Igneous Geology of the Keystone Window, Simpson Park Mountains, Eureka County, Nevada: Age, Distribution, Composition, and Relationship to Carlin-style Gold Mineralization
Keystone is an early-stage gold exploration project operated by U.S. Gold Corp. located in the northern Simpson Park Mountains, in the Battle Mountain-Eureka mineral belt of north-central Nevada. Dating of ore-stage minerals and cross-cutting relationships between mineralization and dikes elsewhere in Nevada shows that most Carlin-type gold deposits (CTDs) formed over a short interval in the Eocene (~42-35 Ma), coeval with a distinct pulse of arc magmatism that supplied heat and permissively, metals for CTDs. The on-trend location of Keystone, 25 km south of the giant Cortez Hills and Goldrush CTDs, together with widespread alteration of a domed lower plate structural window in the Roberts Mountains allochthon (RMA) that is cored by an Eocene intermediate pluton, make it ideal for the study of proximal to distal styles of Eocene mineralization, including CTDs. U-Pb zircon and 40Ar/39Ar dating in conjunction with mapping of igneous rocks and alteration provide constraints on the timing and style of magmatism and mineralization.A major normal fault along Keystone’s west flank tilted units moderately east, exposing a 2 km thick crustal section that includes Eocene volcanic rocks, the RMA, lower plate carbonates, and abundant intrusions. Eocene igneous rocks comprise the 4.1 km2 intermediate composite Walti pluton, which domed surrounding wall rocks, and abundant porphyritic, intermediate to silicic stocks, dikes, lavas, and tuffs. All Eocene rocks are shoshonitic to high-K calc-alkaline, metaluminous to weakly peraluminous, and were emplaced from ~36-34.5 Ma. A ~25 km2 magnetic anomaly coincides with the Walti pluton and indicates much larger intrusions at depth. The anomaly is of similar size to the magnetic anomaly associated with the Eocene Bullion intrusion (22 km2) in the southern Carlin trend and the Copper Canyon stock (11 km2), which is associated with the Phoenix-Fortitude gold skarns. Trace-element geochemistry, petrography, crosscutting relations, and isotopic dating of Eocene igneous rocks at Keystone support a complex magmatic evolution with likely varied sources. Older andesite of McClusky Creek, Mud Springs diorite and Gund diorite are interpreted as a separate magmatic system from the later Walti granodiorite to diorite intrusion and its related rocks.The composition and timing of igneous rocks at Keystone are consistent with the last stages of mid-Cenozoic, dominantly intermediate composition magmatism in northeast Nevada, which was active from ~44 to 34 Ma and migrated rapidly southwestward. This Eocene magmatism was characterized by shallow intrusion and mostly effusive volcanism. After ~34 Ma and generally at or immediately south of Keystone, magmatism became dominantly silicic and pyroclastic, part of the Oligocene ignimbrite flare-up of central Nevada (e.g. the nearby ~34 Ma Caetano and Hall Creek calderas). The change in style and composition of mid-Cenozoic magmatism at this latitude coincides with a change in the distribution of Eocene Carlin-type gold deposits from major deposits (+5 Moz contained Au) to the north, to relatively smaller deposits to the south. Whether the change in the magnitude of Carlin-type gold mineralization is directly or indirectly related to changes in mid-Cenozoic magmatism is uncertain and an area of future research.40Ar/39Ar dating of biotite from an alkaline basalt in the Valmy Formation yielded a plateau age of 466.1±0.7 Ma, which is Middle Ordovician. This age is consistent with biostratigraphic ages from conodonts and radiolaria at Keystone. Alkaline mafic rocks at Keystone solely occur in the Ordovician Valmy and Cambrian-Ordovician Comus formations as sills, volcanogenic debris flows, and pillows, typically intercalated with limestone. These rocks, in addition to unusual mineralogy, have distinctive and unusually high concentrations of both compatible and incompatible trace elements consistent with ocean island basalts (OIB) and unlike depleted mid-ocean ridge basalts (MORB) or arc-related basalts. OIBs likely reflect intraplate or hotspot-related magmatism that developed as a series of seamounts during deposition of rocks of the Valmy and Comus formations and which were later tectonically transported eastward to the Keystone area during Late Devonian/Early Mississippian Antler thrusting. The age, lithologic, and geochemical similarities of these rocks to the Comus Formation in the Osgood Mountains is permissive for their correlation and is particularly important, since the Comus is the principal host unit in the Getchell trend Carlin-type gold deposits.U-Pb zircon dating of conglomerate resting on Paleozoic rocks of the RMA yielded a maximum depositional age of 35.62±0.32 Ma. Conglomeratic rocks elsewhere are commonly correlated with the Paleozoic Antler overlap sequence based on the abundance of RMA-derived chert and quartzite clasts. The recognition of this Eocene conglomerate is important not only for regional mapping but also for understanding the development of Eocene basins, some of which are hydrocarbon-bearing, and for characterizing the switch from late Cretaceous Sevier contraction to an early Cenozoic extensional regime. The basal Eocene conglomerate is the oldest exposed Cenozoic unit at Keystone and is used to constrain the Eocene paleosurface and estimate the depth that Eocene intrusions were emplaced and that mineralization occurred. Hydrothermal activity at Keystone was constrained by field relations and 40Ar/39Ar of illite in altered igneous rocks. The Walti pluton is associated with proximal Pb-Zn-Cu skarn, at the site of the historic Keystone mine. Distal epithermal and Carlin-style mineralization occurs outboard of hornfels halos in overlying and adjacent strata. Drilling by U.S. Gold Corp. commonly intercepted mineralized breccia and jasperoid at the Paleozoic upper- to lower-plate transition, and argillized dikes adjacent to decarbonatized Paleozoic lower-plate carbonate rocks, both of which contain strongly anomalous gold and high concentrations of As, Sb, Hg, Bi, W, Se, and/or Tl. Illite from two samples of altered andesite dikes of McClusky Creek did not yield 40Ar/39Ar plateaus but nonetheless produced geologically reasonable weighted mean ages of 35.71±0.12 Ma and 35.54±0.06 Ma, which are slightly younger than the ~35.99 to 35.85 Ma 40Ar/39Ar igneous hornblende ages. Thus, the illite ages from the altered andesite dikes are considered the best approximate for the age of Carlin-type gold mineralization in the northeastern area of Keystone. A rhyolite porphyry intrusion exhibited the most sericitic and argillic alteration after the altered dikes, and 40Ar/39Ar dating of sanidine from the unaltered core of the rhyolite yielded an age of 35.43±0.06 Ma. This rhyolite porphyry is possibly the youngest intrusion at Keystone, and is preferentially altered over adjacent intrusions. This suggests the rhyolite intruded an extensional structure that also controlled hydrothermal fluids. Abundant quartz porphyry rhyolite dikes at the nearby Cortez Hills CTD have been interpreted as syn-mineralization because they are altered and locally mineralized; these pre-Caetano caldera dikes are of similar age and composition to the rhyolite porphyry at Keystone
