Mechanics of formation of gold-bearing quartz veins, Nova Scotia, Canada

Abstract

Abstract The Meguma Terrane of Nova Scotia, Canada, is composed largely of low-grade, clastic metasediments. Swarms of structurally-complex, bedding-concordant quartz veins are found across the whole terrane, partly or wholly outlining anticlinal domes. These veins exhibit spectacular features due to intermittent hydraulic fracturing and healing (“crack-seal”), such as phyllosilicate inclusion bands, “columnar” quartz grains, and included sheets of host rock. Overprinting all the precipitation features are a suite of deformation structures which include pinch-and-swell, tectonic stylolites, and cross-cutting extension and hybrid fractures. The veins were formed syntectonically, in folded and foliated strata, the vein-precipitating fluids pooling beneath less-permeable beds in anticlinal domes. Fluid migration paths were controlled by the two fundamental mechanical anisotropies in the host rocks: bedding and foliation. A simple two-dimensional vein model is considered. Using fracture propagation and quartz precipitation rates, estimates of rates of vein formation are determined. For example, it seems possible that a quartz vein 100 m long and 175 cm wide could form in less than 10 yrs. Such times are effectively geologically instantaneous, and have considerable implications with respect to physical and chemical processes in deformation and metamorphism.