Magmatism and mineralization in the Oregon Plateau and northern Great Basin: Mid-Miocene volcanism and associated bonanza ore deposits and their relations to the inception of the Yellowstone hotspot

Mid-Miocene epithermal precious metal deposits and their genetically associated
volcanic rocks crop out across the Oregon Plateau and its boundary with the
northern Great Basin. The recently established chronology of many of these deposits,
in conjunction with Pb-isotope data, indicate that the genesis of these bonanza
deposits is intimately linked to voluminous tholeiitic Colombia-River Steens flood
basalt magmatism commonly associated with initiation of the Yellowstone hotspot.
Incipient lithospheric extension (e.g. the northern Nevada rift and related features)
and widespread development of silicic volcanic centers also accompanied flood basalt
volcanism and precious metal mineralization; together, these four phenomena
define a regional magmatic and mineralization event. Voluminous NGB and OP
flood basalt volcanism (e.g. the Steens Basalt) occurred from 16.7 to 14 Ma but the
main phase is restricted to 16.5–15.5 Ma. Steens Basalt lava flows erupted from discrete
centers disbursed across the OP and NGB, though the most voluminous eruptions
occurred in the vicinity of Steens Mountain, OR. During ~2.5 million years of
mafic magmatism, effusive and explosive eruptions of genetically associated silicic
magmas occurred throughout this region; many of these silicic rocks host mineral
deposits. Recent geochronologic results for the silicic volcanic rocks indicate that
this mid-Miocene magmatism primarily occurred during an ~1 m.y. period coeval
with the peak of Steens magmatism and was coincident with most ore-forming
events. Contrasting styles of volcanism across the region correlate with variations in
local geologic features. For example, in the Santa Rosa-Calico volcanic field local
subalkaline silicic rocks reflect the presence of multiple chemically, physically, and
temporally diverse silicic magmatic systems that formed via upper crustal anatexis
associated with input of Steens basalt into the lithosphere. Coeval lithospheric extension
enabled mafic magma ascent to upper crustal levels where crustal melting
initiated. Extensional structures served as conduits for magmas, magmatic fluids,
andmagmatic volatiles (±Au,Ag, Se,As, Sb, andHg).Mineralizing events in theNational
District also appear to have occurred between 16.1 and 15.6 Ma, coincident
with local magmatism. The temporal relationship between bonanza ore formation
events and episodes of voluminous Steens Basalt volcanism appears to be a regional
phenomenon and supports recent Pb-isotope evidence suggestive of precious metal
derivation from flood basalt magma.