Formation age and genesis of the Nasigatu greisentype beryllium mineralization in the southern Great Xing'an Range

This paper presents monazite U-Pb ages, major and trace elements and Nd isotope compositions for the newly discovered Nasigatu greisen-type beryllium deposit from the Linxi region of the southern Great Xing'an Range ( SGXR). These data, compared with monazite geochemistry of the coeval Jingpeng highly fractionated alkali -feldspar granite in the SGXR, allow us to explore the formation time and genesis of the greisen-type beryllium mineralization. The Nasigatu greisens occur as strips within the roof of the Early Cretaceous ( similar to 140Ma) alkali -feldspar granite pluton, with a mineral assemblage of quartz, muscovite, fluorite, monazite and rare metal minerals including cassiterite and beryl. The whole rock beryllium contents of the beryl -bearing greisens can reach 397 x 10(-6) similar to 13908 x 10(-6). The monazites from the Jingpeng beryllium -depleted alkali -feldspar granite and the Nasigatu beryllium-riched greisen and greisenization granite all have relatively high Th02 ( averaged at 3. 01% similar to 6.88%) and low UO2 ( < 0. 55%) contents, with geochemical features of LREEs-enrichments, HREEs-depletions and strong negative Eu anomalies (8Eu < 0. 08). Compared with the Jingpeng magmatic monazites ( TE1,3 = 0. 95 similar to 1. 00), the Nasigatu greisenization-related monazites have obviously higher TE,,, values of 0. 98 similar to 1. 26. Petrology and geochemistry results indicate the Nasigatu monazites formed in a magmatic-hydrothermal transition stage, and the Na-, Ca-, F-riched fluids derived from the highly evolved granitic magma play a key role in the generation of the Nasigatu monazites. LA-ICP-MS U-Pb dating results of the monazite reveal that the Nasigatu greisen-type beryllium deposit was formed during the Early Cretaceous with the age of similar to 139Ma, which is consistent with the ages of the ore -related alkali -feldspar granite. The Nasigatu monazite epsilon(Nd) (t) values range from -0. 29 to 0. 95, mostly overlapping with the Jingpeng monazite ENd (t) values (-2.38 similar to -1. 68) and the whole rock epsilon(Nd) ( t) values ( 2. 51 similar to 2. 11) of the Early Cretaceous highly fractionated granites from the SGXR. Monazite ages and Nd isotopes suggest that the Nasigatu beryllium-riched greisens were formed closely associated with the melt-fluid interaction during the late stage of high differentiation processes of Early Cretaceous granitic magmas, and their sources mainly consist of both juvenile and old crustal materials. Combined with the regional geological information, it is determined that the Early Cretaceous is an important period of rare metal ( e. g., Sn-W-Li-Be) mineralization within the SGXR, and this mineralization peak occurred in a lithosphere extensional environment induced by the roll -back of the Paleo-Pacific Plate. The discovery of the Nasigatu greisen-type beryllium deposit shows that the SGXR has good prospects for beryllium mineralization, and the Linxi area is a key target area for future beryllium mineral exploration, which deserves full attention in the future research.