Mineralogical and geochemical characteristics of the Miaoya REE prospect, Qinling orogenic Belt, China: Insights from Sr-Nd-C-O isotopes and LA-ICP-MS mineral chemistry

Abstract

Most carbonatite-related REE (rare earth element) deposits record two stages of REE enrichment: magmatic and magmatic–hydrothermal. It is generally accepted that the first stage of enrichment, which occurs in magmas associated with carbonatite–syenite complexes, is a prerequisite to the formation of REE deposits. The magmatic–hydrothermal process is also important, as demonstrated by the fact that many fertile carbonatite–syenite complexes do not produce REE deposits.The Miaoya carbonatite–syenite complex is prospective for REE and is ideal for studies of the formation of REE deposits. The Miaoya REE prospect lies in the western member of the Wudan Terrane of the Qinling Belt, China, and is hosted by a carbonatite–syenite complex that was intruded along a fault zone between schist of the lower Silurian Meiziya Group and adjacent Proterozoic quartzite. Mineralization at the Miaoya REE prospect includes carbonatite-, syenite-, and mixed-type, all low grade (about 1%). Results of X-ray diffraction (XRD) and electron probe micro-analyzer (EMPA) analyses reveal that modes of REE minerals are low in all samples (<5%), which is consistent with the fact that less monazite, bastnäsite and other REE minerals have been found in the Miaoya REE prospect. REE mineralization is less likely to occur as an overprint on gangue minerals. Results of Photon Laser Ablation Inductively-Coupled-Plasma Mass-Spectrometer (LA–ICP–MS) analyses reveal that apatite and calcite in carbonatite have the highest REE concentrations which are responsible for the relatively high concentration in carbonatite rather than because of the presence of REE minerals. The consistence of Sr-Nd isotopes ratios between altered host rocks and fresh hosted rocks suggested REE mineralization originates directly from the unmineralized carbonatite–syenite complex rather than other host rocks. Carbon and oxygen isotope ratios of hydrothermal calcite are consistent with low-temperature alteration subsequent to ore.Trace element ratios for the Miaoya carbonatite–syenite complex lie in the barren carbonatite field (REEs vs. CaO/MgO, FeO/MgO, Ba and Sr/Ba) compared with those of other giant or large carbonatite–syenite complex related REE deposits, just below the boundary between fields for fertile and barren carbonatites. This suggests that the carbonatite–syenite complex at the Miaoya prospect did not have the potential to produce large or giant REE deposits. The low REE of the Miaoya prospect compared with other carbonatite-syenite hosted deposits may reflect: 1) as supported by petrography, minimal tectonic deformation in the area resulting in 2) restricted cycling of hydrothermal solutions that led to 3) minimal fluid scavenging from REE-rich apatite and calcite for local REE re-deposition and concentration.