Harley E. Hutchinson

B. Sc. Honours Thesis

Geology, Geochemistry, and Genesis of the Brazil Lake Pegmatites, Yarmouth County, Nova Scotia

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The Brazil Lake pegmatites, located about 25 km northeast of Yarmouth, intrude amphibolites and orthoquartzites of the White Rock Formation on the eastern flank of the Yarmouth syncline. The pegmatites are of igneous origin and were forcefully injected into fractures subparallel to the strongly developed regional schistosity. The following mineralogy was determined for the lithium-rich pegmatites: quartz, albite, microcline, spodumene, muscovite, beryl, tourmaline, zircon, apatite, columbite, tantalite, biotite, epidote and cassiterite.

Three distinct chemical/mineralogical/textural phases are recognized within the pegmatite. The three phases are asymmetrically distributed in zones. These zones, in order of emplacement, are: zone A - a coarse-grained, K-rich microcline-quartz-muscovite phases; zone B - a very coarse- grained, Li-rich spodumene-quartz-muscovite phase; and zone C - a Na-rich aplitic albite-quartz phase. The strong partitioning of the alkali elements Li, Na and K, is interpreted to have resulted from differential rates of element transport and crystallization, owing to the presence of a fluid phase coexisting with the silicate magma. Metasomatic effects from the pegmatite resulted in tourmaline and holmquistite ( a rare Li-rich amphibole) being emplaced in the host quartzite rocks.

A K-Ar date of 333 + 7 Ma was determined for the pegmatites, and this is interpreted as a cooling age, possibly resulting from thermal overprinting. This date is in agreement with dates from other southern Nova Scotia plutons, including the Brenton, which is just 2 km southwest of the Brazil Lake pegmatites. These are plutons all record a Late Carboniferous tectonic or thermal event dated at about 320 Ma.

The genetic evidence available is inconclusive, but it is very unlikely that the Brazil Lake pegmatites, which contain an average of 1.16% Li2O, were derived from the nearby Brenton pluton, which is, in fact, depleted in lithium (averaging 71 ppm Li), when compared to other southern Nova Scotia plutons.

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Pages: 106
Supervisor:  Gunter Muecke