Water-sill hypothesis for the origin of certain veins in the Meguma Group, Nova Scotia, Canada

Abstract

Certain bedding-parallel quartz veins (some gold-bearing) in the Cambrian-Ordovician Meguma Group, Nova Scotia, are believed to indicate the former presence of water sills. The veins contain primary subhedral quartz crystals, which tend to be oriented normal to the vein walls and display Brazil twins. Twinned crystals commonly show hexagonal sections with dark solid-inclusion-rich centers and white fluid-inclusion-rich rims. Viewed in a plane perpendicular to bedding, the dark centers of crystals exhibit millimetre-thick quartz plates bounded by bedding-parallel micrometre-thick inclusion bands of white micas. The inclusion bands are aligned parallel to the vein walls and terminate within tapered crystals that have an outer zone of milky quartz. The quartz veins are on limbs and hinges of folds, particularly anticlines. From these observations we conclude that: (1) the solid-inclusion banded quartz resulted from episodic hydraulic extension fracturing and incremental growth of dispersed quartz bridges across water sills; (2) the milky quartz filled the vein by growing in optical continuity with the solid-inclusion banded quartz; (3) localization of certain bedding-parallel quartz veins is due to ponding of water beneath aquacludes in very broad domes formed by differential compaction in the Meguma Group; (4) during tectonic shortening these intensely veined rocks localized some of the major anticlines.