Geological Characteristics And Genesis Of The Lizhuang REE Deposit In Sichuan

Lizhuang REE deposit is a typical carbonate-type rare earth elements deposit.In whole REE belt,Lizhuang deposit is the one with stringer-disseminated characteristic,the causes of mineralization characteristics and main features of altered carbonatite and syenite have not been study in detail,and there is no consistent mineralization age.This paper will summarize the ore type and the characteristics of carbonatite and syenite alteration through detailed field investigation and indoor research,reveal the genesis of stringer-disseminated mineralization and determine the age of mineralization.This study found the carbonatite in the mine area was subjected to alteration and mineralization,and the original self-formed calcite was altered,and lots of bastnasite was superimposed on the altered calcite.The syenite develops a typical fenitization.The original K-feldspar is replaced by fine-grained albite and biotite,and a small amount of subhedral bastnasite is superimposed on albite and biotite.The fenite is enriched in LREE(7227×10^-67331×10^-6),and the distribution characteristics of rare earth elements is similar to those of carbonatite.The ore type in Lizhuang deposit is dominated by stringer-disseminated and minor breccia type.The stringer-disseminated ore is densely massive,and the mineral assemblage is mainly bastnasite+calcite+fluorite+celestite+phlogopite.The large-scale bastnasite is formed in the late stage of the hydrothermal stage and overlay on the early calcite,fluorite and celestite.The stringer-disseminated ore is strongly enriched in LREEs(33430×10^-646530×10^-6).The distribution characteristics of REE of the whole rock are similar to those of carbonatite and syenite,the latter two are lower in total amounts of rare earth elements.The mineralization characteristics of Lizhuang mainly is stringer-disseminated type,the reason may be due to the weak tectonic activities and undeveloped extension fracture.In the Lizhuang deposit,a large number of phlogopite is closely associated with bastnasite In this paper,the phlogopite formation age of phlogopite is?26.0±1.1?Ma,which is consistent with the reported syenite zircon SHRIMP U-Pb age and the bastnasite SIMS Th-Pb age.26.0±1.1 Ma can be considered as the mineralization age of the Lizhuang deposit.The Hf-O analysis results of zircon in the syenite of Lizhuang deposit is that?Hf?t?is between-3.8³.2‰,and?¹⁸O is between 5.36⁶.55‰,indicating the heterogeneity of syenite magma source area,The main body of the source area of magma is enriched mantle,and minor of crust-derived materials.Combined with this study and the existing research results,it is believed that syenite magmas were formed by the partial melting of subcontinental lithospheric mantle that had been metasomatized by fluids derived from subducted marine sediments during subduction of the Paleo-Tethyan Ocean.These magmas contaminated the crust and released REE-rich fluids during their ascent.All of the available inclusion data suggest that the inflow of meteoric waters during the late-stage of mineralization processes rapidly lowered the temperature of hydrothermal fluids,which severely diminished the stability of REE-bearing complexes and therefore triggered the crystallization of REE minerals.