Continuous carbonatitic melt–fluid evolution of a REE mineralization system: Evidence from inclusions in the Maoniuping REE Deposit, Western Sichuan, China

Abstract

The Maoniuping REE deposit is a world-class deposit with 1.2 Mt of REO grading on average 2.89 wt.% REO. It is the largest in the 270-km long Mianning–Dechang REE belt and is associated with Himalayan carbonatite–alkalic complexes in the eastern Indo-Asian collisional zone, Western Sichuan Province, China. The deposit is hosted by nordmarkite stocks and carbonatite sills that display a radiometric age of 40 to 24 Ma and which intruded a Yanshanian granite pluton. The 40.3 to 27.8 Ma REE mineralization occurs as vein systems hosted in nordmarkite and carbonatite with minor altered granite and rhyolite. Four ore types are recognized based on ore texture and mineral assemblage: (1) disseminated; (2) pegmatitic; (3) brecciated; and 4) stockwork (stringer) types. Five mineralizing stages are confirmed according to vein crosscutting relationships, mineral assemblage and microthermometric results, these are: 1) carbonatite stage, 2) pegmatite stage, 3) barite–bastnaesite stage, 4) later calcite stage and 5) epigenetic oxidation stage. Varied inclusion assemblages are found in fluorite, quartz, bastnaesite, barite and calcite from stages 1 through to stage 4. The dominant inclusion types include: melt, melt–fluid, CO 2-rich fluid and aqueous-rich fluid inclusions. Fluid, melt–fluid and melt inclusion studies indicate that the ore-forming fluid resulted from the unmixing of carbonatite melt and carbonatitic fluid. Initial ore-forming fluids were high-temperature (600 to 850 °C), high-pressure (> 350 MPa) and high-density supercritical orthomagmatic fluids, characterized by SO 4-rich and multi-component composition (e.g. K, Na, Ca, Ba, Sr and REE). The dominant anion is not Cl, but SO 4. The evolution of the ore-forming fluid is from a melt–fluid at high temperature, through a CO 2-rich fluid at high to medium temperature to aqueous-rich fluid at low temperature. REE precipitation occurred from a high to medium temperature CO 2-rich fluid. The main mechanism for REE precipitation was phase separation of CO 2 and aqueous fluids resulting in a decrease of temperature and pressure.