Igneous geology of the Carlin trend, Nevada: The importance of Eocene magmatism in gold mineralization

Igneous rocks of five ages are present in the Carlin trend, Nevada, and include: (1) Paleozoic basalt of the Roberts Mountains allochthon, (2) the Jurassic (∼158 Ma) Goldstrike intrusive complex, which includes the Goldstrike diorite laccolith and abundant dikes and sills, (3) a Cretaceous (112 Ma) granite stock, (4) lavas and intrusions of the Emigrant Pass volcanic field and widespread epizonal plugs and dikes of Eocene (∼40-36 Ma) age that range from rhyolite through basalt, and (5) Miocene (15 Ma) rhyolite lava and tuff. Jurassic and Eocene igneous rocks are by far the most important volumetrically and are spatially associated with nearly all ore deposits of the Carlin trend.; This study focuses on the field relations, isotopic dating, and geochemistry of Eocene dikes that intrude sedimentary rocks in many deposits of the Carlin trend, because they are the youngest pre-mineral rocks and have simpler alteration histories than other host rocks. In the Beast, Genesis, Deep Star, Betze-Post, Rodeo-Goldbug, Meikle-Griffin, and Dee-Storm deposits, Eocene dikes are altered, commonly mineralized, and locally constitute ore. Gold-bearing dikes and sedimentary rocks have similar ore mineralogy, including arsenian pyrite, marcasite, and arsenopyrite, with late barite and stibnite. At Beast, as much as half the ore is hosted in a 37.3 Ma rhyolite dike. Post-gold alunite is ∼18.6 Ma. At Meikle and Griffin, porphyritic dacite dikes yield concordant U/Pb zircon and 40Ar/39Ar biotite emplacement ages of ∼39.2 Ma, and illite from the same QSP-altered dacite, with as much 9 ppm Au, yields similar, although imprecise 40Ar/39Ar ages. Thus, gold mineralization at these deposits closely followed emplacement of Eocene dikes.; Carlin-type gold deposits in northeastern Nevada have been variously interpreted as partly syngenetic with Paleozoic carbonate rocks, products of Mesozoic contraction and metamorphism with or without significant magmatism, and of Tertiary age and related or not to magmatism, metamorphism, and/or large-scale extension. A recently established Eocene age for major gold introduction narrows the possibilities, and two principal models have emerged: one involving Eocene magmatism as the heat source to drive shallow hydrothermal circulation and the other advocating deeply sourced metamorphic fluids released into the upper crust during regional extension. Critical to the latter argument is the temporal association of extension to gold mineralization, which as yet, is not demonstrated. We argue that Eocene magmatism in the form of large underlying plutons, was the major recognized process that affected the Carlin trend during gold mineralization. These plutons supplied the heat that drove discrete hydrothermal systems. (Abstract shortened by UMI.)...