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GEOLOGY OF THE WILLOW CREEK RESERVOIR SE QUADRANGLE

The Willow Creek Reservoir SE Quadrangle is in the northeastern Sheep Creek Range in southwestern Elko County and northernmost Eureka and Lander Counties, 60 km northeast of Battle Mountain and 25 km east-southeast of Midas (fig. 1). The quadrangle includes the southern part of the Ivanhoe mining district, from which mercury ores were mined intermittently between 1915 and the mid 1970s and gold ores in the early 1990s. Mapping in the quadrangle is a continuation of a broader study of Tertiary volcanism, extension, and gold-mercury mineralization related to the northern Nevada rift and the Yellowstone hot spot (Wallace, 1991, 1993; Wallace and John, 1998; John and others, 2000). The quadrangle also is near the northwestern end of the Carlin trend of gold deposits. Those deposits formed largely in the late Eocene (Ressel and others, 2000) in Paleozoic sedimentary rocks similar to those that underlie the quadrangle at depth. Although many exploration companies have mapped in parts of the quadrangle, little has been published on the geology of the study area. The general geology is shown on the Elko County geologic map (Coats, 1987). Bailey and Phoenix (1944) described the mercury deposits in the district, using unpublished mapping and data from J.M. Nelson and R.J. Roberts of the U.S. Geological Survey. During an exploration program for gold in the late 1980s, the geology of parts of the quadrangle were studied and reported by Bartlett and others (1991) and Deng (1991), with an emphasis on the area surrounding the Hollister gold deposit near the north edge of the quadrangle. LaPointe and others (1991) summarized the mining history of the Ivanhoe district. Hollister and others (1992) described the hot-spring nature of the Hollister gold deposit. Gold exploration in the late 1990s and 2000s has produced new unpublished data on the geology of the north-central part of the quadrangle. The present study shows a complex suite of middle Miocene volcanic and volcaniclastic units that depositionally overlie a basement composed of the Ordovician Vinini Formation and a small Eocene pluton (fig. 2). The oldest volcanic rocks are middle Miocene tuffs and tuffaceous sediments, and, in the northern half of the quadrangle, andesite and various rhyolite flows were erupted during sedimentation. Units in the northern part of the quadrangle were cut and tilted by high-angle, northeastand northnorthwest-striking faults, whereas volcanic and sedimentary rocks in the southern part of the study area are flat lying. Mercuryand gold-bearing hydrothermal solutions rose along some of the high-angle faults, forming mercury deposits in massive silica replacement and sinter bodies in the tuffs, disseminated gold deposits in various Miocene rocks, and high-grade gold-silver veins in Paleozoic and deeply buried rhyolitic rocks. Reactivation of many faults in the late Miocene exposed the mercury deposits, whereas the gold deposits remain concealed. Geochronologic data for the Ivanhoe district and nearby areas are given in table 1. Major-oxide and trace-element compositions of volcanic rocks in the district are provided in table 2.