Mineralogical evidence for the role of deep magmatic fluids in beryllium enrichment in magmatic-hydrothermal systems

Abstract

Vein-type and greisen-type beryllium (Be) mineralization is closely associated with the magmatic-hydrothermal evolution in highly fractionated granites. However, the source of ore-forming fluids and their role in the migration and enrichment of Be is poorly understood. Although whole rock Ba-Rb isotope researches highlight that even deep magmatic fluids beneath the shallow magmatic-hydrothermal systems could provide rare metals and promote mineralization, it is still challenging to determine the influences of such fluids via using conventional geochemical indicators. Here we present zircon-monazite-xenotime-wolframite U-Pb ages, whole-rock geochemistry, monazite Nd isotopes, as well as muscovite and beryl geochemistry for the Zhujiayingzi quartz vein-type Be mineralization in the southern Great Xing’an Range (SGXR), NE China, and introduce the neighboring Nasigatu gresien-type Be mineralization for comparison. Geochronological results reveal that the Zhujiayingzi beryl-wolframite-bearing quartz veins formed in the Early Cretaceous (149–147 Ma), much younger than the host Permian (270–267 Ma) crystal tuffs, but coeval with the Nasigatu highly fractionated alkali-feldspar granite (144–139 Ma). Similar muscovite-beryl compositions and monazite Nd isotopes, combining with their close temporal-spatial relationships indicate that the Zhujiayingzi and Nasigatu Be mineralization have consistent hydrothermal assemblages and enrichment processes. Higher silica, K, and lower Al contents of muscovites from Be-rich veins (M2) and greisens (M4) than those recorded in barren crystal tuffs (M1) and alkali-feldspar granite samples (M3) at both Zhujiayingzi and Nasigatu are consistent with the evolution of the Early Cretaceous granitic magma, proving the involvement of shallow magmatic fluids exsolved from the Nasigatu-like highly evolved granitic melts. However, ore-related M2 and M4 samples have higher Mg (>1 wt%) and Ti contents than those of M1 and M3 samples, which violates the conventional hydrothermal evolution trend. Trace element compositions as well as K/Rb, K/Cs, and Rb/Sr ratios of four-type muscovites, together with the core-rim geochemical variations of muscovites at Zhujiayingzi and beryls at Nasigatu, indicate the existence of deep magmatic fluids rather than fluid-rock interaction to account for the anomalous mineralogical evidence. Such deep magmatic fluids were exsolved from large silicic magmatic reservoirs beneath the shallow magmatic-hydrothermal system, and thus likely display low evolution degrees and high Mg-Ti contents. They can not only provide heat and promote fluid-mineral interactions, but also can efficiently extract and transport Be enriched in deep crystal mushes at different depths. Thus we invoke that the Early Cretaceous Be mineralization in the SGXR was achieved under the combined action of both shallow and deep magmatic fluids derived from the crystal mush-dominated transcrustal magmatic system.