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Results for “Geochemical Society Special Publication”

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9 recordsLinked to original sources
USGS2004

Geochemistry of coastal tarballs in southern California—A tribute to I. R. Kaplan

In the southern offshore California borderland, natural oil seeps occur mainly in the Santa Barbara Channel and Santa Monica Bay. Coastal tar residues (tarballs) from beaches bordering these water bodies were analyzed for six geochemical parameters: stable carbon isotopic compositions (δ 13 C) and four biomarker ratios (C 28 IC 29 hopane, sterane/hopane, refractory index, bisnorhopane index), and the presence or absence of trisnorhopane. The objectives of this study were to group these residues and infer possible sources and transport directions. Three major groups were established. Two groups are likely from natural seeps near the Channel Islands, whereas the third group probably comes from seeps within Santa Monica Bay. Residues from all groups occur on the Channel Islands and on mainland beaches from as far south as San Diego to Point Reyes north of San Francisco.

California↗
USGS1987

The significance of observations at active volcanoes; A review and annotated bibliography of studies at Kilauea and Mount St. Helens

Study of active volcanoes yields information of much broader significance than to only the discipline of volcanology. Some applications are 1) interpretation of lava-flow structures, stratigraphic complexities, and petrologic relations in older volcanic units; 2) interpretation of bulk properties of the mantle and constraints on partial melting and deep magma transport; 3) interpretation of geophysical characteristics of potentially active volcanic systems; 4) direct determination of physical properties of molten and solidified basalt, and of intensive variables (e.g., oxygen fugacity and temperature) accompanying cooling and crystallization; 5) quantitative assessment of crystal fractionation and magma mixing, 6) tests of theoretical and experimental geochemical, geophysical, and rheologic models of volcanic behavior; and 7) confirmation in nature of laboratory experiments related to crystallization in igneous systems. The critical factors that make real-time study of volcanic activity valuable are that the location and timing of events are known, and that molten rock and gases are available for direct observation and sampling for subsequent study. Observations made over a period of time make it possible to calculate rates of magma transport, storage, and crystallization, as well as to quantitatively determine elastic and inelastic deformation and the build up and decay of stress within the active volcanic system. Discussion of these topics is keyed to an annotated bibliography from which quantitative information on properties and processes may be obtained. Emphasis is on Hawaii's active basaltic volcanoes for which the most information is available. Additional references are made to research at Mount St. Helens, one of the first real-time studies of an active volcano of dacitic composition.

Hawaii, Washington↗
USGS2021

Geochemical and palaeomagnetic characteristics of the Vestfold Hills mafic dykes in the Prydz Bay region: implications of a Paleoproterozoic connection between East Antarctica and Proto-India

The Archean age granite gneiss basement along the Prydz Bay coastline in East Antarctica hosts north–south-, east–west-, NE–SW- and NW–SE-trending mafic dyke swarms in the Vestfold Hills region that intruded between 2420 and 1250 Ma. The dyke trends do not show a direct correlation with the dyke geochemistry but can be broadly discriminated into high-Mg and Fe-rich tholeiites. The former type are more siliceous, are enriched in large ion lithophile elements (LILEs), high field strength elements (HFSEs) and light REEs (LREEs), and crystallized from a fractionated melt with a notable crustal component or fluid enrichment through the previous subduction process. The Fe-rich tholeiites are less siliceous, have lower abundances of LILEs and REEs, and were derived from an undifferentiated, primitive melt. The geochemical characteristics of both types underline a shallow level and a high degree of melting in the majority of cases, and a broadly island arc basalt (IAB) affinity. Palaeomagnetic analysis of hand samples shows directional groups consistent with geochemical groupings. The Vestfold Hills dykes show a possible linkage with the coeval mafic dykes in the Eastern Dharwar and Bastar cratons of the South Indian Block, based on the similarity in the Paleoproterozoic palaeolatitudes

Geological Society, London, Special Publications↗
USGS1981

Organic geochemistry in the Deep Sea Drilling Project

Since the beginning of the Deep Sea Drilling Project DSDP in 1968 and extending through 1975 organic geochemical studies have been undertaken on about 2300 samples recovered on Legs I through 44 from sediments beneath the ocean floors These studies have provided fundamental information regarding the distribution of carbon in oceanic sediments and have yielded a better understanding of the processes that alter and transform organic matter in the marine environment. Beginning with Leg 38 organic geochemists have been included as members of the scientific staffs of those legs of particular organic geochemical interest They have advised operations personnel on matters concerning potential hazards such as drilling into significant accumulations of oil and gas and have collected samples for the scientific community The shipboard organic geochemical laboratory is equipped with sophisticated instrumentation to provide those meassurements helpful in advising the drilling operation. Since Leg 15 samples for organic geochemical studies have been maintained at 180Cat the DSDP sample repository at the Scripps Institution of Oceanography La Jolla California Samples aredistributed to the scientific community but at least one quarter of all cores is retained for possible future studies. Organic geochemical studie sgenerally relate to one of these topic areas (I) petroleum potential source rock evaluation (2) thermal history and gradients (3) diagenesis (4) gas hydrates (5) novel geochemical fossils (6) sources of organic materials (7) paleo depositional environments and (8) geochronology Much of the organic geochemical work on hydrocarbons and kerogen has been directed to the potential of sediments as sources of petroleum The organic matter found thus far during drilling is dominantly of terrestrial origin and thermally immature but much of the organic matter could be a potential source of petroleum mainly gas if buried more deeply. Three new technologies will help advance organic geochemical knowledge of ocean sediments I pressure core barrel for evaluation of gas hydrates 2 hydraulic piston core to study in detail sources and early diagenesis of organic matter and 3 riser for the study of late diagenesis and catagenesis

Society of Economic Geologists Special Publication↗
USGS2016

New insights into the Kawah Ijen hydrothermal system from geophysical data

Volcanoes with crater lakes and/or extensive hydrothermal systems pose significant challenges with respect to monitoring and forecasting eruptions, but they also provide new opportunities to enhance our understanding of magmatic–hydrothermal processes. Their lakes and hydrothermal systems serve as reservoirs for magmatic heat and fluid emissions, filtering and delaying the surface expressions of magmatic unrest and eruption, yet they also enable sampling and monitoring of geochemical tracers. Here, we describe the outcomes of a highly focused international experimental campaign and workshop carried out at Kawah Ijen volcano, Indonesia, in September 2014, designed to answer fundamental questions about how to improve monitoring and eruption forecasting at wet volcanoes.

Kawah Ijen↗
USGS2006

Metallogeny of the Great Basin: Crustal evolution, fluid flow, and ore deposits

The Great Basin physiographic province in the Western United States contains a diverse assortment of world-class ore deposits. It currently (2006) is the world’s second leading producer of gold, contains large silver and base metal (Cu, Zn, Pb, Mo, W) deposits, a variety of other important metallic (Fe, Ni, Be, REE’s, Hg, PGE) and industrial mineral (diatomite, barite, perlite, kaolinite, gallium) resources, as well as petroleum and geothermal energy resources. Ore deposits are most numerous and largest in size in linear mineral belts with complex geology. U.S. Geological Survey (USGS) scientists are in the final year of a research project initiated in the fall of 2001 to increase understanding of relations between crustal evolution, fluid flow, and ore deposits in the Great Basin. Because of its substantial past and current mineral production, this region has been the focus of numerous investigations over the past century and is the site of ongoing research by industry, academia, and state agencies. A variety of geoinformatic tools was used to organize, reinterpret, and display, in space and time, the large amounts of geologic, geophysical, geochemical, and hydrologic information deemed pertinent to this problem. This information, in combination with concentrated research on (1) critical aspects of the geologic history, (2) an area in northern Nevada that encompasses the major mineral belts, and (3) important mining districts and deposits, is producing new insights about the interplay between key tectonic events, hydrothermal fluid flow, and ore genesis in mineral belts. The results suggest that the Archean to Holocene history of the Great Basin was punctuated by several tectonic events that caused fluids of different origins (sea water, basinal brine, meteoric water, metamorphic water, magmatic water) to move through the crust. Basement faults reactivated during these events localized deformation, sedimentation, magmatism, and hydrothermal fluid flow in overlying rocks to form mineral belts that contain ore deposits of different types and ages that are locally superimposed (demonstrating inheritance). Fluid flow in these systems also was influenced by the distribution of permeable lithologies and paleotopographic highs and lows. Hydrothermal fluids evolved from their initial chemistries towards compositions that reflect the ƒ O 2 and ƒ S 2 buffering capacity of, and the ligands and metals present in, the rocks (±older mineralization) through which they moved. In northern Nevada, where gold deposits are relatively common, carbonaceous, pyritic strata buffered fluids of diverse origins to H 2 S-rich compositions so they could transport gold repeatedly over Paleozoic-Cenozoic time (convergent evolution). Ore formed where metal-laden fluids encountered effective physicochemical traps. Maps of Neogene basin fill and erosion surfaces identify areas where preexisting ore deposits have been progressively exposed or concealed. Comparisons with analogous terrains and deposit types in other parts of the world provide global context. The initial findings and some of the databases, geologic maps, sections, reconstructions, hydrogeologic models, topical syntheses, regional overviews, short courses, field guides, and deposit comparisons produced by project staff and associated managers, contractors, and collaborators have been presented in numerous abstracts, symposia, USGS publications, and professional journals over the last 5 years (see the extensive bibliography). Notable among these was the 2005 Geological Society of Nevada symposium in Reno, Nevada, and the 2005 Geological Society of America annual meeting in Salt Lake City, Utah, where project results were presented to audiences from around the nation and world. The final results of the project will be submitted for publication in 2007 to appropriate USGS and professional journals. A special issue of GEOSPHERE, scheduled for publication in 2007, will be devoted to the results of this project and related work. This special issue will reach an international audience and be available worldwide on the internet. Much of the research for this project has concentrated on areas that will receive the focused attention of the mining industry in the future. As such, the data and interpretations generated by this project have direct use for land-use managers in Federal, State, and local agencies. Improved hydrogeologic models developed by this project will considerably enhance ongoing and future water resource investigations in the region. The increased understanding of when, where, and how hydrothermal systems produce significant economic deposits has direct uses for mineral exploration and for future USGS mineral resource assessments in the Great Basin and other parts of the world.

Great Basin↗
USGS2003

Distinguishing palagonitized from pedogenically-altered basaltic Hawaiian tephra: Mineralogical and geochemical criteria

Palagonitization is a common, but imperfectly defined process that greatly modifies the physical and chemical properties of glassy basaltic tephra deposited in subaquatic/subglacial environments on Earth and perhaps Mars. It also results in textures and mineralogies that are distinct from other forms of (mainly pedogenic) low temperature alteration. Specifically, the process of palagonitization (1) initially results in the formation of 'palaginitized glass', a quasi- or nano-crystalline, rind-like material that contains smectite, as well as lesser amounts of other clays (e.g. serpentine), and (2) eventually results in consolidation of tephra, mediated through the accretion of palagonitized glass and later- formed authigenic cements. Conversely, pedogenic weathering of glassy basaltic tephra is characterized by disaggregation of tephra, and formation of a wide range of pedogenic products, including layer silicates (although not primarily smectite), short-range-order aluminosilicates and oxyhydroxides, whose composition reflects the intensity of the weathering environment. These mineralogical and textural properties can be readily recognized through a variety of techniques including electron microscopy/microprobe analysis, reflectance spectroscopy, X-ray diffraction and soil chemistry. Analyses of samples collected from the summit regions of Kilauea and Mauna Kea volcanoes on the island of Hawaii are presented here in order to illustrate differences between palagonitization and pedogenic weathering of glassy basaltic tephra. In the young Hawaiian tephras studied, palagonitization has occurred in response to hydrothermal activity shortly after deposition. Although some, non-hydrothermally affected tephras may eventually become palagonitized, those that have been strongly desilicated by intense pedogenic weathering will probably never become palagonitized.

Geological Society Special Publication↗
USGS2003

Seismic evidence for a mantle source for mid-Proterozoic anorthosites and implications for models of crustal growth

Voluminous anorthosite intrusions are common in mid-Proterozoic crust. Historically, two end-member models have been proposed for the origin of these anorthosites. In the first model anorthosites derive from fractionation of a mantle source leaving a residue of metagabbro in the lower crust; in the second model anorthosites are the product of partial melting of the lower crust with residual pyroxene and high-grade minerals (i.e. a pyroxenitic and/or metapelitic lower crust). Although a general consensus has developed that the first model provides the best fit to petrological and geochemical constraints, the sparse evidence for mafic and ultramafic counterparts to the anorthosites leaves the issue still unresolved. We use the absolute P-wave velocity and the ratio between P- and S-wave velocities ( V P / V S ) to infer the composition of the lower crust beneath the Marcy Anorthosite (New York State, USA). Seismic refraction data reveal a lower crust 20 km thick, where V P and V P /V S range from top to bottom between 7.0 km s −1 and 7.2 ± 0.1 and 1.84 km s −1 and 1.81 ± 0.02, respectively. Laboratory measurements on rock samples indicate that these seismic properties are typical of plagioclase-rich rocks. Magmatic underplating of basaltic melts is a mechanism to form plagioclase-rich bulk composition for the Grenville crust. At the bottom of the lower crust, increase of P-wave velocity, slight decrease of V P / V S ratios and the presence of a low-reflective seismic Moho are additional observations supporting crustmantle interactions related to magmatic underplating. High P-wave velocity (8.6 km s −1 ) in the upper mantle may indicate that the ultramafic portion (e.g. pyroxenites) of the underplated magma has become eclogite. High average P-wave velocity (6.7 km s −1 ) and V P / V S (1.81), and the exceptional abundance of anorthosites-norites-troctolites among the rocks exposed at the surface, indicate that the Grenville Proterozoic crust may have a unique plagioclase-rich bulk composition. We suggest magmatic underplating, occurring either over a wide time span or with separate syn- and post-collisional magmatic pulses, as being a major crust-forming mechanism operating in mid-Proterozoic time.

Geological Society Special Publication↗
USGS2013

Tectonic evolution and Cretaceous gold metallogenesis of southwestern Alaska

Cretaceous gold metallogenesis in southwestern Alaska comprises three distinct episodes related to the accretionary evolution of northwestern North America. The oldest mineralizing event is characterized by 112 Ma Cu-Au-Bi-Te porphyry-type(?) veining in the zoned Bonanza and adjacent plutons that intruded rocks of the Nyac terrane. Tectonic reconstructions and limited geological and geochemical data suggest that Cu-Au mineralization in the Nyac district may be related to terminal subduction during accretion of the Togiak-Koyukuk arc. The subsequent 100 to 89 Ma metallogenic event is a product of subduction-related magmatism immediately following accretion of the Peninsular-Alexander-Wrangellia superterrane and includes formation of the giant Pebble porphyry Cu-Au-Mo deposit. Pebble underwent a complex history of highly oxidized magmatism that is isotopically linked to enriched lithosphere or metasomatized mantle sources. Pebble and other porphyryrelated plutons were emplaced as a consequence of changes in plate motion and onset of dextral transpression along the continental margin to the southeast of their present-day locations. The final 75 to 65 Ma metallogenic event is regionally the most extensive. It followed a ~15-m.y.-long magmatic lull and is related to enigmatic subduction-related magmatism in the western part of the Alaska Range and the Kuskokwim basin. This event resulted in the formation of porphyry, reduced pluton-related, orogenic, and possible epithermal Au deposits. In the better-studied relatively low-relief areas of the Kuskokwim basin, many of the mineralized systems are spatially associated with ilmenite-series monzonite to quartz monzonite composite plutons that have isotopic signatures consistent with derivation from crustally contaminated mantle sources. These pluton-hosted Au deposits comprise low-sulfide stockworks, sheeted veins, and/or breccias in the cupolas of moderately differentiated intrusions that contain high Au; anomalous As, Bi, Sb, and/or Te; and have variable, but not uneconomic, Cu concentrations. In addition, Au, Sb, and/or Hg deposits hosted by competent NE-trending granite porphyry dike-sill complexes or along faults in flysch are widespread in the Kuskokwim basin. These deposits, including the giant Donlin Creek Au deposit, formed at shallow crustal levels and are classified here as epizonal orogenic Au deposits and related Hg and Sb lodes. Mesozonal orogenic Au deposits were formed along the margins of the uplifting Willow Creek pluton that is now exposed along the southern side of the Talkeetna Mountains batholith. Recent discoveries in the rugged Alaska Range comprise less well-studied porphyry Au-Cu (Whistler, Island Mountain), epithermal(?) Au (Terra), and reduced pluton-hosted Au deposits (Estelle). The cause of the broad latest Cretaceous magmatism associated with the final metallogenic event is enigmatic, but indicates widespread high heat flow possibly related to flat-slab subduction, lithospheric mantle delamination, or escape tectonics. Plutonism postdated regional folding and coincided with periods of movement along regional faults and formation of the NE-trending structural fabric. Magmatism resulted from initiation of transpressional faulting in more landward positions during oblique subduction of the Kula plate. Crustal contamination of mantle melts, either near their source or during ascent through the thick flysch of the Kuskokwim basin, produced low oxidation state magmas and controlled much of the metallogeny, particularly for those deposits that display similarities to both reduced porphyry Au-Cu deposits and, to a lesser degree, reduced intrusion-related Au systems. High heat flow also induced crustal melting and metamorphic devolatilization to form orogenic Au deposits. A transition to extension at ~60 Ma recorded elsewhere in Alaska temporally corresponds to termination of gold deposit formation in southwestern Alaska. The multiple periods of gold metallogenesis in southwestern Alaska offer a wide variety of targets for exploration within discrete parts of the region.

Society of Economic Geologists Special Publication↗