Chapter 3: Structural Controls on Ore Localization in Epithermal Gold-Silver Deposits: A Mineral Systems Approach
Authors: Rhys David A. , Lewis Peter D. , Rowland Julie V. 
Published: March 2021

Epithermal deposits form in tectonically active arc settings and magmatic belts at shallow crustal levels as the products of focused hydrothermal fluid flow above, or lateral to, magmatic thermal and fluid sources. At a belt scale, their morphology, geometry, style of mineralization, and controls by major structural features are sensitive to variations in subduction dynamics and convergence angle in arc and postsubduction settings. These conditions dictate the local kinematics of associated faults, influence the style of associated volcanic activity, and may evolve temporally during the lifetime of hydrothermal systems. Extensional arc settings are frequently associated with arc-parallel low- to intermediate-sulfidation fault-fill and extensional vein systems, whereas a diversity of deposit types including intermediate-sulfidation, high-sulfidation, and porphyry deposits occur in contractional and transtensional arc settings. Extensional rift and postsubduction settings are frequently associated with rift-parallel low-sulfidation vein deposits and intermediate- and high-sulfidation systems, respectively.

At a district scale, epithermal vein systems are typically associated with hydrothermal centers along regional fault networks, often coexistent with late fault-controlled felsic or intermediate-composition volcanic flow domes and dikes. Some districts form elliptical areas of parallel or branching extensional and fault-hosted veins that are not obviously associated with regional faults, although veins may parallel regional fault orientations. In regional strike-slip fault settings, dilational jogs and stepovers and fault terminations often control locations of epithermal vein districts, but individual deposits or ore zones are usually localized by normal and normal-oblique fault sets and extensional veins that are kinematically linked to the regional faults. Faults with greatest lateral extent and displacement magnitude within a district often contain the largest relative precious metal endowments, but displacement even on the most continuous ore-hosting faults in large epithermal vein districts seldom exceeds more than several hundred meters and is minimal in some districts that are dominated by extensional veins.

Veins in epithermal districts typically form late in the displacement history of the host faults, when the faults have achieved maximum connectivity and structural permeability. While varying by district, common unidirectional vein-filling sequences in low- and intermediate-sulfidation veins comprise sulfide-bearing colloform-crustiform vein-fill, cockade, and layered breccia-fill stages, often with decreasing sulfide-sulfosalt ± selenide abundance, and finally late carbonate-fill; voluminous early pre-ore barren quartz ± sulfide fill is present in some districts. These textural phases record cycles associated with transient episodes of fluid flow triggered by fault rupture.

The textural and structural features preserved in epithermal systems allow for a field-based evaluation of the kinematic evolution of the veins and controlling fault systems. This can be achieved by utilizing observations of (1) fault kinematic indicators, such as oblique cataclastic foliations and Riedel shear fractures, where they are preserved in silicified fault rock on vein margins, (2) lateral and vertical variations in structural style of veins based on their extensional, fault-dominated, or transitional character, (3) extensional vein sets with preferred orientations that form in the damage zones peripheral to, between, or at tips of fault-hosted veins, and (4) the influence of fault orientation and host-rock rheology and permeability on vein geometry and character. Collectively, these factors allow the prediction of structural settings with high fracture permeability and dilatancy, aiding in exploration targeting.

Favorable structural settings for the development of ore shoots occur at geometric irregularities, orientation changes, and vein bifurcations formed early in the propagation history of the hosting fault networks. These sites include dilational, and locally contractional, steps and bends in strike-slip settings. In extensional settings, relay zones formed through the linkage of lateral fault tips, fault intersections, and dilational jogs associated with rheologically induced fault refraction across lithologic contacts are common ore shoot controls. Upward steepening, dilation, and horsetailing of extensional and oblique-extensional fault-hosted vein systems in near-surface environments are common and reflect decreasing lithostatic load and lower differential stress near surface. In these latter settings, the inflection line and intersections with branching parts of the vein system intersect in the σ2 paleostress orientation, forming gently plunging linear zones of high structural permeability that coincide with areas of cyclical dilation at optimal boiling levels to enhance gangue and ore precipitation.

The rheological character of pre- and syn-ore alteration also influences the structural character, morphology, and position of mineralized zones. Adularia-quartz-illite–dominant alteration, common to higher-temperature upflow zones central to intermediate- and low-sulfidation epithermal vein deposits, behaves as a brittle, competent medium enabling maintenance of fracture permeability. Lateral to and above these upflow zones, lower-temperature argillic alteration assemblages are less permeable and aid formation of fault gouge that further focuses fluid flow in higher-temperature upflow zones. Fault character varies spatially, from entirely breccia and gouge distally through progressively more hydrothermally lithified fault rocks and increasing vein abundance and diminishing fault-rock abundance proximal to ore shoots. In poorly lithified volcaniclastic rocks or phreatic breccia with high primary permeability, fault displacement may dissipate into broader fracture networks, resulting in more dispersed fluid flow that promotes the formation of disseminated deposits with low degrees of structural control.

In disseminated styles of epithermal deposits, mineralization is often associated with synvolcanic growth faults or exploits dikes and phreatic breccia bodies, feeding tabular zones of advanced argillic and silicic alteration that form stratabound replacement mineralized zones. In lithocap environments common to high-sulfidation districts, early, laterally continuous, near-surface barren zones of advanced argillic alteration and silicification form near the paleowater table above magmatic-hydrothermal systems. In many high-sulfidation deposits, these serve as aquitards beneath which later hydrothermal fluids may localize mineralization zones within permeable stratigraphic horizons, although deeper mineralization may also be present within or emanating from faults unrelated to lithocap influence. Silicified lithocaps may contain zones with high secondary structural permeability that localize ore through the formation of zones of vuggy residual quartz and/or elevated fracture densities in the rheologically competent silicified base of the lithocap, often along or emanating laterally outward from ore-controlling faults. Syn-ore faults in such settings may form tabular, intensely silicified zones that extend downward below the lithocap.