Zirconosilicates in the kakortokites of the Ilímaussaq complex, South Greenland: Implications for fluid evolution and high-field-strength and rare-earth element mineralization in agpaitic systems

Abstract

The layered agpaitic nepheline syenites (kakortokites) of the Ilímaussaq complex, South Greenland, host voluminous accumulations of eudialyte-group minerals (EGM). These complex Na-Ca-zirconosilicates contain economically attractive levels of Zr, Nb and rare-earth elements (REE), but have commonly undergone extensive autometasomatic/hydrothermal alteration to a variety of secondary mineral assemblages. Three EGM alteration assemblages are recognized, characterized by the secondary zirconosilicates catapleiite, zircon and gittinsite. Theoretical petrogenetic grid models are constructed to assess mineral stabilities in terms of component activities in the late-stage melts and fluids. Widespread alteration of EGM to catapleiite records an overall increase in water activity, and reflects interaction of EGM with late-magmatic Na-, Cl- and F-rich aqueous fluids at the final stages of kakortokite crystallization. Localized alteration of EGM and catapleiite to the rare Ca-Zr silicate gittinsite, previously unidentified at Ilímaussaq, requires an increase in CaO activity and suggests post-magmatic interaction with Ca-Sr bearing aqueous fluids. The pseudomorphic replacement of EGM in the kakortokites was not found to be associated with significant remobilization of the primary Zr, Nb and REE mineralization, regardless of the high concentrations of potential transporting ligands such as F and Cl. We infer that the immobile behaviour essentially reflects the neutral to basic character of the late-magmatic fluids, in which REE-F compounds are insoluble and remobilization of REE as Cl complexes is inhibited by precipitation of nacareniobsite-(Ce) and various Ca-REE silicates. A subsequent decrease in F– activity would furthermore restrict the mobility of Zr as hydroxyl-fluoride complexes, and promote precipitation of the secondary zirconosilicates within the confines of the replaced EGM domains.

Fenites associated with carbonatite complexes: A review

“…Within the intrusion, autometasomatism can be seen as hundreds of square meters of albitized, analcimized and hematitized zones (Ferguson, 1964;Marks and Markl, 2015); mineralogically diverse hydrothermal veins (Engell et al, 1971;Markl, 2001;Markl and Baumgartner, 2002); and secondary mineral assemblages of albite, aegirine, fluorite, analcime, pectolite, catapleite, gittingsite and other HFSE and REE phases. These features result from the release of aqueous fluids variably enriched in Na, Cl − , F − , Ca and Sr during the later stages of crystallization (Markl and Baumgartner, 2002;Graser and Markl, 2008;Borst et al, 2016). Fenitization of the granitic country rock itself is limited however, extending no further than 120 m from the intrusion-country rock contact (Ferguson, 1964).…”

“…Secondary assemblages of minerals are formed at the expense of primary magmatic minerals (see Fig. 12) Chakhmouradian and Mitchell, 2002;Marks et al, 2003;Graser and Markl, 2008;Schilling et al, 2009;Borst et al, 2016), often causing mobilization of REE and Nb-rich phases (Chakhmouradian and Mitchell, 2002). This is the same process termed 'internal fenitization' by .…”

The Rare Earth Elements: Demand, Global Resources, and Challenges for Resourcing Future Generations

“…However, many large agpaitic nepheline syenite complexes are currently being explored for the REE: these include Ilímaussaq and Motzfeldt in Greenland, Norra Kärr in Sweden, and Red Wine and Kipawa in Canada (Sørensen 1992;Mariano and Mariano 2012;Dostal 2016). At the majority of these complexes the ore mineral is eudialyte (and its alteration products (Borst et al 2016)) although at the Kvanefjeld prospect steenstrupine is the main ore mineral (Sørensen et al 2011). Agpaitic nepheline syenite deposits may be large in size, but typically have relatively low total rare earth oxide (TREO) grades (generally < 1 wt%).…”

Neural Network Modeling for the Extraction of Rare Earth Elements from Eudialyte Concentrate by Dry Digestion and Leaching

“…Eudialyte is a somewhat complicated cyclosilicate mineral, which forms in alkaline igneous rocks, such as nepheline syenites. It tends to be rich in zirconium, beryllium, cerium, niobium, barium, yttrium, and other rare earth elements [18,19]. The typical chemical formula of Eudialyte is Na 4 (Ca, Ce, Fe) 2 ZrSi 6 O 17 (OH, Cl) 2 , but the component of the mineral may be sometimes different because of the zeolite crystal structure, which possesses ion-exchange properties of eudialyte [20,21].…”