Mechanisms of element precipitation in carbonatite-related rare-earth element deposits: Evidence from fluid inclusions in the Maoniuping deposit, Sichuan Province, southwestern China

Abstract

Carbonatite-related rare-earth element (REE) deposits (CARDs) are the major global source of REEs. The ore-forming fluids of CARDs usually comprise multiple components and record complicated evolutions. The Maoniuping REE deposit, located in the eastern Tibetan Plateau, is the second-largest CARD in China and contains total reserves of 3.17 Mt of light rare-earth oxides (REOs). Geochronological and geological data show that the deposit was formed at ∼25 Ma and was only moderately affected by tectonic and hydrothermal activities, thereby allowing us to study the evolution of ore fluids as well as the mechanisms of REE mineralization. The Maoniuping REE deposit is spatially associated with a carbonatite–syenite complex and includes two sections: Guangtoushan and Dagudao. The Dagudao section is the main focus of exploration and hosts well-developed vein systems. In the uppermost vein system, minerals are zoned from the syenite wall-rock contact to the vein centers in the order of biotite, aegirine-augite, arfvedsonite, calcite, quartz, barite, fluorite, and bastnäsite-(Ce). Based on geological observations and the petrography of fluid inclusions, the mineralization processes are classified into magmatic, pegmatitic, hydrothermal I, hydrothermal II, and REE stages. The inclusions in these stages include melt (M), melt–fluid (M–L), pure CO2 (C), aqueous–CO2 (L–C), aqueous–CO2 with crystals (L − C + S), liquid–vapor aqueous with crystals (L − V + S), and liquid–vapor (L–V) type inclusions. The magmatic stage is marked by a carbonatite–syenite complex with minor bastnäsite-(Ce), whereas the pegmatitic stage consists of coarse-grained calcite, barite, fluorite, and quartz that contain M, M–L, and L–C type inclusions with a fluid system of NaCl–Na2SO4–H2O–CO2 at high temperature (>600 °C) and high salinity (>45 wt% NaCl equiv.). The hydrothermal I stage is characterized by fenitization and is marked by aegirine-augite and arfvedsonite containing abundant L–V and few L–C type inclusions. This stage is characterized by high temperatures (∼480 °C) and moderate salinity (10.2–17.9 wt% NaCl equiv.), with a fluid system of NaCl–Na2SO4–H2O and minor CO2 and CH4 + C2H6. The hydrothermal II stage is dominated by L–C, L − C + S, L − V + S, and L–V type inclusions that are hosted in barite, calcite, fluorite, and quartz, and formed at moderate to high temperatures (260–350 °C), with a wide range of salinity (9.4–47.8 wt% NaCl equiv.), a fluid system of NaCl–Na2SO4–CO2–H2O, and abundant CH4 + C2H6. During the REE stage, pervasive bastnäsite-(Ce) containing abundant L–V type and few L–C type inclusions crystallized under low temperatures (160–240 °C) and low salinities (8.8–13.1 wt% NaCl equiv.) with a fluid system of NaCl–H2O and minor CO2 and CH4 + C2H6.The results of ion-chromatographic analysis show that the ore fluids are rich in Na+, K+, Cl−, F−, and (SO4)2−, and have low Cl−/(SO4)2− ratios (0.78–2.00), showing a marked contrast with the fluids of granite-related REE deposits (Cl−/(SO4)2− > 50) and a similarity to subcontinental lithospheric mantle (SCLM). The δD and δ18Ofluid values and the high N2/Ar ratios indicate that the ore fluids originated from carbonatitic magma and were dominated by magmatic water during the hydrothermal I stage, whereas magmatic and meteoric water co-existed during the hydrothermal II and REE stages. Moreover, the higher ratios of CO2/N2 (9–64) and CO2/CH4 (17–472) and the higher concentrations of CO2, CH4, C2H6, and N2 in the hydrothermal II stage compared with the hydrothermal I stage are attributed to intense immiscibility that resulted from decompression and is constrained to temperatures of 310–350 °C and pressures of 2.0–2.4 kbar. In contrast, microthermometric data and low CH4, C2H6, and N2 contents for the REE stage show that fluid cooling and mixing with meteoric water played an important role during the intensive mineralization of this stage, which occurred under shallow open-system conditions at temperatures of ∼200 °C and pressures of <0.5 kbar. The mineral assemblages, together with experimental petrology results, suggest that the REE transport capability of the hydrothermal fluids was due to the high contents of (SO4)2−, Cl−, and F− complexes. In addition, CO2 that separates during immiscibility is known to act as a buffer that constrains the pH of ore fluids. Thus, immiscibility during the hydrothermal II stage could have provided favorable conditions for the migration of REEs. The subsequent cooling of fluids, the involvement of meteoric water, and increased fluid pH, favored the precipitation of REEs in the Maoniuping deposit.