Interrelation of Eocene magmatism, extension, and Carlin-type gold deposits in northeastern Nevada
Christopher D. Henry, Michael W. Ressel, 2000. "Interrelation of Eocene magmatism, extension, and Carlin-type gold deposits in northeastern Nevada", Great Basin and Sierra Nevada, David R. Lageson, Stephen G. Peters, Mary M. Lahren
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The Eocene of northeastern Nevada is distinguished by the inception of Cenozoic magmatism and extension and the formation of the huge Carlin-type gold deposits. This field trip examines the characteristics of, and temporal and spatial relation between, Eocene magmatism and extension and the influence of both on the formation of Carlin-type deposits. Cenozoic magmatism began in northeastern Nevada and Utah ~43 Ma, part of a southward sweep of activity from Washington and Idaho beginning in the early Cenozoic. Eocene igneous centers are widespread in northeastern Nevada and dominated by andesite and dacite or granodiorite. Rhyolite is also common as late intrusions or voluminous ash-flow tuffs, but only one caldera has been found.
Proposed ages and driving mechanisms of Carlin-type deposits have ranged from Mesozoic to Miocene and from magmatism to extension to metamorphism. Our work demonstrates that deposits are spatially and temporally related to Eocene magmatism. Eocene plutons were the heat source and may have been the source of some metals and fluids.
New mapping, published maps, and aeromagnetic data indicate that the largest Eocene igneous centers in Nevada are in and around the area of Carlin-type deposits and west of the Ruby Mountains core complex. The northern Carlin Trend-Emigrant Pass igneous complex (NCEP), which adjoins the Carlin trend, is the largest center; its size is best indicated by its corresponding 700 km2 aeromagnetic anomaly. The NCEP was active between 40 Ma and 36 Ma in several pulses that were contemporaneous with mineralization in the trend. Along the Carlin trend, Eocene rocks consist only of numerous dikes; extrusive rocks may never have been present, but the dikes require the presence of nearby stocks. At Emigrant Pass, andesitic to dacitic lavas and vents are abundant.
The NCEP is distinctive in several ways that may be significant for the origin of deposits of the Carlin trend. It was the largest and most long-lived (~4 Ma) Eocene igneous center in Nevada. Magmatism transferred immense amounts of heat to the upper crust, which was then available to generate hydrothermal systems. Rocks are notably hornblende rich (up to 12% hornblende phenocrysts) indicating H2O-rich magmas. Pyroclastic rocks are absent, and possibly no lavas erupted in the northern Carlin trend; therefore, magmatic volatiles and any contained metals were likely trapped in the subsurface and would have been available to generate deposits.
Northeastern Nevada has undergone several episodes of Cenozoic extension. The greatest extension in and around the NCEP was between 25 Ma and 15 Ma, possibly correlative with the 23 Ma age of most rapid cooling and presumed greatest extension in the Ruby Mountains core complex. One possible and one definite episode of minor extension occurred in the Eocene, during initial development of the core complex and broadly contemporaneous with Eocene magmatism and Carlin-type ore formation. However, neither magmatism nor ore formation occurred at the same time as high magnitude extension.
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Great Basin and Sierra Nevada, the second volume of the Geological Society of America Field Guide Series, focuses on the dynamic and spectacular geology of this region, providing the inspiring backdrop for the 2000 GSA Annual Meeting in Reno. This volume gives complete coverage of field trips held in conjunction with that meeting, and contains 20 chapters organized into three sections. The first section consists of 16 chapters arranged in geochronological order, beginning with the active tectonics of Lake Tahoe and the historical surface faulting and paleoseismicity of the central Nevada seismic belt, and ending with the Neoproterozoic glacial record of Death Valley. In between are chapters dealing with Basin and Range extension, Eocene magmatism, Mesozoic plutonism in the Sierra Nevada, Paleozoic subduction, and Ordovician stratigraphy, to name a few. The second section covers the geology of the Nevada Test Site and the nuclear-waste repository at Yucca Mountain. The last section is an invited field guide from the 1999 GSA Cordilleran Section meeting that covers the wines and geology of Napa Valley, California. Overall, Great Basin and Sierra Nevada is a comprehensive compilation of new and exciting research on this amazingly diverse region, with well-crafted guides to field localities of special interest. Full-color plates in some chapters make this guide an especially appealing and useful volume.