New Geologic Map: Lookout Mountain, Ratto Ridge, and Rocky Canyon, Eureka County

Open-File Report 14-8
Preliminary geologic and alteration maps of Lookout Mountain, Ratto Ridge and Rocky Canyon, southern Eureka mining district, Eureka County, Nevada

by Russell V. Di Fiori, Sean P. Long, John L. Muntean, and Gary P. Edmondo, 2014



Identification of favorable structural settings for Carlin-type gold deposits is fundamental for future exploration.  In this study, mapping and structural analysis were performed in the southern part of the Eureka mining district in east-central Nevada, in order to understand geometric and temporal relationships between structural systems and Carlin-type mineralization.  Geologic and alteration maps at a scale of 1:10,000 of a ~3.5 km (east-west) by ~8 km (north-south) region were generated, along with cross sections that illustrate pre- and post-extensional deformation geometry.  This project bridges a gap between recent 1:24,000-scale mapping and <1:500-scale mapping performed in an active exploration campaign.

The stratigraphy of the map area consists of ~4 km of Cambrian-Devonian carbonate and siliciclastic rocks, which are unconformably overlain and intruded by late Eocene silicic volcanic rocks.  Four structural systems are identified, consisting of Early Cretaceous contractional structures and three separate sets of normal faults: 1) 1st-order, kilometer-scale offset, down-to-the-west normal faults, including the Lookout Mountain and Dugout Tunnel faults, 2) 2nd-order, 10’s to 100’s meter-scale offset, north-striking normal faults, including the Rocky Canyon, Oswego, and East Ratto Ridge fault systems, and 3) a set of 3rd-order, meter-scale offset, east-striking normal faults that cut jasperoid bodies of presumed late Eocene age.  The 1st- and 2nd-order faults are interpreted to be contemporary, cut Late Cretaceous intrusions and an associated contact metamorphic aureole, and are overlapped by a late Eocene, subvolcanic unconformity.

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.  Carlin-type replacement mineralization, primarily hosted within Cambrian carbonate rocks, occurs in a series of deposits in the southern part of the map area.  The deposits are associated with decarbonatization, silicification and jasperoid formation, and argillization, and are constrained to late Eocene or older by the overlap and intrusion of dated volcanic rocks.

The map area contains a km-scale, faulted relay-ramp of 2nd-order faults that transfer slip between synthetic 1st-order faults. Within accommodation zones, wall-damage zones are predicted to provide hydrothermal fluid pathways and therefore localize mineralization.  The footwall of the Lookout Mountain fault, which contains the majority of Carlin-type deposits identified in the map area, contains a set of antithetic, 2nd-order normal faults, which is interpreted as a wall-damage zone that was responsible for controlling fluid flow that led to mineralization.

The southern Eureka mining district contains several favorable structural conditions for Carlin-type gold mineralization, including: 1) normal fault systems that predate or are contemporary with late Eocene gold mineralization, and 2) complex normal fault interactions in an accommodation zone, including zones of dense fault intersections, antithetic normal faults, and fault-damage zones.  These structural conditions were fundamental for generating a network of open-system fluid pathways, which created an ideal structural architecture for Carlin-type mineralization, and can be used as predictive tools for exploration elsewhere.

This project was supported by Timberline Resources Corporation and the Geological Society of America.

Open-File Report 14-8, two plates, scale 1:10,000; plate one: 56 x 41 inches, plate two: 29 x 41 inches; folded or rolled, $35.00

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