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.
Key Words: Epithermal Au-Ag deposits, Great Basin, Cenozoic, magmatic-tectonic setting