A Description of the World-Class Rare Earth Element Deposit, Tanbreez, South GreenlandHans K. Schønwandt, ... Thomas Ulrich, inRare Earths Industry, 20164.1 Primary Ore Mineral: EudialyteAll of the metal commodities of the Tanbreez deposit are contained in the mineral eudialyte (REE, Nd, Ta, and Zr). Eudialyte is a sodium-rich zirconosilicate with varying amounts of cations including calcium (Ca), iron (Fe), manganese (Mn), REE, Nb, Ta, Y, and Ti. The kakortokite at the Tanbreez deposit is the type locality for eudialyte where it was first described from Kangerdluarssuk in South Greenland (Stromeyer, 1819).Chemical investigations over the years of eudialyte from various localities have shown a wide variation in chemical composition. The basic crystal structure of eudialyte was independently solved by Golyshev et al. (1971) and Giuseppetti et al. (1971). Although subsequent structural refinement has given information on some of the eudialyte phase relationship between structure and chemical composition, there are still unresolved aspects of the crystal chemistry of the eudialyte group minerals (Johnsen et al., 2003).Knowledge of the crystal chemistry of eudialyte s.l. resulted in redesigning the mineral eudialyte into a group of minerals, all with the same fundamental crystal structure. To date, 27 species have been accounted for within the eudialyte group of minerals (www.mindat.org) and there is the potential for many more species (end-members). Johnsen and Gault (1997)studied chemical variation in the eudialyte group and concluded that a solid-solution series exists between two end-members of the group: one relatively rich in silicon, Ca, Fe, and chlorine (the classic type of eudialyte) and another relatively rich in Nb, REE, Mn, and F, known as kentbrooksite (Johnsen et al., 1998).Pfaff et al. (2008) studied the chemical variation in eudialyte and amphibolethrough the kakortokite series and parts of the overlying lujavrites of the Ilímaussaq intrusion.A surprisingly small variation in chemical composition was found within the kakortokite, but a strong variation occurred in the overlying lujavrites. The lack of or negligible cryptic variation in the kakortokite was previously noted by Ferguson (1970). The slight change in chemical composition of eudialyte s.s. toward kentbrooksite indicated by an evolution from Ca- and Fe-dominated composition to a composition slightly richer in Mn- and REE is insignificant regarding practical magnetic processing of the Tanbreez ore. The chemical composition of eudialyte in this deposit has given eudialyte magnetic susceptibility between the two other main constituents of the ore (arfvedsonite and feldspar/nepheline), rendering it possible to produce a magnetic concentrate of eudialyte.Following is a description of the contact relationships of the kakortokite sequence to the upper, lower, and marginal boundaries of the intrusion.
Expand
