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At least 865 records · Page 48Linked to original sources
USGS2025

Carbonatite-hosted residual REE deposits

Rare earth elements (REEs) occur in magmatic rocks but are especially enriched in carbonatite and alkaline silicates. If these rocks are chemically weathered, then the REEs may become further enriched within the regolith developed from these rocks. Primary magmatic REE minerals, as well as the various carbonate minerals and apatite, provide the REEs which, under pervasive chemical weathering, are incorporated within low-temperature REE minerals forming within the regolith. Many of these minerals, as well as their textures, are characteristic of this mode of formation. Lateritic conditions of weathering are instrumental in producing a thick, weathered, or regolith, profile, and the roles of sulfide oxidation, fluctuating groundwater tables, and downward mass wasting due to carbonate dissolution are identified as the most important controls on REE enrichment in the regolith.

Book chapter↗
USGS1992

Petrology of lavas from episodes 2-47 of the Puu Oo eruption of Kilauea Volcano, Hawaii: Evaluation of magmatic processes

The Puu Oo eruption of Kilauea Volcano in Hawaii is one of its largest and most compositionally varied historical eruptions. The mineral and whole-rock compositions of the Puu Oo lavas indicate that there were three compositionally distinct magmas involved in the eruption. Two of these magmas were differentiated (<6.8 wt% MgO) and were apparently stored in the rift zone prior to the eruption. A third, more mafic magma (9-10 wt% MgO) was probably intruded as a dike from Kilauea's summit reservoir just before the start of the eruption. Its intrusion forced the other two magmas to mix, forming a hybrid that erupted during the first three eruptive episodes from a fissure system of vents. A new hybrid was erupted during episode 3 from the vent where Puu Oo later formed. The composition of the lava erupted from this vent became progressively more mafic over the next 21 months, although significant compositional variation occurred within some eruptive episodes. The intra-episode compositional variation was probably due to crystal fractionation in the shallow (0.0-2.9 km), dike-shaped (i.e. high surface area/volume ratio) and open-topped Puu Oo magma reservoir. The long-term compositional variation was controlled largely by mixing the early hybrid with the later, more mafic magma. The percentage of mafic magma in the erupted lava increased progressively to 100% by episode 30 (about two years after the eruption started). Three separate magma reservoirs were involved in the Puu Oo eruption. The two deeper reservoirs (3-4 km) recharged the shallow (0.4-2.9 km) Puu Oo reservoir. Recharge of the shallow reservoir occurred rapidly during an eruption indicating that these reservoirs were well connected. The connection with the early hybrid magma body was cut off before episode 30. Subsequently, only mafic magma from the summit reservoir has recharged the Puu Oo reservoir. ?? 1992 Springer-Verlag.

Bulletin of Volcanology↗
USGS1974

U-Th-Pb and Rb-Sr systematics of Apollo 17 boulder 7 from the North Massif of the Taurus-Littrow Valley

Portions of highland breccia boulder 7 collected during the Apollo 17 mission were studied using U Th Pb and Rb Sr systematics. A Rb Sr internal isochron age of 3.89 ± 0.08b.y. with an initial 87 Sr/ 86 Sr of0.69926 ± 0.00008 was obtained for clast 1 (77135,57) (a troctolitic microbreccia). A troctolitic portion of microbreccia clast 77215,37 yielded a U Pb internal isochron of 3.8 ± 0.2b.y. and an initial 206 Pb/ 207 Pb of 0.69. These internal isochron age are interpreted as reflecting metamorphic events, probably related to impacts, which reset Rb Sr and U Pb mineral systems of older rocks. Six portions of boulder 7 were analyzed for U, Th, and Pb as whole rocks. Two chemical groups appear to be defined by the U, Th, and Pb concentration data. Chemical group A is characterized by U, Th, and Pb concentrations and 238 U/ 204 Pb values which are higher than those of group B. Group A rocks have typical 232 Th/ 238 U ratios of ∼ 3.85, whereas-group B rocks have unusually high Th/U values of ∼ 4.1. Whole-rock U Pb and Pb Pb ages are nearly concordant. Two events appear to be reflected in these data — one at ∼ 4.4 b.y. and one at ∼ 4.5 b.y. The chemical groupings show no correlation with documented ages. The old ages of ∼ 4.4 b.y. and ∼ 4.5 b.y. may, like the younger ∼ 4.0 b.y. ages, be related to basin excavation events.

Earth and Planetary Science Letters↗
USGS1967

The petrography of some Illinois Pleistocene and recent sands

Some Recent and Pleistocene sands of Illinois and the nearby Missouri River were separated into three groups by petrographic characteristics that reflect source material. The sands derived largely or entirely from the glacial material of Illinois and the upper Mississippi, Wabash, and Lake Michigan drainage basins contain types of feldspars and rock fragments that indicate derivation from the Precambrian metamorphic rocks of the Canadian Shield. The sands of the Ohio River at the southern boundary of Illinois contain relatively large amounts of polycrystalline quartz and nonfeldspathic rock fragments that may have been derived from Paleozoic sedimentary rocks largely of Appalachian derivation, from glacial drift of the eastern states, or from both sources. A significant portion of the Missouri River sands and the Mississippi River sands below the mouth of the Missouri River consists of feldspars and rock fragments derived from the Cretaceous and Tertiary igneous rocks of the western United States. The volcanic rock fragments are especially indicative of a western source. Petrographic characteristics of 23 samples of these sands were determined. The sources of the sands were interpreted principally from their rock fragments and light minerals, especially the feldspars, taking into account the variation in composition with changing grain size. Much of the plagioclase was untwinned, but certain varietal features proved useful in its identification.

Illinois↗
USGS2008

Application of multiple isotopic and geochemical tracers for investigation of recharge, salinization, and residence time of water in the Souss-Massa aquifer, southwest of Morocco

Groundwater and surface water in Souss-Massa basin in the west-southern part of Morocco is characterized by a large variation in salinity, up to levels of 37 g L-1. The high salinity coupled with groundwater level decline pose serious problems for current irrigation and domestic water supplies as well as future exploitation. A combined hydrogeologic and isotopic investigation using several chemical and isotopic tracers such as Br/Cl, ??18O, ??2H, 3H, 87Sr/86Sr, ??11B, and 14C was carried out in order to determine the sources of water recharge to the aquifer, the origin of salinity, and the residence time of water. Stable isotope, 3H and 14C data indicate that the high Atlas mountains in the northern margin of the Souss-Massa basin with high rainfall and low ??18O and ??2H values (-6 to -8??? and -36 to -50???) is currently constitute the major source of recharge to the Souss-Massa shallow aquifer, particularly along the eastern part of the basin. Localized stable isotope enrichments offset meteoric isotopic signature and are associated with high nitrate concentrations, which infer water recycling via water agricultural return flows. The 3H and 14C data suggest that the residence time of water in the western part of the basin is in the order of several thousands of years; hence old water is mined, particularly in the coastal areas. The multiple isotope analyses and chemical tracing of groundwater from the basin reveal that seawater intrusion is just one of multiple salinity sources that affect the quality of groundwater in the Souss-Massa aquifer. We differentiate between modern seawater intrusion, salinization by remnants of seawater entrapped in the middle Souss plains, recharge of nitrate-rich agricultural return flow, and dissolution of evaporate rocks (gypsum and halite minerals) along the outcrops of the high Atlas mountains. The data generated in this study provide the framework for a comprehensive management plan in which water exploitation should shift toward the eastern part of the basin where current recharge occurs with young and high quality groundwater. In contrast, we argued that the heavily exploited aquifer along the coastal areas is more vulnerable given the relatively longer residence time of the water and salinization processes in this part of the aquifer. ?? 2008 Elsevier B.V. All rights reserved.

Journal of Hydrology↗
USGS2021

Long-term African dust delivery to the eastern Atlantic Ocean from the Sahara and Sahel regions: Evidence from Quaternary paleosols on the Canary Islands, Spain

Africa is the most important source of dust in the world today and dust storms from that continent frequently deposit sediment on the nearby Canary Islands. Many investigators have inferred African dust inputs to Canary Islands paleosols based only on the presence of quartz. However, some local rocks do contain this mineral, so quartz alone is insufficient proof of dust deposition. Further, it is not known whether the Sahara Desert or the Sahel region is more important as a dust source. We address these issues by study of sequences of Pleistocene aeolian sands on the islands of Lanzarote and Fuerteventura. Aeolian sands are composed mostly of marine carbonate minerals and locally derived volcanic minerals. They date from the early-middle Pleistocene to the Holocene. Trace element geochemistry shows that the soils formed from both locally derived basalt and African dust. Major element geochemistry and clay mineralogy indicate that dust additions to the Canary Islands likely come from both the Sahara and Sahel. Dust delivered from the Sahel indicates that droughts in that region have had a history extending through much of the Quaternary. Accretionary-inflationary profile development, from dust accretion, is evident in the upward growth of Canary Islands paleosols.

Quaternary Science Reviews↗
USGS2003

Fracture network of the Ferron Sandstone Member of the Mancos Shale, east-central Utah, USA

The fracture network at the outcrop of the Ferron Sandstone Member of the Mancos Shale was studied to gain an understanding of the tectonic history of the region and to contribute data to studies of gas and water transmissivity related to the occurrence and production of coal-bed methane. About 1900 fracture readings were made at 40 coal outcrops and 62 sandstone outcrops in the area from Willow Springs Wash in the south to Farnham dome in the north of the study area in east-central Utah.Two sets of regional, vertical to nearly vertical, systematic face cleats were identified in Ferron coals. A northwest-striking set trends at a mean azimuth of 321??, and a northeast-striking set has a mean azimuth of 55??. Cleats were observed in all coal outcrops examined and are closely spaced and commonly coated with thin films of iron oxide.Two sets of regional, systematic joint sets in sandstone were also identified and have mean azimuths of 321?? and 34??. The joints of each set are planar, long, and extend vertically to nearly vertically through multiple beds; the northeast-striking set is more prevalent than the northwest-striking set. In some places, joints of the northeast-striking set occur in closely spaced clusters, or joint zones, flanked by unjointed rock. Both sets are mineralized with iron oxide and calcite, and the northwest-striking set is commonly tightly cemented, which allowed the northeast-striking set to propagate across it. All cleats and joints of these sets are interpreted as opening-mode (mode I) fractures. Abutting relations indicate that the northwest-striking cleats and joints formed first and were later overprinted by the northeast-striking cleats and joints. Burial curves constructed for the Ferron indicate rapid initial burial after deposition. The Ferron reached a depth of 3000 ft (1000 m) within 5.2 million years (m.y.), and this is considered a minimum depth and time for development of cleats and joints. The Sevier orogeny produced southeast-directed compressional stress at this time and is thought to be the likely mechanism for the northwest-striking systematic cleats and joints. The onset of the Laramide orogeny occurred at about 75 Ma, within 13.7 m.y. of burial, and is thought to be the probable mechanism for development of the northeast-striking systematic cleats and joints. Uplift of the Ferron in the late Tertiary contributed to development of butt cleats and secondary cross-joints and probably enhanced previously formed fracture sets. Using a study of the younger Blackhawk Formation as an analogy, the fracture pattern of the Ferron in the subsurface is probably similar to that at the surface, at least as far west as the Paradise fault and Joe's Valley graben. Farther to the west, on the Wasatch Plateau, the orientations of Ferron fractures may diverge from those measured at the outcrop. ?? 2003 Elsevier B.V. All rights reserved.

International Journal of Coal Geology↗
USGS2003

Typing mineral deposits using their grades and tonnages in an artificial neural network

A test of the ability of a probabilistic neural network to classify deposits into types on the basis of deposit tonnage and average Cu, Mo, Ag, Au, Zn, and Pb grades is conducted. The purpose is to examine whether this type of system might serve as a basis for integrating geoscience information available in large mineral databases to classify sites by deposit type. Benefits of proper classification of many sites in large regions are relatively rapid identification of terranes permissive for deposit types and recognition of specific sites perhaps worthy of exploring further. Total tonnages and average grades of 1,137 well-explored deposits identified in published grade and tonnage models representing 13 deposit types were used to train and test the network. Tonnages were transformed by logarithms and grades by square roots to reduce effects of skewness. All values were scaled by subtracting the variable's mean and dividing by its standard deviation. Half of the deposits were selected randomly to be used in training the probabilistic neural network and the other half were used for independent testing. Tests were performed with a probabilistic neural network employing a Gaussian kernel and separate sigma weights for each class (type) and each variable (grade or tonnage). Deposit types were selected to challenge the neural network. For many types, tonnages or average grades are significantly different from other types, but individual deposits may plot in the grade and tonnage space of more than one type. Porphyry Cu, porphyry Cu-Au, and porphyry Cu-Mo types have similar tonnages and relatively small differences in grades. Redbed Cu deposits typically have tonnages that could be confused with porphyry Cu deposits, also contain Cu and, in some situations, Ag. Cyprus and kuroko massive sulfide types have about the same tonnages. Cu, Zn, Ag, and Au grades. Polymetallic vein, sedimentary exhalative Zn-Pb, and Zn-Pb skarn types contain many of the same metals. Sediment-hosted Au, Comstock Au-Ag, and low-sulfide Au-quartz vein types are principally Au deposits with differing amounts of Ag. Given the intent to test the neural network under the most difficult conditions, an overall 75% agreement between the experts and the neural network is considered excellent. Among the largestclassification errors are skarn Zn-Pb and Cyprus massive sulfide deposits classed by the neuralnetwork as kuroko massive sulfides—24 and 63% error respectively. Other large errors are the classification of 92% of porphyry Cu-Mo as porphyry Cu deposits. Most of the larger classification errors involve 25 or fewer training deposits, suggesting that some errors might be the result of small sample size. About 91% of the gold deposit types were classed properly and 98% of porphyry Cu deposits were classes as some type of porphyry Cu deposit. An experienced economic geologist would not make many of the classification errors that were made by the neural network because the geologic settings of deposits would be used to reduce errors. In a separate test, the probabilistic neural network correctly classed 93% of 336 deposits in eight deposit types when trained with presence or absence of 58 minerals and six generalized rock types. The overall success rate of the probabilistic neural network when trained on tonnage and average grades would probably be more than 90% with additional information on the presence of a few rock types.

Natural Resources Research↗
USGS2020

The chemostratigraphy of the Murray formation and role of diagenesis at Vera Rubin ridge in Gale crater, Mars, as observed by the ChemCam instrument

Geochemical results are presented from Curiosity’s exploration of the Vera Rubin ridge (VRR), in addition to the full chemostratigraphy of the predominantly lacustrine mudstone Murray formation up to and including VRR. VRR is a prominent ridge flanking Aeolis Mons (informally Mt. Sharp), the central mound in Gale crater, Mars, and was a key area of interest for the Mars Science Laboratory mission. ChemCam data show that VRR is overall geochemically similar to lower-lying members of the Murray formation, even though the top of VRR is characterized by strong hematite spectral signatures as observed from orbit. While overall geochemically similar, VRR is characterized by a prominent decrease in Li abundance and Chemical Index of Alteration across the ridge. This decrease follows the morphology of the ridge rather than elevation and is inferred to reflect a non-depositionally controlled decrease in clay mineral abundance in VRR rocks. Additionally, a notable enrichment in Mn above baseline levels is observed on VRR. While not supporting a single model, the results suggest that VRR rocks were likely affected by multiple episodes of post-depositional groundwater interactions that made them more erosionally resistant than surrounding Murray rocks – thus resulting in the modern-day ridge after subsequent erosion.

Journal of Geophysical Research: Planets↗
USGS2012

Implications of the Surveyor 7 results

The chemical and television data from the Surveyor 7 mission are discussed. The significance of the α-scattering analysis is found to depend on whether the flow unit underlying the spacecraft was laid down by mass flow at the time of the Tycho-forming impact. The chemical composition of the site is significantly lower in Fe than the composition of the previously analyzed mare sites; no other resolvable distinctions can be made. The higher albedo of the highlands is attributed to the lower iron content in the silicate minerals. By assigning the atomic composition to minerals common in basaltic rocks, we estimate-the highland rock density to be 3.0 and the mare density to be 3.2. The densities may be incorporated into a discussion of recent information on the gravitational field of the moon, yielding estimates of crustal thickness in the highlands around 30 to 50 km. A number of television pictures of rocks and fragments are shown, and we offer some suggestions about the type and origin of these objects. They compose a heterogeneous population in which the brighter rocks seem to be more resistant to mechanical breakdown than the dark rocks. The characteristics of the fine-grained component of the surface mantle are found to be similar to those at earlier Surveyor sites. The process of eroding exposed rocks and covering them with a blanket of fine particles appears to be relatively less advanced than at the sites previously studied; A bright line of light was seen on the western horizon after sunset, as was observed during Surveyor 5 and 6; this rather puzzling phenomenon is discussed again.

Journal of Geophysical Research↗
USGS2014

Recharge and discharge of the ground‐water reservoirs on the High Plains in Texas

The High Plains in Texas occupy an area of about 35,000 square miles extending from the northern boundary of the Panhandle southward about 300 miles, and from the New Mexico line eastward an average distance of about 120 miles to a boundary which in most places is sharply defined by a bold escarpment several hundred feet in height. The region is noted for its abundant supply of ground‐water, most of which is found in the Ogallala formation, a sandy deposit lying at or near the surface throughout most of the region, and reaching a depth of between 200 and 300 feet. The Ogallala formerly extended over a much greater area, but it has been removed by erosion from much of the territory it once occupied. The areas in which it remains are the High Plains, which are bounded by prominent escarpments both on the east and on the west, and are traversed in Texas by the Canadian River, which is deeply entrenched in the older rocks. The water‐bearing sands of the Ogallala in both segments are cut off in all directions from any underground connection, except through the underlying older rocks which contain highly mineralized water unlike the fresh water in the Ogallala. The source of the fresh water, therefore, is within the High Plains themselves, and is from the rain and snow that falls on the surface of the Plains.

Eos, Transactions, American Geophysical Union↗
USGS1989

Salinity variations in submarine hydrothermal systems by layered double-diffusive convection

Various mechanisms have been proposed to explain the salinity variations in vent fluids of seafloor geothermal systems. New experiments reacting diabase and evolved seawater were carried out to reproduce earlier published observations of Cl depletions attributed to formation of an ephemeral Cl-bearing mineral. The absence of any Cl depletions in the present study suggests that the formation of Cl-bearing minerals is not sufficiently widespread to account for the observed salinity variations in the vent fluids. A re-evaluation of both field and laboratory evidence has led to a new model for subseafloor circulation that accounts for salinity variations as well as other chemical and mineralogic observations. In place of a simple single-pass convection system, we propose that the seafloor systems consist of two vertically nested convection cells in which a brine layer at depth heats and drives an overlying seawater cell. Such layering of salinities, a process known in fluid mechanics as double-diffusive convection, is an expected result when convection is induced in saline fluids. The process provides for stable high-temperature heat transfer upward from the cracking front adjacent to the magma, and for limited chemical exchange of the brine with the overlying seawater to explain salinity variations and high metal contents in the vent fluids. The brine also provides an effective medium to produce the secondary mineral assemblages observed in rocks from the mid-ocean ridges and ophiolites unsuccessfully produced in laboratory studies using seawater. The brine originates from the two-phase separation of seawater during magmatic/tectonic events and accumulates and remains relatively stable in the region immediately above the magma chamber.

Journal of Geology↗
USGS1968

Isotopic age of the Nevadan Orogeny and older Plutonic and metamorphic events in the Klamath Mountains, California

Several metamorphic and plutonic events have been recognized in the Klamath Mountains utilizing potassium-argon and rubidium-strontium mineral and whole-rock ages. The oldest known metamorphic event in the region produced the Abrams Mica Schist and the co-extensive Salmon Hornblende Schist. Strontium evolution diagrams indicate that the age of primary metamorphism of the Abrams Mica Schist is approximately 380 m.y. (Devonian). The Stuart Fork Formation of Davis and Lipman (1962), the schists of Condrey Mountain, and related schists were produced during a Middle and Late Jurassic metamorphic event. Granitic plutonic rocks are divided into four groups that are different in age and somewhat different in chemical characteristics. The oldest pluton, the Pit River stock, has a minimum age of 246 m.y. (Permian) and is grouped with the Castle Crags pluton. The other three plutonic groups are Middle and Late Jurassic and are characterized by the following ranges in isotopic age: 165 to 167 m.y., 145 to 155 m.y., and 127 to 140 m.y. The principal lode gold deposits in the California part of the Klamath Mountains seem to be related to the youngest group of plutons. If the Nevadan orogeny in this area is restricted to a Middle and Late Jurassic deformational, metamorphic, and plutonic event, the orogeny includes the emplacement of plutons of the three younger groups and the metamorphic development of the Stuart Fork Formation, schists of Condrey Mountain, and related schists.

California↗
USGS1985

40Ar/39Ar and K-Ar data bearing on the metamorphic and tectonic history of western New England

40 Ar/ 39 Ar ages of coexisting biotite and hornblende from Proterozoic Y gneisses of the Berkshire and Green Mountain massifs, as well as 40 Ar/ 39 Ar and K-Ar mineral and whole-rock ages from Paleozoic metamorphic rocks, suggest that the thermal peak for the dominant metamorphic recrystallization in western New England occurred 465 ± 5 m.y. ago (Taconian). Although textural data indicate a complex metamorphic-tectonic history for Paleozoic rocks, no evidence in rocks at least as high as kyanite grade dictates an Acadian age for the Barrovian metamorphism. Available 40 Ar/ 39 Ar and K-Ar data suggest that the low-grade metamorphism and cleavage formation in Taconic allochthons and the higher-grade metamorphism and emplacement of the Berkshire massif allochthon are Taconian. 40 Ar/ 39 Ar age data from a poorly defined terrane beginning near the east margin of the Green Mountain massif and extending along the eastern one-third of the Berkshire massif as far south as Otis, Massachusetts, suggest that the area has been retrograded during a metamorphism that peaked at least 376 ± 5 m.y. ago (Acadian). Available age and petrologic data from western New England indicate the presence of at least three separate metamorphic-structural domains of Taconian age: (1) a small area of relict high-pressure and low-temperature metamorphism in northern Vermont (T-1 domain), (2) a broad area in Vermont and eastern New York of normal Barrovian metamorphism from chlorite to garnet grade and characterized by a gentle metamorphic gradient (T-2 domain), and (3) a rather narrow belt of steep-gradient, Barrovian series metamorphic rocks extending from near the Cortlandt Complex northeastward through Dutchess County, New York, to the Berkshire massif in western Massachusetts (T-3 domain). Areas of maximum metamorphic intensity within the T-3 domain coincide with areas of maximum crustal thickening resulting from imbricate thrusting (Berkshire massif) or from recumbent folding (Manhattan Prong) of remobilized North American continental crust in the later stages of the Taconic orogeny.

Geological Society of America Bulletin↗
USGS1992

40 Ar/ 39 Ar dating of Quaternary feldspar: Examples from the Taupo Volcanic Zone, New Zealand

Using a continuous laser and resistance furnace, we have measured ages on Quaternary plagioclase with an absolute precision of about ±30 ka and on Quaternary sanidine with a relative precision of better than 1%. Such precision was achieved by using low-temperature heating steps to remove much of the nonradiogenic argon contamination. Plagioclase is one of the most common mineral phases in volcanic rocks; thus, these procedures will be widely applicable to many problems for which precise radiometric age control has not been available. We studied plagioclase and plagioclase-sanidine concentrates from the oldest and the three largest silicic ash-flow deposits of the Taupo Volcanic Zone, New Zealand, one of the world's largest and most active volcanic systems. The results are in close agreement with new magnetostratigraphic data, suggest that existing fission-track age determinations significantly underestimate the age of older units, and shift the inception of Taupo Volcanic Zone volcanism back to at least 1600 ka. The improved precision has permitted the first correlations between proximal and distal units; previous correlations with deep-sea ash flows in the western Pacific require major revision.

Geology↗
USGS1970

Geology and mineral evaluation of the Wadi Bidah district, southern Hijaz quadrangle, Kingdom of Saudi Arabia

The Wadi Bidah district in southwest Saudi Arabia contains several ancient mines and mineral-prospects. The Precambian rocks of the district are steeply dipping and highly folded and faulted. They are divided into three major units: (1) older metavolcanic rocks, (2) metasedimentary rocks, and (3) younger metavolcanic rocks. Massive sulfide-type deposits show stratigraphic control and are found in metasedimentary rocks and in the younger metavolcanic rocks. There appears to be a close genetic relationship between the sulfide deposits and volcanism. Deposits containing copper, zinc, gold, and silver are indicated by geologic mapping and by shallow diamond drill holes in two of the ancient mine localities. The ore estimate for the district is 2.55 million short tons computed to depths that range from 79 to 150 meters. It seems probable that deeper drilling of the indicated deposits and drilling of other outlined target areas would add substantially to the ore reserves.

Open-File Report↗