Abstract

Graftonite-beusite occurs in only three localities of granitic pegmatites in the Superior Province of the Canadian Shield. The three occurrences illustrate the broad crystal-chemical, geochemical and genetic flexibility of this phosphate series. (i) In the granulite-facies terrane at Conifer Road in northwestern Ontario, a geochemically primitive, barren pegmatite of the Treelined Lake granitic batholith carries graftonite with inclusions of Sc-bearing, Mg-rich johnsomervilleite and exsolved sarcopside lamellae; all three phases rank with the most Fe- and Mg-rich compositions analyzed to date in terrestrial samples. (ii) In the YITT-B pegmatites, atypical members of the beryl-columbite-phosphate subtype at Bernic Lake, southeastern Manitoba, homogeneous Ca-rich beusite is Mn-dominant and associated with manganoan fluorapatite and traces of triplite and triphylite. (iii) In the #22 pegmatite, of the beryl-columbite-phosphate subtype, at Cross Lake in north-central Manitoba, homogeneous Ca-poor beusite shows the most manganoan composition known to date, and is associated with similarly Mn-rich fillowite, fluorapatite and triploidite, plus two unidentified phosphates. Low activities of Na, Li and F, combined with high concentrations of Ca, Fe, Mn (and locally Mg), are required to stabilize graftonite-beusite. These conditions cannot be expected to be widespread in the rare-element pegmatites of the Superior Province, as most of them show early enrichment in Li and F in their regional fractionation trends. Consequently, graftonite-beusite is not stable, as triphylite-lithiophilite tends to be a relatively early phase, and fluorapatite consumes all phosphate-bound Ca.

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