Controls on critical metals in magmatic and hydrothermal systems

Abstract

Metals with high demand and a supply risk are called critical metals. One solution to the criticality problem is source diversification. We do not fully understand critical metal enrichment in ore deposits. I focus on two groups: the rare earth elements (REE) and the platinum group elements (PGE). At Nolans, NT, REE are hosted in fluorapatite veins up to several metres thick. I show experiments testing two formation hypotheses. First, I tested whether REE are hydrothermally mobile. I ran layers of rock-like compositions, REE-free apatite, REE, and a saline solution in piston cylinder experiments at 2-5 kbar and 550-700 C. The results show that the REE layer bonded with the more soluble elements (Si, Al, Mg, Fe) to produce insoluble REE minerals such as allanite or britholite. Formation of REE-bearing apatite was limited to a thin zone between the REE and apatite. In a P-F dominated fluid, the solubility of REE is negligible. Second, I tested the reaction between a REE-P-F-bearing carbonatite layer and a silicate layer, at 6 kbar and 650-900 C. The experiments resulted in a reaction zone consisting of diopside and REE-rich apatite. This indicates that carbonatites can carry P and REE, and form REE-rich apatite in reaction with silicate rocks. These textures closely reproduce the Nolans ore. The Nolans REE are now hosted in alteration products consisting of carbonates, phosphates and silicates. This study documents the alteration mineralogy. I show a decoupling between Ce and La, caused by oxidation of Ce(III) to Ce(IV), leading to its incorporation in Th-bearing minerals. La, Nd and Pr are concentrated in Ce-free phases. This has implications for mineral processing. Thorium is an unwanted by-product of REE production, and Ce is a lower-value product. The concentration of Ce and Th in a single mineral may allow its separation before processing, increasing the monetary value and reducing the environmental hazard. Analysis methods for fluorapatite compositions typical for carbonatite (LREE-enriched, carbonate bearing) are discussed. Rhenium is another critical metal that commonly occurs in molybdenite. Carbonatite-hosted molybdenites commonly host 100s ppm of Re. Carbonatites are usually considered as deposits for REE or Nb, and the occurrence of Re-rich molybdenite is perplexing. I performed experiments which tested whether (1) carbonatites can recrystallise molybdenite powder, and (2) whether rheniite can crystallise in carbonatites. The powder was recrystallised to coarse crystals, and similarly sized rheniite crystals formed by reaction of perrhenate and sulfate. Thus, carbonatites flux molybdenite and rheniite growth. It is a first step in understanding the reason for the occurrence of molybdenite in carbonatites. These experiments also tested the behaviour of other PGE in peralkaline melts. It is uncertain whether PGE must be concentrated from silicate melts by sulfide saturation, or can they be transported as nanonuggets in silicate melts independently of any presence of a sulfide phase. Nanonuggets are well known from PGE solubility experiments in which they are treated as experimental artefacts. I exploited the tendency of these metals to form nanonuggets to further explore their behaviour. These experiments were conducted in Ag-Pd, Au, or Pt capsules at 5 kbar and 1050-1100 C. Textural evidence indicates that the nanonuggets formed by reduction of an initially oxidised melt. They preferentially stick to magnetite and coarsen by consumption of existing nanonuggets. PGE can be transported as nanonuggets in silicate melts regardless of the presence of sulfide, and their concentration can be greater than that allowed by equilibrium solubility. Re-bearing sodalite that formed in those experiments was in equilibrium with Re metal, suggesting the role of the peralkaline melt for stabilising the higher oxidation state