Hunting for Rare-Earth-Element (REE)-bearing minerals in Northern Labrador: MLA-SEM analysis of surficial sediments within the glacial dispersion zone from the Strange Lake main zone deposit

Miller, Mikayla (2021) Hunting for Rare-Earth-Element (REE)-bearing minerals in Northern Labrador: MLA-SEM analysis of surficial sediments within the glacial dispersion zone from the Strange Lake main zone deposit. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Strange Lake area hosts important Zr-Nb-Y-REE deposits, associated with a small peralkaline granite intrusion. The deposits and the host rocks contain unusual minerals, some of which are essentially unique to this site. Geochemical data from glacial sediments or “tills” define dispersion from the deposits for at least 35 km, and the Strange Lake area is regarded as a “type example” of linear glacial dispersion from a point source. This thesis study uses Mineral Liberation Analysis – Scanning Electron Microscopy (MLA-SEM) methods to investigate the mineralogy of glacial sediments and document the dispersion of unusual (indicator) minerals. It is in part an assessment of the MLA-SEM technique for use in indicator-mineral studies, which are increasingly important in mineral exploration. Seventy-six samples of till were collected from an area extending for 35 km ENE of the Strange Lake Main Zone deposit, aligned with the inferred direction of ice movement. Samples were processed to separate the 0.125 - 0.18 mm size fraction for direct analysis, without any preferential separation of denser minerals. MLA-SEM results thus directly document the abundances of 55 minerals, ranging from common silicates to rare accessory minerals diagnostic of the Strange Lake deposits. This large database was then evaluated using statistical and geographical analysis methods. Common silicates (e.g., quartz, feldspars, garnet and amphiboles) collectively make up > 90% of typical till samples, but the rarest indicator minerals occur at levels < 10 ppm. The reliability of data degrades at such low abundances (in part due to probability effects) but systematic geographic variation patterns can still be discerned for many such rare minerals. Numerous diagnostic minerals from Strange Lake were detected, although their abundance was lower than expected from previous MLA-SEM analyses of drill core samples. Many minerals show linked abundance variation (correlation or anti-correlation) and such variation commonly has a geographic component. Systematic geographic variations for major minerals and many minor minerals seem to correspond with regional contrasts in bedrock geology from west to east, suggesting that patterns mostly record local provenance. Accessory minerals that are diagnostic of Strange Lake also show systematic geographic abundance variations, which are superimposed on these regional trends, but in some cases the patterns appear superficially similar. The most abundant and persistent indicator minerals are the Ca-Zr silicate gittinsite and the Y-Ca-REE silicate gerenite, which are also the most abundant in the Strange Lake source rocks. However, geographic variation patterns for these minerals are rather different. Gerenite abundance diminishes in a down-ice direction, as expected, but gittinsite seems to increase in abundance, which is unexpected. Other indicator minerals mostly diminish in abundance in a down-ice direction but even some of the rarest (e.g., stetindite, gadolinite and bastnaesite) remain sporadically detectable at 35 km from the source. The controls on dispersion patterns are not fully understood, but likely involve mineralogical factors as well as aspects of the glacial environment. MLA-SEM data suggest that many indicator minerals from Strange Lake typically form small domains within larger particles of common minerals, and these host minerals may thus influence dispersion patterns. In this context, it is interesting that the most persistent indicator minerals seem to be preferentially associated with quartz, which is the most durable of common rock-forming minerals. Like most research studies, this project did not answer all questions posed at the outset, and it did not always follow the intended plan. However, the results indicate that the MLA-SEM method has considerable potential for use in indicator-mineral studies, and point to interesting future research directions connected to development of the method and its application to other geological problems.

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