Byron R. Berger

Geology and structural evolution of the Muruntau gold deposit, Kyzylkum desert, Uzbekistan

Abstract The Muruntau gold deposit in the Kyzylkum desert of Uzbekistan is the largest single deposit (⪢ 1100 tonnes of gold) of the class of low-sulfide syndeformation/synigenous gold deposits formed in the brittle/ductile transition zone of the crust within transpressional shear zones. Hosted by the Cambrian to Ordovician Besopan Suite, the ores were deposited in pre-existing thrust-fault- and metamorphism-related permeabilities and in synmineralization dilational zones created in a large fault-related fold. The Besopan Suite is a 5,000-m-thick sequence of turbiditic siltstones, shales and sandstones. The ore is primarily localized at the base of the Besopan-3 unit, which is a 2,000-m-thick series of carbonaceous shales, siltstones, sandstones and cherts. Initial gold deposition took place within the Sangruntau-Tamdytau shear zone, which was developed along the stratigraphic contact between the Besopan-3 and Besopan-4 units. During the mineralization process, folding of the Besopan Suite and a left-step adjustment in the Sangruntau-Tamdytau shear zone were caused by two concurrent events: (1) the activation of the left-lateral Muruntau-Daugyztau shear zone that developed at nearly a 90° angle to the preceding shear zone and (2) the intrusion of granitoid plutons. These structural events also resulted in the refocusing of hydrothermal fluid flow into new zones of permeability.

Epithermal gold-siver deposits in the western United States: time-space products of evolving plutonic, volcanic and tectonic environments

Abstract The western United States has been the locus of considerable subaerial volcanic and plutonic igneous activity since the mid-Mesozoic. After the destruction of the Jurassic-Cretaceous magmatic arc-trench system, subduction was re-established in the Late Mesozoic with low-angle underthrusting of the oceanic plate beneath western North America. This resulted in crustal shortening during the Late Cretaceous to Early Tertiary and removal of the mantle lithosphere west of the Rocky Mountains. Commencing in the Eocene, flat subduction ceased, the volcanic arc began to re-establish itself along the continental margin, and the hingeline along the steepening subducting plate migrated from east to west. The crust east of the migrating hingeline was exposed to hot asthenosphere, and widespread tectonics and volcanic activity resulted. Hydrothermal activity accompanied the volcanism resulting in numerous epithermal gold-silver deposits. The temporal and spatial distributions of epithermal deposits in the region are therefore systematic and can be subdivided into discrete time intervals which are related to widespread changes in magmatic activity. Time intervals selected for discussion are Pre-Cenozoic, 66-55 Ma, 54-43 Ma, 42-34 Ma, 33-24 Ma, 23-17 Ma, and deep rifting that alunite-kaolinite ± pyrophyllite type epithermal deposits are formed. Adularia-sericite type deposits are most common, being related to all compositions and styles of volcanic activity. Therefore, the volcano-tectonic context of the western United States provides a unified framework in which to understand and explore for epithermal type deposits.

Pre-Cenozoic normal faulting in the Osgood Mountains, Humboldt County, Nevada

The Getchell fault system consists of several major faults extending for more than 25 km along the eastern side of the Osgood Mountains. Movement on the fault system has been complex and largely dip slip in character. The emplacement of igneous rocks and subsequent gold deposits was controlled by the fault system; dating of the igneous rocks indicates that the fault system existed in Late Cretaceous time.

A Framework for Quantitative Assessment of Impacts Related to Energy and Mineral Resource Development

Natural resource planning at all scales demands methods for assessing the impacts of resource development and use, and in particular it requires standardized methods that yield robust and unbiased results. Building from existing probabilistic methods for assessing the volumes of energy and mineral resources, we provide an algorithm for consistent, reproducible, quantitative assessment of resource development impacts. The approach combines probabilistic input data with Monte Carlo statistical methods to determine probabilistic outputs that convey the uncertainties inherent in the data. For example, one can utilize our algorithm to combine data from a natural gas resource assessment with maps of sage grouse leks and piñon-juniper woodlands in the same area to estimate possible future habitat impacts due to possible future gas development. As another example: one could combine geochemical data and maps of lynx habitat with data from a mineral deposit assessment in the same area to determine possible future mining impacts on water resources and lynx habitat. The approach can be applied to a broad range of positive and negative resource development impacts, such as water quantity or quality, economic benefits, or air quality, limited only by the availability of necessary input data and quantified relationships among geologic resources, development alternatives, and impacts. The framework enables quantitative evaluation of the trade-offs inherent in resource management decision-making, including cumulative impacts, to address societal concerns and policy aspects of resource development.

Crustal controls on magmatic-hydrothermal systems: A geophysical comparison of White River, Washington, with Goldfield, Nevada

The White River altered area, Washington, and the Goldfield mining district, Nevada, are nearly contemporaneous Tertiary (ca. 20 Ma) calc-alkaline igneous centers with large exposures of shallow (<1 km depth) magmatic-hydrothermal, acid-sulfate alteration. Goldfield is the largest known high-sulfidation gold deposit in North America. At White River, silica is the only commodity exploited to date, but, based on its similarities with Goldfield, White River may have potential for concealed precious and/or base metal deposits at shallow depth. Both areas are products of the ancestral Cascade arc. Goldfield lies within the Great Basin physiographic province in an area of middle Miocene and younger Basin and Range and Walker Lane faulting, whereas White River is largely unaffected by young faults. However, west-northwest–striking magnetic anomalies at White River do correspond with mapped faults synchronous with magmatism, and other linear anomalies may reflect contemporaneous concealed faults. The White River altered area lies immediately south of the west-northwest–striking White River fault zone and north of a postulated fault with similar orientation. Structural data from the White River altered area indicate that alteration developed synchronously with an anomalous stress field conducive to left-lateral, strike-slip displacement on west-northwest–striking faults. Thus, the White River alteration may have developed in a transient transtensional region between the two strike-slip faults, analogous to models proposed for Goldfield and other mineral deposits in transverse deformational zones. Gravity and magnetic anomalies provide evidence for a pluton beneath the White River altered area that may have provided heat and fluids to overlying volcanic rocks. East– to east-northeast–striking extensional faults and/or fracture zones in the step-over region, also expressed in magnetic anomalies, may have tapped this intrusion and provided vertical and lateral transport of fluids to now silicified areas. By analogy to Goldfield, geophysical anomalies at the White River altered area may serve as proxies for geologic mapping in identifying faults, fractures, and intrusions relevant to hydrothermal alteration and ore formation in areas of poor exposure.

Report of Working Group III

This report summarizes the discussions of the Working Group on Natural Resource Assessments and Resource Management that were held as part of the NATO Advanced Study Institute on “Deposit and Geoenvironmental Models for Resource Exploitation and Environmental Security.” The Working Group was made up of experts who represented 12 countries and had expertise in such fields as the geosciences and natural resources. The discussions focused on the scope of natural-resource assessments, the formats for presenting assessments, and the relation with geoenvironmental assessments. The Working Group stressed the importance of effective communication to the users of assessments and the responsibilities of the scientists who make assessments.

Application Of The Porphyry Copper/Polymetallic Vein Kin Deposit System To Mineral-Resource Assessment In The Mátra Mountains, Northern Hungary

The application of the tectonic model for the porphyry copper/polymetallic vein kin-deposit system, proposed by the authors and used to assess the undiscovered metallic mineral resources of northern Hungary, is illustrated here for the Matra Mountains, northern Hungary. This model is based on the evolution of strain features (duplexes and flower structures) developed in the strike-slip fault systems in continental crust above a subducting plate and the localization of mineral deposits within them.

Mineral-Deposit models

Mineral-deposit models are the basis for consistent resource-assessment, exploration, and environmental risk analysis methodologies. To reduce uncertainties in these methodologies, improved predictability of deposit occurrence is essential. Advances in understanding about structure and tectonics and the geology of earthquakes, together with improved insights as to how fluid flow is coupled with active deformation, heat transport, and solute transport, provide the framework for integrating mechanical phenomena into deposit models. With this framework, it is possible, for a given deposit type, to predict where in structural systems hydrothermal systems may occur, the chances that significant concentrations of ore may be expected, and where in larger vein arrays ore bodies may be localized. Discussions of porphyry copper and related polymetallic veins illustrate the value of this new generation of deposit models.

Timing, magnitude, and style of Miocene deformation, west-central Walker Lane belt, Nevada

The timing, magnitude, style, and kinematics of deformation in the Eastern California shear zone and Walker Lane belt are important in defining interactions between shear-dominated plate-boundary tectonics and extension-dominated plate-interior tectonics. Geologic studies of middle Miocene strata in a 50-km-long north-south belt along the west margin of the Wassuk Range, west-central Walker Lane belt, reveal a pattern of folding on subhorizontal axes associated with displacements on convex-upward faults, a pattern that greatly reduces estimates of extension. We report an array of previously unrecognized shortening structures, including east-west folds, reverse faults, and steep-axis folds that formed at high angle to, and synchronous with, regional extension in the Coal Valley portion of the Walker Lane belt west of the Wassuk Range. We also report new radiometric age data and field relationships that support a reinterpretation of the history, magnitude, and distribution of extensional deformation. Instead of extreme (150%–200%) extension at 15–13 Ma, the new data support moderate (perhaps 30%) extension beginning ca. 10 Ma, coincident with the development of an inboard component of plate-boundary transtensional deformation. Also, new age data from moderately tilted pre-extension volcanic rocks forming the lower part of the Tertiary stratigraphic section in the southern Singatse Range, west of the northern Wassuk Range, show a lack of the extreme extension reported for coeval strata in the central part of that range. Published tectonic models that assume that extreme middle Miocene extension was uniformly distributed from the central Singatse Range through the Coal Valley area and southward to the Mina Deflection are invalid, as are models of 100 km of westerly extensional strain migration from the Basin and Range into the Sierra Nevada. Our reinterpretation reflects a return to a Miocene history, developed almost four decades ago, of formation of a volcanic highland, followed by sedimentation in broad basins controlled partly by east-west structures, followed in turn by extensional deformation that formed the existing ranges and basins after ca. 10 Ma. Following these early studies, the history was revised based on geologic mapping and thermochronologic studies in the Gray Hills–Wassuk Range directly north of the Coal Valley area. The published thermochronologic studies of a transect across the Wassuk Range show approximate invariance across 4 km of the central range. Previously, these data had been modeled as recording rigid, whole-block west tilting of ∼50° at ca. 15 Ma, and the approximate invariance resulted in geologically instantaneous uplift of ∼6 km. However, middle Miocene strata lack evidence for such rapid large uplift; that is, large volumes of proximal coarse clastic rocks are not found, and the strata do not exhibit a pattern of growth-fault fanning expected in the tilted fault-block model. The invariance is more consistent with arching during and following magmatism than with fault-related whole-block tilting. During main-phase extension, which began at ca. 10 Ma, the range was again flexed during uplift, similar to smaller structural blocks directly to the west. How or whether the extension-normal shortening structures we describe accommodated plate-boundary strain is not clear. Northwest-striking dextral faults are not reported in the Coal Valley area or to the northwest, so there is no clear association between the shortening structures and strain accommodation at terminations, left bends, or stepovers of such faults. We speculate that the complex heterogeneous Miocene strain of the Coal Valley area records coupling of approximately east-west regional extension with extension-normal shortening. The shortening could record midcrustal flow, possibly responding to lateral gravity gradients.

Geochemical studies in the Indian Pass and Picacho Peak Bureau of Land Management Wilderness study areas, imperial county, Southern California

Abstract The U.S. Geological Survey has conducted geochemical studies in the Indian Pass (CDCA-355), 124 km 2 , and Picacho Peak (CDCA-355A), 23 km 2 , Wilderness Study Areas (WSAs) as part of a program to evaluate the mineral resource potential of designated areas in the California Desert Conservation Area. These two WSAs are of particular interest because they lie within a region which has intermittently produced significant quantities of Au since the mid-1800s, and is currently the site of much exploration activity for additional Au resources. Within a 15-km radius of the WSAs, there is one actively producing gold mine, a major deposit which began production in 1986, and one recently announced discovery. In the reconnaissance geochemical surveys of the two WSAs - 177 μm (-80 mesh) stream sediments, heavy-mineral concentrates from stream sediments, and rocks were prepared and analyzed. Four areas of possible exploration interest were identified within the WSAs. The first area is characterized by anomalous W and Bi in nonmagnetic heavy-mineral concentrates, and is underlain primarily by the Mesozoic Orocopia Schist which has been intruded by monzogranite of Oligocene age. Alteration and mineralization appear to be localized near the intrusive contact. The mineralized rock at the surface contains secondary Cu and Fe minerals where the monzogranite intrudes the metabasite horizons of the Orocopia Schist and scheelite where the monzogranite intrudes marble within the Orocopia Schist. The second area is characterized by anomalous As, Sb, Ba, B, and Sr in nonmagnetic heavy-mineral concentrates and by anomalous As in - 177 μm stream sediments. Geologically, this area is underlain by metasedimentary and metavolcanic rocks of Jurassic(?) age; a biotite monzogranite of Jurassic(?) age; and Tertiary volcanic and hypabyssal rocks composed of flows, domes, and tuffs of intermediate to silicic composition. All these rock types are cut by a set of north-south-striking normal faults. The anomalies in the heavy-mineral concentrates are believed to be related to silica-clay alteration observed in the vicinity of some of these faults.