Mineralogy and geochemistry of the Zhuxi Nb-rich trachytic rocks, South Qinling (China): Insights into the niobium mineralization during magmatic-hydrothermal processes

Abstract

Series of trachytic rocks, containing significantly high Nb contents, were identified in the Zhuxi region of South Qinling Orogenic Belt (SQB). An integrated investigation, mainly focusing on Nb variations in rocks and minerals, was conducted to constrain the enrichment mechanism of niobium. The Zhuxi trachytic rocks are predominantly composed of trachyte ignimbrites and trachytes, with pronounced Nb contents of 213–627 ppm and 116–384 ppm, respectively. Apatite U–Pb dating has revealed that the rocks were formed during the Silurian (434 ± 10 Ma), consistent with the associated alkali basaltic rocks. The trachytic rocks are chemically characterized by high total alkali, HFSEs (Nb, Ta, Zr, and Hf), and LREE, along with a marked depletion of Ba, Sr, P, and Ti, similar to the oceanic island basalt (OIB)-type series. The Zhuxi trachytic rocks are most probably generated by partial melting of a metasomatic lithospheric mantle followed by extensive fractional crystallization. The whole rock fluorine contents gradually decreased with decreasing Nb contents from trachyte ignimbrites (F = 123–3090 ppm) to trachytes (F = 90–1820 ppm), indicating that substantial quantities of halogen played an important role in Nb enrichment.The major mineral reservoirs of niobium in Zhuxi trachytic rocks are columbite, titanite, and aeschynite of magmatic and hydrothermal origins. The magmatic titanite generally contains lower Nb2O5 (0.71–1.68 wt%) than the hydrothermal titanite (0.67–7.27 wt%). In contrast, the Nb2O5 contents are indistinguishable in magmatic and hydrothermal columbites (70.16–74.44 wt%). Aeschynite is essentially a hydrothermal product, comprising high Nb2O5 contents (25.64–39.67 wt%). The high Nb contents in early-crystallized magmatic columbites and titanites indicate that the parental magma was initially Nb-enriched. The pronounced Nb contents in hydrothermal mineral phases (such as titanite) reveal that hydrothermal processes likely led to further Nb enrichment.