Christopher S. Henry | USGS Science

USGS2020

Magmatic-tectonic settings of Cenozoic epithermal gold-silver deposits of the Great Basin, western United States

Numerous epithermal gold-silver deposits formed during the past 40 Ma are irregularly distributed across the Great Basin. These deposits formed in six major magmatic-tectonic settings that varied during the complex evolution of the continental margin of western North America: (1) slab rollback–ignimbrite flareup (~45–17 Ma), (2) slab rollback–ancestral Cascade arc (~35 Ma–present), (3) Yellowstone hotspot– bimodal (~16.7–3 Ma), (4) slab window (~16 Ma–present), (5) Basin and Range bimodal extensional (≤ 7 Ma), and (6) amagmatic extensional (≤ 5 Ma). Most large (> 1 Moz gold produced) deposits are Miocene (~20–8 Ma), low-, intermediate-, and high-sulfidation deposits in the southern part of the ancestral Cascade arc; late Miocene post-subduction, low-sulfidation deposits formed over the slab window; and lowsulfidation deposits related to early (16.7–15 Ma) Yellowstone hotspot magmatism formed along the northern Nevada rift and related fracture zones to the west. The world-class Round Mountain low-sulfidation deposit is the only large deposit in ignimbrite flareup rocks despite these rocks constituting the largest eruptive volume of Cenozoic magmas in the Great Basin. Intermediate to silicic composition lava dome complexes are the most common setting for epithermal deposits in the western Great Basin, whereas deposits formed in a wide range of settings and rock types during Yellowstone hotspot activity. With exception of the Round Mountain caldera, the dozens of calderas of the ignimbrite flareup do not host large epithermal deposits. Several young (≤ 5 Ma), “amagmatic” low-sulfidation deposits formed along Basin and Range fault zones in sedimentary rocks that lack proximal magmatic activity. The types and characteristics of epithermal gold-silver deposits in the Great Basin systematically vary with magmatic-tectonic setting and magma composition, and their distribution reflects the combined effects of tectonic setting of magma genesis; magma source, composition and eruptive style; crustal thickness and composition; presence of crustal-scale structural zones; climate; and preservation of deposits.

Great Basin↗

USGS2011

The regional structural setting of the 2008 Wells earthquake and Town Creek Flat Basin: implications for the Wells earthquake fault and adjacent structures

The 2008 Wells earthquake occurred on a northeast-striking, southeast-dipping fault that is clearly delineated by the aftershock swarm to a depth of 10-12 km below sea level. However, Cenozoic rocks and structures around Wells primarily record east-west extension along north- to north-northeast-striking, west-dipping normal faults that formed during the middle Miocene. These faults are responsible for the strong eastward tilt of most basins and ranges in the area, including the Town Creek Flat basin (the location of the earthquake) and the adjacent Snake Mountains and western Windermere Hills. These older west-dipping faults are locally overprinted by a younger generation of east-dipping, high-angle normal faults that formed as early as the late Miocene and have remained active into the Quaternary. The most prominent of these east-dipping faults is the set of en-échelon, north-striking faults that bounds the east sides of the Ruby Mountains, East Humboldt Range, and Clover Hill (about 5 km southwest of Wells). The northeastern-most of these faults, the Clover Hill fault, projects northward along strike toward the Snake Mountains and the approximately located surface projection of the Wells earthquake fault as defined by aftershock locations. The Clover Hill fault also projects toward a previously unrecognized, east-facing Quaternary fault scarp and line of springs that appear to mark a significant east-dipping normal fault along the western edge of Town Creek Flat. Both western and eastern projections may be northern continuations of the Clover Hill fault. The Wells earthquake occurred along this east-dipping fault system. Two possible alternatives to rupture of a northern continuation of the Clover Hill fault are that the earthquake fault (1) is antithetic to an active west-dipping fault or (2) reactivated a Mesozoic thrust fault that dips east as a result of tilting by the west-dipping faults along the west side of the Snake Mountains. Both alternatives are precluded by the depths of the earthquake and aftershocks, about 8 km and as deep as 12 km, respectively. These depths are below where an antithetic fault would intersect any main fault, and a tilted, formerly shallow and sub-horizontal thrust fault would not extend to depths of more than about 5–6 km. The east-dipping, high-angle, earthquake fault cuts older west-dipping faults rather than reactivating them, highlighting a change in the structural style of Basin and Range extension in this region from closely-spaced, west-dipping faults that rotated significantly during slip and accommodated large-magnitude extension, to widely-spaced, high-angle faults that accommodate much less total strain over a long time span.

Nevada↗