IRON- AND ALUMINUM-RICH SERPENTINE AND CHLORITE FROM THE BOUNDARY ULTRAMAFIC COMPLEX, CAPE SMITH BELT, NEW QUEBEC

GEORGE V. ALBINO*
U.S. Geological Survey, Reno Field Office, Mackay School of Mines, University of Nevada, Reno, Nevada 89557, U.S.A.
* Present address: U.S. Geological Survey, P.O. Box 1488, Jeddah 21431, Saudi Arabia.

Abstract

Iron- and aluminum-rich serpentines (maximum 15.8% Al2O3 and 17.1% FeO, by weight) are common in the Boundary ultramafic complex, a komatiitic intrusive-extrusive body in the eastern part of the Aphebian Cape Smith Belt, northern Quebec. Most of the pervasive serpentinization occurred during peak (biotite-grade) metamorphism, which was contemporaneous with regional thrusting. Later retrogression, locally accompanied by CO2 metasomatism, was associated with later phases of deformation. Early-formed serpentines are typically pseudomorphic, whereas later serpentines include interpenetrating or vein antigorite and, lastly, vein chrysotile. Formation of the nonpseudomorphic serpentine textures is associated with other features indicative of element migration, including tremolitization of serpentine, serpentinization of clinopyroxene, and replacement of sulfides by silicates, and vice versa. Aluminum contents of lizardite are variable; nonpseudomorphic multilayer varieties have the highest Al contents. In contrast to the highly variable compositions of the serpentine- group minerals, chlorite compositions are relatively constant (Fe- rich clinochlore). Elevated abundance of Fe is most clearly associated with sulfide accumulations. Formation of Al- and Fe- enriched serpentine may be a function of the relatively aluminous bulk-composition (as much as 10 wt.% Al2O3) of the komatiitic rocks, and a later metamorphic history that involved considerable migration of elements on a local scale.

Keywords: lizardite, antigorite, chrysotile, chlorite, aluminous- serpentine, serpentinization, peridotite, electron-microprobe analyses, Cape Smith Belt, New Quebec.