Strange Lake

Quebec, Canada

Main commodities: REE Zr Nb
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The Strange Lake Rare Earth Element deposit is located on the border between Quebec and Labrador 235 km NE of Schefferville, 150 km west of Nain, 125 km west of the Voisey's Bay nickel-copper mine and 1100 km NE of Québec City in NE Canada.

The Strange Lake area is located within the Paleoproterozoic Rae or Southeastern Churchill Province in the northeastern Canadian Shield of Québec and Labrador. The Southeastern Churchill Province is interpreted to have formed as a result of oblique collisions involving the Superior and Nain cratons with a third intervening Archaean block. It comprises a 300 km wide, north-south trending, tectonically assembled group of lithotectonic domains that separate the Superior and Nain cratons, commencing with the Labrador Trough to the west, interpreted to be a passive margin wedge located along, and overlying, the eastern edge of the Superior craton. Strange Lake is within the two easternmost of these domains, the Mistastin in the east and the Mistinibi-Raude domain to the west. The intervening domains are mostly heavily metamorphosed and include a series of major north-south shear zones accommodating regional dextral shearing as well as contraction.

The Strange Lake deposit is part of a ~6 km wide, Mesoproterozoic (1240 Ma; Pillet et al., 1989; Miller et al., 1997) post-tectonic, anorogenic (A-type), peralkaline granite complex. Mineralisation occurs within peralkaline granite-hosted pegmatites and aplites and, to a lesser degree, within the host granites, particularly in intra-pegmatitic granites.

The pluton at Strange Lake is surrounded by a fluorite/hematite breccia, and intruded along the contact between Palaeoproterozoic (Aphebian) heterolithic gneiss and Elsonian (1500 to 1400 Ma) aged Napeu Kainiut monzonite pluton. A shallow emplacement is indicated by the presence of roof pendants of gneiss and monzonite in the Strange Lake granite and low trapping pressures estimated from orthomagmatic fluid inclusions (Salvi and Williams-Jones, 1992). Two different granite types have been recognized, based on their feldspar mineralogy (Nassif, 1993). The rocks in the central and southern part of the pluton comprise hyper-solvus granite (with perthitic K feldspar), surrounded by a later sub-solvus granite (characterised by two feldspars, albite and microcline), which is host to two pegmatite-rich mineralised zones. This reverse granite zoning has been attributed to feldspar fractionation and fluid saturation during crystallisation (Nassif, 1993). However, Gysi et al. (2016) suggest a similar reverse zoning could result from a later batch of melt forcing its way to the margins during multiphase intrusion cycles (e.g., Vigneresse, 2007). This interpretation is supported by field observations, which indicate that the first batch of melt was less hydrous, as evidenced by the occurrence of late interstitial amphibole, and the second batch of melt was saturated with H2O at an early stage of crystallisation, with amphibole occurring as early phenocrysts (Siegel and Williams-Jones, 2015). This indicates that successive intrusions of variably evolved melts likely affected H2O saturation, alkali content, and HFSE enrichment (i.e., REE, Zr, Nb and Ti).

The Subsolvus granite is the most voluminous unit in the Strange Lake Alkali Complex, and is the principal host to REE-bearing pegmatites. It typically contains very fine-grained dark grey to black rounded inclusions of hyper-solvus granite, locally surrounded by minor white-grey mm-scale reaction rims. It is typically fine- to medium-grained (i.e., <1 cm) comprising variably altered feldspar (sodic>potassic), intergranular white-grey quartz, subhedral variably altered arfvedsonite (a sodium amphibole), interstitial/poikilitic gittinsite [CaZrSi
2O7] and euhedral ghosts of narsarsukite [Na4(Ti,Fe)2(Si8O20)(O,OH,F)2], with ubiquitous wispy pale purple or interstitial dark purple fluorite. The groundmass of the granite has an overall granular to sugary appearance as a result of extensive albitisation, while arfvedsonite, which commonly exhibits a bimodal grain size of fine mm-scale anhedral grains and relatively coarser-grained euhedral crystals, is variably altered or may be fresh. Similar to the pegmatites, arfvedsonite is commonly altered either by aegirine, particularly proximal to pegmatites, or earthy brown-red hematite. Large portions of the B Zone (see below) exhibit fresh arfvedsonite in a variably altered matrix. Narsarsukite, which is grey when unaltered, is often tan-beige, suggesting replacement by titanite. Gittinsite is variable in colour, but is commonly partially replaced by dark grey-green LREE-bearing allanite. This replacement may take the form of 'salt and pepper' spotting or as amorphous patches. Alteration typically developed in the host sub-solvus granite is not typically developed in the inclusions (description from Gowans et al., 2014).

The pegmatites and minor aplite are composed of feldspar (potassic>sodic), glassy to white quartz, arfvedsonite, gittinsite, fluorite and various minor accessory minerals including titanite [CaTiSiO
5], allanite [(CaCe)(Al2Fe2+)(Si2O7)(SiO4)O(OH)], pyrochlore [(Na,Ca)2Nb2O6(OH,F)] and gadolinite [(Ce,La,Nd,Y)2FeBe2Si2O10]. Euhedral to subhedral gittinsite and amphibole are apparently generally coeval. Whilst feldspar exhibits a variable paragenetic relationship relative to arfvedsonite and gittinsite, it is commonly somewhat later in complex pegmatites and earlier in simpler, late pegmatites. Quartz is late and interstitial whilst fluorite, which is commonly dark purple to black, is generally later than quartz. Arfvedsonite is typically strongly replaced by either coarse bottle green aegirine or red-brown earthy hematite and may be strongly leached to form vugs that are sometimes quartz-hematite lined. Gittinsite is typically altered to a mottled orange-pink to beige colour and spotted with very fine grey-green LREE-bearing allanite, giving a spotted 'salt and pepper' texture. Feldspar is often altered as concentric oscillating zones or mixed hematite and fluorite, giving a mottled, often fractured appearance (description from Gowans et al., 2014).

Two zones of mineralisation have been delineated:
• the Main zone, located in the central part of the pluton, (detail not encountered) and
• the B zone, located along the northwestern margin of the Strange Lake pluton, which consists of a lens-shaped pegmatite-rich domain hosted by subsolvus granite, and covers an area of ~400,000 m
2. The pegmatites occur as a NE-SW elongated ~1400 x 300 m stack of flat-lying sheets dipping gently to the NE, distributed over a thickness of ~50 m. The pegmatites form sheets that range from a few cm to 10 m in thickness and are mineralogically zoned into border and core zones. Most of the pegmatite sheets are highly altered and primary features are rarely preserved. The least altered border pegmatites, however, are similar in mineralogy to the granite. Thin pegmatite dykes (~3 to 5 cm thick) are unzoned and have a mineralogy similar to the border pegmatite, albeit with a higher modal proportion of quartz.

The pegmatites of the B zone are characterised by a core enriched in quartz, variable proportions of zircon and fluorite, and light rare earth elements (LREE) fluoro-carbonates REE-F-(CO
2), encased between a granitic border enriched in zircono-silicates and granitic minerals.The border pegmatites contain large (>1 to 5 cm) to medium (~1 cm) subhedral K feldspar crystals (between ~40 and 60 vol.%). Quartz forms subhedral to interstitial crystals (>40 vol.%).

Gysi et al. (2016) conclude that the pegmatite sheets and surrounding granites evolved in three essential stages, as follows:
Stage I a magmatic stage, involving a progressive enrichment in REE and HFSE, from a relatively low abundance in the hypersolvus granites, to relative enrichment in the subsolvus granites;
Stage II a near-neutral hydrothermal (alkali metasomatism) stage involving interaction with high temperature (≥350°C) NaCl-bearing orthomagmatic fluids, resulting in Na metasomatism and albitisation of the granite (primary K-feldspar replaced by albite) and a relative depletion in Zr, Y and REE in the Na enriched sub-solvus granite, mobilised by Ca-free, fluorine rich fluids, forming REE-fluorine complexes that were concentrated in the carapace of the intrusion. Following the replacement of K by Na in the granite, K metasomatism occurred in the pegmatites with albite lamellae replaced by microcline;
Stage III an acidic hydrothermal stage (comprising high- IIIa and low-temperature IIIb substages) that resulted from their interaction with HCl-HF bearing fluids. Stage IIIa led to pseudomorphic mineral replacement reactions (e.g., Na-Ca exchange during replacement of zircono-silicates) and formation of an aegirine/hematite halo around the pegmatites. In contrast, low temperature ≤200°C stage IIIb Ca-rich fluids, which were responsible for the hydrothermal mobilisation of Zr and REE, is manifested by fluorite and quartz veins, zircon spherules, gadolinite-group minerals, gittinsite, ferriallanite-(Ce), and a pervasive replacement of the granite by these minerals. The distribution of REE, Zr, Nb, and Ti was controlled by the competition between hydrothermal fluids and the stability of primary REE-F-(CO
2) minerals (e.g., bastnäsite-(Ce) host to LREE), zirconosilicates (i.e., Na zirconosilicates and zircon host to HREE and Zr), and Nb-Ti minerals (i.e., pyrochlore host to Nb and narsarsukite host to Ti), and the stability of secondary LREE silicates (i.e., ferriallanite-(Ce)), HREE silicates (i.e., gadolinite-(Y)), zirconosilicates (i.e., gittinsite and zircon), and Nb-Ti minerals (i.e., titanite and pyrochlore).

The mineral resources of the B zone at October, 2012 comprises (Quest Rare Minerals Limited website, 2016):
  TOTAL indicated resource - 278 Mt @ 0.93% TREO, comprising 0.57% LREO, 0.36% HREO, 1.92% ZrO
2, 0.05% HfO2 and 0.18% Nb2O5.
        Enriched zone - 20.02 Mt @ 1.44% TREO, comprising 0.72% LREO, 0.72% HREO, 2.59% ZrO
2, 0.06% HfO2 and 0.34% Nb2O5, and
        Granite - 258 Mt @ 0.89% TREO, comprising 0.55% LREO, 0.33% HREO, 1.87% ZrO
2, 0.05% HfO2 and 0.16% Nb2O5.
  Inferred resource - 214 Mt @ 0.85% TREO, comprising 0.55% LREO, 0.30% HREO, 1.71% ZrO
2, 0.04% HfO2 and 0.14% Nb2O5.

LREO = light rare earth oxides (La, Ce, Pr, Nd, Sm); HREO = heavy rare earth oxides (Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu); TREO = total rare earth oxides. All percentages are wt.%.

This summary is drawn from reference(s) listed below and from "Gowans, R.M., Lewis, W.J., Shoemaker, S., Spooner, J. and Zalnieriunas, R.V., 2014 - Technical report on the preliminary economic assessment (PEA) for the Strange Lake property, Quebec, Canada; an NI 43-101 Technical Report prepared by Micon International Limited for Quest Rare Minerals Limited, 258p."

The most recent source geological information used to prepare this summary was dated: 2016.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Gysi, A.P., Williams-Jones, A.E. and Collins, P.,  2016 - Lithogeochemical Vectors for Hydrothermal Processes in the Strange Lake Peralkaline Granitic REE-Zr-Nb Deposit: in    Econ. Geol.   v.111, pp. 1241-1276.

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