Age, Distribution, and Composition of Igneous Rocks of the Pequop Mountains, Northeast Nevada: Association with Carlin-type Gold Deposits

摘要

Carlin-type gold deposits at Long Canyon and West Pequop in the Pequop Mountains are two of Nevada's newest and most significant exploration discoveries. These discoveries have generated great interest, because they lie far east of the known gold trends in the Paleozoic carbonate platform previously considered less prospective for large gold deposits. Host rocks were deformed and variably metamorphosed, probably during the Late Cretaceous Sevier Orogeny. Recent work demonstrates the presence of abundant, previously unrecognized and still poorly documented intrusions, mostly of Jurassic (-159 to 165 Ma) and Eocene (~39 to 41 Ma) age, with fewer late Cretaceous dikes (Bedell et al., 2010). The range in age and composition of igneous rocks of the Pequops is consistent with that observed regionally. The primary goals of this study are to establish the character, absolute and relative timing, and spatial relationships between tectonism, magmatism, and hydrothermal activity that led to significant gold deposition in this geologic setting. Abundant dikes, sills, and small, irregular, mafic to silicic intrusions are recognized in four areas of the Pequop Mountains. In addition, the Nanny Creek area~5 km north of Long Canyon contains locally sourced Eocene volcanics. Jurassic intrusions are widespread and compositionally diverse, ranging from lamprophyre and monzodiorite through granodiorite and granite. The Long Canyon gold deposit contains a swarm of Jurassic lamprophyre and monzodiorite dikes and sills, and identical dikes are present at West Pequop. Our preliminary ~(40)Ar/~(39)Ar dating of hornblende and biotite phenocrysts from chloritized lamprophyre dikes at Long Canyon provide imprecise Jurassic ages, which are consistent with previous U/Pb ages from other Pequop Mountains locations. Distinctive textures, mineralogy, and geochemistry of Long Canyon lamprophyres are indistinguishable from those of Jurassic lam-prophyres of the Carlin trend and Cortez area. Small, irregular lenses of Late Creta-ceous(?) garnet-muscovite leucogranite occur in areas of highest metamorphic grades along the west flank of the Pequops Mountains. Leucogranite is peraluminous, which, together with its close spatial association with high-temperature metamorphic rocks, suggests a crustal melt origin. Eocene igneous rocks occur in at least three, possibly four, areas of the Pequop Mountains. Intrusions in two areas in and near West Pequop are high-silica, spher-ulitic rhyolite (41-39 Ma), whereas an undated but probably Eocene dike at Long Canyon is basaltic. High-silica rhyolite dikes differ compositionally from andesite through low-silica rhyolite lavas of the~39 Ma Nanny Creek volcanic center suggesting two loci for Eocene activity. Spherulitic textures in the Eocene rhyolites where they intrude high-grade metamorphic rocks suggest: 1) very shallow level of emplacement~40 Ma, and 2) significant exhumation pre-40 Ma. In seeming conflict with this interpretation, Eocene volcanic rocks at Nanny Creek and regionally everywhere rest on uppermost Paleozoic rocks, indicating little exhumation by that time. Source plutons for widespread Jurassic and Eocene igneous rocks remain uncertain. A large (265 km~2) magnetic anomaly centered in Goshute Valley and extending west into the southern Pequop Mountains may represent source plutons for both Jurassic and Eocene dikes and sills. A small part of this anomaly overlaps with the Jurassic Silver Zone Pass pluton in the neighboring Toano Range. Other sources, especially for silicic compositions, may not be resolvable because of their inherently weak magnetism. Gold mineralization at Long Canyon postdates the swarm of Jurassic mafic dikes and sills, which host ore and provide some structural controls to mineralization. Mineralization at West Pequop is likely more tightly constrained, because spherulitic rhyolite dikes dated at~40 Ma nearby are hydrothermally altered and host gold.