Controls on the Distribution of Gallium in the Nechalacho REE Deposit, NWT, Canada

Abstract

Despite the numerous industrial and scientific applications of gallium, its behavior in nature and the processes that concentrate it to potentially economic levels are poorly understood. Although the main supply of this metal is as a by-product of the mining of bauxite, it is also concentrated by magmatic-hydrothermal processes in peralkaline igneous systems. Here we report the results of a study of the distribution of gallium and the controls on this distribution in the Nechalacho rare metal deposit, Northwest Territories, Canada, which has been shown to contain significant reserves of this critical metal. Electron microprobe analyses and X-ray element maps of gallium-bearing minerals were used to determine the mineralogical distribution of gallium in the Nechalacho intrusive suite. Elevated gallium concentrations were identified in albite, biotite, orthoclase, chlorite, and allanite. Of these aluminum-bearing minerals, the most important hosts of gallium are albite, biotite, and orthoclase. Ferric iron-bearing minerals, including magnetite and aegirine, which were considered potential candidates for gallium sequestration, contain relatively low concentrations of the metal. This behavior of gallium, at least from a magmatic perspective, is consistent with its predicted partitioning between phenocrysts and melt. However, there is also evidence that gallium was redistributed by hydrothermal fluids. Chloritization of biotite resulted in the enrichment of gallium in the secondary mineral (chlorite), and the development of secondary albite (albitization) led to a depletion of gallium in primary albite. On the basis of these results, we argue that the overall distribution of gallium within the Nechalacho deposit was controlled by magmatic crystal fractionation, whereas hydrothermal processes led to local remobilization of the metal. During fractional crystallization, gallium was moderately compatible in minerals such as albite and biotite, whereas it bordered between compatibility and incompatibility in minerals such as magnetite and orthoclase, and was incompatible in aegirine. This resulted in a relatively constant bulk gallium concentration in the Nechalacho deposit, although locally, gallium was remobilized hydrothermally, particularly within the most altered parts of the intrusion, notably, the albitite.