Vein morphology, host rock deformation and the origin of the fabrics of echelon mineral veins

Abstract

A system of sigmoidal echelon veins from a sample of sandstone from the Upper Carboniferous Culm sequence of southwest England is described. Veins are separated from one another by strips of sandstone, and divided internally by thin seams with crack—seal fabrics. The latter extend as thin veins into the sandstone host rock without change of fabric. Seams appear to be merely parts of crack—seal veins formed in a first phase of deposition in only minutely opened fractures. This phase ended as rates of fracture opening greatly increased. To allow for this widespread opening host rock between dilatating fractures (sandstone strips and seams) had to be deformed. This deformation was limited, however, to rotation, bending and fracture. Shear displacement was a function of dilatation, not zone-parallel ductile shear strain. The textures of the quartz and carbonate aggregates filling the sigmoidal veins show that second-phase crystallization took place into cavities opening more rapidly than growth was able to fill them. Growth for the greater part took place from fibres in seams and not off vein walls of the sandstone host rock. Coarsest aggregates fill the arcs of folds in seams, where rates of vein opening might be expected to have been highest and the scope for competitive cavity growth greatest.