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At least 559 records · Page 31Linked to original sources
USGS2016

Geologic and geochemical results from boreholes drilled in Yellowstone National Park, Wyoming, 2007 and 2008

Between 2007 and 2008, seven Earthscope Plate Boundary Observatory (PBO) boreholes ranging in depth from about 200 to 800 feet deep were drilled in and adjacent to the Yellowstone caldera in Yellowstone National Park, for the purpose of installing volcano monitoring instrumentation. Five of the seven boreholes were equipped with strainmeters, downhole seismometers, and tiltmeters. Data collected during drilling included field observations of drill cuttings, stratigraphy within the boreholes, water temperature, and water and drill cuttings samples from selected depths. Six of the seven boreholes encountered rhyolite lavas and tuffs. The rhyolite lavas compose the Canyon flow, the Gardner River flow, the Gibbon River flow, the Hayden Valley flow, the Nez Perce Creek flow, and the West Thumb flow. Boreholes also penetrated a vertical sequence through the Lava Creek Tuff and the Tuff of Bluff Point. In addition, one borehole drilled through a Swan Lake Flat Basalt sequence and terminated in a rhyolite lava flow. After drilling the seven PBO boreholes, cuttings were examined and selected for preparation of grain mounts, thin sections, and geochemical analysis. Major ions and trace elements (including rare earth elements) of selected cuttings were determined by x-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS); the ICP-MS provided more precise trace-element analysis than XRF. A preliminary interpretation of the results of geochemical analyses generally shows a correlation between borehole cuttings and previously mapped geology. The geochemical data and borehole stratigraphy presented in this report provide a foundation for future petrologic, geochemical, and geophysical studies.

Wyoming↗
USGS2020

Calibration of the U.S. Geological Survey National Crustal Model

The U.S. Geological Survey National Crustal Model (NCM) is being developed to include spatially varying estimates of site response in seismic hazard assessments. Primary outputs of the NCM are continuous velocity and density profiles from the Earth’s surface to the mantle transition zone at 410-kilometer (km) depth for each location on a 1-km grid across the conterminous United States. Datasets used to produce the NCM may have a resolution of better than 1 km near the Earth’s surface in some regions, but, with increasing depth, NCM resolution decreases to tens to hundreds of kilometers in the mantle. Basic subsurface information is provided by the NCM geologic framework, thermal model, and petrologic and mineral physics database. In this report, the velocities and densities that can be extracted from the NCM are calibrated through the development of a porosity model based on Biot-Gassmann theory and more than 2,000 compressional- and (or) shear-wave velocity profiles less than 10 km deep from across the conterminous United States and southwestern Canada. Sediment and rock porosities are derived from shear-wave velocity and are found to depend on effective pressure, rock type, and age (for sedimentary and extrusive volcanic deposits). Porosity-effective pressure functions are then estimated for each rock type (and age for sedimentary and extrusive volcanic deposits). Unconsolidated sediments are found to have higher porosities than consolidated units, which have higher porosities than unweathered igneous units; young sedimentary units (for example, Quaternary age units) tend to have higher porosities than older sedimentary units (for example, pre-Cenozoic age units); porosity decreases with increasing effective pressure; and porosities can decrease quickly through the weathered layer of intrusive rocks. Comparing two Los Angeles area velocity models and the U.S. Geological Survey Bay Area velocity model with the NCM, the NCM does a better job on average of reproducing observed shear-wave velocities below 1 km per second because it has less bias and uncertainty. Approaching and above 1 km per second, the NCM tends to underpredict observed shear-wave velocity. Whereas several factors could contribute to this, the primary factor is probably bias in the NCM geologic framework. For example, the NCM will predict lower velocities in places where the depth to bedrock and basement appear shallower in the measured velocity profiles than specified in the NCM geologic framework. With regard to observed compressional-wave velocity and density, the NCM has significantly less bias than California models for the former, especially below 2 km per second, and all models tend to overpredict density for densities less than about 2,200 kilograms per cubic meter.

Open-File Report↗
USGS2025

Gravity and magnetic surveys of the Skaergaard intrusion, East Greenland

Aeromagnetic and gravity surveys of the Skaergaard intrusion in East Greenland were carried out in July–August 1971 as part of a grant to the University of Oregon Center for Volcanology to refine the models of crystallization and differentiation of the intrusion, specifically to test whether the intrusion is underlain by dense rocks of a reservoir 20 kilometers (km) thick (referred to as a “hidden zone”). The Skaergaard intrusion is a source of platinum group elements that are critical mineral resources for many technologies, and because no new data have been collected these legacy datasets remain a valuable asset. The total-intensity aeromagnetic survey was flown in early July 1971 with a proton precession magnetometer at a constant barometric altitude of 1.5 km (5,000 feet) with a nominal line spacing of 1 km. Two gravimeters were used to acquire 168 stations of which 86 were at known altitudes (mainly sea level) and 82 had altitudes measured by altimetry in late July–August 1971. Finally, a north-south ground vertical-intensity magnetic traverse was completed across the intrusion together with collection of oriented hand specimens. The hand specimens were measured for remnant magnetization and density, along with density measurements of more specimens collected by expedition geologists for other purposes. The intrusion is composed of layered gabbro with extensive crystal fractionation that is dense and strongly reversely polarized. After terrain correction and standard Bouguer gravity reduction, the gravity anomaly dataset was corrected for all rock above sea level using the density measurements of the various zones of the intrusion and the topographic and geologic maps (variable density Bouguer gravity reduction). A large regional gradient in the gravity anomaly data was removed using orthogonal polynomial fitting to the gridded data. The zonal volumes of rock below sea level were calculated from the dipping polygonal layer gravity model of the intrusion below sea level and combined with elliptic cross–section cylinders for the various zones above sea level to approximate the original zonal volumes of the intrusion. The residual gravity anomaly of 18–20 milligals (mGal) was only about half of the expected anomaly if a large hidden zone proposed from petrologic considerations were present, and both two-dimensional and three-dimensional models imply that the exposed series of intrusion zones explain the gravity anomaly by their down-dip extension below sea level together with a small hidden-zone volume. A three-dimensional model of the exposed rocks and their down-dip extension below sea level also can account for the aeromagnetic anomaly with little or no requirement for hidden-zone rock. The middle and upper zone units of the intrusion contain the most magnetite and account for most of the aeromagnetic anomaly.

Skaergaard intrusion↗
USGS1951

Rocks and structure of the Quartz Spring area, northern Panamint Range, California

The Quartz Spring area covers about 50 square miles in the northern part of the Panamint Range, Inyo County, California. It lies near the southwestern border of the Basin and Range Province. The paper describes the stratigraphy, igneous petrology, and the main structures of the area. The formations, as defined by the stratigraphy, and the structure are shown on a large scale topographic map and in structure sections. The stratigraphy, except for Quaternary deposits that cover about one-fourth of the area, consists of Paleozoic sedimentary rocks preponderantly dolomite and limestone, representing each system from Cambrian to Carboniferous, as outlined below. No surface of erosion (unconformity) has been recognized between any of the formations or systems within the area.

Open-File Report↗
USGS1971

Cretaceous plutonic rocks of St. Lawrence Island, Alaska; a preliminary report

Reconnaissance mapping on St. Lawrence Island, Alaska has revealed seven epizonal granitic plutons of mid-Cretaceous age with a combined outcrop area of about 350 square miles. The plutonic rocks are dominantly quartz monzonite, but include minor amounts of granodiorite, monzonite, syenite, syenodiorite, and alaskite. The plutons are structurally and petrologically similar to and about the same age as plutons on the Chukotsky Peninsula, U.S.S.R. and in western Alaska. They suggest tectonic continuity between Siberia and Alaska since at least Cretaceous time. Several mineralized areas containing molybdenum, copper, lead, and zinc sulfides are associated with the plutonic bodies in the western part of the island.

Alaska↗
USGS1979

Preliminary report on the petrography and geochemistry of basalt from the Pribilof Islands and vicinity, southern Bering Sea

Alkali basalt from the Pribilof Islands has been described by several workers (Washington and Keyes, 1930; Barth, 1956; Cox and others, 1966; and Kay, 1977). This report complements the previous reports by providing new petrographic and geochemical data for samples from St. Paul and St. George Islands and from two dredge hauls near St. George Island (Fig. 1). The geologic setting of the Pribilof Islands region is given by Barth (1956), Hopkins and Einarsson (1965), Cox and others (1966), Hopkins and others (1969), Marlow and others (1976), and Hopkins and Silberman (1978). Quaternary basalt (Table 1) (Cox and others, 1966; Hopkins, 1976; Simpson and others, 1979) is extruded onto the structurally high Pribilof Ridge (Marlow and others, 1976) which consists of rocks as old as late Jurassic (Vallier and others, 1979). Barth (1956) recognized the structural implication of alkalic volcanism in back-arc environments. We plan to interpret alkalic volcanism in the Pribilof Islands within the context of recent tectonic and petrologic findings in the southern Bering Sea. Funds for this project were provided jointly by the U.S. Geological Survey and Bureau of Land Management. Funds from the Bureau of Land Management were managed by the Outer Continental Shelf Environmental Assessment Program (OCSEAP) office of the National Oceanic and Atmospheric Administration (NOAA).

Alaska↗
USGS1979

Formation and resulfidization of a South Texas roll-type uranium deposit

Core samples from a roll type uranium deposit in Live Oak County, south Texas have been studied and results are reported for Se, Mo, FeS2 and organic-carbon distribution, sulfide mineral petrology, and sulfur isotopic composition of iron-disulfide phases. In addition, sulfur isotopic compositions of dissolved sulfate and sulfide from the modern ground water within the ore bearing sand have been studied. The suite of elements in the ore sand and their geometric relationships throughout the deposit are those expected for typical roll-type deposits with well-developed oxidation-reduction interfaces. However, iron-disulfide minerals are abundant in the altered tongue, demonstrating that this interval has been sulfidized after mineralization (resulfidized or rereduced). Iron disulfide minerals in the rereduced interval differ mineralogically and isotopically from those throughout the remainder of the deposit. The resulfidized sand contains dominantly pyrite that is enriched in 34S, whereas the sand beyond the altered tongue contains abundant marcasite that is enriched in the light isotope, 32S. Textural relationships between pyrite and marcasite help to establish relative timing of iron disulfide formation. In reduced rock outside the altered tongue, three distinct generations of iron disulfide are present. The oldest of these generations consists largely of pyrite with lesser amounts of marcasite. A major episode of marcasite formation contemporaneous with ore genesis postdates the oldest pyrite generation but predates a younger pyrite generation. Resulfidization probably led to the final pyrite stage recognized beyond the altered tongue. Stable isotope data establish that the source of sulfur for the resulfidization was fault-leaked H2S probably derived from the Edwards Limestone of Cretaceous age which underlies the deposit. The deposit formed in at least two stages: (1) a pre-ore process of host rock sulfidization which produced disseminated pyrite as the dominant iron disulfide phase; and (2) an ore-stage process which led to the development of the uranium roll with emplacement of the characteristic suite of minor and accessory elements and which produced abundant isotopically light marcasite. The host rock was modified by a post-ore stage of resulfidization which precipitated isotopically heavy pyrite. Sulfur isotopic compositions of sulfide and sulfate present in modern ground water within the host sand differ greatly from sulfur isotopic composition of iron disulfides formed during the resulfidization episode. Iron disulfide minerals formed from the sulfur species of modern ground water have not been unequivocally identified.

Open-File Report↗
USGS1979

Calc-alkaline plutonism along the Pacific rim of southern Alaska

Field, petrology, and age data on southern Alaska plutonic rocks now enable the delineation of eight calc-alkaline plutonic belts. These belts of plutons or batholithic complexes are curvilinear to linear and trend parallel or subparallel to the continental margin. The belts represent the principal loci of emplacement for plutons of specific ages and although there is spatial or temporal overlap in some cases, they are more commonly spatially and temporally distinct. Intermediate lithologies such as quartz diorite, tonalite, and granodiorite dominate in most of the Belts but granodiorite and granite characterize one. The belts are of Mesozoic or Cenozoic age and plutonism began in six of them at about 195, 175, 120, 75, 60, and 40 m.y. ago; age relations in two are poorly known. Recognition of the belts is important for future studies of regional geology, tectonism, and magmatism along the Pacific rim of southern Alaska.

Open-File Report↗
USGS1982

Geology of areas marked by geophysical anomalies (B-35 and B-34), Wadi Bidah District, Kingdom of Saudi Arabia

Geophysical surveys in the Wadi Bidah district, in the southwestern part of the Kingdom of Saudi Arabia, have revealed anomalies of the type that may be caused by deposits of massive sulfides. Accordingly, a study was undertaken to investigate the geologic environment associated with the geophysical anomalies and to determine whether mineral exploration of the anomalous areas might be warranted. An extensive geochemical sampling program did not disclose any evidence of ore mineral concentrations in the rocks. Detailed geologic mapping shows the anomalous areas to be underlain by Precambrian carbonaceous graywacke, the outcrop patterns of which closely fit the anomaly patterns. Field evidence and petrologic studies indicate that carbonaceous material in the rocks is the cause of the geophysical anomalies. Nothing was found in the anomalous areas that suggests the presence of mineral deposits of commercial grade, and further mineral investigations or exploration in the areas is not recommended.

Open-File Report↗
USGS1983

Oxygen and strontium isotopic studies of basaltic lavas from the Snake River plain, Idaho

The Snake Creek-Williams Canyon pluton of the southern Snake Range crops out over an area of about 30 km2, about 60 km southeast of Ely, Nev. This Jurassic intrusion displays large and systematic chemical and mineralogical zonation over a horizontal distance of 5 km. Major-element variations compare closely with Dalyls average andesite-dacite-rhyolite over an SiO2 range of 63 to 76 percent. For various reasons it was originally thought that assimilation played a dominant role in development of the Snake Creek-Williams Canyon pluton. However, based on modeling of more recently obtained trace element and isotopic data, we have concluded that the zonation is the result of in-situ fractional crystallization, with little assimilation at the level of crystallization. This report summarizes data available for each of the mineral species present in the zoned intrusion. Special attention has been paid to trends We present oxygen and strontium isotopic data for olivine tholeiites, evolved (that is, differentiated and (or) contaminated) lavas, rhyolites, and crustal- derived xenoliths from the Snake River Plain. These data show that the olivine tholeiites are fairly uniform in d80 (5.1 to 6.2) and 87Sr/86Sr (0.7056 to 0.7076) and reveal no correlation between these ratios. The tholeiites are considered representative of mantle-derived magmas that have not interacted significantly with crustal material or meteoric water. The evolved lavas display a wider range in d 80 (5.6 to 7.6) and 87Sr/86Sr (0.708 to 0.717) with positive correlations between these ratios in some suites but not in others. Crustal xenoliths have high and variable 8?Sr/86Sr (0.715 to 0.830) and d80 values that vary widely (6.7 to 9.2) and are a few permil greater than d80 values of the Snake River basalts. Thus, isotopic data for the evolved lavas are permissive of small degrees of contamination by crustal rocks similar to the most d80-depleted xenoliths. The d80 enrichments in some evolved lavas also are consistent with crystal fractionation processes and do not necessarily require bulk interaction with crustal rocks. Enrichment in d80 but not in 87Sr/86Sr in one suite of evolved lavas suggests that crustal contamination may not be essential to the petrogenesis of those lavas. Other suites of evolved lavas display large variations in 87Sr/86Sr that reflect at least some selective contamination with 87St. Bulk solid/liquid oxygen-isotope fractionation factors (a's) calculated for the evolved lavas from Craters of the Moon National Monument are comparatively large. These a's are dependent upon the nature and proportions of phases removed by crystal fractionation; basaltic lava a's differ from latitic lava a?s in accordance with different phenocryst assemblages in these rocks. Snake River Plain rhyolites are isotopically distinct from both the analyzed crustal xenoliths and olivine tholeiites. Their origin remains poorly understood, but crustal or sub-crustal sources may be viable. In the first case, they must be derived by anatexis of material distinct from the analyzed crustal xenoliths. In the second case, they must be derived from material unlike the source for tholeiites. No cogenetic relation with the tholeiites seems likely on the basis of available data. that might relate to the variation in the chemical petrology of the pluton.

Open-File Report↗
USGS1983

Mineral potential of selected felsic plutons in the eastern and southeastern Arabian Shield, Kingdom of Saudi Arabia

Sixteen well-defined geochemical anomalies associated with felsic plutons and (or) muscovite-bearing plutons of Proterozoic age have been identified in the eastern and southeastern Arabian Shield. The majority of these plutons are highly evolved, granitophile-element-enriched muscovite-bearing granites that are weakly peraluminous. A followup study of the mineral potential of these plutons included detailed geologic mapping, extensive geochemical sampling, ground traverses, and petrographic and petrologic studies. A total of 212 samples were collected for geochemical analysis, and a lesser number were collected for petrographic studies. Most of the plutons studied are similar to metallogenically specialized granites described elsewhere in the world and found associated with deposits of tin, tungsten, or molybdenum and other rare metals. Only four of the plutons, the Sitarah, Tarban, and Gaharra monzogranites and the Bwana granite, contained highly anomalous amounts of tin, tungsten, or molybdenum and altered zones- and (or) numerous quartz veins and otherwise appeared to have noteworthy mineral potential. Additional work on at least these four plutons is suggested.

Open-File Report↗
USGS1983

Reconnaissance geology of the Jabal Saq Quadrangle, sheet 26/43 C, Kingdom of Saudi Arabia

The Jabal Saq quadrangle is located in the northeastern part of the Arabian Shield at the northern end of the Najd region between lat 26?00' and 26?30' N. and long 43?00' and 43?30' E. The northeastern two-thirds of the quadrangle is underlain by Paleozoic sedimentary rocks, the southwestern one-third by Proterozoic metamorphic and igneous rocks. The oldest rocks in the quadrangle are weakly metamorphosed immature sandstones that may correlate with the Murdama group. They have been intruded by several large plutons ranging in composition from mafic granodiorite to syenogranite. A densely developed fracture cleavage cuts the metamorphic rocks; otherwise, no structural features were recorded. The quadrangle has low mineral potential; no ancient mines were identified. The Dharaymeeah syenogranite forms a large pluton and bears some petrologic resemblance to alkali granites identified elsewhere in the Arabian Shield that have documented potential for containing deposits of lithophile rare metals. Analyses of wadi sediment samples collected in the quadrangle suggest that the syenogranite may also be enriched in some of these elements. The anomalously radioactive Usba monzogranite resembles highly evolved peraluminous granite plutons that elsewhere in the world are associated with deposits of tin and tungsten.

Open-File Report↗
USGS1983

GRANNY, a data bank of chemical analyses of Laramide and younger high-silica rhyolites and granites from Colorado and north-central New Mexico

GRANNY is a data bank containing information on 507 chemically analyzed Laramide or younger high-silica rhyolites and granites from Colorado and north-central New Mexico. The data were compiled from both published and unpublished sources. The data bank is designed to aid in the recognition of igneous rocks with a high exploration potential for the discovery of molybdenum (and other lithophile element) deposits. Information on source reference, geographic location, age, mineralogic and petrologic characteristics, major constituent analyses, and trace element analyses for each sample are given. The data bank is available in two formats: 1) paper- or microfiche-hardcopy, and 2) fixed format computer readable magnetic tape.

Open-File Report↗
USGS1990

Tectonic history of the northern Nabitah fault zone, Arabian Shield, Kingdom of Saudi Arabia

Geologic mapping combined with structural, petrographic, and geochemical investigations were used to evaluate the motion and paleogeographic setting of the northern Nabitah fault zone. The orientation and asymmetry of small structures in three areas along the fault zone suggest that motion was primarily left-lateral strike slip. East of the fault zone, the Afif terrane is underlain by a north-trending granodioritic batholith that is interpreted to be the plutonic core of a volcanic arc (herein named the Nabitah arc) whose activity was coincident with Nabitah faulting. Available geochronologic data indicate that the Nabitah fault zone and arc were active at 710 Ma and that activity may have continued until about 670 Ma. Relicts of a prebatholithic island-arc ophiolite crop out as roof pendants and septa east of the fault zone. Analysis of petrologic, geochemical, and geochronologic data suggests that these rocks may have formed a terrane older than 800 Ma that was continuous from the Nabitah fault zone to the Nuqrah belt. These rocks may have been moved northward by Nabitah faulting from an original position opposite the Asir terrane. Scale-model comparisons with Recent and Phanerozoic volcanic arcs suggest that the Nabitah fault zone may have been a transform fault that developed as a result of sinistral oblique subduction east of the Al Amar fault. According to this model, the Abt schist would have formed in an accretionary prism and the Murdama group would have been deposited in a fore-arc basin. Analogs for thrust faults in the northern Arabian Shield are found in modern arcs developed behind oblique subduction zones. The orientations of the Najd faults may reflect initial formation as normal faults in the fore-arc region of such an arc system. Based on the presence of similar lithologies, similar structure, and analogous tectonic setting, the Mother Lode District in California is reviewed as a model for gold occurrences near the Nabitah fault zone in this report.

Open-File Report↗
USGS1992

Geologic and hydrologic data collected at test holes NC-4 and NC-14, Manati and Vega Baja, Puerto Rico

Test holes NC-4 and NC-14 were drilled in the municipalities of Manati and Vega Baja, respectively, in northern Puerto Rico as part of a study of the ground-water resources in the Northern Coastal Province of Puerto Rico. This study was conducted from 1986 to 1988 by the U.S. Geological Survey in cooperation with the Puerto Rico Department of Natural Resources. Test holes NC-4 and NC-14 were drilled to depths of 1,898 and 1,837 feet below land surface, respectively. Geologic and hydrologic data collected during drilling included continuous core lithology, water-quality measurements, water levels, and estimates of aquifer yields from the water-bearing zones. Detailed petrological and microfaunal analyses of the cores were used to determine the mineralogical content, ages, and paleoenvironments of deposition. Analysis of the core recovered from test hole NC-4 indicated that four geologic formations of middle Tertiary age were penetrated, these were in descending order: the Aymam6n Limestone, the Los Puertos Limestone, the Cibao Formation, and the Lares Limestone. The Tertiary formations are capped by surficial deposits of Quaternary age. Test hole NC-14 penetrated rocks of middle Tertiary age that included three geologic formations, these were in descending order: the AymamOn Limestone, the Los Puertos Limestone, and the Cibao Formation. These formations are overlain by surficial deposits of Quaternary age. In both test holes the Cibao Formation consisted of the undifferentiated Quebrada Arenas and the Rio Indio Limestone Members of the Cibao Formation and an unnamed mudstone unit. Test hole NC-4 penetrated three water-bearing units: a water-table aquifer and two artesian aquifers. The specific conductance in the water-table aquifer ranged from 500 to 38,000 microsiemens per centimeter at 25 degrees Celsius. The specific conductance ranged from 740 to 800 microsiemens per centimeter at 25 degrees Celsius in the two artesian aquifers. The relative yield ranged from 120 to 195 gallons per minute in the water-table aquifer and from 60 to 195 gallons per minute in the artesian aquifers. The water level ranged from 81 to 89 feet below land surface in the water-table aquifer. In the artesian aquifers, water levels ranged from 22 feet below land surface to 88 feet above land surface. Test hole NC-14 penetrated four water-bearing units: a water-table aquifer and three artesian aquifers. The specific conductance in the water-table aquifer ranged from 590 microsiemens per centimeter at 25 degrees Celsius at a depth of 40 feet to 49,500 microsiemens per centimeter at 25 degrees Celsius at 1,010 feet below land surface. In the artesian aquifers specific conductance was about 700 microsiemens per centimeter at 25 degrees Celsius. The relative yield ranged from 58 to 370 gallons per minute in the water-table aquifer, and from 15 to 60 gallons per minute in the artesian aquifers. The water level in the water table aquifer ranged from 20 to 32 feet below land surface. In the artesian aquifers the water levels ranged from 18 feet below land surface to 19 feet above land surface.

Open-File Report↗
USGS1995

Trace metals and major and rare earth elements in cuttings from five high-temperature wells in the northwest region of The Geysers, California, vapor-dominated geothermal system

Temperatures within the main vapor-dominated steam reservoir at The Geysers geothermal field generally are in the range 238°C to 244°C. A few deep wells in the northwestern part of the field have penetrated beneath this reservoir into a second vapor-dominated reservoir where temperatures are >315°C, while vapor pressure remains nearly constant at about 35.9 bars (Walters et al., 1992). Vapor-dominated reservoirs generally are thought to operate like heat pipes in which steam formed near the base of the system convects upward (along with other gases, such as CC>2 and H2S), while liquid that has condensed from steam near the top of the reservoir counterflows downward (White et al., 1971). To the extent that this steam condensate carries H2S in solution, it may dissolve gold from the surrounding rock during the counterflow. Re-evaporation of the down-flowing condensate and precipitation of dissolved material might occur at the base of the upper reservoir where there is a relatively sharp increase in temperature while vapor pressure remains nearly constant. In addition, brine that once was present throughout the system (Moore, 1992) may have deposited a variety of ore minerals when and where boiling was vigorous during the transition from previous hot water-dominated to present-day vapor-dominated conditions. The investigation reported here was a geochemical reconnaissance survey looking for evidence of accumulation of Au and other metals in the transition zone between the two reservoirs. The petrology of the cuttings was not examined as part of the investigation.

Open-File Report↗
USGS1999

An evaluation of alternative chemical classifications of sandstones

Chemical classification of sandstone is worth attempting because modern analytical methods are producing extensive data sets on rock composition. Previous compilations and statistical studies have demonstrated the application of chemical data to problems in sandstone petrology, but classification has been largely ignored. Chemical data has value for classification, but the limitations of using chemical data for classification must be identified. Consideration of the processes of sandstone formation permits formulation of classification guidelines and systems for classification. Alternative systems for chemical classification can be then evaluated for their power to identify major types of sandstones and to facilitate genetic interpretation.

Open-File Report↗