"Focused geologic mapping and structural analysis of the southern Eureka mining district; assessing structural controls and spatial patterns of mineralization"
AuthorDi Fiori, Russell V.
AdvisorLong, Sean P
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The Eureka Mining District is located in the northern part of the Fish Creek Range in east-central Nevada. In this study, 1:6,000-scale geologic mapping and structural analysis were performed in the southern part of the district in order to identify structural controls on mineralization. This project bridges a gap between recent regional-scale mapping studies and detailed (<1:500) mapping performed in an active gold exploration campaign. A geologic map of a ~3.5 km (east-west) by ~8 km (north-south) region was generated, along with five deformed and restored cross-sections that illustrate the post- and pre-extensional deformation geometry. The stratigraphy of the map area consists of ca. 4 km of Cambrian to Devonian rocks that are dominated by carbonates, which, in turn, are unconformably overlain and intruded by Late Eocene silicic volcanic rocks. The map area is composed of four distinct structural systems, including Early Cretaceous contractional structures which include the Eureka culmination and blind Ratto Canyon thrust, and three separate sets of normal faults: 1) 1st-order, km-scale offset, down-to-the-west normal faults, which include the Lookout Mountain and Dugout Tunnel faults, 2) 2nd-order, 10's to 100's meter-offset normal faults, including the Rocky Canyon, Oswego, and East Ratto Ridge fault systems, and 3) a 3rd-order set of sub-meter scale offset, east-striking extensional faults that offset the presumed Late Eocene jasperoid bodies. The 1st- and 2nd-order faulting can be bracketed between Late Cretaceous (ca. 86 Ma), the age of contact metamorphism in northern Rocky Canyon that is cut by the Dugout Tunnel fault, and Late Eocene (ca. 37 Ma), based on the overlapping relationship of a sub-volcanic unconformity. The Eureka district is characterized by silver-lead polymetallic carbonate replacement and Carlin-type gold deposits, which are the two primary deposit-types in the project area. In addition to lithology and structure, specific types of hydrothermal alteration and mineralization were mapped, including silicification, decarbonatization, dolomitization, quartz/calcite-veining, argillization, and the introduction of sulfides and their limonite weathering products. Through this method two distinct mineralization events were identified. Polymetallic mineralization, characterized by dolomitization, argillization, and quartz-veining, resides in the northern part of the map area in Rocky Canyon. It is interpreted to be genetically-related to Late Cretaceous granitic magmatism, which indicates that it pre-dates timing extension along 1st- and 2nd-order normal faults. The Carlin-type mineralization occurs as a series of deposits, mainly along Ratto Ridge. The deposits are associated with strong decarbonatization, strong silicification and jasperoid formation, and argillization. This Carlin-type deposit is temporally constrained to be pre- or syn-Late Eocene, due to overlap and intrusion of dated silicic-volcanic rocks. The map area contains a kilometer-scale, synthetically-faulted relay-ramp of 2nd-order faults that transfer slip between the 1st-order, synthetic Dugout Tunnel and Lookout Mountain faults. Within accommodation zones, wall-damage zones are predicted to exert a first-order control on hydrothermal fluid pathways and localization of mineralization. The footwall of the Lookout Mountain fault contains a set of antithetic, 2nd-order normal faults, the East Ratto Ridge fault system, which is interpreted as a wall-damage zone that was fundamentally responsible for controlling fluid-flow that led to Carlin-type mineralization.