Exploring for structurally concealed Carlin-type mineralization: A case study from the northern Shoshone Range, Nevada, USA

Abstract

This study integrates results from previous studies with new reconnaissance-scale mapping of rock types, structures, and hydrothermal alteration at Goat Ridge and Tenabo to provide a district-scale cross section and palinspastic restoration of an area in Nevada with high potential for Carlin-type mineralization in the footwall of a dismembered thrust fault. Reinterpretation of legacy drill data in light of recent advances in the understanding of Cenozoic normal faulting in the area with the expected stratigraphy of the Roberts Mountains autochthon allows for deciphering whether previous drill holes piercements of faults are the Roberts Mountains thrust or are tilted normal faults that down-dropped the deep-water siliciclastic rocks of Roberts Mountains allochthon onto the coeval carbonate rocks of the Roberts Mountains autochthon. A cross-sectional reconstruction shows that ∼11.1 km (120 %) of extension was accomplished along one set of faults that initiated at 60-70°and tilted 40°E. Previously underappreciated Mesozoic folds are present, which likely formed with horizontal fold hinges. Because of Cenozoic normal faulting and associated tilting, the hinge lines currently plunge at moderate angles and are offset as they step across each crosscutting fault. The anticlinal fold hinges produced the modern exposures of lower plate rocks in the range. The Eocene paleogeologic map that results from a plan-view structural restoration shows that the hinge lines of the original folds trended northwest-southeast in the southern half of the range but turned northward toward the northern end of the range.Mineralization in the range includes silver-dominated polymetallic veins/lodes, weak porphyry-type, Carlin-type, and siliciclastic-hosted gold deposits of uncertain origin. Porphyry-style and silver-dominated polymetallic vein mineralization is associated with ∼39–40 Ma granodiorites, with the silver-dominated polymetallic veins interpreted as the distal veins around weakly mineralized and/or deeply buried porphyry system(s), probably of the quartz monzonitic-granitic porphyry Mo-Cu subclass of porphyry molybdenum deposits. Crosscutting relationships demonstrate that at least one of “upper plate” gold deposits is Eocene, but a Miocene age cannot be ruled out for the other gold deposits.