Comparison of fluorite geochemistry from REE deposits in the Panxi region and Bayan Obo, China

摘要

Panxi region in west Sichuan province is one of the most economically significant REE mineralization belts in China, and includes the large Maoniuping and Daluxiang deposits and the minor Lizhuang deposit. The REE mineralization in these deposits is spatially and temporally associated with carbonatite-syenite complexes. Large proportional fluorites and REE minerals occurring as veins intrude Cretaceous granite and Oligocene syenite in Maoniuping, and Oligocene syenite and carbonatite in Lizhuang, and Miocene syenite in Daluxiang. Fluorite is also one of main gangue minerals in the world-class Bayan Obo REE deposit. We present a comparison of the trace element and isotopic compositions of fluorites from four REE deposits in the Panxi region and Bayan Obo. The fluorites from Maoniuping and Daluxiang are characterized by variable REE patterns, with either LREE enrichment or LREE depletion relative to MREE. Typically they have a larger range in La/Ho compared to Y/Ho ratios, and pronounced positive Y anomaly relative to chondrite-normalized REE patterns. Their REE distribution patterns are controlled by fluoride-complexes and the loss of separate LREE-rich minerals. Different Y/Ho (ca. 73 vs. 108) and initial Sr isotopic (ca. 0.7061 vs. 0.7077) ratios are observed between the fluorites from Maoniuping and Daluxiang, reflecting their different source compositions. This contrasts with the fluorites from Maoniuping and Lizhuang, which have similar initial Sr isotopes, and appear to be cogenetic. However, the Lizhuang fluorite shows a consistent depletion of LREE relative to MREE, as well as lower Y/Ho ratios and higher HREE content than that in Maoniuping. In this respect the Lizhuang fluorite may have precipitated from a late-stage fluid following abundant fluorite and REE mineral deposition in Maoniuping. Carbonate, more than fluoride complexing, appears to have a stronger control on REE fractionation in the Lizhuang fluorites. The fluorites from three deposits in Panxi region show uniform initial Sr and Nd isotopic compositions similar to their associated carbonatites, but differ from ore-veins found intruding wall rocks, e.g. granite in Maoniuping and syenite in Daluxiang. This is not consistent with a model for fluorite formation involving interaction of F-rich, carbonatite-exsoloved fluid with wall rocks. Instead, the fluorite in Panxi region may precipitate from a residual carbonthermal fluid, which was dominated by Ca, CO_2 but also contained F, H_2O and REE, and derived from the fractioned carbonatitic magma. Fluorite deposition produced a sharp drop in the activity of F~-, which destabilized the REE fluoride complexes and caused deposition of REE minerals. In Bayan Obo, the fluorite typic ally has higher La/Ho than that in Panxi region and is characterized by a consistent LREE enrichment relative to MREE and negligible to positive Y anomalies. This is consistent with the compositional change of the hydrothermal fluids, which were infiltrated by external F-, LREE-rich fluids. The ~(87)Sr/~(86)Sr of Bayan Obo fluorite is relatively low radiogenic, and has a large range (0.7038-0.7065): similar characteristics to the carbonatite dykes found near the ore bodies. This supports a model for fluorite and REE mineral genesis involving the interaction of a carbonatite-derived fluid and the ore-hosted dolomitic marble.