Shannon Broughm

Granitoids enriched in incompatible elements have been the focus of ongoing
exploration for rare earth elements around the world (REE’s). High concentrations
of REE’s were discovered in 2010 at the contact zone between two Late Devonian to
Early Carboniferous felsic igneous units in the Debert Lake area, northeastern
Cobequid Highlands, Nova Scotia. Combined REE-concentrations range from 1800 to
>7000 ppm within the REE-mineralized dykes. The main rock types present in the
Debert Lake area include granitic rocks of the Hart Lake-Byers Lake (HLBL) pluton,
felsic volcanic and volcaniclastic rocks of the Byers Brook Formation and late diorite
bodies and diabase dykes. REE-mineralization is associated with a coarse-grained
and pegmatitic arfvedsonite-bearing granitoid that is elevated in incompatible
elements compared to the rest of the HLBL pluton. REE-mineralization occurs as
late granitic dykes that cross cut all other rock types in the area and range from 1-50
cm wide. Less common are 1-25 cm wide segregated pods, within the arfvedsonitebearing
granitoid. In order to study the relationship between the HLBL pluton, the
REE-enriched granitoids and the REE-mineralized dykes and pods, 19 samples were
selected for petrographic study of differences in mineralogy and textures.
Appropriate samples were analyzed using an electron-microprobe (EMP) for
geochemical trends recorded by amphibole.
Petrographic studies show a change in amphibole composition based on
color of pleochroism between each granitoid phase. Amphiboles within the
incompatible-enriched granitoids consistently show dark-blue to black or greenishbrown
to dark-blue pleochroism, indicative of arfvedsonite, compared to green to
yellow pleochroism in the HLBL granite. This evidence is consistent with EMP
results on amphiboles which show higher sodium and iron (arfvedsonite) content
within the REE-enriched granitoids compared to the HLBL pluton, which have
kataphorite amphibole. Harker variation plots of amphibole chemistry show
discontinuous trends between the HLBL pluton and the REE-enriched/mineralized
granite. Based on geochemical trends recored by the amphibole data, it is unlikely
that the REE’s were concentrated due solely to magma differentiation of the HLBL
pluton.

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Pages: 72
Supervisor: Richard Cox / Trevor MacHattie