Chemographic modelling of zirconosilicates Insight in late-stage fluid evolution and HFSE mineralisation in the Ilímaussaq Complex, South Greenland

Anouk M. Borst1,2*, Henrik Friis3, Tom Andersen4, Troels F.D. Nielsen1, Tod E. Waight2, Matthijs Smit5

1) Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 København 2) Department of Geoscience and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1350

København 3) Natural History Museum, University of Oslo, P.O. 1172 Blindern, N-0318, Norway 4) Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, N-0316, Norway , 5) Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, 2020-2207 Main Mall, Vancouver, BC , V6T 1Z4, Canada *Corresponding author: amb@geus.dk

N Abstract The Ilímaussaq Complex (1.16 Ga1) in the Gardar Rift Province, South Greenland, is renowned for its

unique rock types, mineralogy and extreme enrichment in incompatible elements such as Zr, Hf, Nb, Ta,

Be, REE, U, Th and Li2. Ilímaussaq has long been recognised for its economic ore potential and is

considered to host some of the largest deposits of REE and U in the world. Due to a growing demand and

potential supply-risk for such metals in the high-tech industry, two areas are currently explored for their

resources: Kringlerne in the south (REE, Zr, Nb, Ta) and Kvanefjeld in the north (REE, U, Zn) (stars on map).

This study focusses on the layered agpaitic nepheline syenites in the lower part of the magma chamber,

which record extensive autometasomatic alteration of the primary ore mineral eudialyte to a wide range

of secondary Zr, REE and Nb phases. We present detailed petrographic studies of eudialyte alteration

assemblages and theoretical phase diagrams for the secondary zirconosilicates, constructed to gain

further insight in the late-stage melt-fluid evolution of the Ilímaussaq Complex. This was done by

chemographic modelling of reported and observed zirconosilicate assemblages using the principles of

Schreinemakers analyses3.

Eudialyte alteration assemblages A large proportion of the eudialyte has been partially or completely replaced by fine-grained aggregates of complex secondary Na-rich silicates and REE- and HFSE-phases. The original grain boundaries of eudialyte are fully preserved. Distinguished pseudomorph assemblages are listed in the paragenetic diagram below. These are dominated by three types of secondary zirconosilicates; catapleiite, gittinsite and zircon (underlined). In contrast to zircon and catapleiite4,5, gittinsite was previously observed in Ilímaussaq, nor in eudialyte alteration assemblages elsewhere. The different assemblages reflect changing chemical conditions and fluid compositions at the late- to post-magmatic stage. The alteration led to strong fractionation of Zr, Nb and REE among the secondary phases, which may complicate potential ore recovery.

Chemographic modelling To evaluate the relative stability of the various eudialyte alteration assemblages as a function of changes in the chemical components of the melts and late-magmatic fluids (log activities of Na2SiO3 (alkalinity), H2O and CaO), we constructed a theoretical 3D chemographic grid model using Schreinemakers analysis3. Divariant reactions were calculated for the 13-component system of NaO0.5 - KO0.5 - CaO - FeO - REEO1.5 - ZrO2 - TiO2 - NbO2.5 - AlO1.5 - SiO2 - HO0.5 - FO-0.5 -ClO-0.5, and a co-existing mineral assemblage of analcime, annite, pectolite and aegirine. The model assumes isobaric and isothermal conditions, which are roughly constrained at 1 kPa and 350 °C6. Variations in these parameters could influence the topology of the diagrams.

Results and conclusions Eudialyte alteration assemblages are invariably dominated by the Na-zirconosilicate catapleiite (type-I). This reaction records a general increase in water activity, and resulted from interaction with H2O-rich interstitial melt, or locally exsolved Na, Cl and F-rich aqueous fluids at the final stages of crystallisation (fluid-I). Stabilisation of the Ca-zirconosilicate gittinsite (type-II) requires an increase in CaO activity and/or decrease in alkalinity (fluid-II). We infer that a general input of Ca and Sr (producing Sr-rich hydrothermal rims on relict eudialyte) was necessary to produce this paragenesis. An increase in Ca activity is in contrast with general evolutionary trends of the complex and would thus require either an anomalous loss in alkalis, or the influx of external Ca-Sr-rich fluids soon after kakortokite consolidation. Zircon-bearing alterations (type-III) are restricted to more heavily altered kakortokites and associated pegmatites4,5, and require a general decrease in alkalinity and H2O activities, and potentially higher CaO activity. Mass balance calculations for the observed eudialyte decomposition assemblages suggest that alteration was not associated with significant remobilisation of Zr, Nb and REE. We infer that the immobility of HFSE and REE during subsolidus eudialyte alteration reflects both the in-situ nature of the metasomatising melts/fluids, merely migrating along crystal boundaries, as well as the overall high pH of the late-magmatic fluids7, which prevents stable complexation of REE and HFSE with fluoride and chloride compounds8.

Layered Kakortokites at Kringlerne Kakortokite Unit #0 ~8 m thick

#0white

#0red

#0black

Magmatic paragenesis: Euhedral and sector-zoned eudialyte in red kakortokites

Late- to post-magmatic paragenesis: Type-I: Typical pseudomorphs dominated by Na-Zr phase catapleiite, nacareniobsite-(Ce), aegirine, albite and analcime

Type-II: Rare pseudomorphs dominated by Ca-Zr phase gittinsite, annite, analcime, pectolite, fluorite and Ca-REE-silicates

Four intrusive events: 1. Marginal augite syenite 2. Alkali granites 3. Pulaskite, Foyaite, Naujaite 4. Kakortokite, Lujavrite

Agpaitic nepheline syenites: • Molar Na+K/Al > 1.2 • Complex Na-HFSE minerals like

eudialyte and rinkite, instead of common Zr-Ti phases like zircon, titanite or ilmenite

References [1] Waight, T.E, et al., 2002, Chem. Geol. [2] Sørensen, H., 1992, App. Geochem. [3] Andersen, T. et al., 2010, J. Pet [4] Karup-Møller, S. et al., 2010, Bul. Geol. Soc. Denmark [5] Karup-Møller, S. & Rose-Hansen, J., 2013, Bul. Geol. Soc. Denmark [6] Markl, G. et al., 2001, J. Pet [7] Markl, G. and Baumgartner, L., 2003, Contr. Min. Pet. [8] Migdisov, A.A. and Williams-Jones, A.E., 2014, Min. Deposita.

Augite syenite

Kringlerne deposit - the kakortokite series The Kringlerne deposit, licensed by TANBREEZ A/S Mining Greenland, covers the full sequence of kakortokites and related rock types in the lower part of the magma chamber (blue on map). These comprise a series of rhythmically layered floor-cumulates with alternating horizons enriched in arfvedsonite (Na-amphibole), eudialyte, nepheline and alkali feldspar. Large volumes of magmatic eudialyte, easily extracted by magnetic separation, provide significant resources of Zr, (H)REE, Nb and Ta. Over 29 repetitive units of black, red and white layers are mapped in the field and numbered from -11 to +16.

Inferred resources: 4,3 billion tons @ 1.8 % ZrO2, 0.2% Nb2O5,

0.5 wt % TREO (of which 27% HREE and 73% LREE)

Main ore mineral: Eudialyte

Formula: Na15Ca6(Fe,Mn)3Zr3(Si,Nb)Si25O72(O,OH,H2O)3(Cl,OH)2

1 mm

*To be submitted to Mineralogical Magazine , April 2015

3D petrogenetic grid Horizontal section: Eudialyte to Catapleite

Vertical section: Catapleiite to Gittinsite

Section below

Section above