First posted October 11, 2023 For additional information, contact: Geology, Minerals, Energy, & Geophysics Science CenterU.S. Geological SurveyBuilding 19, 350 N. Akron Rd.P.O. Box 158Moffett Field, CA 94035 Rattlesnake Knoll is a small, 30-meter-high mound of igneous breccia in the center of Spring Valley, east-central Nevada. In the past, researchers have disagreed as to whether the unusual-looking outcrop is intrusive or volcanic. The breccia possesses a normal magnetic polarity, but this is not apparent in aeromagnetic survey data. These data instead show that the knoll lies within a small aeromagnetic low that partially overlaps the extent of a small gravity high. The small gravity anomaly associated with the knoll, combined with an initial, limited ground magnetic survey taken at the knoll, indicates that the knoll rocks extend northward in the subsurface. A second, more extensive ground magnetic traverse was also done north of the knoll. Taking into consideration these new survey data and preexisting data, a two and one-half dimensional modeling program based on Webring (1985) was used to produce a geophysical model that accounts for gravity and magnetic properties, satisfies available geologic information, and conforms to current estimates of basin thickness. This model and the field observations support the interpretation that the knoll consists of gently west-dipping beds of Tertiary volcanic flow breccia, mudflow breccia, and conglomerate.

Increasing demands on the Colorado River system within the arid Southwestern United States have focused attention on finding new, alternative sources of water. Particular attention is being paid to the eastern Great Basin, where important ground-water systems occur within a regionally extensive sequence of Paleozoic carbonate rocks and in the Cenozoic basin-fill deposits that occur throughout the region. Geophysical investigations to characterize the geologic framework of aquifers in eastern Nevada and western Utah began in a series of cooperative agreements between the U.S. Geological Survey and the Southern Nevada Water Authority in 2003. These studies were intended to better understand the formation of basins, define their subsurface shape and depth, and delineate structures that may impede or enhance groundwater flow. We have combined data from gravity stations established during the current study with previously available data to produce an up-to-date isostatic-gravity map of the study area, using a gravity inversion method to calculate depths to pre-Cenozoic basement rock and to estimate alluvial/volcanic fill in the valleys.

The origin of Bayan Obo, the world's largest known rareearth-element (REE) ore deposit, has been a subject of geologic interest and speculation. Recent field and laboratory observations support the early conclusion that the Middle Proterozoic host rock of the Bayan Obo ores is not a carbonatite, but a metamorphosed, sedimentary dolomitized limestone. Petrographic textural analysis indicates that the ores are epigenetic, hydrothermal and metasomatic in origin. Radioisotopic data together with the textural analysis show that the principal episodes of mineralization for this unique deposit probably began about 800 Ma and continued to at least 425 Ma, a span of about 400 million years. Introduction. The use of rare earth elements (REEs) in high technology (for example high-strength ceramics, catalytic functions, permanent magnets, phosphors in video displays, and high-temperature superconductors) (1) makes the occurrence of these elements of increasing economic interest worldwide. Bayan Obo, the world's largest known REE deposit, with inferred reserves of more than 30 million metric tons of rare earth oxides (RE20^)(2), has until recently been of extremely limited access for study by the international scientific community. Five hypotheses for the origin of the Bayan Obo deposit have been proposed (3): a) high-temperature hydrothermal metasomatism related to Hercynian granitic rocks that are widespread in the mine region, b) and c) sabkha sedimentary syngenesis preceding regional metamorphism with or without additional hydrothermal metasomatism, d) magmatic carbonatite, and e) exhalative volcanogenic carbonatite sedimentation. Results reported below support a sixth hypothesis, a late Proterozoic to Caledonian age epigenetic, hydrothermal, metasomatic origin. Location and geologic setting. The Bayan Obo REE deposit was discovered in 1927 as a Fe deposit (3). Currently both Fe and REEs are mined; Nb ore is also present. The deposit is located in Inner Mongolia, at 41°45'N and 110°E. The mine is owned and operated by the Baotou Iron and Steel Corporation of the Ministry of Metallurgical Industry of the People's Republic of China. The rocks of the Middle Proterozoic Bayan Obo Group, which hosts the ore, represent a platform sequence on the northern flank of the Archean Sino-Korean craton. Sedimentary rocks of the sequence were probably deposited in an E-W-trending graben or trench, with the open sea to the north (4), and then uplifted and regionally metamorphosed at about 1.4 Ga (5). Geologically, the Bayan Obo region (Fig. 1) can be divided into two parts: (a) north of the Kuanggou fault zone is an unmineralized, weakly regionally metamorphosed limestone and shale sequence H1-H10 (Hu in Fig. 1) (6); and (b) south of the Kuanggou fault zone the mine region is more strongly regionally metamorphosed and includes the Bayan Obo Group, which is 1,880 m thick and consists of a sequence of quartzite, slate and shale, and crystalline dolomite (H1-H9, Fig. 1). The H9 black slate and shale south of the fault zone is in the trough of a syncline, and is underlain and flanked on both sides by the highly mineralized H8 dolomite. The striking differences in lithology, structural pattern, and mineralization north and south of the fault zone indicate a postoreformation age for the Kuanggou fault. Both REE ores and Fe-REE ores are stratabound within the H8 dolomite (Fig. 1). The dolomite ranges from 240 to 540 m in thickness, and is overlain by the relatively impervious, 340-360 m thick, H9 black slate and shale (Fig. 1), which may have acted as a caprock for the mineralizing solutions. The dolomite strikes E-W for about 16 km and is approximately 1-2 km in width in the N-S direction. The Main Ore Body and East Ore Body (Fig. 1) are located where the H8 dolomite is thickest. In the field, the ores were not found to be associated with any plutonic or volcanic rocks, and there has been no observed occurrence of alkalic granitic or subsilicic alkalic rocks in and adjacent to the mine region. Field and laboratory evidence indicates that the ore-bearing rocks have suffered both preand post-ore regional metamorphism. Mineral paragenetic sequence and ages of mineralization. Four independent methods were used to determine the mineral paragenetic sequence and ages of mineralization of the deposit: (a) field observations, (b) detailed petrographic textural analysis, (c) radioisotopic determination of mineral ages, and (d) chemical analysis of minerals determined by the preceding three methods to be of different generations in the mineral paragenetic sequence. Field and laboratory evidence indicates that the H8 host dolomite is of sedimentary origin (4, 7, 8), and not a carbonatite as previously suggested (3, 9, 10). This evidence includes: (a) conformable contacts of the dolomite with the overlying H9 black slate and shale and the underlying H7 mica schist and dolomite; (b) general massive appearance of the H8 dolomite, with occasional interbeds and lenses of quartzite, scattered detrital quartz, and rounded detrital apatite grains in planes parallel to the bedding; (c) presence, although rare, of algae microfossils in the dolomite (7); and (d) nature of the dolomite: ferroan (FeO 3-7 wt %), dominantly very fine-grained and well recrystallized (11). The 0 values for the H8 dolomite range from +12 to +16 0/00 (SMOW), and presumably were induced by interaction with hydrothermal solutions of light 0 isotopic composition. The mineralogy of the Bayan Obo ores is extremely complex, as shown by the fact that more than 100 minerals have been described from this deposit (12). The most important REE ore minerals of Bayan Obo are monazite ((Ce,La,Nd)PO^), bastnaesite ((La,Ce)(C03 )F), and huanghoite (BaCe(C03)2F); the most important Fe minerals are magnetite (Fe^O^) and hematite ^620^); and the most important Nb ore minerals appear to be fergusonite (YNbO^), aeschynite ((Ce,Ca,Fe,Th)Nb2 (0,OH) 6 ), and columbite (FeNb206 ) (see (12)). The two major types of dolomite-hosted REE and REE-Fe ores are: (a) disseminated REE ore, and (b) finely laminated and banded high-grade REE and REE-Fe ore that occurs in pods and lenses (Fig. 2). In addition, massive Fe ores with very low REE content occur in the central part of the major ore bodies and in the western part of the mine region. Mineral zoning is not apparent in the ore bodies (13). RE203 contents in the disseminated ores range from 2 to 6 wt %. In the banded ores and some massive Fe ores, ore grades vary from 2 to 25 wt % RE20-J and from 20 to 55 wt % Fe (averaging about 34 wt % Fe) (13). Replacement textures are widespread and are observable both megascopically and microscopically. The earliest introduction of stringers and sheaths of very fine-grained monazite aggregates, which preceded the introduction of magnetite in the mineral paragenetic sequence, came after recrystallization of the dolomite into a marble. For example, in the field, yellow streaks and irregular lacy networks of very fine-grained monazite were seen in essentially pure dolomite. Irregular patches of magnetite within dolomite and irregular boundaries between magnetite-rich areas and the dolomite host were also widely observed. Microscopically, fine-grained granular monazite occurs interstitially along dolomite triple-junction grain boundaries, destroying the host fabric by replacing the dolomite crystals (Figs. 3A and 3B). Where magnetite occurs in H8 dolomite that contains early monazite, the magnetite is interstitial between grains of dolomite and monazite (Fig. 4A). Where stongly twinned dolomite crystals occur with magnetite, the irregularly shaped relict dolomite remains in continuous optical orientation surrounded by the replacing magnetite (Fig. 4B). Similar destruction of the triple-junction grain boundary fabric of the host dolomitic marble by other minerals, such as magnesio-arfvedsonite, aegirine, phlogopite, fluorite, barite, apatite, and hematite, is also widespread. Banded ores are cut by veins of younger aegirine, with or without magnesio-arfvedsonite, and contain large huanghoite and aeschynite crystals. No evidence indicating a syngenetic origin for the ores, such as oolitic textures in Fe ores, was found. On the basis of field and microscopic textural analysis of disseminated and banded ores and radioisotopic dating of minerals, the generalized principal mineral paragenetic sequence (8) was determined to be: (a) magnesio-arfvedsonite, 802 + 19 Ma (1, Table 1); (b and c) disseminated monazite, earlier than magnesio-arfvedsonite with an age of 628 + 15 Ma (2, Table 1); (d) disseminated monazite, 594 + 4 Ma (5, Table 1); (e) magnesio-arfvedsonite, 440 + 11 Ma (3, Table 1); (f) early magnetite; (g) granular hematite; (h) magnesio-arfvedsonite, monazite, and bastnaesite, 425 + 10 Ma (4, 6, and 7, Table 1); (i) late magnetite; (j) vein huanghoite and aeschynite, 438 + 25 Ma (see below); and (k) Hercynian late-stage mineral assemblages including sulfides, microcline, albite, phlogopite, fluorite, barite, quartz, calcite, fergusonite, rare Ba and Sr REE fluorocarbonates, and Ba and Sr carbonates. Episodes of REE mineralization appear to be separate in time from those of Fe mineralization. Niobium mineralization occurred during and after the second stage of magnetite mineralization (stages i, j, and k). In general, radioisotopic ages (Table 1) are consistent with the texturally determined mineral paragenetic sequence. Mineral ages listed in Table 1 are considered to be minimum ages because later heating events may have resulted in a slight reduction of the radioisotopic ageS> 40 39 It is important to note that the K/Ar and Ar/ Ar ages of the four magnesio-arfvedsonites (Table 1) cover a span of about 400 million years and that these samples have distinctly different chemical characteristics, varying from MgOFeO to MgCKFeO, coupled with MnC^ content ranging from 0.06 to 5.68 wt %. Such distinct compositions suggest that each genera

Cenozoic basins in eastern Nevada and western Utah constitute major ground-water recharge areas in the eastern part of the Great Basin and these were investigated to characterize the geologic framework of the region. Prior to these investigations, regional gravity coverage was variable over the region, adequate in some areas and very sparse in others. Cooperative studies described herein have established 1,447 new gravity stations in the region, providing a detailed description of density variations in the middle to upper crust. All previously available gravity data for the study area were evaluated to determine their reliability, prior to combining with our recent results and calculating an up-to-date isostatic residual gravity map of the area. A gravity inversion method was used to calculate depths to pre-Cenozoic basement rock and estimates of maximum alluvial/volcanic fill in the major valleys of the study area. The enhanced gravity coverage and the incorporation of lithologic information from several deep oil and gas wells yields a much improved view of subsurface shapes of these basins and provides insights useful for the development of hydrogeologic models for the region.

The Newark Valley area, eastern Nevada is one of thirteen major ground-water basins investigated by the BARCAS (Basin and Range Carbonate Aquifer Study) Project. Gravity data are being used to help characterize the geophysical framework of the region. Although gravity coverage was extensive over parts of the BARCAS study area, data were sparse for a number of the valleys, including the northern part of Newark Valley. We addressed this lack of data by establishing seventy new gravity stations in and around Newark Valley. All available gravity data were then evaluated to determine their reliability, prior to calculating an isostatic residual gravity map to be used for subsequent analyses. A gravity inversion method was used to calculate depths to pre-Cenozoic basement rock and estimates of maximum alluvial/volcanic fill. The enhanced gravity coverage and the incorporation of lithologic information from several deep oil and gas wells yields a view of subsurface shape of the basin and will provide information useful for the development of hydrogeologic models for the region.

A project to study ground-water and surface-water interactions in the desert southwestern United States was initiated in 2001 by the Tucson, Arizona office of the Water Resources Division, U.S. Geological Survey (USGS). One of the goals of the Southwest Ground-water Resources Project was to develop a regional synthesis that includes the use of available digital geologic data, which is growing rapidly due to the increasing use of Geographic Information Systems (GIS). Included in this report are the digital maps and databases of geologic information that should have a direct impact on the studies of ground-water flow and surface-water interaction. Ground-water flow is governed by many geologic factors or elements including rock and soil permeability, stratigraphy and structural features. These elements directly influence ground-water flow, which is key to understanding the possible inter-connectivity of aquifer systems in desert basins of the southwestern United States. We derive these elements from the evaluation of regional geology and localized studies of hydrogeologic basins. These elements can then be applied to other unstudied areas throughout the desert southwest. This report presents a regional perspective of the geologic elements controlling ground-water systems in the desert southwest that may eventually lead to greater focus on smaller sub-regions and ultimately, to individual ground-water basins.

4 Introduction 5 Geologic Summary 7 Field Methods and Data Reduction 8

A 3-dimensional caldera model based on gravity inversion, drill-hole data, and geologic mapping offers the framework for a hydrogeologic evaluation of the Silent Canyon caldera in the central part of Pahute Mesa, Nevada. It has been recognized for several decades that the central part of Pahute Mesa is the site of a buried caldera called the Silent Canyon caldera. Conceptually, the structural framework of the Silent Canyon caldera is based on the idea of collapse of the caldera roof over a shallow magma chamber to form a structural basin following violent volcanic eruptions. Calderas are common in certain volcanic regions of the world, and most well-exposed calderas are broadly similar to each other, particularly the arcuate or circular shape of their collapse depression. There are other reasons for modeling the Silent Canyon caldera as a circular feature in addition to knowledge that calderas throughout the world are generally circular features. The Silent Canyon caldera is the site of one of the largest gravity lows in the Western United States, indicating a thick accumulation of low-density rocks such as lavas and tuffs—a fact confirmed by drilling on Pahute Mesa. This gravity low is bowl-shaped, and the uppermost volcanic units on Pahute Mesa form a circular outcrop pattern of inward-dipping tuff interpreted to be the result of their filling the upper part of the bowl-shaped depression. Together, these features are consistent with, and indicative of, a circular collapse structural model for the Silent Canyon caldera. The collapse depression of the Silent Canyon caldera, bounded by arcuate faults, is filled with as much as 6 km (19,800 ft) of volcanic and sedimentary rocks that are considerably less dense than the underlying and surrounding basement rocks. The boundary surface between less dense caldera fill and more dense basement is modeled as the caldera ring fault. Rocks in the upper part of the caldera fill are penetrated by drilling, and the drill-hole data are the basis for 3-dimensional computer modeling of the thickness and distribution of the rock units. The displacement on younger N-S faults that cut the caldera is also determined by offset of the computer derived surfaces defined by the drill-hole intercepts of stratigraphic units.

Paleomagnetic and geochronologic data combined with geologic mapping tightly restrict the timing and character of a late Oligocene to early Miocene episode of large magnitude extension in the southern Stillwater Range and adjacent regions of west central Nevada. The southern Stillwater Range was the site of an Oligocene to early Miocene volcanic center comprising (1) 28.3 to 24.3 Ma intracaldera ash flow tuffs, lava flows, and subjacent plutons associated with three calderas, (2) 24.8 to 20.7 Ma postcaldera silicic dikes and domes, and (3) unconformably overlying 15.3 to 13.0 Ma dacite to basalt lava flows, plugs, and dikes. The caldera‐related tuffs, lava flows, and plutons were tilted 60°–70° either west or east during the initial period of Cenozoic deformation that accommodated over 100% extension. Directions of remanent magnetization obtained from these extrusive and intrusive, caldera‐related rocks are strongly deflected from an expected Miocene direction in senses appropriate for their tilt. A mean direction for these rocks after tilt correction, however, suggests that they were also affected by a moderate (33.4° ±11.8°) component of counterclockwise vertical axis rotation. Paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24.8 to 20.7 Ma postcaldera dikes and domes. In detail, an apparent decrease in rotation with decreasing age of individual, isotopically dated bodies of the postcaldera group indicates that most tilting occurred between 24.4 and 24.2 Ma. The onset of tilting immediately following after the final caldera eruptions suggests that the magmatism and deformation were linked. Deformation was not driven by magma buoyancy, however, because tilting equally affected the caldera systems of different ages, including their plutonic roots. It is more likely that regional extension was focused in the southern Stillwater Range due to magmatic warming and reduction of tensile strength of the brittle crust. Faults that accommodated deformation in the southern Stillwater Range initially dipped steeply and cut deeply to expose more than 9 km of crustal section. The exposed crustal sections are probably rotated blocks above an unexposed basal detachment that lay near the early Miocene brittle‐ductile transition.

Structures in the pre-Tertiary basement of Yucca Flat, Nevada Test Site, Nevada, are interpreted using the basement topography and basement gravity anomaly derived from an isostatic gravity inversion model. A new fault is proposed which eliminates some of the Paleozoic carbonate section just west of the Halfpint Range. Proposed faults that offset basement surface correlate closely with magnetic anomalies caused by the offset of Tertiary volcanic rocks.

Geophysical data relating the dynamic processes of plate motion and subduction to Andean orogenesis are interpreted in terms of a new model for magmatic and tectonic development of the central Andes. The model is based on changing subduction geometry—from normal to flat to normal—and the attendant magmatic and tectonic effects of slab dewatering, continental lithospheric hydration, and asthenospheric flow during closing and opening of the subduction zone mantle wedge. The model includes five stages:1. Normal subduction extended into Eocene time.2. A slab transition from normal to flat subduction occurred in late Eocene-early Oligocene time, coincident with extensive crustal deformation in the eastern Altiplano and Eastern Cordillera.3. Flat subduction during much of Oligocene time was accompanied by a volcanic null throughout the central Andes, when water from the slab infiltrated and hydrated the overlying continental lithosphere, resulting in advective cooling and abnormally low heat flow values. Lithospheric hydration was concentrated not only in the usual fore-arc region but also within the inner arc, in the zone of resubduction where amphibole is presumed to break down and the slab dips steeply into the mantle.4. The transition from flat to normal subduction in late Oligocene-earliest Miocene time brought about an influx of asthenospheric material from depth into the growing mantle wedge above the slab. Hot asthenospheric mantle in contact with hydrated lithosphere of the inner arc produced widespread melting of both mantle and crust beneath the eastern Altiplano-Eastern Cordillera and ushered in a period of ductile deformation associated with oroclinal formation. The magmatic activity and orogenic uplift that began in the inner arc broadened westward as hot asthenospheric material flowed into the mantle wedge above the sinking slab.5. The westward broadening of volcanic activity culminated in a resumption of calc-alkaline volcanism all along the main volcanic arc by at least 20 to 15 Ma. The crust beneath the main arc, probably thickened by previous magmatic and deformational events, was further thickened and uplifted by the intrusion or underplating of massive volumes of mantle-derived magmas. Eruptive activity in the inner arc, much of it anatectic and correlated with periods of crustal deformation, gradually waned, with migration of minor magmatic centers eastward almost to the present day. The thermally thinned and weakened lithosphere of the Eastern Cordillera and sub-Andean belt formed a ductile block in which compressive stresses have been concentrated in Neogene time. The tectonic collapse of the inner

A three-dimensional inversion of gravity data from the Amargosa Desert and Pahrump Valley reveals a topographically complex pre-Cenozoic basement surface concealed by younger sedimentary and volcanic deposits.The Amargosa Desert is underlain by a deep, steep-sided trough extending from the southwest Nevada volcanic complex to the Nevada-California state line.The linear margins of the Amargosa Desert trough and its internal topography suggest that it formed as a series of transtensional basins that transferred strain between right-stepping, northwest-striking, right-lateral, strike-slip faults.Pahrump Valley is underlain by two deep, steep-sided sub-basins separated by a narrow basement ridge aligned parallel to the state line.The Pahrump Valley sub-basins also formed as transtensional pull-apart basins, accommodated in part by displacement along the northwest-striking State Line fault zone.The state-line ridge at Pahrump Valley is on strike with a narrow basement ridge beneath Ash Meadows, also lying along the state line and within the State Line fault zone.Both ridges are associated with late Cenozoic faulting.The ridges may have formed as transpressional structures, caught slightly askew of the northwest-directed strain that formed the sub-basins.Carbonate rocks probably compose the basement beneath most of the Amargosa Desert and Pahrump Valley.Because carbonate rocks are important aquifers in this region, the three-dimensional aspects of the concealed basement surface strongly influence ground-water flow paths and transport rates.For example, the deeper parts of the Amargosa Desert trough, or faults that bound the western margin of the trough, may impede the westward flow of ground water through the carbonate aquifer.If so, the gravity analysis predicts that water discharging at Ash Meadows originates entirely from the carbonate flow path north and northeast of Ash Meadows, whereas water discharging at Furnace Creek originates from the volcanic flow path north and northwest of Furnace Creek.

In part I, we described the characteristics and paragenesis of the giant Bayan Obo REE-Fe-Nb ore deposit: 1. Bayan Obo is on the northern edge of the North China craton, near the suture of Caledonian subduction of the Mongolian plate beneath the craton2. The host H8 marble and quartzite are sedimentary rocks that were metamorphosed prior to mineralization3. The deposit is mineralogically very complex, and ore classification is based on host rocks, ore type (disseminated, banded, or massive), and mineral assemblages. The Main and the East Orebodies are surrounded by disseminated ores and have intermediate zones of banded ores and massive Fe cores. 4. Metamorphic and replacement textures provide evidence for the epigenetic, hydrothermal, metasomatic origin of the Bayan Obo ore deposit5. Precise ages of episodes of REE mineralization, supported by chemical compositions of various generations of monazites and bastnaesites, indicate that the episodes of REE mineralization lasted from about 555 to 395 Ma, a duration of about 150 m.y. 6. Textural relations among the three main Fe ore minerals, precise ages of REE minerals, and minimum ages of associated alkali amphiboles of metasomatic origin indicate that most Fe mineralization occurred between about 430 and 390 Ma. Most Nb mineralization resulted in disseminated ores associated with the West Orebodies. 7. Textural relations, mineral ages, and chemical composition of various generations of REE's and alkali amphiboles indicate three periods of regional metamorphism: Late Proterozoic, early Paleozoic (Caledonian), and late Paleozoic (Hercynian). These data also provide evidence of hydrothermal activities ranging from 1.26 Ga to beyond 343 Ma. 8. The complex history of the Bayan Obo deposit, from the deposition of the H8 carbonates (>1.26 Ga) to the end of Hercynian (Permian) granitic intrusion (about 260 Ma), is illustrated by the generalized mineral paragenetic sequence (table 13). In part II, we described and discussed evidence for Caledonian subduction, origin of the Caledonian Hejao granitic rocks 50 km south of the Bayan Obo mine region, and the probability that Caledonian subduction and regional metamorphism provided a repeated activation mechanism and heat source that helped to generate not only REE- and Fe-rich hydrothermal ore fluids of Bayan Obo, but also the A-type and S-type anorogenic Hejao granitic rocksIn part III, we discussed the giant Bayan Obo REE-FeNb ore deposit as a cornerstone example of a giant ore deposit in terms of the following parameters: (1) the Early Proterozoic and crustal sources of REE's, Fe, and Mb on the basis of (a) occurrence of large amounts of LREE-rich allanites in Early Proterozoic gneisses and pegmatites, and (b) isotopic indicators of large negative ENd values; (2) the favorable tectonic setting for mineralization during the Caledonian orogeny; (3) the probable mechanism for concentrating ore metals in the hydrothermal solutions in source reservoirs; (4) the geochemical evolution, based on the mineral paragenetic sequence; (5) the mode of transport of REE complexes from the lower sialic crust to the H8 unit of the Bayan Obo Group; (6) the ages of episodes of mineralization; (7) the Middle Proterozoic H8 marble as the favorable reservoir rocks hosting the Bayan Obo ores; and (8) the less permeable H9 pelitic biotite and albitized biotite schist as cap rocks that probably preserved the ore within the upper part of the H8 marble host rocks. Bayan Obo and the Andean giant and super giant porphyry Cu-Mo ore deposits were both formed in subduction zone environments over long periods of time. Although the ore types differ, comparison of these deposits may help to increase our understanding of the paragenesis of giant polymetallic deposits.

Ar-40/Ar-39 incremental heating analysis and conventional K-Ar age determinations on plutonic rocks of the White Mountains define two stages of magmatic emplacement: Late Cretaceous, between ca, 90 Ma and 75 Ma, and Middl-Late Jurassic, between ca. 180 and 140 Ma. The Jurassic stage can be divided into two substages, 180-165 Ma and 150-140 Ma. Thermal effects of the younger plutons on the older granitoids partially to completely reset ages, making it difficult to determine the age of emplacement and cooling of several of the plutons even by Ar-40/Ar-39 incremental heating analyses. New data together with published ages and regional geochronological synthesis of the Sierra Nevada batholith indicate that regions within the batholith have coherent periods or episodes of magmatic activity. In the White Mountains and Sierra Nevada directly to the west there mas little or no activity in Early Jurassic and Early Cretaceous time; magmatism took place during relatively short intervals of 15 m.y. or less in the Middle and Late Jurassic and Late Cretaceous periods. The new K-Ar and Ar-40/Ar-39 analyses of granitoids from the White Mountains help, but do not completely clarify the complex history of emplacement, cooling, and reheating of the batholith.