EPITHERMAL GOLD-SILVER DEPOSITS ASSOCIATED WITH NEOGENE BIMODAL MAGMATISM IN THE NORTHERN GREAT BASIN

Numerous low-sulfidation epithermal Au-Ag deposits are associated with middle Miocene to Holocene bimodal basalt-rhyolite magmatism (BM) in the northern Great Basin (NGB). Beginning at about 17 Ma, BM magmas erupted across large parts of the NGB, SE Oregon, and SW Idaho, with magmatism focused along the northern Nevada rift (NNR), the McDermitt volcanic complex, and areas farther north inundated by the Columbia River and Steens Basalts. Early BM magmas erupted east of the Western Cascades magmatic arc and were related to the Yellowstone hot spot, a possible small mantle plume that impinged the crust in northern NV or SE Oregon. Early BM magmas (ca. 17 to 14 Ma) dominantly were reduced, water-poor tholeiites with basalt to mafic andesite and rhyolite compositions. Most early mafic magmas in BM are tholeiitic with compositions and isotopes suggesting mantle lithosphere sources, whereas some young (<5 Ma) basalts have OIB characteristics and may have asthenospheric mantle sources. In contrast time-equivalent arc magmas in Western Cascades part of NGB were subduction-related, oxidized, water-rich, and calc-alkaline, with mostly andesite to dacite compositions.

Most BM-related epithermal Au-Ag deposits formed between 17 and 14 Ma and after ~4 Ma and include deposits formed from magmatically heated (MH) and non-magmatically heated (NMH) hydrothermal systems. Most MH deposits are closely associated with middle Miocene rhyolite flow-dome complexes, or mafic flow and dike complexes such as the NNR. Mineralization during WSW-ENE extension produced veins, replacement deposits, and near-surface sinter deposits; levels of exposure vary widely. Most NMH deposits are associated with NE-striking faults related to Pliocene and younger transtension and extension; coeval magmatic rocks are rare. Hot-springs-style alteration and ore mineralogy characterize many of the younger NMH deposits, for which exposure levels are very shallow. Magmatically heated deposits have smaller tonnages and higher grades (Midas, Sleeper, Mule Canyon), whereas NMH deposits have lower grades but larger tonnages (Lewis-Crofoot, Florida Canyon). Both deposit types share a clear epithermal geochemical signature. Non-magmatically heated systems were active over several m.y., whereas MH deposits apparently formed more quickly.