The 45 km 2 map area is situated at the south end of the Hot Creek Range in central Nevada, ~16 km east of the buried leading edge of the Mississippian Roberts Mountains thrust. Three eastward-trending left-slip faults divide the area into four structural blocks. The southernmost block is occupied solely by upper Oligocene volcanic rocks. The narrow northernmost block, now occupied surficially by valley fill and volcanic rocks, represents the south end of the main part of the Hot Creek Range, from which the study area is offset. The middle two blocks display different aspects of the eastward-traveled outer crater rim created by the ca. 382 Ma (early Late Devonian, middle Frasnian) Alamo impact. The Alamo impact was produced by a 5-km-diameter bolide, most likely a comet, which excavated a transient submarine crater 44–65 km in diameter. Comparison of thin (8–12 m) Alamo Breccia deposits in the northern of the middle two blocks with a more easterly, thick (35–42 m) Alamo deposit in the main Hot Creek Range, 4 km north of the map area, suggests that these blocks traveled many kilometers eastward. The northern of the middle two blocks contains a large olistolith capped by the thin breccia, whereas the southern block contains a larger olistolith lacking an Alamo Breccia cap. Three Devonian pulses of the Antler orogeny are better documented in the chapter on the Bisoni-McKay area. Here, the first Antler pulse in latest Middle Devonian time is obscured within an ~9 m.y. hiatus enlarged by excavation of the Alamo impact crater. The second Antler pulse is recorded by the ~4 m.y. hiatus produced by the regional unconformity between the lower and upper members of the Woodruff Formation. The third Antler pulse is documented by an ~8 m.y. regional hiatus between the Mississippian Webb Formation and Upper Devonian Woodruff Formation. In previous papers, we had interpreted this pulse to initiate the Antler orogeny.

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Cenozoic rocks include about 15,000 feet of continental sedimentary and volcanic rocks of Oligocene age. Volcanic rocks consist of ignimbrite flows, plugs and dikes, lavas, and air-fall tuffs. Ignimbrite plugs and dikes fill vents from which some of the surrounding ignimbrite sheets were probably discharged. Plug rocks have vitroclastic textures and textures transitional between vitroclastic and flow banding. Dike rocks have vitroclastic textures and show various stages of vesiculation. Ignimbrites forming the basal sheet are crystal-rich rhyodacites, and have less silica and alkalies than the rhyodacites forming the top sheet. The ignimbrite plugs are more siliceous and alkalic than their surface-equivalent ignimbrite sheets. These plugs are modally and chemically “stratified”; their upper parts are crystal-rich rhyodacites and contain less silica and alkalies than the basal, moderately crystal-rich rhyolites. The origin of these variations is not well understood, but available data suggest that the basal portion of a differentiating magma chamber may have been tapped, so that the magma became progressively more siliceous and alkalic; continued pulses of increasingly more siliceous material may account for the “stratification” of the plugs. Cognate crystals in the ignimbrite plugs, dikes, and sheets were broken prior to extrusion; some were broken during compaction and welding. Most of the vesiculation took place prior to extrusion. Fluidization of the constituents may account for the mobility of the ignimbrites. Chemical compositions and textures have been modified in most sheets after they were emplaced. Secondary sanidine of a devitrification origin locally crystallized to a more sodic sanidine, and the replaced potassium may be accounted for by the K-feldspar rims on cognate crystals. Textural changes include welding, devitrification, and vapor-phase crystallization. The ignimbrite plugs and thick ignimbrite sheets are welded and devitrified from base to top. Thin sheets are welded only in their middle portions, and devitrification is confined to their upper zones. Ignimbrite plugs and their equivalent ignimbrite sheets contain vapor-phase minerals from base to top in vesicles that formed after welding. These ignimbrites were probably richer in volatiles than those in which vapor-phase crystallization was confined to the upper zones.

Abstract In central Nevada, a series of angular unconformities records protracted orogenic activity between middle Mississippian and late Permian time. These unconformities are regional, and can be correlated with lithofacies boundaries at their distal edges. Both the unconformities and the tectonically created sedimentary basins they bound are best expressed in a north-south belt of localities from Winnemucca south to the Las Vegas area. This paper briefly describes seven localities where rocks display both structural and stratigraphic features related to one or more of these unconformities and their related tectonic events. At Edna Mountain, the record is both stratigraphic and structural, and is mostly from the Pennsylvanian. At Carlin Canyon, we will look at both Mississippian and Pennsylvanian folding, thrusting, and unconformities. In the Diamond Range, we will see evidence that Pennsylvanian folding is regionally important. At Secret Canyon, the record is mostly of Permian deformation and sedimentation. In the Hot Creek Range, we will see southern versions of Mississippian stratigraphy, and thrusting that is late Paleozoic in age. In the Timpahute Mountains, complex faulting is also believed to be late Paleozoic.

The early Late Devonian (early Frasnian) Alamo Impact targeted an oceanic, off-platform site in southern Nevada, excavating a crater with a final diameter of 44–65 km. The original crater is now dismembered and buried beneath younger rocks. Consequently, its size and site must be deduced through multiple converging lines of geological and paleontological evidence. Previous and new evidence includes the catastrophically emplaced Alamo Breccia, tsunamites, shock-metamorphosed quartz grains, carbonate accretionary lapilli, an iridium anomaly, sub-Breccia clastic injection, deep-water Breccia channels, and ejecta material. We now demonstrate, on the basis of conodont microfossils in carbonate ejecta clasts within lapillistone blocks and widely distributed shocked-quartz and lithic-clast ejecta within the upper part of the Breccia, that the Alamo Impact excavated down at least into Upper Cambrian strata, at a depth of 1.7 km, and possibly into the underlying Proterozoic–Lower Cambrian Prospect Mountain Quartzite, ∼2.5 km beneath the Late Devonian seafloor. Distal tsunamites and probable ejecta are now documented as far north as Devils Gate, northern Nevada, and as far northeast as the Confusion Range, western Utah. A charcoal-bearing, early Frasnian estuarine deposit in the Bighorn Mountains, Wyoming, may provide the first evidence of an Alamo Impact fallout-generated forest fire. Our new data further document the widespread effects and size of the Alamo Impact, and constrain the likely present position of the tectonically transported crater to an area between the Timpahute and Hot Creek Ranges, southern Nevada.

Abstract Hot springs along Sulphur Creek in Colusa County, California, have been recognized for about 130 years. Several researchers have proposed that the hot spring fluid there is derived from mixing of “connate” or “evolved connate” water which is derived from ancient seawater deposited in the Mesozoic sedimentary rocks. This water, which is similar in composition to Complexion Spring, mixes with meteoric water to form Wilbur Springs and other hot spring waters along Sulphur Creek. A δD - δ 18 O plot shows that Complexion Spring really does not plot along this trend; it must be isotopically modified to plot along the trend. Tuscan Springs, which is located 140 km NNE of Wilbur Springs, just NE of Red Bluff, has chemical and isotopic characteristics which are similar to the Sulphur Creek hot springs. Tuscan Springs vent from the Chico Formation of the Great Valley sequence and indicate that Tuscan Springs and Wilbur Springs are both derived from waters originating in the Great Valley sequence. Also δ 11 B correlates well with Cl, δD and δ 18 O, which originate in the Great Valley sequence, suggesting a similar source for the higher d 11 B values. Chemical geothermometry of the Sulphur Creek hot springs indicates a reservoir temperature of ~ 180 °C. This temperature agrees with measured homogenization temperatures from fluid inclusion which range from 150 to 180 °C. The calculated cation geothermometer temperatures are affected by the presence of dissolved Mg, even though the concentrations appear low.

Abstract A well-preserved fault scarp resulting from recurrent Quarternary normal faulting occurs at the western edge of the northern Sangre de Cristo Mountains of south-central Colorado, at the mouth of west-draining Major Creek (Fig. 1). The site is approximately 56 mi (90 km) north-northeast of Alamosa, Colorado, and may be approached from Colorado 17 between Alamosa and Poncha Springs, Colorado. Turn east off of Colorado 17, 50 mi (80 km) north of Alamosa, opposite the junction with U.S. 285, onto a dirt road that leads due east for 6 mi (10 km) across the valley floor toward Valley View Hot Springs. Instead of turning off to the hot springs, bear right and follow the road as it turns south to parallel the range front. The road will continue south past the mouth of Garner Creek [1 mi (1.7 km) south of the hot springs turnoff] to the upper part of the Major Creek alluvial fan [2 mi (3.2 km) south of the hot springs turnoff]. Approximately 0.25 mi (400 m) north of the crossing of Major Creek (Fig. 2) turn onto a dirt driveway leading due east to the head of the fan where the fault scarps occur. Entry through the locked gate will require permission from the landowner on the north side of the fanhead, Dr. Ben Eismann, Dept. of Surgery, University of Colorado Health Science Center, Denver, CO 80262. The fault scarps on the south side of the road (including the 1980 trench site, Profile 35 on Fig.

Abstract Epithermal precious-metal and mercury deposits are present in the Sonoma and Clear Lake volcanic fields of central California and several hot springs in the Clear Lake volcanic field are presently depositing mercury and gold. The deposits and hot springs are associated with late Miocene to Holocene volcanic centers developed above a zone of thin crust and hot asthenosphere termed a slab window (Dickinson and Snyder, 1979, Benz and others, 1992) as the end of Pacific plate subduction was marked by the passage of the Mendocino triple junction along the California coast. Mercury deposition is actively occurring at the Sulphur Bank mercury mine, but no precious metals are present there because the geothermal system is vapor-dominated. In the water dominated geothermal systems at Wilbur Springs (Peters, 1990, Donnelly and others, 1993) and springs near the Cherry Hill gold deposit, both cinnabar and gold are being deposited (Pearcy and Petersen, 1990). Transport of mercury and gold is in a fluid which also contains high concentrations of petroleum and associated methane and CO2 derived from thermal degradation of organic matter in sedimentary rocks (Peabody, 1989). Chemical and isotopic analysis of oxygen and deuterium of the hot springs indicate that three types of fluid are present: moderate chloride, isotopically heavy, evolved formation fluid equilibrated with oceanic sedimentary rocks; evolved meteoric water; and isotopically light meteoric water (Peters, 1990,1991, Sherlock and Jowett, 1992, and Donnelly-Nolan and others, 1993). High concentrations of Hg, As, Sb, Au, and Ag occur in precipitates from hot springs composed dominantly of the isotopically heavy fluid, but not in the moderate-temperature, oxidized springs that are mixtures of these two fluid types (Peters, 1990, Donnelly-Nolan and others, 1993). The McLaughlin gold deposit (initial reserves of 2.9 million oz of gold) is economically the most important deposit in the Clear Lake and Sonoma volcanic fields. This precious metal-mercury hydrothermal system developed within and adjacent to andesitic vents and dikes emplaced along the Stony Creek fault zone (Lehrman, 1986). Gold occurs in opal, chalcedony, and quartz veins, and the highest gold values typically occur in amber to brown opal containing petroleum. Gold occurs in several sites within the petroleum-bearing opal: as a filling of 2050 micron- diameter oval voids representing large fluid inclusions; as 2-4 micron size crystals that coalesce to form dendrites of gold along primary vein banding; and in syneresis cracks which cut the vein banding. Oxide phases of Ga, In, Sn, and Ni are present within the petroleum-bearing opal. The isotopically heavy McLaughlin ore fluid plots in the field of andesitic magma volatiles (Hedenquist and Aoki, 1990, Giggenbach, 1987) and evolved formation waters (Sherlock and Jowett, 1992) suggesting that these two components are present Andesitic vents and dikes at the McLaughlin gold deposit suggest that a larger intrusion underlies the area and provided the heat source for the hydrothermal system. Andesitic vents along the Stony Creek fault provided a conduit for volatiles degassing from the intrusion to become entrained within the hydrothermal fluid composed of gas-oil-field water derived from the Great Valley sequence. The McLaughlin gold deposit reflects the complex interaction of three types of fluid each transporting a different elemental suite: evolved gas-oil field formation water transporting petroleum, Ga, In, Sn, Ni, and Hg; andesitic magmatic fluid transporting Au, Ag, Hg, Sb, and As; and near-surface meteoric water. Prospective areas for precious metal hot-spring deposits occur in the volcanic-structural environment above the thin crust and hot asthenosphere within the slab window in the Coast Ranges and parts of the Great Valley sequence where blind thrusts and associated faults are intruded by Pliocene to Holocene intrusive rocks. Mercury deposits with little or no gold content form along major structures from gas-oil field fluids with little or no magmatic component in the fluid and contain petroleum, Ni, Ga, In, Sn, and other transition elements. Epithermal gold deposits contain a significant magmatic component characterized by Au, Ag, As, Sb, and Hg as well as a gas-oil- field fluid component characterized by petroleum and transition metals. Both deposit types may occur along the same structures.

Abstract Results from a comprehensive U-Pb geochronology study of the Kidd Creek deposit and the surrounding Kidd Volcanic Complex are presented. Eleven new zircon and two titanite ages are reported and integrated with U-Pb age results on five related samples, which were published in a previous study. Zircon ages for rhyolite volcanism of the Kidd Volcanic Complex range from 2717.0 + –2 2 . . 5 6 to 2711.5 ± 1.2 Ma. This age range is established on immediate footwall and hanging-wall rhyolites of the Kidd Creek orebody. Since both footwall and hanging-wall rhyolites can be linked to ore formation, the conclusion must be that the giant Kidd Creek deposit is the product of unusually long-lived, albeit episodic, sea-floor hydrothermal activity. Our most precise estimate for the age of footwall rhyolites at Kidd Creek is provided by results on two nearby rhyolites that have been dated at 2716.1 ± 0.6 and 2716.0 ± 0.5 Ma, respectively. The age of at least one large massive sulfide lens at Kidd Creek, the North orebody, has been tightly bracketed between the age of footwall rhyolites and the age of an overlying rhyolite lapillistone horizon dated at 2715.8 ± 1.2 Ma. Hence, ore formation of individual massive sulfide lenses appears to have been rapid and well within the resolution limits of current U-Pb dating techniques. Based on the large number of ages, it appears that volcanic activity of the Kidd Volcanic Complex can be divided into four general phases, each of which is supported by at least one or more high-precision ages: phase I, onset of bimodal komatiite and rhyolite volcanism, probably as early as 2717.7 ± 1.1 Ma, and extrusion of the footwall assemblage at Kidd Creek at ca. 2716 Ma; phase II extrusion of ca. 2714 Ma rhyolites; phase III, extrusion of ca. 2711 Ma rhyolite, including the quartz porphyritic hanging-wall rhyolite at Kidd Creek; and phase IV, extrusion of the hanging-wall basalt sequence and intrusion of subvolcanic gabbro sills sometime after 2711 Ma. The Kidd Creek deposit probably formed along the axial zone of a slow-spreading rift basin that developed during extension of an older volcanic-arc assemblage. This older arc assemblage, which included ca. 2723 and 2735 Ma components, is probably represented by the Deloro Group and correlative assemblages exposed to the south of Timmins. Rifting of the older volcanic substrate, and partial melting to produce the rhyolites, was induced by the arrival of a hot mantle plume that gave rise to the komatiites of the Kidd-Munro assemblage. Graywacke turbidites in the Kidd Creek area are all younger than ca. 2699 Ma and do not form the deeper stratigraphic footwall to the deposit. Instead, the graywackes probably overlie, unconformably to disconformably, all the volcanic assemblages in the region. From the sub -sequent protracted structural-metamorphic evolution in the area, two discrete events have been dated: the 2663.3 ± 3.3 Ma intrusion of the Prosser porphyry granitoid stock and a discrete 2639.1 ± 7.2 Ma metamorphic-hydrothermal event. The timing of both events corresponds closely to ages for granulite facies metamorphic events in lower crustal rocks of the nearby Kapuskasing structural zone.