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

Abstract

The structural geology and tectonic setting of hydrothermal gold deposits are paramount in 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 time scales 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. Geometrical analysis relates ore body 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 pre-existing 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 time scales, so that it is inadvisable to infer local tectonics from deposit-scale patterns, and visa versa. It is essential to place mineralizing events within a complete geological history in order to distinguish pre-and post-mineralizing structures from syn-mineralization deformation features.

Revised Geology and Geological Structures of the Northeastern Chungnam Basin in the Southwestern Korean Peninsula

“…Notably, the inversion-related fault system in the study area is filled with gold-bearing quartz veins, indicating that the hydrothermal fluid migration was contemporaneous with the contractional deformation. Although it has been suggested that mesothermal quartz veins ubiquitously form by fault-valve behavior related to unfavorable high-angle reverse faulting and resultant crustal fluid redistribution in inverted extensional basins (Sibson et al, 1988;Sibson and Ghisetti, 2018;Blenkinsop et al, 2020;Kwak et al, 2022 and references therein), the characteristics and processes of fluid circulation related to Mesozoic orogenic events in the southwestern Korean Peninsula remain unsolved. Interestingly, the reported sericite and K-feldspar K-Ar isotopic ages from hydrothermally altered basements and basin-fills in the inverted Chungnam Basin and adjacent area (ca.…”

Shallow Composition and Structure of the Upper Part of the Exhumed San Gabriel Fault, California: Implications for Fault Processes

“…Entire thin-section scans were made using an Epson Perfection Pro scanner in plane-polarized and cross-polarized light. We use our microscopic observations to search for evidence for grain-scale brittle, crystal-plastic, diffusive mass transfer, and alteration-related deformation (c.f., Rutter, 1986;Blenkinsop et al, 2020;Wang, 2021). Systematic X-ray diffraction (XRD) analyses were done to determine whole-rock mineralogy and to constrain rock compositions, possible depth ranges, and fluid-rock interactions in the fault-related rocks.…”

Hydrodynamic Links between Shallow and Deep Mineralization Systems and Implications for Deep Mineral Exploration

“…Most hydrothermal deposits, including different types formed in various geological settings, are controlled by structures (Richards and Tosdal, 2001;Vearncombe et al, 2004;Zhai et al, 2010;Chauvet, 2019a;Rowland and Rhys, 2020). Even though many studies of structural control of mineralization are mainly concerned with the geometric relationships between structures and ores as well as kinematic and stress analyses of the ore-hosting structures (Wang, 2010;Wang Z M et al, 2020;Chen et al, 2021), it is well understood that the nature of structural control of mineralization is actually about the structural control of ore-forming fluid flow (McCaffrey et al, 1999;Cox, 2005;Xu et al, 2019;Blenkinsop et al, 2020). It can be shown that the amount of ore-forming fluid flowing through a mineralization site, which determines the size and economic value of the deposit, is controlled by three factors: hydraulic potential gradient, rock permeability, and duration, all of which are related to structures.…”