Genesis of the uranium-rich pyrochlore in the giant Huayangchuan U-Nb-REE polymetallic deposit, Qinling Orogen, Central China

Pyrochlore is an important Nb-bearing ore mineral, which supplies over 90% niobium resource in the world. Abundant pyrochlore as the principal U-Nb ore mineral have been found in the Huayangchuan U-Nb-REE polymetallic deposit located in the Qinling Orogenic Belt, Central China. Based on geology and geochemistry, three types of pyrochlore have been identified, namely carbonatite-hosted pyrochlore (Ip), altered pyrochlore (IIp) and skarn-hosted pyrochlore (IIIp), respectively. The carbonatite-hosted pyrochlore usually occurs with calcite, apatite, magnetite, aegirine-augite, titanite, and monazite. The composition of carbonatite-hosted pyrochlore at Huayangchuan is relatively stable, mainly containing abundant UO2 (avg. 30.34 wt%) and Nb2O5 (avg. 32.05 wt %). The altered pyrochlore is hosted in altered carbonatite that have experienced metasomatic alteration, and mainly occurs in the northwest part of the Huayangchuan deposit. The altered pyrochlore can be divided into five distinct compositional subgroups: II1p, which is compositionally similar to Ip; U-rich II2p (43.40-54.93 wt% UO2); U-poor II3p (11.01-15.76 wt% UO2); Pb-rich II4p (18.03-48.87 wt% PbO); and Si-Y-rich II5p (11.38-20.33 wt% SiO2; 0.18-2.16 wt% Y2O3). The skarn-hosted pyrochlore is usually small, occurring in garnet growth zones with epidote and calcite. Compared with Ip, IIIp has higher Nb2O5 (avg. 47.84 wt%) and relatively lower TiO2 UO2 (avg. 17.04 wt%). The occurrence of the Triassic carbonatite-hosted pyrochlore and its coexistence with apatite and monazite (PF-bearing system) indicates that carbonatitic magma reacted with feldspar wall rocks facilitating mineralization. Later F-rich hydrothermal fluid, which may have been responsible for the formation of the Cretaceous skarn at Huayangchuan, released Pb and Si, remobilized U and Ca to form the altered pyrochlore varieties, and precipitated uraninite. Meanwhile, the Nb-Ca-rich pyrochlore formed during the skarn formation. Multi-type U-rich pyrochlore at Huayangchuan well coincides with regional multi-stage thermal events, and records the evolution of an unusual and complex uranium mineralizing process in carbonatite systems. In this study, the enrichment of uranium and niobium through primary to most altered pyrochlore also provides an exceptional example to understand the remobilization of pyrochlore group minerals during hydrothermal fluid alteration.