr mineralization/enrichment with the phyllite containing the highest grades, while the latter one has REE mineralization as well as Nb-Ta-Zr mineralization/enrichment. Our geochemical and mineralogical results suggested that rare metal enrichment in these trachytic rocks was initially attributed to the partial melting of an enriched mantle source and subsequent protracted fractional crystallization of alkaline basaltic magmas. Effects of late-stage hydrothermal alteration on rare metal upgrading are highly dependent on rock structures and distinct on the intensively altered phyllite and ignimbrite.