The behavior of niobium and tantalum is poorly understood in rocks that have undergone significant hydrothermal alteration, and niobium-tantalum minerals of hydrothermal origin are rarely mentioned in the literature. Consequently, the mobility of these critical metals, although widely considered to be negligible, has not been evaluated. In this paper, we present the results of a study of the genesis of niobium and tantalum mineralization in the Nechalacho rare metal deposit, Northwest Territories, Canada, which contains one of the largest known resources of these metals in rocks that have undergone intense hydrothermal alteration.
Analyses and examination of samples using the electron microprobe has led to the identification of a variety of niobium- and tantalum-bearing minerals in the Nechalacho deposit. Niobium-bearing zircon, columbite-(Fe), fergusonite-(Y), and samarskite-(Y) were identified in the ore zones of the deposit, uranopyrochlore, and columbite-(Fe) were found outside the ore zones, and magmatic fluornatropyrochlore was shown to be the sole niobium-tantalum mineral in relatively unaltered syenites below the Basal ore zone.
Based on the paragenetic relationships among the above minerals, variations in the composition of the columbite group minerals as a function of location in the Nechalacho Layered Suite and the distribution of niobium, tantalum, zirconium, and uranium in the bulk rocks, we have developed a model to explain the occurrence of niobium and tantalum in the Nechalacho deposit. The first step in the concentration of these elements was the crystallization of niobium- and tantalum-bearing zircon and eudialyte in the subhorizontal Upper and Basal ore zones, respectively. This was accompanied by the crystallization of magmatic columbite-(Fe) in the Upper ore zone. Fergusonite-(Y) crystallized in the Basal ore zone and also formed due to the breakdown of eudialyte. Outside the ore zones, there was crystallization of pyrochlore and to a lesser extent magmatic columbite-(Fe). This step led to the development of strong spatial associations among niobium, zirconium, and uranium that are evident as strong positive correlations in the bulk-rock concentrations of these elements at the meter scale. During the ensuing intense and widespread hydrothermal alteration, niobium was locally remobilized. Hydrothermal columbite-(Fe) and fergusonite-(Y) formed at the cores of altered zircon grains. Wholesale replacement of magmatic columbite-(Fe) and fergusonite-(Y) by hydrothermal anhedral crystals occurred in the two ore zones. The estimated relative proportions of the sources of these minerals in the ore zones, although varying to some extent because of a dependence on the amount of niobium mobilized from zircon, is ~40/60. Outside the ore zones, columbite-(Fe) and uranopyrochlore are the present manifestations of the former pyrochlore. With the exception of magmatic fluornatropyrochlore in the fresher syenites below the Basal ore zone and a single example of magmatic columbite-(Fe) in an Upper ore zone sample, all niobium and tantalum minerals have a hydrothermal origin as a result of this pervasive alteration.