Sigmoidal wall-rock fragments: application to the origin, geometry and kinematics of en échelon vein arrays

Abstract

Wall-rock fragments found within en échelon vein arrays may provide valuable information on the origin of vein systems, shear zone kinematics, and vein geometry. These fragments are sigmoidal and comprise intact rock between adjacent veins or fragments enclosed within individual veins. We present field examples of sigmoidal rock fragments within en échelon veins that grew as opening-mode fractures in response to an applied simple shear. By virtue of their unique shape and alignment of pre-existing fabric, we demonstrate how small wall-rock fragments may be used as kinematic indicators as well as to predict geometries of individual veins and entire vein arrays. Such a tool may be especially useful in mining exploration where the precise boundaries of large veins are unknown, or in stockworks where the rock is composed primarily of vein material.