Chapter 1: Structural Geology Applied to the Evaluation of Hydrothermal Gold Deposits

Abstract

Abstract The structural geology and tectonic setting of hydrothermal gold deposits are paramount for understanding their genesis and for their exploration. Strong structural control on mineralization is one of the defining features of these deposits and arises because the permeabilities of crustal rocks are too low to allow the formation of hydrothermal deposits on realistic timescales unless rocks are deformed. Deformation zones and networks of deformation zones are the fundamental structures that control mineralization. Systematically analyzing deposit geometry, kinematics, and dynamics leads to the most thorough comprehension of a deposit. Geometric analysis relates orebody shape to controlling structures, and networks of deformation zones can be analyzed using topology to understand their connectivity and mineralizing potential. Kinematic analysis determines the location of permeability creation and mineralization. New views of shear zone kinematics allow for variable ratios of pure to simple shear, which change likely directions of mineralization. Multiple orientations of mineralized deformation zones may form simultaneously and symmetrically about the principal strain axes. Dynamic analysis is necessary for a mechanical understanding of deformation, fluid flow, and mineralization and can be achieved through numerical modeling. The relationship between deformation (kinematics) and stress (dynamics) constitutes the rheology; rheological contrasts are critical for the localization of many deposits. Numerous gold deposits, especially the largest, have evidence for multiple mineralizing events that may be separated by tens to hundreds of millions of years. In these cases, reactivation of structures is common, and a range of orientations of preexisting structures are predicted to be reactivated, given that they are weaker than intact rock. Physical and chemical processes of mineralization can be integrated using a nonequilibrium thermodynamics approach. Hydrothermal gold deposits form in contractional, strike-slip, and extensional tectonic settings. However, there may be great variation in the spatial scale over which the tectonic setting applies, and tectonic settings may also change on rapid timescales, so that it is inadvisable to infer local tectonics from deposit-scale patterns, and vice versa. It is essential to place mineralizing events within a complete geologic history in order to distinguish pre- and postmineralizing structures from synmineralization deformation features.