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At least 1,783 records · Page 99Linked to original sources

USGS1983

Ground water in the northern part of Clackamas County, Oregon

Northern Clackamas County is part of the rapidly growing Portland metropolitan area. Population of this 250-square-mile area increased about 50 percent between 1970 and 1976. The study area includes a small segment of the Willamette River alluvial valley near Canby, and extends northward to the Clackamas River and eastward to the western boundary of Mount Hood National Forest. Also included is the narrow, well-populated corridor along U.S. Highway 26 from Cherryville to Government Camp. The main part of the study area is largely a rolling upland underlain by volcanic and stream-deposited rocks ranging in age from Eocene to Holocene. In most places, these rocks yield water in quantities adequate for individual homes. Locally, ground-water supplies are adequate for small irrigation, industrial, or public-supply uses. Depths of wells range from 50 to 1,000 feet; wells generally are shallowest in lowlands near streams and deepest in upland locations near deeply incised stream valleys. All aquifers receive recharge at sufficient rates to sustain present rates of natural and man-produced discharge. All aquifers could supply additional water, but careful engineering is needed to avoid overdevelopment problems for the Columbia River Basalt Group near Oregon City. Chemical analyses indicate that the ground water in the study area is of good quality for drinking and other uses. Mineralized water reported from some wells northeast of Canby may be from rocks of the Skamania Volcanics, which locally are at shallow depth. Although ground water locally may be subject to bacterial contamination where it occurs at shallow depths in alluvium and other unconsolidated deposits, no contaminated water was identified during the study.

Oregon↗

USGS2000

Environmental geochemical studies of selected mineral deposits in Wrangell-St. Elias National Park and Preserve, Alaska

Environmental geochemical investigations at Wrangell-St. Elias National Park and Preserve, Alaska, between 1994 and 1997 included studies of the Kennecott stratabound copper mines and mill area; historic mines and mill in the Bremner District, gold placer mines at Gold Hill; the undisturbed porphyry, Cu-Mo deposits at Orange Hill and Bond Creek, and the historic mines and mill at Nabesna, The study was in cooperation with the National Park Service and focused on sample media including surface water, bedload sediment, rock, mine waste, and mill tailings samples. Results demonstrate that bedrock geology and mineral deposit type must be considered when environmental geochemical effects of historic or active mine areas are evaluated.

Alaska↗

USGS1968

Geology and geochemistry of the Wanamu-Blue Mountains area, Waini SW, Guyana

The Wanamu-Blue Mountains area is situated in the Northwest District of Guyana, about 110 miles northwest of Georgetown. The Blue Mountains are low, but rugged, hills that have developed on mafic metamorphic rocks that occur in an arcuate "mantle" around a granite batholith, the Aranka-Wanamu Granite. The area lies within the Precambrian Guiana Shield and is entirely covered with tropical rain forest. The geology of the area is complex and is made up of an assemblage of metamorphic rocks consisting of phyllites, quartzites, amphibolites, and epidiorites that are intruded by serpentinites. The serpentinites occur as thin, tabular, dike-like bodies that trend northwest across the area. The serpentinites contain abundant magnetite, but very little chromite or sulfide minerals. Lletasomatic reaction zones have developed around the serpentinite bodies at their contacts with country rock. These reaction zones consist of aureoles of actinolite schist, chloritic hornfels, and talcose zones in the phyllites.

Wanamu-Blue Mountains↗

USGS1962

The origin of jasperoid in limestone

The name jasperoid has been applied to rocks that consist mainly of silica and that have formed by replacement. This paper considers only those jasperoids formed by replacement of limestone . Major problems involved in the origin of such jasperoid include: source of the silica; nature of solutions that dissolve, transport, and precipitate silica; and the mechanism of replacement of limestone by silica. The answers to these problems are of practical as well as scientific interest because many jasperoid bodies are closely related to mineralization. Silica may be derived from: juvenile silica of magmatic origin ; silica leached from underlying rocks by hydrothermal solutions; silica locally derived from enclosing rocks by circulating solutions; and silica carried downward in ground water from the weathering of overlying rocks. The nature and the concentration of other substances in solutions influence, in a complex manner, the ability of these solutions to dissolve, transport, and precipitate silica. Nevertheless, the following generalizations can be made. The solubility and rate of solution of silica in water at moderate pressure increase slowly with temperature up to 200 degrees C.; from 200 degrees to 360 degrees C. they increase rapidly; above 360 degrees C. solubility is pressure dependent, increasing steadily at high pressure and decreasing slightly at moderate pressure due to the formation of a vapor phase. The pH has slight effect on the ionic solubility of silica in the range from pH 1 to ph 9 at low temperature. The effect of other components on the solubility of silica is probably subordinate to that of temperature above 200 degrees C., but becomes increasingly important as the temperature falls below that point. Most jasperoid bodies form by both replacement and silica deposition in voids, with replacement dominant during the early phase, and precipitation dominant during later phases. Replacement of limestone by silica is favored by relatively low temperature acid solutions, and the presence of CO 2 . As limestone dissolves, Ca ions are released to promote the precipitation of colloidal silica. Acid solutions then diffuse through this gelatinous film to continue dissolving limestone behind it; the Ca ions diffusing outward cause the precipitation of more colloidal silica at the solution-gel interface. As the gel mass ages, it shrinks, hardens, and ruptures. More silica is then deposited in the fractures. Eventually the gel crystallizes to a dense mass of aphanitic quartz and chalcedony, with shrinkage cracks and vugs filled or coated with younger coarse-grained quartz and other minerals that have been deposited directly from solution. The theory that relatively low temperature favors the formation of jasperoid replacement bodies in carbonate rocks, and high temperature inhibits their formation, offers an explanation for the gap that is observed in some districts between contact metasomatic lime silicates and siliceous replacement of limestone . This gap is characterized by the lack of any reaction between limestone and silica-bearing solutions moving through it.

Economic Geology↗

USGS2025

Critical minerals in orogenic (gold) and Coeur d’Alene-type mineral systems of the United States

Orogenic and Coeur d’Alene-type mineral systems are produced by metamorphic devolatilization of thick volcanic or siliciclastic sedimentary rock sequences and the focused flow of hydrothermal fluids upwards along crustal-scale faults. Most orogenic systems are found along the Cordilleran orogen, stretching from California northwards into Alaska, whereas most Coeur d’Alene-type systems occur in the Proterozoic Belt Basin in Idaho and Montana. Although the deposit types in these systems are exploited for precious and base metals, potential exists for the production of critical minerals necessary for current (2025) societal needs in the United States. Publicly available geochemical data compiled for these mineral systems, coupled with mineralogical characteristics, indicate that several critical minerals could potentially be recovered from unmined resources and processed mine waste: arsenic, antimony, tellurium, cobalt, and tungsten from orogenic gold deposits and zinc, antimony, arsenic, and manganese from Coeur d’Alene-type systems. These critical minerals reside primarily in arsenopyrite (arsenic), scheelite (tungsten), siderite (manganese), sphalerite (zinc), tetrahedrite (antimony and arsenic), stibnite (antimony), and telluride (tellurium) minerals.

continental United States↗

USGS2002

Gold deposits in the Xiaoqinling-Xiong'ershan region, Qinling mountains, central China

The gold-rich Xiaoqinling-Xiong'ershan region in eastern Shaanxi and western Henan provinces, central China, lies about 30-50 km inland of the southern margin of the North China craton. More than 100 gold deposits and occurrences are concentrated in the Xiaoqinling (west), Xiaoshan (middle), and Xiong'ershan (east) areas. Late Archean gneiss of the Taihua Group, and Middle Proterozoic metavolcanic rocks of the Xiong'er Group are the main host rocks for the deposits. Mesozoic granitoids (ca. 178-104 Ma) are present in most gold districts, but deposits are typically hosted in the Precambrian basement rocks hundreds of meters to as far as 10 km from the intrusions and related hornfels zones. Deposits in the Xiaoqinling and Xiaoshan areas are best classified as orogenic gold deposits, with ores occurring in a number of distinct belts both in quartz veins and disseminated in altered metamorphic rocks. Alteration assemblages are dominated by quartz, sericite, pyrite, and carbonate minerals. The ore-forming fluids were low salinity, CO2-rich, and characterized by isotopically heavy ??18O. Four deposits (Dongchuang, Wenyu, Yangzhaiyu, and Dahu) in the Xiaoqinling area each contain resources of about 1 Moz Au. Some of the gold deposits in the Xiong'ershan area represent more shallowly emplaced tellurium-enriched orogenic systems, which include resources of approximately 1-1.5 Moz Au at Shanggong and Beiling (or Tantou). Others are epithermal deposits (e.g., Qiyugou and Dianfang) that are hosted in volcanic breccia pipes. Isotopic dates for all gold deposits, although often contradictory, generally cluster between 172-99 Ma and are coeval with emplacement of the post-kinematic granitoids. The gold deposits formed during a period of relaxation of far-field compressional stresses, clearly subsequent to the extensive Paleozoic-early Mesozoic accretion of are terranes and the Yangtze craton onto the southern margin of the North China craton. Hydrothermal and magmatic events occurred locally where extension-related Precambrian basement uplifting took place along the craton margin. Fluids for the orogenic gold deposits in the Xiaoqinling, Xiaoshan, and Xiong'ershan areas may have been released from evolving magmas or resulted from prograde metamorphic reactions within the uplift zones. Alternatively, for the epithermal gold deposits at shallower levels in the Xiong'ershan area, gold-transporting fluids were mainly exsolved from coeval magmas, although meteoric water was also involved in these hydrothermal systems.

Mineralium Deposita↗

USGS1989

Geochemical and paleoenvironmental variations across the Cretaceous/Tertiary boundary at Braggs, Alabama

The Cretaceous/Tertiary (K/T) boundary in southern Alabama occurs in a sequence of interbedded shallow-marine limestones and marls deposited during a Late Maastrichtian regression and subsequent Danian transgression. The presence of a diverse assemblage of Cretaceous and Paleocene benthic micro- and macrofossils has allowed detailed examination of paleoenvironmental changes in this shallow-marine setting at the time of the K/T boundary extinctions. Although extensive diagenesis has resulted in the recrystallization and cementation of whole rock carbonate, a few molluscan macrofossils have retained their original calcitic structure. The oxygen isotopic record of these macrofossils shows a gradual cooling of 2–3°C over a 3 m.y. period which began in the late Maastrichtian and continued into the Danian. The characteristic depletion in δ 13 C across the K/T boundary displayed by planktonic microfossils from pelagic sequences is not recorded at Braggs. Major changes in whole rock trace and minor element chemistry reflect the decrease in terrigenous mineral fluxes to this location brought about by the rapid Early Paleocene transgression.

Alabama↗

USGS2021

Compositional evolution of organic matter in Boquillas Shale across a thermal gradient at the single particle level

The molecular composition of petroliferous organic matter and its compositional evolution throughout thermal maturation provides insight for understanding petroleum generation. This information is critical for understanding hydrocarbon resources in unconventional reservoirs, as source rock organic matter is highly dispersed, in contact with the surrounding mineral matrix, and may occur as multiple organic matter maceral types. Here, Raman spectroscopy and optical microscopy approaches were applied to a marginally mature (vitrinite reflectance ~0.5%) sample of the Late Cretaceous Boquillas Shale before and after hydrous pyrolysis (HP) at 300 °C and 330 °C for 72 h. This analytical approach allowed for correlative examination of micro-scale changes in organic matter compositional properties (e.g., aromaticity) for a variety of organic matter macerals across a thermal gradient (from marginally mature into the late oil/wet gas window) at the single particle level. Results indicate that while the examined amorphous organic matter, solid bitumen, and vitrinite particles exhibit different aromatic signatures in the unheated shale, they effectively progress along a similar trend through composition space with thermal maturation. Examined inertinite fragments were generally insensitive to the applied thermal stress, reinforcing the idea that reservoir temperature may be secondary for dictating the molecular composition of inertinite. Additional analysis of Raman spectra for individual organic matter macerals was performed using multivariate curve resolution (MCR) and correlation of standard Raman and reflectance-derived thermal maturity proxies against MCR parameters shows consistent trends. This trend suggests that MCR may be a fast and statistically robust method for extracting compositional information from Raman spectra of sedimentary organic matter, and can be used to construct thermal maturity relationships. These findings inform our understanding of how different petroliferous organic matter maceral types evolve throughout thermal reactions and further demonstrate that Raman spectroscopy combined with petrographic analysis can provide complementary estimates of organic matter composition and thermal maturity.

International Journal of Coal Geology↗

USGS2005

Neocrystallization, fabrics and age of clay minerals from an exposure of the Moab Fault, Utah

Pronounced changes in clay mineral assemblages are preserved along the Moab Fault (Utah). Gouge is enriched up to ???40% in 1Md illite relative to protolith, whereas altered protolith in the damage zone is enriched ???40% in illite-smectite relative to gouge and up to ???50% relative to protolith. These mineralogical changes indicate that clay gouge is formed not solely through mechanical incorporation of protolith, but also through fault-related authigenesis. The timing of mineralization is determined using 40Ar/39Ar dating of size fractions of fault rocks with varying detrital and authigenic clay content. We applied Ar dating of illite-smectite samples, as well as a newer approach that uses illite polytypes. Our analysis yields overlapping, early Paleocene ages for neoformed (1Md) gouge illite (63??2 Ma) and illite-smectite in the damage zone (60??2 Ma), which are compatible with results elsewhere. These ages represent the latest period of major fault motion, and demonstrate that the fault fabrics are not the result of recent alteration. The clay fabrics in fault rocks are poorly developed, indicating that fluids were not confined to the fault zone by preferentially oriented clays; rather we propose that fluids in the illite-rich gouge were isolated by adjacent lower permeability, illite-smectite-bearing rocks in the damage zone. ?? 2005 Elsevier Ltd. All rights reserved.

Journal of Structural Geology↗

USGS1963

Matureland of northern Chile and its relationship to ore deposits

The old surface that extends over a long north-striking belt in northern Chile probably reached the mature stage of erosion by middle Tertiary time. Low areas are mantled by the piedmont deposits of the pampa, and the higher parts exhibit rock decay, oxidation, leaching, and super-gene enrichment of sulfide mineral deposits as a result of deep weathering. The surface and its deposits are deeply eroded south of the main pampa by the Río Copiapó and its tributaries. Numerous ore deposits in the thick secondary zones have not been eroded, but ore deposits are few in the youthful valleys where the primary zones extend to the surface.

northern Chile↗

USGS2016

Transformations to granular zircon revealed: Twinning, reidite, and ZrO 2 in shocked zircon from Meteor Crater (Arizona, USA)

Granular zircon in impact environments has long been recognized but remains poorly understood due to lack of experimental data to identify mechanisms involved in its genesis. Meteor Crater in Arizona (United States) contains abundant evidence of shock metamorphism, including shocked quartz, the high pressure polymorphs coesite and stishovite, diaplectic SiO2 glass, and lechatelierite (fused SiO2). Here we report the presence of granular zircon, a new shocked mineral discovery at Meteor Crater, that preserve critical orientation evidence of specific transformations that occurred during its formation at extreme impact conditions. The zircon grains occur as aggregates of sub-µm neoblasts in highly shocked Coconino Formation Sandstone (CFS) comprised of lechatelierite. Electron backscatter diffraction shows that each grain consists of multiple domains, some with boundaries disoriented by 65°<110>, a known {112} shock-twin orientation. Other domains have crystallographic c-axes in alignment with {110} of neighboring domains, consistent with the former presence of the high pressure ZrSiO4 polymorph reidite. Additionally, nearly all zircon preserve ZrO2 + SiO2, providing evidence of partial dissociation. The genesis of CFS granular zircon started with detrital zircon that experienced shock-twinning and reidite formation from 20 to 30 GPa, ultimately yielding a phase that retained crystallographic memory; this phase subsequently recrystallized to systematically oriented zircon neoblasts, and in some areas partially dissociated to ZrO2. The lechatelierite matrix, experimentally constrained to form at >2000 °C, provided an ultra high-temperature environment for zircon dissociation (~1670 °C) and neoblast formation. The capacity of granular zircon to preserve a cumulative P-T record has not been recognized previously, and provides a new method for retrieving histories of impact-related mineral transformations in the crust at conditions far beyond which most rocks melt.

Arizona↗

USGS2002

Age and origin of base and precious metal veins of the Coeur d'Alene mining district, Idaho

Ore-bearing quartz-carbonate veins of the Coeur d'Alene mining district yield 87Sr/86Sr ratios of 0.74 to >1.60 for low Rb/Sr, carbonate gangue minerals, similar to current ranges measured in Middle Proterozoic, high Rb/Sr rocks of the Belt Supergroup. Stable isotope and fluid inclusion studies establish a genetic relationship between vein formation and metamorphic-hydrothermal systems of the region. These extraordinary 87Sr/86Sr ratios require accumulation of radiogenic 87Sr in a high Rb/Sr system over an extended period prior to incorporation of Sr into the hydrothermal veins. Evaluation of the age and composition of potential sources of highly radiogenic Sr indicates that the ore-bearing veins of the Coeur d'Alene district formed during the Cretaceous from components scavenged from rocks of the Belt Supergroup, the primary host rocks of the district. Proterozoic Pb isotope ratios observed in galena from many Coeur d'Alene veins were established when Pb separated from uranium during deposition or diagenesis of the Belt Supergroup at 1400 to 1500 Ma, possibly as disseminated syngenetic deposits. K-Ar and Rb-Sr apparent ages and ??18O values of Belt Supergroup rocks decrease from the Coeur d'Alene district toward the Idaho and Kaniksu batholiths, approximately normal to the trends of metamorphic isograds, fold axes, foliation, and the major reverse faults of the district. Isoclinal folding, thrust faulting, high-temperature metamorphism, granitic plutonism, and regional-scale metamorphic-hydrothermal activity is documented in the region between 140 and 45 Ma, representing the only such combination of events in the Coeur d'Alene region subsequent to about 1300 Ma. The Sr and oxygen results and geologic evidence favor formation of the ore-bearing carbonate veins by fluids related to a complex metamorphic-hydrothermal system during the Cretaceous. Pb with Proterozoic isotopic compositions was probably mobilized and incorporated like other metals into the hydrothermal veins during this event. The ore-bearing veins were sheared and displaced during early Tertiary northwest-trending dextral strike-slip faulting along the Osburn fault and related structures of the Lewis and Clark line.

Economic Geology↗

USGS1984

U-Pb isotope systematics and apparent ages of uranium ores, Ambrosia Lake and Smith Lake districts, Grants mineral belt, New Mexico

Orebodies occur in continental sandstones of the Upper Jurassic Morrison Formation and comprise two main types of mineralization; primary ore formed early in the history of the host rock and resulted in tabular, peneconcordant orebodies rich in organic material; redistributed ore apparently was formed by oxidative destruction and reconcentration of primary ore at some time after Laramide deformation and resulted in roll-type orebodies lacking organic material. U-Pb isotope ages of four redistributed ores from the Ambrosia Lake District give concordant to nearly concordant ages of 3.3 to 12.5 m.y., supporting the concept that most redistributed ore formed in the late Tertiary or later. U-Pb isotope ages of primary ores from the Ambrosia Lake District show pronounced normal discordance, evidently due to coupled, continuous loss of Pb and radioactive daughters of 238 U. Data for ores from the Smith Lake District consistently give pre-Tertiary apparent ages.

Economic Geology↗

USGS1998

Remote sensing in the USGS Mineral Resource Surveys Program in the eastern United States

Mineral deposits commonly occur within special geologic units or structures, such as fault zones, which can be detected and mapped from aircraft and satellite images. Modern techniques analyze multispectral images that record the way solar energy is reflected or emitted by the materials exposed at the Earth's surface. In sparsely vegetated regions, including most of the Western United States, mineral composition is determined directly by analyzing the spectral properties of rock outcrops. In more densely vegetated terrain, such as the Eastern United States, rock and soil composition can be determined directly in manmade exposures, such as plowed fields and construction sites, or much more general determinations can be made indirectly by analyzing the distribution and apparent health of naturally occurring plants. The association of certain plants with particular rock or soil types has been known for decades. For example, coniferous trees grow preferentially on well-drained sandy soil, whereas deciduous trees dominate on shaly bedrock. These two forest types reflect solar radiation quite differently and, therefore, are distinguished readily in conventional aerial photographs. More subtle plant-bedrock associations require digital multispectral image analysis to infer compositional information from the spectral characteristics of the forest canopy.

Information Handout↗

USGS2009

Climax-Type Porphyry Molybdenum Deposits

Climax-type porphyry molybdenum deposits, as defined here, are extremely rare; thirteen deposits are known, all in western North America and ranging in age from Late Cretaceous to mainly Tertiary. They are consistently found in a postsubduction, extensional tectonic setting and are invariably associated with A-type granites that formed after peak activity of a magmatic cycle. The deposits consist of ore shells of quartz-molybdenite stockwork veins that lie above and surrounding the apices of cupola-like, highly evolved, calc-alkaline granite and subvolcanic rhyolite-porphyry bodies. These plutons are invariably enriched in fluorine (commonly >1 percent), rubidium (commonly >500 parts per million), and niobium-tantalum (Nb commonly >50 parts per million). The deposits are relatively high grade (typically 0.1-0.3 percent Mo) and may be very large (typically 100-1,000 million tons). Molybdenum, as MoS2, is the primary commodity in all known deposits. The effect on surface-water quality owing to natural influx of water or sediment from a Climax-type mineralized area can extend many kilometers downstream from the mineralized area. Waste piles composed of quartz-silica-pyrite altered rocks will likely produce acidic drainage waters. The potential exists for concentrations of fluorine or rare metals in surface water and groundwater to exceed recommended limits for human consumption near both mined and unmined Climax-type deposits.

Open-File Report↗

USGS1969

Preliminary geologic interpretation of aeromagnetic data in the Nixon Fork district, Alaska

An aeromagnetic map covering 480 square miles was compiled for the Nixon Fork district, which is located approximately 35 miles northeast of McGrath, Alaska. The survey was flown in search of concealed intrusive rocks which may have produced contact metamorphic deposits in limestone similar to the known lode deposits which have been the principal source of gold in the district. The exposed quartz monzonite stocks with which the lode deposits are associated produce negative magnetic anomalies. Slight to moderately strong positive anomalies correlate with granitic intrusives in contact with Upper Cretaceous rocks in the Iditarod-Nixon Fork fault zone. No significant mineral deposits have been found in conjunction with these granitic bodies. Positive anomalies, delineating buried intrusives, occur near the east and west boundaries of the mapped area. The nature of the westernmost intrusive is unknown. An area of possible economic interest lies between Limestone Mountain and Whirlwind-Canyon Creeks in the eastern sector of the mapped area. An elliptical positive anomaly is superimposed on an elongate, slightly negative anomaly. This negative anomaly may represent an intrusive similar to the quartz monzonite with which the lode deposits are affiliated. The positive anomaly may be a near-vertical mafic dike intruded to within 50 feet of the surface of a limestone ridge. Limestone in the vicinity of the dike may be a favorable area for prospecting for lode deposits similar to the known gold-producing deposits of the district.

Alaska↗

USGS1982

Eruptive history, petrology, and petrogenesis of the Joe Lott Tuff Member of the Mount Belknap Volcanics, Marysvale volcanic field, west-central Utah

The Joe Lott Tuff Member of the Mount Belknap Volcanics is the largest rhyolitic ash-flow tuff sheet in the Marysvale volcanic field. It was erupted 19 m.y. ago, shortly after the changeover from intermediate-composition calc-alkalic volcanism to bimodal basalt-rhyolite volcanism. Eruption of the tuff resulted in the formation of the Mount Belknap Caldera whose pyroclastic intracaldera stratigraphy parallels that in the outflow facies. The Joe Loft Tuff Member is a composite ash-flow sheet that changes laterally from a simple cooling unit near the source to four distinct cooling units toward the distal end. The lowest of these units is the largest and most widespread; it is 64 m thick and contains a basal vitrophyre. Eruption of the lower unit led to the initial collapse of the caldera. The lower unit is followed upward by a 43 m middle unit, a 26 m pink-colored unit which is separated by a prominent air- fall layer, and a 31 m upper unit. The Joe Loft Tuff Member is an alkali rhyolite with 75.85-77.31 wt. % silica and 8.06-9.32 wt. % K2O+Na2O; the agpaitic index (Na2O+ K2O/Al2O3) is .77-.98. The tuff contains about I% phenocrysts of quartz, sanidine, oligoclase, augite, apatite, zircon, sphene, biotite, and oxidized Fe-Ti oxides. The basal vitrophyre contains accessory allanite, chevkinite, and magnesiohastingsite. The main cooling units are chemically and mineralogically zoned indicating that the magma chamber restratified prior to each major eruption. Within each of the two thickest cooling units, the mineralogy changes systematically upwards; the Or content and relative volume of sanidine decreases and An content of plagioclase increases. The basal vitrophyre of the lower unit has a bulk composition that lies in the thermal trough near the minima of Or-Ab-Q at 1 kb PH2O. Microprobe analyses of feldspar and chemical modeling on experimental systems indicate that pre-eruption temperatures were near 750?C and that the temperature increased during the eruption of the cooling units. The chemical gradients in the apatite and whole-rock data in the Joe Loft Tuff Member and the consistent mineral assemblages throughout the ash-flow cannot be explained by crystal settling. The fractionation of the Joe Lott Tuff Member appears to closer fit the model of convection-driven thermogravitational diffusion.

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↗