Scapolite Pegmatite from the Minas Fault, Nova Scotia: Tangible Manifestation of Carboniferous, Evaporite-Derived Hydrothermal Fluids in the Western Cobequid Highlands?

J. V. Owen1 and J. D. Greenough2
1 Department of Geology, Saint Mary's University, Halifax, N.S. Canada B3H 3C3
2 Department of Geological Sciences, Okanagan University College, 3333 College Way, Kelowna, B.C., Canada V1V IV7

Abstract: Pegmatite cutting chlorite schist in the Minas fault at McKay Head, Nova Scotia, consists of Cl-rich (2.7–3.8 wt.% Cl) marialitic scapolite (EqAn21−32) with interstitial, apparently primary analcite, hematite and rutile, and later (including vug-lining) analcite, pyrite, chlorite, titanite and calcite, and cross-cutting epidote veins. Some of the latter phases might have crystallized from residual pegmatitic fluids. Unlike many other primary scapolite-bearing igneous rocks, the McKay Head occurrence has compositional affinities with mafic (rather than felsic) systems: it is enriched in transition metals (e.g. Cr≤53 ppm), and has very low LILE concentrations (e.g. Rb<10 ppm; U<1 ppm; Th<2 ppm; Ba<20 ppm) and Rb/Sr ratios (∼0.05). The presence of interstitial rutile and hematite rather than ilmenite indicates that the pegmatitic fluid was oxygenated late (T∼400°C) in its crystallization history.

The pegmatite is interpreted to be related to highly sodic hydrothermal solutions derived from (or affected by) early Carboniferous evaporites of the Windsor or Horton groups. Compositionally-similar fluids, perhaps also related to an evaporite source, may be responsible for a regional, early Carboniferous Na-metasomatic event that altered a suite of alkaline granitoid intrusions shortly after their emplacement.

Keywords: scapolite • pegmatite • evaporates • Minas fault • Nova Scotia

Mineralogical Magazine; June 1999 v. 63; no. 3; p. 387-397; DOI: 10.1180/002646199548484
© 1999, The Mineralogical Society
Mineralogical Society (