Hydrogeochemical exploration for Mississippi valley-type deposits, Arkansas, U.S.A.

Three areas located in northern Arkansas, U.S.A., representing differing host rock and mineralization, were selected to investigate the usefulness of hydrogeochemical exploration for Mississippi Valley-type Pb-Zn mineralization. Despite the geologic differences among the areas, there were no great differences in groundwater chemistry and threshold values. Anomalous Pb concentrations, and also anomalous Zn concentrations to a lesser extent, are useful in detecting the Pb-Zn mineralized areas; however, specific deposits could not be located. Because of the low threshold values (about 20 μg/1) for Pb and Zn, spring water must be utilized in order to avoid plumbing contamination.     

Holocene lake-level trends in the Rocky Mountains, U.S.A.

The availability of water shapes life in the western United States, and much of the water in the region originates in the Rocky Mountains. Few studies, however, have explicitly examined the history of water levels in the Rocky Mountains during the Holocene. Here, we examine the past levels of three lakes near the Continental Divide in Montana and Colorado to reconstruct Holocene moisture trends. Using transects of sediment cores and sub-surface geophysical profiles from each lake, we find that mid-Holocene shorelines in the small lakes (4–110 ha) were as much as 10 m below the modern lake surfaces. Our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000–2000 years ago. We also discuss evidence for millennial-scale moisture variation, including an abruptly-initiated and -terminated wet period in Colorado from 4400 to 3700 cal yr BP, and find only limited evidence for low-lake stands during the past millennium. The extent of low-water levels during the mid-Holocene, which were most severe and widespread ca 7000–4500 cal yr BP, is consistent with the extent of insolation-induced aridity in previously published regional climate model simulations. Like the simulations, the lake data provide no evidence for enhanced zonal flow during the mid-Holocene, which has been invoked to explain enhanced mid-continent aridity at the time. The data, including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium, confirm the ability of large boundary-condition changes to push western water supplies beyond the range of recent natural variability.     

Geochemical exploration for mineralized breccia pipes in northern Arizona,U.S.A.

Thousands of solution-collapse breccia pipe crop out in the canyons and on the plateaus of northern Arizona. Over 80 of these are known to contain U or Cu mineralized rock. The high-grade U ore associated with potentially economic concentrations of Ag, Pb, Zn, Cu, Co and Ni in some of these pipes has continued to stimulate mining and exploration activity in northern Arizona, despite periods of depressed U prices. Large expanses of northern Arizona are comprised of undissected high plateaus; recognition of pipes in these areas is particularly important because mining access to the plateaus is far better than to the canyons. The small size of the pipes, generally less than 600 ft (200 m) in diameter, and limited rock outcrop on the plateaus, compounds the recognition problem. Although the breccia pipes, which bottom in the Mississippian Redwall Limestone, are occasionally exposed on the plateaus as circular features, so are unmineralized near-surface collapse features that bottom in the Permian Kaibab and Toroweap Formations. The distinction between these two classes of circular features is critical during exploration for this unique type of U deposit.Various geochemical and geophysical exploration methods have been tested over these classes of collapse features. Because of the small size of the deposits, and the low-level geochemical signatures in the overlying rock that are rarely dispersed for distances in excess of several hundred feet, most reconnaissance geochemical surveys, such as hydrogeochemistry or stream sediment, will not delineete mineralized pipes.Several types of detailed geochemical surveys made over collapse features, located through examination of aerial photographs and later field mapping, have been successful at delineating collapse features from the surrounding host rock: (1) Rock geochemistry commonly shows low level Ag, As, Ba, Co, Cu, Ni, Pb, Se and Zn anomalies over mineralized breccia pipes; (2) Soil surveys appear to have the greatest potential for distinguishing mineralized breccia pipes from the surrounding terrane. Although the soil anomalies are only twice the background concentrations for most anomalous elements, traverses made over collapse features show consistent enrichment inside of the feature as compared to outside; (3) B. Cereus surveys over a known mineralized pipe show significantly more anomalous samples collected from within the ring fracture than from outside of the breccia pipe; (4) Helium soil-gas surveys were made over 7 collapse features with discouraging results from 5 of the 7 features.Geophysical surveys indicate that scaler audio-magnetotelluric (AMT) and E-field telluric profile data show diagnostic conductivity differences over mineralized pipes as compared to the surrounding terrane. These surveys, coupled with the geochemical surveys conducted as detailed studies over features mapped by field and aerial photograph examination, can be a significant asset in the selection of potential breccia pipes for drilling.     

Kinematics of late Mesozoic thrusting,Pilot Mountains,west-central Nevada,U.S.A.

The northern Pilot Mountains of west-central Nevada, consist of a complexly deformed terrane of imbricate thrust nappes composed of rocks of Permian(?), Triassic through Jurassic, and possible Cretaceous ages. Three episodes of fold and thrust generation are recognized on the basis of folded thrusts and thrusted folds, and deformation and emplacement of the nappes is constrained as having occurred during the late Mesozoic. Folds are apparently coeval with thrust faults, and fold geometry is used in determining approximate directions of thrust displacement. The history of thrust displacement is complex and involves three directions of motion on a regionally extensive detachment surface, the Luning thrust. The first motion, from NW to SE, results in displacements of the order of several tens of kilometres and is the probable result of NW-SE regional compression. The final two episodes of motion are NE-SW followed by E-W; they result in small displacements and are possibly the product of gravity sliding of the thrust sheet into depressions in the autochthon. Sites of downwarp in the autochthon may have been formed either by load induced subsidence or regional compression.     

Geochemistry of Precambrian mafic dikes,central Bighorn Mountains,Wyoming, U.S.A.

Precambrian quartz dolerites and metadolerites of the central Bighorn Mountains form dikes that intrude a Precambrian metamorphic and igneous terrane typical of the Laramide uplifts of the middle Rocky Mountains. They have a restricted range of major- and trace-element compositions and are typical of basalts in the middle stages of tholeiitic fractionation. Fractionation in the direction of iron enrichment occurred by removal of plagioclase. Average element concentrations of the two groups are nearly identical to one another, are comparable to those in Archean metabasalts from numerous shield areas, and are intermediate between those of modern oceanic tholeiites and continental tholeiites. These average concentrations suggest a depth of magma generation and thickness of crust intermediate between those for the oceanic and continental environments.     

Primitive and contaminated basalts from the Southern Rocky Mountains,U.S.A.

Basalts in the Southern Rocky Mountains province have been analyzed to determine if any of them are primitive. Alkali plagioclase xenocrysts armored with calcic plagioclase seem to be the best petrographic indicator of contamination. The next best indicator of contamination is quartz xenocrysts armored with clinopyroxene. On the rocks and the region studied, K₂O apparently is the only major element with promise of separating primitive basalt from contaminated basalt inasmuch as it constitutes more than 1 % in all the obviously contaminated basalts. K₂O: lead (> 4 ppm) and thorium (> 2 ppm) contents and Rb/Sr (> 0.035) are the most indicative of the trace elements studied. Using these criteria, three basalt samples are primitive (although one contains 1.7% K₂O) and are similar in traceelement contents to Hawaiian and Eastern Honshu, Japan, primitive basalts.Contamination causes lead isotope ratios, ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb, to become less radiogenic, but it has little or no effect on ⁸⁷Sr/⁸⁶Sr. We interpret the effect on lead isotopes to be due to assimilation either of lower crustal granitic rocks, which contain 5–10 times as much lead as basalt and which have been low in U/Pb and Th/Pb since Precambrian times, or of upper crustal Precambrian or Paleozoic rocks, which have lost much of their radiogenic lead because of heating prior to assimilation. The lack of definite effects on strontium isotopes may be due to the lesser strontium contents of granitic crustal rocks relative to basaltic rocks coupled with lack of a large radiogenic enrichment in the crustal rocks.Lead isotope ratios were found to be less radiogenic in plagioclase separates from an obviously contaminated basalt than in the primitive basalts. The feldspar separate that is rich in sodic plagioclase xenocrysts was found to be similar to the whole-rock composition for ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb whereas a more dense fraction probably enriched in more calcic plagioclase phenocrysts is more similar to the primitive basalts in lead isotope ratios.The primitive basalts have: ²⁰⁶Pb/²⁰⁴Pb 18.09–18.34, ²⁰⁷Pb/²⁰⁴Pb 15.5, ²⁰⁸Pb/²⁰⁴Pb 37.6–37.9, ⁸⁷Sr/⁸⁶Sr 0.704–0.705. In the primitive basalts from the Southern Rocky Mountains the values of ²⁰⁶Pb/²⁰⁴Pb are similar to values reported by others for Hawaiian and eastern Honshu basalts and abyssal basalts, whereas ²⁰⁸Pb/²⁰⁴Pb tends to be equal to or a little less radiogenic than those from the oceanic localities. ⁸⁷Sr/⁸⁶Sr appears to be equal to or a little greater than those of the oceanic localities. These ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios are distinctly less radiogenic and ⁸⁷Sr/⁸⁶Sr values are about equal to those reported by others for volcanic islands on oceanic ridges and rises.Publication authorized by the Director, U.S. Geological Survey     

Geochemical exploration methods for gold in areas with mountain glaciation in Siberia,U.S.S.R.

In the mountainous areas of Siberia intensive destruction of ore deposits occurs as a result of Pleistocene mountain glaciation and, as a consequence, element dispersion trains form in the eroded material and in the stream sediments. In these areas, cirques and glaciated valleys are incised as much as 250–600 m into ore deposits. Geochemical anomalies in the stream sediments in the glaciated valleys form as water washes through the till and lacustrine sediments. The surface erosion processes result in the removal of fine gold from talus-covered cirque walls from where it is deposited in channels and lakes. Secondary dispersion halos may either be localized entirely within cirques, or they may be found in the drift within the outwash or glaciolacustrine environments downstream of the cirques. Intermittent accumulations of gold are specifically concentrated in glacially-fed stream sediments in those glaciated valleys characterized by smooth, flat bottoms and abundant lakes: gold in such cases tends to be concentrated in the lake muds. After the lakes have silted up, gold will be found in the fluvial deposits. In general, the complete elemental composition of the ore under discussion (Au, Ag, Pb and Zn) is reflected in only short dispersion trains within the fluvial and lacustrine deposits. Gold anomalies, on the other hand, are extensive (up to 6 km) in the stream sediments within the glaciated valleys and will occur sorbed onto hydrous iron oxides and clays, as well as in the form of flattened (thin) gold particles. In the glaciated valleys, the gold geochemical anomalies in moraine, lacustrine and stream sediments properly reflect the mineralization in the drainage basin.     

Chronology of latest Pleistocene mountain glaciation in the western Wasatch Mountains, Utah, U.S.A.

Understanding the timing of mountain glacier and paleolake expansion and retraction in the Great Basin region of the western United States has important implications for regional-scale climate change during the last Pleistocene glaciation. The relative timing of mountain glacier maxima and the well-studied Lake Bonneville highstand has been unclear, however, owing to poor chronological limits on glacial deposits. Here, this problem is addressed by applying terrestrial cosmogenic ¹⁰Be exposure dating to a classic set of terminal moraines in Little Cottonwood and American Fork Canyons in the western Wasatch Mountains. The exposure ages indicate that the main phase of deglaciation began at 15.7 ± 1.3 ka in both canyons. This update to the glacial chronology of the western Wasatch Mountains can be reconciled with previous stratigraphic observations of glacial and paleolake deposits in this area, and indicates that the start of deglaciation occurred during or at the end of the Lake Bonneville hydrologic maximum. The glacial chronology reported here is consistent with the growing body of data suggesting that mountain glaciers in the western U.S. began retreating as many as 4 ka after the start of northern hemisphere deglaciation (at ca. 19 ka).     

Hydrogeochemical exploration for gold in the Osilo area,Sardinia, Italy

A study to test the use of hydrogeochemical methods for gold prospecting was carried out in the Osilo area, northern Sardinia. The study area, covering about 30 km² is characterised by Tertiary andesitic rocks. Gold concentrations up to several ppm, associated with abundant pyrite, arsenopyrite, stibnite, tetrahedrite and electrum, and subordinate galena, sphalerite and chalcopyrite, are present in quartz veins associated with a polyphase, incipient and pervasive alteration of the andesitic rocks.Forty-eight water samples (17 streams, 29 springs and 2 boreholes) were analysed for Au and a wide range of major and trace elements, both in solution (< 0.4 μm) and in suspension. Background values for dissolved Au were below the detection limit of the methods used (between 0.3 and 0.5 ng L⁻¹ Au). Gold concentrations in solution up to 3 ng L⁻¹ were found in waters draining the mineralised vein system. The observed dispersion of Au in surface waters was restricted to about 500 m from the auriferous veins. Dissolved Au anomalies do not vary significantly in water samples, taken monthly over a one year period, suggesting that the dispersion of Au is unaffected by seasonal conditions in the Osilo area. For samples where Au was detected both in solution and in suspension, the Au content of the suspended matter was usually lower than that in solution.The best indicators of Au mineralisation, apart from Au itself, both in solution and in suspension, were As and Sb which showed a dispersion clearly related to the known auriferous veins.     

Trace element distribution in the soils above deeply weathered pegmatites, Virginia, U.S.A.: implications for exploration

Chemical partitioning data are of fundamental interest to exploration geochemists. This paper is one of the few studies which has investigated the relative proportions of the rare elements in various soil extracts. The dispersion of trace elements from weathering pegmatites in Powhatan Country, Virginia, was found to be restricted to the immediate vicinity of the pegmatites. A sequential extraction procedure was used to measure the distribution of Be, La, Nb, Sn, U, Li, and Ni,among the following fractions of the B soil horizons: exchangeable, Fe and Mn oxyhydroxide, residual, and aqua regia digestion. The elements Sn, Be, Li, and U were found to be associated with soils over the complex Herbb No. 2 pegmatite, whereas La and Ni were generally associated with the background soils.A geochemical exploration model was developed using stepwise discriminant function analysis to determine the combination of elements and soil extracts that best differentiates between complex pegmatitic, simple pegmatitic, and background soils. Log-transformed aqua regia extract concentrations of Sn, La, U, and Li were the most effective variables when used to separate complex pegmatitic from simple pegmatitic soils.     

Hydrogeochemical exploration for uranium within the Athabasca Basin, northern Saskatchewan

As part of our exploration programs for unconformity-related uranium deposits, we have studied the chemistry of deep ground waters from throughout the Athabasca Basin. Samples have been collected from routine, small-diameter, exploration diamond drillholes, and we have placed considerable emphasis on ensuring that they represent true ground waters.We have measured both major and minor constituents, including various uranium daughter products. Ground-water samples collected from the vicinity of uranium mineralization have consistently high levels of uranium, radium, radon and helium. Although we have not been able to establish the maximum extent of the hydrogeochemical anomalies in this environment, we have detected anomalies at distances of up to tens of meters from known mineralization.     

Stable isotope geochemistry of a porphyry-style hydrothermal system,West Silverton District,San Juan Mountains,Colorado, U.S.A.

A porphyritic monzonite (25.1 Ma) intrudes Tertiary volcanic rocks of pre- and post-San Juan Caldera age along the western margin of the Silverton Caldera and the northern side of the Sultan Mountain monzonite stock (25.9 Ma), within an area of strong sericitic to argillic alteration (25.0 Ma). These alteration facies, which are genetically related to the monzonite, are characterized by quartz-MoS₂ mineralization (Type 1), are surrounded by pyritic and propylitic facies alteration within which quartz/ base-metal sulfide (Type 2), quartz pyrite (Type 3), and barren quartz (Type 4) veining occurs. Whole-rock H- and O-isotope compositions are variable throughout the district (δD= − 148 to −72%; δ¹⁸O= − 3.5 to +9.3%). The propylitized country rocks are strongly depleted in¹⁸O and D but more highly altered samples have “normal” δD and δ¹⁸O values. Coarse-grained sericites that also exhibit a very large range of δD values to −60%) with respect to biotites and clay-sized sericites that also exhibit a very large range of δD values (−137 to −98% and −139 to −84%, respectively). Type 1 veins have a very restricted range of¹⁸O/¹⁶O ratios (+7.6 to 9.3%), whereas veins of Type 2 and Types 3 and 4 have very large, overlapping, δ¹⁸O ranges of −2.7 to +12.0% and −2.8 to +9.0%, respectively. Type 1 veinlets are characterized by halite-bearing and vapour-rich types, whereas liquid-rich types (excluding halite-bearing types) are dominant in quartz and sphalerite of vein Types 2, 3, and 4. Homogenization and salinity data indicate higher temperatures and much higher salinities of quartz fluids in the central alteration zones (∼350°C, ∼40 ≡wt%NaCl) ompared with those of the peripheral pyritic and propylitic (∼60°C, ≦5 ≡wt%NaCl). The δD values for quartz fluid inclusion waters are quite variable: Type 1 (−125 to −79%₀); Type 2 (−130 to −67%₀); and Types 3 and 4 (−128 to −98%₀). A “magmatic” fluid component is indicated by the calculated composition of sericitic fluids. Of the quartz fluids, those associated with MoS₂-mineralization lie closest to the field of “Primary Magmatic Water”; those associated with the other vein types lie within a large field which ranges from the composition of nearly unexchanged, local meteoric water to compositions reflecting extensive hydrothermal exchange and mixing.     

Ammonium halos in lithogeochemical exploration for gold at the Horse Canyon carbonate-hosted deposit,Nevada, U.S.A.: use and limitations

Rock samples taken from the oxidized, open pit (to depths of 55 m) of the carbonate-hosted Horse Canyon gold deposit contain no NH₄⁺. Samples from the unoxidized zone at the bottom of the pit in the “protore”, however, contain as much as 1.44% NH₄⁺ (1.12% N); the presence of NH₄⁺ was confirmed by mass spectrometry. From these results it is concluded that samples from any oxidized environment (e.g. outcrops, soils) likely will have lost all, or most, of any NH₄⁺ originaly present in minerals such as buddingtonite of NH₄⁺-containing clays. This limits the use of NH₄⁺ halos in lithogeochemical exploration programs. The most reliable application of lithogeochemical studies based on NH₄⁺ halos will likely be based on samples from drill cores and other situations where the samples have always been in a reducing environment.Analysis for NH₄⁺ by IR spectroscopy, the method preferred by most previous workers, is subject to several inherent problems, the main one being the presence of the CO₃²⁻ ion whose absorbance at about 1430 cm⁻¹ coincides with that used to determine NH₄⁺. The removal of CO₃²⁻ and other phases (e.g. organic matter) which may cause analytical difficulties by chemical treatment is not recommended because it is not possible to predict the effect of such treatment on any one of several possible minerals which may contain NH₄⁺. It is suggested that the most efficient method of analysis of rock samples for NH₄⁺ (or N which can be assumed to represent NH₄⁺) will be to analyze the samples after heating to 550°C, at which temperature all organic NH₄⁺ (and N) is removed by oxidation. Any remaining NH₄⁺ (N) determined on this residue can be assumed to originate in NH₄⁺-containing mineral structures.     

Hydrogeochemical characteristics of spring water in the Harz Mountains,Germany

Spring water samples of the Harz Mountains were taken in several seasons of 2010, 2011, and 2012. The samples have been analysed for main components (Na⁺, K⁺, Ca²⁺, Mg²⁺, SO₄²⁻, Cl⁻, HCO₃⁻ and NO₃⁻), trace elements (Fe, Cu, Pb, Zn, Y and REE), DOC, δ¹⁸O and δ²H of water. Meteoric water is indicated as the main source of the springs sampled. High precipitation rates lead to a dilution of the measured elemental concentrations. Furthermore, regional differences of rock and water interactions were found. REE concentrations and patterns of the spring waters vary between the distinct geological units and reflect the geochemical characteristics of the surrounding rocks. The actual data compared to measured data from the seventies and nineties of the last century indicate a decrease of the sulphate concentrations in the spring waters which is typical of many European mountain catchments.     

Hydrogeochemical exploration for volcanic-hosted massive sulfide deposits in semi-arid Australia

Research on hydrogeochemistry for mineral exploration for inland Australia includes development of weathering models and extensive mine-scale and regional groundwater data. Mineral saturation indices for groundwater, activity–activity plots and reaction modelling simulate weathering of volcanic-hosted massive sulfide (VHMS) deposits in deeply weathered environments. At 10 m or more below surface, dissolved O₂ is very low and other solutes such as sulfate, carbonate and nitrate are more likely oxidants. Modelling indicates that these processes differ from oxic weathering of highly eroded terrains, and provide the framework to develop robust hydrogeochemical exploration procedures in covered terrains. Sulfide weathering potentially occurs in two or more phases that effect surrounding groundwaters in differing manners. Deeper oxidative alteration of sulfides (e.g. bornite to chalcopyrite), occurring tens to hundreds of metres below surface, uses sulfate and carbonate as oxidants, causing neutral to alkaline conditions. In this zone, only pyritic massive sulfides potentially generate acidic conditions. Thus, deep sulfide-rich rocks are indicated by sulfate-depleted groundwater. Closer to the surface, sulfides are oxidised to soluble sulfates by dissolved nitrate, with much less acid production than if dissolved oxygen was the main oxidant. Thus, in shallow groundwater, sulfides are indicated by sulfate enrichment and nitrate depletion. Elements are released from sulfides and wall rocks by acid or alkaline conditions. The derived FeS (pH–Eh + Fe + Mn) and AcidS (Li + Mo + Ba + Al) indices distinguish sulfide systems through tens of metres of cover. VHMS systems are distinguished from other non-economic sulfide deposits where there is little transported cover, using various dissolved elements, including Zn, Pb and Cu. Elsewhere, ‘patchiness’ and limited aerial extent of metal signals are due to adsorption effects, that intensify with depth. Other elements such as Mn and Co have lesser diminution effects, but are less selective indicators for VHMS. There is exploration potential for elements such as Pt or Ag. These varying sulfide indicators have moderate utility, even for large-scale (～5 km spacing) sampling. Results indicate that hydrogeochemistry can add value to greenfields exploration for VHMS ore deposits in deeply weathered terrains. It is also moderately successful at indicating the presence of sulfide-rich systems (whether magmatic or hydrothermal) under >100 m cover, thus providing a rapid and cost-effective regional prospectivity tool for deeply buried terrains. Such mineral exploration tools will encourage exploration investment for more difficult regions of Australia and in other deeply weathered regions of the world.     

Hydrogeochemical characteristics of mine water in the Harz Mountains,Germany

Water samples (springs, creeks, mine adits) from different former mining districts of the Harz Mountains and the nearby Kupferschiefer (copper shale) basin of Sangerhausen were analysed for major ions and trace metals. Due to more intensive water rock interactions including the ore minerals, the mine water concentrations of main components and trace metals are generally higher compared to non mining affected surface waters of the mountain range. Furthermore, the content of major ions in mine water is enriched by mixing processes with saline waters from Permian layers in the Kupferschiefer district and at the deeper levels of the mines in the Upper Harz Mountains. The waters of the different mining districts can be distinguished by trace metal occurrences and concentrations derived from the different ore bodies. Water from the Kupferschiefer mines shows the highest Na, Cl, Cu, Mo and U concentrations, whereas a combination of elevated As and Se concentrations is typical for most of the samples from the mines around St. Andreasberg. However, there are exceptions, and some water samples of all the investigated mining districts do not follow these general trends. Despite the influence of mining activities and ore mineralisation, hydrochemical effects due to rain water dilution can be seen in most of the waters. According to the elevation of the mountain range, higher precipitation rates decrease the ion concentrations in the waters of springs, creeks and mine adits.     

Retrograde metamorphism of eclogite in the southern Appalachian Mountains,U.S.A.–A case involving seamount subduction?

This work supports a growing body of evidence that the Ashe Metamorphic Suite (AMS) of the eastern Blue Ridge province in North Carolina has an ensimatic origin and is part of a subduction‐related accretionary mélange, marking the Taconic suture between the North American craton and the Inner Piedmont. In a palinspastic reconstruction, the thrust fault at the base of the AMS appears to have intercepted the greatest depths (i.e. highest‐P metamorphic rocks) beneath parts of the AMS now exposed adjacent to the Grandfather Mountain window. The greatest volume of mafic rock is found in these same areas. We suggest that the nascent, subduction‐related, basal thrust fault was deflected downward by an obstacle in the form of an isolated, mafic volcanic edifice on the oceanic crust–a sea mount. Pelitic and mafic rocks dominate the AMS. North of the Grandfather Mountain window, retrograded eclogite occurs in the amphibolite near the base of the AMS. Textures and mineralogy indicate that an original eclogite assemblage was subjected to the following sequence of parageneses: (a) Eclogite(I) facies: omphacite+garnet+quartz, (b) Eclogite(II) facies: omphacite+garnet+epidote+quartz, (c) Symplectic (diopside+plagioclase)+garnet+epidote+quartz, (d) Amphibolite facies: (diopside+plagioclase)+garnet+epidote+hornblende+quartz, (e) Amphibolite facies: plagioclase+garnet+epidote+hornblende+quartz. P–T conditions, estimated from geothermobarometry applied to relevant mineral compositions, are c.720 °C and c.16 kbar for (b) eclogite(II) facies; c.655 °C and c.8.5 kbar for (e) amphibolite facies.     

Seismic hazard evaluation for dams in northern Colorado, U.S.A.

A seismic hazard evaluation for three dams in the Rocky Mountains of northern Colorado is based on a study of the historical seismicity. To model earthquake occurrence as a random process utilizing a maximum likelihood method, the catalog must exhibit random space-time characteristics. This was achieved using a declustering procedure and correction for completeness of recording. On the basis of the resulting a- and b-values, probabilistic epicentral distances for a 2 × 10^–5 annual probability were calculated. For a random earthquake of magnitude M _L 6.0–6.5, this distance is 15 km. Suggested ground motion parameters were estimated using a probabilistic seismic hazard analysis. Critical peak horizontal accelerations at the dams are 0.22g if median values are assumed and 0.39g if variable attenuation and seismicity rates are taken into account. For structural analysis of the dams, synthetic acceleration time series were calculated to match the empirical response spectra. In addition, existing horizontal strong motion records from two Mammoth Lakes, California earthquakes were selected and scaled to fit the target horizontal acceleration response spectra.     

Sterane distribution of solid bitumen pyrolyzates. Changes with biodegradation of crude oil in the Ouachita Mountains,Oklahoma

Solid bitumens (grahamite and impsonite) of southeastern Oklahoma have been shown to originate from near-surface alteration of crude oil (Curiale, 1981; Curiale and Harrison, 1981). Pyrolysis of these solids has been employed to compare the sterane distribution of geographically proximate oils to that of the bitumens. The ratio of rearranged to regular steranes is higher in the pyrolyzates than in the oils, a finding consistent with a bitumen origin due to biodegradation of oil. The remaining presence of steranes, particularly regular steranes, in the bitumens suggests that sterane occlusion may have occurred prior to or during the alteration process, thus removing tetracyclic compounds from the influence of microbial attack. These data suggest that pyrolysis-$\text{GC}\text{MS}$ offers a viable approach to correlation problems involving solid bitumens.