Chemical and boron isotopic compositions of tourmaline at the Baiyanghe Be[sbnd]U deposit, northwest China: Implications for Be[sbnd]U mineralization

Abstract

Tourmaline records the physical and chemical conditions of the crystallization environment and preserves the information through younger magmatic-hydrothermal processes. Therefore, tourmaline is a favorable geochemical tracer and can be used to reveal processes that have contributed to the metallogenesis of hydrothermal deposits. The Baiyanghe deposit, located in the Xiemisitai Mountains, Northwest China, is the largest volcanogenic BeU deposit in Asia. The deposit has three types of ores: U type, Be type, and BeU type. These ores either occur separately or co-exist but have cross-cutting relationships. Major ore bodies occur as fracture fillings along contact zones between the Yangzhuang granite porphyry and the underlying Devonian volcanic rocks. Three types of tourmaline (e.g., hosted in the granite porphyry, rhyolitic crystal tuff, and basalt) were identified based on their occurrences, and seven sub-type tourmalines were clarified according to their textures and chemical compositions. They all belong to the alkali schorl group and rich in F. The low Al concentration (< 6 apfu) and Fe + Mg > 3 apfu tourmaline hosted in the rhyolitic crystal tuff suggest the substitution at Z site by Fe3+. The tourmalines hosted in the granite porphyry are rich in Li, Be, U, Ca, Ti, Cr, Y, Zr, Nb, Mo, Sn, Sb, Ta, and Th, whereas the tourmalines hosted in the rhyolitic crystal tuff and the basalt are rich in Mg, Sc, V, Co, Cu, Ni, Ga, and Sr, respectively. The δ11B value ranges from −5.94‰ to −4.48‰, from −8.01‰ to −6.93‰, and from −8.31‰ to −4.94% in the tourmalines hosted in the granite porphyry, the rhyolitic crystal tuff and the basalt, respectively. These data indicate that the chemical components that have contributed to the growth of tourmaline were derived from granitic magmas essentially. The oscillating zonation and the apparent variations of trace element compositions (e.g., Fe/(Fe + Mg) and Na/(Na + Ca) ratios) and REE patterns of the tourmalines indicate the involvement of periodic fluids and significant fluid-rock interaction, which are responsible for the precipitation of sub-type tourmalines. It is proposed that the tourmalines at the Baiyanghe deposit are the products of fluids exsolved from either a deep-rooted crystallizing magma chamber or one of the younger intrusions of the hosting composite granitic pluton in addition to the crystallizing the Yangzhuang granitic magma. The chemical and boron isotopic composition of tourmaline can provide a fingerprint to decipher the magmatic-hydrothermal process during the cyclic UBe mineralization.