Experimental study of REE, Ba, Sr, Mo and W partitioning between carbonatitic melt and aqueous fluid with implications for rare metal mineralization

Abstract

Carbonatites host some unique ore deposits, especially rare earth elements (REE). Hydrothermal fluids have been proposed to play a significant role in the concentration and transport of REE and other rare metals in carbonatites, but experimental constraints on fluid–melt equilibria in carbonatitic systems are sparse. Here we present an experimental study of trace element (REE, Ba, Sr, Mo and W) partitioning between hydrous fluids and carbonatitic melts, bearing on potential hydrothermal activity associated with carbonatite ore-forming systems. The experiments were performed on mixtures of synthetic carbonate melts and aqueous fluids at 700–800 °C and 100–200 MPa using rapid-quench cold-seal pressure vessels and double-capsule assemblages with diamond traps for analyzing fluid precipitates in the outer capsule. Starting mixtures were composed of Ca, Mg and Na carbonates spiked with trace elements. Small amounts of F or Cl were added to some of the mixtures to study the effects of halogens on the element distribution. The results show that REE, Ba, Sr, Mo and W all preferentially partition into carbonatite melt and have fluid–melt distribution coefficients (D f/m) below unity. The REE partitioning is slightly dependent on the major element (Ca, Mg and Na) composition of the starting mixtures, and it is influenced by temperature, pressure, and the presence of halogens. The fluid–melt D values of individual REE vary from 0.02 to 0.15 with $$D_{\text{Lu}}^{{{{\text{f}} \mathord{\left/ {\vphantom {{\text{f}} {\text{m}}}} \right. \kern-0pt} {\text{m}}}}}$$ being larger than $$D_{\text{La}}^{{{{\text{f}} \mathord{\left/ {\vphantom {{\text{f}} {\text{m}}}} \right. \kern-0pt} {\text{m}}}}}$$ by a factor of 1.1–2. The halogens F and Cl have strong and opposite effects on the REE partitioning. Fluid–melt D REE are about three times higher in F-bearing compositions and ten times lower in Cl-bearing compositions than in halogen-free systems. $$D_{\text{W}}^{{{{\text{f}} \mathord{\left/ {\vphantom {{\text{f}} {\text{m}}}} \right. \kern-0pt} {\text{m}}}}}$$ and $$D_{\text{Mo}}^{{{{\text{f}} \mathord{\left/ {\vphantom {{\text{f}} {\text{m}}}} \right. \kern-0pt} {\text{m}}}}}$$ are the highest among the studied elements and vary between 0.6 and 0.7; $$D_{\text{Ba}}^{{{{\text{f}} \mathord{\left/ {\vphantom {{\text{f}} {\text{m}}}} \right. \kern-0pt} {\text{m}}}}}$$ is between 0.05 and 0.09, whereas $$D_{\text{Sr}}^{{{{\text{f}} \mathord{\left/ {\vphantom {{\text{f}} {\text{m}}}} \right. \kern-0pt} {\text{m}}}}}$$ is at about 0.01–0.02. The results imply that carbonatite-related REE deposits were probably formed by fractional crystallization of carbonatitic melts rather than from exsolved hydrothermal fluids. The same appears to be true for a carbonatite-related Mo deposit recently discovered in China.