Stephen B. Castor

Eocene–Early Miocene paleotopography of the Sierra Nevada–Great Basin–Nevadaplano based on widespread ash-flow tuffs and paleovalleys

The distribution of Cenozoic ash-flow tuffs in the Great Basin and the Sierra Nevada of eastern California (United States) demonstrates that the region, commonly referred to as the Nevadaplano, was an erosional highland that was drained by major west- and east-trending rivers, with a north-south paleodivide through eastern Nevada. The 28.9 Ma tuff of Campbell Creek is a voluminous (possibly as much as 3000 km 3 ), petrographically and compositionally distinctive ash-flow tuff that erupted from a caldera in north-central Nevada and spread widely through paleovalleys across northern Nevada and the Sierra Nevada. The tuff can be correlated over a modern area of at least 55,000 km 2 , from the western foothills of the Sierra Nevada to the Ruby Mountains in northeastern Nevada, present-day distances of ∼280 km west and 300 km northeast of its source caldera. Corrected for later extension, the tuff flowed ∼200 km to the west, downvalley and across what is now the Basin and Range–Sierra Nevada structural and topographic boundary, and ∼215 km to the northeast, partly upvalley, across the inferred paleodivide, and downvalley to the east. The tuff also flowed as much as 100 km to the north and 60 km to the south, crossing several east-west divides between major paleovalleys. The tuff of Campbell Creek flowed through, and was deposited in, at least five major paleovalleys in western Nevada and the eastern Sierra Nevada. These characteristics are unusual compared to most other ash-flow tuffs in Nevada that also flowed great distances downvalley, but far less east and north-south; most tuffs were restricted to one or two major paleovalleys. Important factors in this greater distribution may be the great volume of erupted tuff and its eruption after ∼3 Ma of nearly continuous, major pyroclastic eruptions near its caldera that probably filled in nearby topography. Distribution of the tuff of Campbell Creek and other ash-flow tuffs and continuity of paleovalleys demonstrates that (1) the Basin and Range–Sierra Nevada structural and topographic boundary did not exist before 23 Ma; (2) the Sierra Nevada was a lower, western ramp to the Nevadaplano; and (3) any faulting before 23 Ma in western Nevada, including in what is now the Walker Lane, and before 29 Ma in northern Nevada as far east as what is now the Ruby Mountains metamorphic core complex, was insufficient to disrupt the paleodrainages. These data are further evidence that major extension in Nevada occurred predominantly in the late Cenozoic. Characteristics of paleovalleys and tuff distributions suggest that the valleys resulted from prolonged erosion, probably aided by the warm, wet Eocene climate, but do not resolve the question of the absolute elevation of the Nevadaplano. Paleovalleys existed at least by ca. 50 Ma in the Sierra Nevada and by 46 Ma in northeastern Nevada, based on the age of the oldest paleovalley-filling sedimentary or tuff deposits. Paleovalleys were much wider (5–10 km) than they were deep (to 1.2 km; greatest in western Nevada and decreasing toward the paleo–Pacific Ocean) and typically had broad, flat bottoms and low-relief interfluves. Interfluves in Nevada had elevations of at least 1.2 km because paleovalleys were that deep. The gradient from the caldera eastward to the inferred paleodivide had to be sufficiently low so that the tuff could flow upstream more than 100 km. Two Quaternary ash-flow tuffs where topography is nearly unchanged since eruption flowed similar distances as the mid-Cenozoic tuffs at average gradients of ∼2.5–8 m/km. Extrapolated 200–300 km (pre-extension) from the Pacific Ocean to the central Nevada caldera belt, the lower gradient would require elevations of only 0.5 km for valley floors and 1.5 km for interfluves. The great eastward, upvalley flow is consistent with recent stable isotope data that indicate low Oligocene topographic gradients in the Nevadaplano east of the Sierra Nevada, but the minimum elevations required for central Nevada are significantly less than indicated by the same stable isotope data. Although best recognized in the northern and central Sierra Nevada, early to middle Cenozoic paleodrainages may have crossed the southern Sierra Nevada. Similar early to middle Cenozoic paleodrainages existed from central Idaho to northern Sonora, Mexico, and persisted over most of that region until disrupted by major Middle Miocene extension. Therefore, the Nevadaplano was the middle part of an erosional highland that extended along at least this length. The timing of origin and location of this more all-encompassing highland indicates that uplift was predominantly a result of Late Cretaceous (Sevier) contraction in the north and a combination of Late Cretaceous–early Cenozoic (Sevier and Laramide) contraction in the south.

Geology, geochemistry, and origin of volcanic rock-hosted uranium deposits in northwestern Nevada and southeastern Oregon, USA

Abstract Northwestern Nevada and southeastern Oregon have the largest uranium (U) deposits in Tertiary volcanic rocks in the US. Most deposits are in or adjacent to calderas or rhyolite lava-dome fields and are hydrothermal. Almost all are associated with rhyolites that have high primary U concentrations (9 to 20 ppm), but the rhyolites range from peralkaline to peraluminous. Caldera-related occurrences are hosted by peralkaline rocks in the McDermitt and Virgin Valley calderas. At McDermitt, major deposits are along or just inboard of the southwestern (Kings River) and northern ring fractures (Bretz–Opalite) of the caldera complex. The Kings River deposits are strongly enriched in trace elements associated with epithermal precious-metal deposits (e.g., As, Au, Ag, Mo, and Sb). Much of the U occurs as hydrothermal uraniferous zircon, and whole rock samples contain as much as 5% Zr. Gangue minerals include quartz, adularia, fluorite, and numerous sulfides. 40 Ar/ 39 Ar dating of adjacent volcanic rocks and adularia indicates that mineralization was contemporaneous with igneous activity. The Bretz–Opalite deposits, which include the large Aurora resource (7.5×106 kg U3O8), are less enriched in epithermal trace elements and contain uraninite, coffinite, chalcedony, opal, and numerous sulfides. At Virgin Valley, deposits are mostly in tuffaceous sediments adjacent to ring-fracture rhyolites. Trace-element enrichment is minor, U is hexavalent in all identified U minerals, and associated phases include opal, organic material, and pyrite. Deposits in the Lakeview U district, Oregon are hosted by weakly peraluminous rhyolite domes and tuffs and are highly enriched in epithermal trace elements; the mineral assemblage includes coffinite, opal, and numerous sulfides. Minor U occurrences at Buff Peak, Nevada, the newly recognized, westernmost topaz (peraluminous) rhyolite in the US, are also trace element enriched; gangue minerals include quartz, adularia, and sulfides. On the basis of their geologic setting, geochemistry, age, and mineralogy, the McDermitt, Lakeview, and Buff Peak deposits are hydrothermal; sparse, published fluid inclusion data indicate temperatures between 200°C and 330°C. Virgin Valley deposits most likely are hydrothermal. Minor deposits in outflow ash-flow tuffs are distant from any intrusive heat source and probably formed from low-temperature groundwater.

Contrasting styles of epithermal precious-metal mineralization in the southwestern Nevada volcanic field, USA

Abstract The southwestern Nevada volcanic field contains epithermal precious-metal deposits hosted by Miocene volcanic rocks and pre-Tertiary sedimentary rocks with production+reserves greater than 60 t of gold and 150 t of silver. The volcanic rocks consist predominantly of ash-flow tuffs erupted between 15 and 7 Ma during three major magmatic stages: the main stage (ca. 15-13 Ma); the Timber Mountain stage (ca. 13-9 Ma); and the late stage (ca. 9-7 Ma). Hydrothermal activity and precious-metal mineralization in the southern part of the field took place between ca. 13 and 8.5 Ma, coinciding with portions of all three magmatic stages. Regional extension during this period produced imbricate normal and detachment faulting that provided structural control for some of the mineralization. Contrasts in the style and geochemistry of mineralization, together with stratigraphic and radiometric age data and differences in geologic setting reflect the variable nature of hydrothermal activity during development of the southwestern Nevada volcanic field. During the main magmatic stage, silver-rich vein mineralization of the adularia-sericite type occurred in an intermediate volcanic center at Wahmonie. Secondary high-salinity fluid inclusions in felsic subvolcanic intrusions, a trace element suite that includes bismuth and tellurium, and geophysical data support the presence of a buried porphyry-type magmatic system at Wahmonie. Hydrothermal activity at Bare Mountain took place during the main magmatic stage, and may have continued into the Timber Mountain magmatic stage. Bare Mountain contains gold-rich, disseminated Carlin-type deposits with high arsenic, antimony, mercury and fluorine in sedimentary and igneous rocks. In northern and eastern Bare Mountain, mineralization is associated with felsic porphyry dikes that contain secondary high-salinity fluid inclusions. A genetic relationship between porphyry magmatism and shallow Carlin-type gold deposits seems likely at Bare Mountain. Sedimentary-rock-hosted mineralization at Mine Mountain is spatially associated with a thrust and was apparently deposited, in part, by a hydrothermal system active during the Timber Mountain magmatic stage. The silver:gold ratio is high and base-metal, arsenic, antimony, mercury and selenium contents are very high. Mine Mountain mineralization shares features with vein and disseminated silver deposits at Candelaria, Nevada. Gold-silver deposits in the areally extensive Bullfrog district comprise the largest known precious-metal resource in the volcanic field. They are mainly quartz-carbonate±adularia veins with alteration and mineralization styles similar to other adularia-sericite-type deposits in the Great Basin. Deposits in the Rhyolite area and at the Gold Bar mine have very low contents of arsenic and mercury compared to other epithermal deposits in the Great Basin, although copper and antimony are locally elevated. Similarities in mineralization style and assemblages, which include two occurrences of the rare gold-silver sulfide uytenbogaardtite, indicate deposition under similar conditions in different parts of the district. Hydrothermal activity in the Bullfrog district was coeval with extensional tectonism and may have continued from the Timber Mountain stage into the late magmatic stage. Mineralization at some deposits in the Bullfrog and Bare Mountain districts is spatially associated with, and, in part, structurally controlled by a regional detachment fault system. However, significant differences in age, mineralization style and geochemistry indicate that mineralization in the two districts is unrelated.