George A. Desborough

U-Th-Pb and Rb-Sr systematics of Allende and U-Th-Pb systematics of Orgueil

U-Th-Pb systematics study of Allende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The 206Pb/204Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the 206Pb/204Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the 207Pb/204Pb vs 204Pb/204Pb plot. The slope of the best fit line is 0.6188 ± 0.0016, yielding an isochron age of 4553 ± 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-Al-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 ± 25 m.y. and 107 ± 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting that the Rb-Sr system was also disturbed by a later event, as suggested by the U-Pb concordia data. The lowest observed 87Sr/86Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (Papanastassiou, Thesis, 1970). The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb.

A biogenic-chemical stratified lake model for the origin of oil shale of the Green River Formation: An alternative to the playa-lake model

A model is proposed which involves biogenic Mg enrichment, a stratified lake environment, and authigenic growth of minerals that led to the development of oil shale in the lacustrine Green River Formation. The chemistry and mineralogy of Ca-Mg-Fe carbonates and other minerals in oil shale are consistent with an authigenic origin. The higher content of magnesium with respect to calcium in kerogen-rich rocks is probably due to the preferential concentration of magnesium with respect to calcium by blue-green algae whose remains released these cations after accumulation on the lake bottom. These elements were available for incorporation in Ca-Mg-Fe carbonates which crystallized in lake-bottom muds, while degradation of admixed algal material led to the development of kerogen. In modern lacustrine environments, primary and secondary Ca-Mg-Fe carbonate development and stability in terms of geologic time are consistent with authigenic development of the Ca-Mg-Fe carbonates present in Green River Formation oil shale. Iron is a significant component of these rhombohedral carbonate assemblages in oil shale, and thus it limits interpretations of origin of oil shale in the context of CaCO 3 -MgCO 3 equilibria. The variable composition and variety of Ca-Mg-Fe carbonates in oil shale also prohibits interpretations in terms of CaCO 3 -MgCO 3 equilibrium diagrams. Similarly, inferences regarding the chemical composition and structure of these carbonates lead to misinterpretations due either to the lack of adequate chemical data or to misunderstanding of crystallographic parameters. Greater amounts of mineral matter in time-stratigraphic intervals in the depositional center of the Piceance Creek basin, compared to the basin margins, suggest strongly that authigenic mineral development is more important than detrital accumulation of minerals in the richer oil-shale sequences. Systematic consideration of all of the arguments developed to promote the playa-lake model to explain the chemical, mineralogical, and geologic aspects of Green River Formation oil shale leads to the conclusion that the biogenic-chemical model proposed here is more appropriate than a strictly chemical model, because the playa-lake model so far does not consider the influence of any biogenic factors.

Osmium, ruthenium, iridium and uranium in silicates and chromite from the eastern Bushveld Complex, South Africa

Abstract Osmium, ruthenium, iridium and uranium contents were determined in eight ortho pyroxene, seven plagioclase, and three chromite mineral separates from the eastern Bushveld Complex. Neutron activation analysis was used to measure the platinum metals, and uranium was determined by a fission track technique. The platinum metals were found to be present within each mineral in the proportions Os : Ru : Ir = 1:7:1, while the concentrations of these metals in the minerals are in the ratios orthopyroxene:plagioclase:chromite = 1:16:700. The concentration of uranium was found to range from 11 to 66 ppb (parts per billion) and not to vary significantly from mineral to mineral. The data for the platinum metals are consistent with a model in which the eastern Bushveld Complex was formed by the fractional crystallization of two separately injected magmas. A computer fit of this model to these data indicates that the initial concentrations of Os, Ru and Ir in the first magma were 0.24, 2.0 and 0.21 ppb and in the second magma were 0.16, 1.1 and 0.18 ppb, respectively. The fit also yields the distribution coefficients for the partitioning between the liquid and cumulus orthopyroxene, cumulus plagioclase and cumulus chromite. These coefficients (mineral/liquid) for osmium are 4.5, 66 and 2700; for ruthenium, they are 5, 65 and 2700; and for iridium, they are 4, 60 and 1600. To make this fit, it was necessary to hypothesize the existence of two types of chromite: one type with a large distribution coefficient, presumably formed as a cumulus phase at high temperature, and another, more prevalent type with a smaller distribution coefficient, which may have been formed by postcumulus growth at a lower temperature. This hypothesis is supported by data for coexisting chromite-silicate pairs, which indicate that the chromite grains expelled these platinum metals as they cooled.

Concentration and mineralogical residence of elements in rich oil shales of the Green River Formation, Piceance Creek basin, Colorado, and the Uinta Basin, Utah — A preliminary report

Abstract Ten samples from drillcore of two rich oil-shale beds from the Parachute Creek Member of the Eocene Green River Formation, Piceance Creek basin, Colorado, and Uinta Basin, Utah, were analyzed for 37 major, minor, and trace elements. For 23 of these elements, principal mineralogical residence is established or suggested and such studies may provide data which are useful for predicting the kinds and amounts of elements and compounds that might be released into the environment by oil-shale mining operations.

The use of synthetic jarosite as an analog for natural jarosite

The presence of jarosite in soil or mining waste is an indicator of acidic sulfate-rich conditions. Physical and chemical properties of synthetic jarosites are commonly used as analogs in laboratory studies to determine solubility and acid-generation of naturally occurring jarosites. In our work we have mineralogically and chemically characterized both natural and synthetic jarosites. Analysis of 32 natural hydrothermal and supergene K- and Na-jarosites indicates no (< 5 mole %) solid solution between K and Na end members. Instead, our detailed study of cell dimensions and composition reveals discrete mixtures of K and Na end members. Hydronium-bearing jarosite was detected in only one natural sample, and it appears that hydronium-bearing jarosites are metastable. Although the presence of hydronium in jarosite cannot be directly measured, we found that when synthetic hydronium-bearing jarosites are heated at 120°C for 78 days or 240°C for 24 hours, Fe(OH)SO4 is formed. The Fe(OH)SO4 is easily detected by X-ray diffraction and, hence, can be used as a post-mortem indicator of the presence of hydronium jarosite. Results from our synthetic jarosite studies indicate that natural metastable hydronium-bearing jarosite or iron-deficient forms of natural jarosite likely play an important role in acid generation in some mining wastes, but are not accurately represented by synthetic jarosite prepared by commonly used methods. The widespread practice of heating to at least 110°C after jarosite synthesis appears to drive off structural waters from protonated hydroxyl sites, which changes the properties of the jarosite. Therefore, synthetic jarosite should not be heated above 95 o C if it is to be used as an analog

Chapter 8 Petrogenesis and mineralogic residence of selected elements in the meade peak phosphatic shale member of the permian phosphoria formation, Southeast Idaho

Abstract The Meade Peak Phosphatic Shale Member of the Permian Phosphoria Formation hosts the ore mined by the phosphate industry of southeast Idaho. It also hosts environmentally sensitive elements (ESE) such as Se, As, Hg, Ni, Cd, Zn, and Cr. Primary chemistry, elemental distribution patterns, and mineralogy within the Meade Peak were modified by element migration and possibly the introduction of elements. Fluids moved within the Meade Peak throughout its history, although the passage of fluids was highly variable in space and time, resulting in small domains of different rock chemistry and different mineralogy. Timing of major events affecting the Meade Peak and mineral habit are used to differentiate among detrital, diagenetic, epigenetic, and supergene mineral assemblages. Cross-cutting relationships among minerals are too rare to provide much paragenetic infor- mation. Carbonate fluorapatite (CFA) occurs in several forms, but dominantly as pelloids, some of which may have formed in situ during diagenesis. The other volumetrically signifi- cant form of CFA is interstitial cement that formed during diagenesis. Beginning during diagenesis and continuing intermittently, multiple generations of carbonate (dolomite and calcite) formed overgrowths and texturally complex carbonate cements. Movement and precipitation of silica followed a similar pattern. The ammonium feldspar buddingtonite, which generally rims orthoclase, also formed during diagenesis. Bacteria apparently played a significant role during diagenesis as well as during supergene processes, resulting in extreme fractionation of S isotopes and the possible bacterially mediated formation of minerals such as glauconite and sphalerite. Catagenesis, apparently culminating in oil generation, was the last significant diagenetic change. Thrusting accompanied by fluid (oil and brine) migration began during catagenesis in the Late Jurassic or Cretaceous and continued into the early Eocene. Fluorite ± carbonate ± barite± bitumen veins formed as a result of brittle deformation and accompanying fluid movement. This fracturing event may have been associated with a period of extension and normal faulting (Neogene to Holocene). Passage of the Yellowstone hot spot to the north of the area during the Neogene is marked by silicic domes and basaltic flows. The enrichment of Hg in fracture coatings might be the result of deposition from warm fluids associated with the emplacement of the silicic domes or a generally elevated, regional thermal gradient associated with the volcanism. Many of the fracture systems are still open and continue to provide fluid pathways that are the primary depositional sites for a wide variety of supergene minerals (such as Se, efflorescent salts) and element associations (such as Hg, Cd-S, Fe-Cr-O) in which many of the ESE are concentrated. Native Se is the most commonly identified host of Se in the studied samples. The largest concentration of Se occurs in open-fracture systems that cross-cut waste rock and ore units. The age(s) of native Se formation is not known; how- ever, the latest period of Se mobility is the present. Direct measurement of efflorescent “salts” forming on new mine faces indicate that several ESE, including both Se and Zn, are concentrated on the faces soon after they are exposed. Zinc is present as hydrous sulfates, but the residence of Se in these “salts” is unknown.

Limitations on analysis of small particles with an electron probe: Pollution studies

Abstract Recent literature concerning the size and composition of airborne lead particles in automobile exhaust emission determined by electron microprobe analysis reports fourteen distinct lead compounds. Particle sizes reported were from 0·2 μm to 2 μm in diameter. The determination of chemical formulae for compounds requires quantitative elemental data for individual particles. It was also assumed that the lead-bearing particles analysed were solid (specifically non-porous or non-fluffy) compounds which occurred as discrete (non-aggregate) particles. Intensity data obtained in our laboratory from the excited volume in a 1 μm-diameter sphere of solid lead chloride indicate insufficient precision and sensitivity to obtain chemical formulae as reported in the literature for exhaust emission products.

Sources of acid and metals from the weathering of the Dinero waste pile, Lake Fork watershed, Leadville, Colorado

Abstract. Two trenches were dug into the south Dinero mine-waste pile near Leadville, Colorado, to study the weathering of rock fragments and the mineralogic sources of metal contaminants in the surrounding wetland and Lake Fork Watershed. Water seeping from the base of the south Dinero waste-rock pile was pH 2.9, whereas leachate from a composite sample of the rock waste was pH 3.3. The waste pile was mostly devoid of vegetation, open to infiltration of precipitation, and saturated at the base because of placement in the wetland. The south mine-waste pile is composed of poorly sorted material, ranging from boulder-size to fine-grained rock fragments. The trenches showed both matrix-supported and clast-supported zones, with faint horizontal color banding, suggesting zonation of Fe oxides. Secondary minerals such as jarosite and gypsum occurred throughout the depth of the trenches. Infiltration of water and transport of dissolved material through the pile is evidenced by optically continuous secondary mineral deposits that fill or line voids. Iron-sulfate material exhibits microlaminations with shrinkage cracking and preferential dissolution of microlayers that evidence drying and wetting events. In addition to fluids, submicron-sized to very fine-grained particles such as jarosite are transported through channel ways in the pile. Rock fragments are coated with a mixture of clay, jarosite, and manganese oxides. Dissolution of minerals is a primary source of metals. Skeletal remnants of grains, outlined by Fe-oxide minerals, are common. Potassium jarosite is the most abundant jarosite phase, but Pb- and Ag-bearing jarosite are common. Grain-sized clusters of jarosite suggest that entire sulfide grains were replaced by very fine-grained jarosite crystals. The waste piles were removed from the wetland and reclaimed upslope in 2003. This was an opportunity to test methods to identify sources of acid and metals and metal transport processes within a waste pile. A series of entrapment ponds, lined with limestone rip rap, was created where the mine waste was once situated. A flooded adit discharges low-pH metal-bearing waters into the ponds. A white (Zn, Mn)-sulfate precipitate was observed in 2003 around the edges of the most distal pond. Key Words: mine waste, dissolution, jarosite, anglesite, microlamination, leachate -----------------------------------------

Characteristics, Performance, and Quantitative Analytical Capability of an Energy-Dispersive Spectrometer on an Electron Microprobe Using Low Operating Voltages

An investigation was made of the analytical capability of a Si (Li) x-ray detector for energy-dispersive techniques in the 1- to 10-keV photon energy range with multiple single channel analyzers using the electron microprobe beam as the excitation source at operating voltages of 10 to 20 kV. This system can provide rapid, accurate, simultaneous, quantitative elemental microanalysis of major constituents in complex solids for all elements heavier than Si where spectral interferences are absent and reference standards are used. Calculated theoretical detection limits are about 0.2 wt %. A relative precision of about 1 to 5% is routine for a 10- or 20-sec counting period for elements in concentrations above about 5 wt %. Data for K, L, and M x-ray spectra of representative elements ranging in energy from 1.25 to 8.64 keV are presented for operating voltages of 10,15, and 20 kV and beam currents of 1 to 10 nA.

Oil and Metals in Ordovician and Devonian Kerogenous Marine Strata of Central Nevada: ABSTRACT

Kerogen-rich mudstone, siltstone, dolomite, and chert units as much as 50 m thick in the Vinini (Ordovician) and Woodruff (Devonian) formations contain potential resources of syncrude oil, V, Zn, Mo, Se, Ag, and Cr. Most kerogenous rocks originally consisted of organic-rich siliceous muds, slimes, and oozes. Organic matter is mostly amorphous, flaky, and stringy sapropel composed of planktonic organisms. The strata are within strongly deformed eugeosynclinal Paleozoic marine rocks of the Roberts Mountains allochthon. Many fresh black rocks are low-grade oil shales which, upon pyrolysis, yield < 40 l of oil per metric ton of rock; some thin layers yield as much as 125 l per metric ton. In these rocks, solid bitumen and liquid oil commonly fill voids and microfractures. Such early-phase hydrocarbons probably were released during diagenesis and formed without any major thermal degradation of the kerogen. Geochemical data suggest that the organic matter is thermochemically immature to mature and has not been subjected to temperatures above 60°C since deposition. Hydrocarbon contents (< 100 to 5,400 ppm) and organic carbon contents (< 1 to 25 weight %) vary widely. V, Mo, Se, Ag, and Cr in fresh black rocks occur chiefly in organic matter; Zn occurs as sphalerite and Ni in iron sulfides. Concentrations are as much as 5,000 ppm V, 18,000 ppm Zn, 1,000 ppm Mo, 100 ppm Se, 20 ppm Ag, 150 ppm Ni, and 600 ppm Cr in unoxidized rocks. Enrichment of V and Se and depletion of Zn, Mo, Ni, and organic matter occur in oxidized rocks. End_of_Article - Last_Page 767------------