Alteration of the Thor Lake layered alkaline complex related to the Nechalacho Deposit

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1 MSc. Proposal: Alteration of the Thor Lake layered alkaline complex related to the Nechalacho Deposit Kent MacWilliam Supervisor: A.E. Williams-Jones Department of Earth and Planetary Sciences McGill University March 22nd, 2010

2 Introduction The analysis of hydrothermal alteration has provided important information for our understanding of the genesis of metallic mineral deposits. Consequently, the alteration of many types of hydrothermal ore deposits, for example porphyry and epithermal deposits, has been documented in considerable detail. In comparison, the alteration of alkaline igneous systems has been poorly documented and its significance to ore deposition is not well understood. The Thor Lake layered alkaline complex (TLLAC) has experienced intense hydrothermal alteration. Located 100km SE of Yellowknife, the TLLAC is the youngest intrusive pulse of the 2.15 Ga Blachford Lake Complex and is located at its core (Sheard et al., in prep.). The alkaline intrusion is host to two currently explored rare-metal (Zr- Nb-REE) horizons containing potentially exploitable concentrations of HREE. Owing to the severe alteration, the TLLAC has been characterized mainly on recognition of igneous textures and on rare primary mineralogy. Sub-horizontal layers of aegirine nephaline syenite, sodalite syenite and lujavrite comprise the primary lithologies of the TLLAC (Sheard et al., in prep.). These layers range in thickness from tens of centimeters to tens of meters, and are divided by sharp intrusive contacts. The dominant lithology is the aegirine nepheline syenite (Sheard et al., in prep.). It is medium- to coarse-grained and pegmatitic lenses are common. Aegirine and nepheline are the main phenocryst minerals and albite commonly forms the groundmass. Lujavrite is a fine-grained rock exhibiting characteristic alignment of albite and aegirine crystals (Sheard et al., in prep.). Nepheline is the main phenocryst phase, occurring as euhedral crystals up to 3mm in diameter. Interstitial albite crystals wrap around nepheline phenocrysts and show a sub-horizontal lamination. The sodalite syenite is mediumgrained, and made up of euhedral to massive sodalite with interstitial aegirine, orthoclase and albite. Sodalite generally forms crystals 0.5cm in diameter, which have been observed to be enclosed in anhedra of orthoclase and aegirine. Interstitial orthoclase and albite are intergrown displaying perthitic exsolution texture. The main Zr-Nb-REE mineralization in the TLLAC is present as two zones, which generally form distinct horizons, though locally the upper zone grades into the lower zone. The two zones are texturally and mineralogically distinct. The upper zone contains abundant disseminated zircon which occurs in undulating laminations. Mineralization of the lower zone occurs within pseudomorphs interpreted to represent former eudialyte phenocrysts. Although the mineralogy of the two zones is similar, the HREE are more strongly concentrated in the lower zone and LREE in the uppper zone (Sheard et al., in prep.). Previous studies have addressed alkali metasomatism of alkaline igneous complexes. The syenites and pegmatites of the Tamazeght complex show evidence of strong hydrothermal alteraton (Salvi et al. 2000). For example: eudialyte in zoned pegmatites is altered to amorphous Fe and Mn hydroxides and Ca-rich catapleiite, and partially altered eudialyte contain zircon and fluorite at their rims. Nepheline is altered to a red colour in syenite by the substitution of K for Ca to form gonnardite. This Ca enrichment is due to contact with carbonate country rocks. Ca metasomatism in alkali complexes has also been documented at Ilimausaq (eg: Graser and Markl 2008).

3 However, there are no descriptions in the literature of alteration akin to that of the TLLAC. Alteration Five main alteration assemblages have been identified as affecting the Thor Lake layered alkaline complex. These assemblages are 1) early deep alteration including sodalite and analcime, 2) biotite/magnetite alteration, 3) intensive albitization, 4) late fluorite-illite alteration and 5) hematization. Currently, no definitive alteration paragenesis has been established and no alteration sections have been synthesised. As a result there are no spatial and temporal constraints on alteration. Until now, my work has been focused on the deep primary alteration. Alteration of the TLLAC is focused upwards in the intrusion. Fresh rocks at depth are essential in understanding primary mineralogy of the intrusion; continued drilling is revealing continually less altered rocks. Thus, most work so far on alteration has been on deep core. Early alteration was intersected near the bottom of studied holes, at depths below 170m. In hand specimen, the principal early alteration mineral, sodalite (massive) fills embayments within interstitial K-feldspar and aegirine phenocrysts. Crystals of sodalite, which are blue in colour, and exhibit strong orange alteration when exposed to UV illumination, have broad, 0.5-1cm white rims where in contact with aegirine and K- feldspar. In thin section, this broad rim is observed to be composed mainly of aphanitic sodalite and bladed albite with analcime rims. The biotite/magnetite alteration (BMA) is of particular importance in the zones of economic Zr-Nb-REE mineralization. In the upper mineralized zone BMA has replaced most of the primary mineralogy and is particularly evident in layers containing abundant altered zircon euhdra and embayed pegmatitic K-feldspar. In the lower mineralized zone, biotite is rare to absent, though magnetite is dominant in the matrix and has replaced the cores of the pseudomorphs interpreted to represent former eudialyte phenocrysts. The pink albitization, or bladed cleavlandite, is a common alteration type, typically occurring within the top 100m of drill holes. It is not laterally continuous, and is intensely destructive of fabric and porous. Most importantly, it is destructive of Zr-Nb- REE mineralization; small relicts of mineralization and BMA or pegmatitic K-feldspar are commonly observed in zones of pervasive albitization. This pink albite is similar in habit to albite deeper in the system but lacks fluorescence. Fluorite/illite alteration occurs in discrete zones commonly several meters thick in which the rock is completely replaced by this purple and green alteration assemblage. It overprints other units and alteration assemblages but is relatively rare compared to the pink albitization. Hematization is an ubiquitous alteration. It is unclear whether this alteration is temporally restricted or occurs throughout the alteration history. Aegirine is particularly susceptible to hematization, as is magnetite. Commonly, the matrix of the nepheline aegirine syenite is completely hematized leaving only feldspar phenocrysts.

4 Research Objectives There is a deficit of knowledge of the alteration of the TLLAC and other alkaline igneous complexes. Alteration is a key component of the economic viability of the Nechalacho deposit as: 1) it is spatially associated with the REE mineralization (i.e., biotite/magnetite in the "upper zone", and magnetite in the "lower zone"); and 2) as hydrothermal fluids are believed to have liberated REE from zircon and eudialyte to form spatially associated more readily extractable REE phases (e.g.,. fergusonite, allanite and Ce-bastnäsite). However, alteration (albitization) also has remobilized and dispersed Zr- Nb-REE mineralization on the scale of meters. Understanding the alteration of this alkaline igneous complex will contribute to the future exploration of HFSE deposits. The goal of this study is to show definitively the effects of alteration on the TLLAC, and expand our knowledge of the variety of alteration styles that affect alkaline complexes. Establishing the spatial and temporal sequence of alteration, reconstructing the primary mineralogy of highly altered zones and determining the physicochemical conditions of alteration, in particular those related to mineralization, are the main objectives of this study. These objectives will be met by methods specified below. Methodology A detailed alteration map and section is central to constraining alteration spatially and temporally. To date no sections or plans have been prepared that show the distribution of alteration types. However, as the primary magmatic mineralogy has been difficult to ascertain, most of the logging of drill holes has taken alteration and primary textures into account, although in many cases their interpretation has led to more confusion than illumination. For example, a former lithocode for the intense albitization alteration considered this a distinct primary unit (felspathite). This exemplifies the need to reinterpret the extensive past logging in light of current understanding in order to produce a comprehensive alteration map. Additionally, there may be a spatial link between the TLLAC and the adjacent T- zone mineralization. At the T-zone, the alteration is hydrothermal and may represent an evolution of ore fluids related to the TLLAC alteration. To date no solid link between the two deposits has been established other than there is an overall enrichment in HFSE in both the T-zone and the mineralized zones of the TLLAC. Establishing the spatial distribution of alteration assemblages may provide insight into the relationship between the two distinct deposits. The fluid chemistry and source of the fluids responsible for alteration of the TLLAC is not known. The presence of abundant ferric iron is clear, and is reflected in the primary mineralogy of the dominant rock type, the aegirine-nepheline syenite. Partial to complete hematization of the aegirine is common throughout the upper 200m of most drill holes. Zones of dominant magnetite alteration, in which aegirine-nepheline syenite has been replaced, implies a reduced fluid. Eudialyte, conversely, has incorporated ferrous iron. Therefore, hematite cores and hematized matrix of eudialyte pseudomorphs may reflect an oxidizing event. Whether, there has been alkali metasomatism (i.e.,

5 addition of K and/or Na) is less clear, although it is apparent that HREE mineralization was accompanied by alteration involving potassic minerals (e.g., biotite) and that the HREE were dispersed in the presence of fluids that albitized the rock. The alteration reactions observed must be quantified. Microprobe analysis of alteration minerals will provide the data needed to reconstruct these reactions. In principle, this will identify the redox mechanisms and also the activities of potentially mobile components like the alkalis. Cathodoluminescence will help qualify alteration by helping in the recognition of temporally different stages of alteration characterized by the same minerals. Mineral luminescence displays the distribution of major and trace elements in alteration minerals by revealing zoning or by differentiating between primary and altered minerals such as albite. In addition to identifying the alteration reactions, the research will also determine the mass changes of elements via whole rock chemistry, thereby evaluating whether or not alteration involved processes metasomatism and if so, which elements were added, removed and conserved. The method of Grant (1986) involving the use of isocon plots will be used for this purpose and will also be helpful in reconstructing them primary mineralogy of the altered rocks. Finally, the source of fluids will be determined isotopically. Ratios of 18 O and D in alteration minerals such as quartz in pseudomorphs interpreted to represent eudialyte phenocrysts will be used to assess the origin of alteration fluids. References: Grant, J.A The isocon diagram-a simple solution to Gresens' equation for metosmatic alteration. Economic Geology 81: Graser, G., Markl, G., Ca-rich ilvaite-epidote-hydrogarnet endoskarns: a record of late magmatic fluid influx into the persodic ilimausaq complex, South Greenland. Journal of Petrology. 49: Salvi, S., Fontan, F., Monchoux, P., Williams-Jones, A.E., Bernard, M Hydrothermal mobilization of high field strength elements in alkaline igneous systems: evidence from the Tamazeght Complex (Morocco). Economic Geology. 95: Sheard, E.R., Williams-Jones, A.E., Helligmann, M., Pederson, C., Trueman, D.L. in prep. Behaviour of zirconium, niobium, yttrium and the rare earth elements in the Thor Lake rare-metal deposit, Northwest Territories, Canada.

The hydrothermal mobility of the rare earth elements

The hydrothermal mobility of the rare earth elements A.E. Williams-Jones 1 1 Department of Earth and Planetary Sciences, McGill University, Montreal, QC, Canada a corresponding author: anthony.williams-jones@mcgill.ca