Application of hydrogeochemistry to uranium exploration in the pine creek geosyncline, northern territory, Australia

In the Pine Creek Geosyncline, fast moving, annually recharged, low-salinity ground waters dissolve uranium- and magnesium-enriched gangue minerals from mineralized aquifer rocks. The level of dissolved uranium depends on prevailing pH, Eh, salinity and degree of adsorption, which limits its effectiveness as an exploration indicator. Near each known deposit, leaching of magnesium-enriched gangue minerals produces ground waters with very similar major-element concentration plots, the shape of which constitutes a mineralized aquifer “signature”. Gangue minerals also supply high levels of Mg²⁺ (expressed as NMg = [Mg²⁺]/[Ca²⁺ + Mg²⁺ + Na⁺ + K⁺] in milliequivalents per litre) to contained ground waters, NMg > 0.8 being common in ground waters from mineralized aquifers at each Pine Creek Geosyncline deposit. Data from Ranger One No. 3 ore body illustrates how progressive mixing of waters from mineralized and unmineralized aquifers causes graded reductions in NMg, which, when plotted onto a ground plan, delineate a hydrogeochemical aureole.High NMg (> 0.8) coincides with high uranium concentration (> 20 μg/l of U) in ground waters near Nabarlek and Ranger. Because pH-Eh conditions in aquifers at Jabiluka depress uranium solution, < 10 μg/l of U is present, although NMg values are generally > 0.8. To date NMg has always been < 0.8 in nonmineralized aquifer waters, whereas uranium may be > 50 μg/l in ground waters from felsic igneous aquifers, which can be identified as uneconomic by low (< 0.4) NMg, and by a fixed relationship between uranium and co-leached species such as F^- and soluble salts.Measurements of pH, Eh, salinity, Fe(II), Ca, Mg, Na, K, Cl, SO₄, total carbonate, phosphate, F^-, Cu, Pb, Zn and U in waters from 48 percussion holes in and near the Koongarra ore bodies have been related to mineralogy recorded in drill logs. The composition of waters from 20 holes near and along strike from known mineralization, fitted the mineralized aquifer “signature”, had NMg > 0.8 and uranium up to 4100 μ/l. These data confirm the use in this region of NMg as a hydrogeochemical indicator of uranium mineralization; they also indicate additional zones of possible mineralization.     

Stress fields about strike-slip faults inferred from stylolites and tension gashes

In general, the long axis of the tension gashes and stylolitic columns developed in limestones during a single phase of compressional deformation occur parallel to the direction of the maximum compressive stress (σ₁). This is the case in the Languedoc for structures developed in the Jurassic limestones during the N-S Pyrenean compression. Exceptionnally, however, these microstructures turn in direction and become oblique (even orthogonal) one to the other, probably as a consequence of a variation in intensity and direction of the stress field at the end of a microfault. This mechanism also occurs in a larger scale structure involving segments of pre-tectonic joints that act as “en échelon” microfaults in a brittle “kink-band” equivalent to a peculiar type of potential wrench-fault.     

Environmental control of carbon isotope variations in Pennsylvania black-shale sequences, Midcontinent, U.S.A.

A reversal of the conventional carbon isotope relationship, “terrestrial-lighter-than-marine” organic matter, has been documented for two Pennsylvanian (Desmoinesian) cyclothemic sequence cores from the Midcontinent craton of the central United States. “Deep” water organic-rich phosphatic black shales contain a significant proportion of algal-derived marine organic matter (as indicated by organic petrography, Rock-Eval hydrogen index and ratios) and display the lightest δ¹³C-values (max −27.80‰ for kerogen) while shallower water, more oxic facies (e.g. fossiliferous shales and limestones) contain dominantly terrestrial organic matter and have heavier δ¹³C_kerogen-values (to −22.87‰ for a stratigraphically adjacent coal). δ¹³C-values for extract fractions were relatively homogeneous for the organic-rich black shales with the lightest fraction (often the aromatics) being only 1‰, or less, more negative than the kerogen. Differences between extract fractions and kerogens were much greater for oxic facies and coals (e.g. saturates nearly 5‰ lighter than the kerogen).A proposed depositional model for the black shales calls upon a large influx of nutrients and humic detritus to the marine environment from the laterally adjacent, extremely widespread Pennsylvanian (peat) swamps which were rapidly submerged by transgression of the epicontinental seas. In this setting marine organisms drew upon a CO₂-reservoir which was in a state of disequilibrium with the atmosphere, being affected by isotopically light “recycled-CO₂” derived from the decomposition of peaty material in the water column and possibly from the anoxic diagenesis of organic matter in the sediments.     

Evidence for cyclic reactions between andalusite, “sericite” and sillimanite, mount franks area, willyama complex N.S.W

In the Mt. Franks area of the Willyama Complex, microfabric evidence suggests that the alteration of andalusite to sillimanite has taken place by a process similar to that suggested by Carmichael (1969). Andalusite is pre- to syn-S₂ in age. Alteration to “sericite” has resulted in the formation of “sericite” laths, some of which are crenulated about S₂, and some which are syn- and post-S₂. “Fibrolite” occurs in these andalusite—“sericite” aggregates within the sillimanite zone and is wholly embedded in “sericite”. “Fibrolite” is pre- to syn-S₂ in age. This evidence is interpreted as suggesting that the formation of sillimanite from andalusite took place via a “sericite” phase.Further microfabric observations are interpreted to imply constant volume for the reaction aluminosilicate → “sericite”. This suggests a situation in which Al³⁺ is relatively mobile but Al⁴⁺ is relatively immobile. This suggestion differs from Carmichael's (1969) idea of Al³⁺ immobility.     

Geomagnetic induction anomalies along the hyderabad—kalingapatnam profile

Observations of H- and Z-variations made at ten temporary field stations along the Hyderabad—Bhadrachalam—Kalingapatnam (east coast) profile during February–March, 1973, are analyzed and discussed. The results are also compared with those of the Alibag (west coast)—Hyderabad profile, completed in May–July, 1970.It is found that the Z-ranges of quiet daily variations are enhanced one and a half times the Hyderabad value, and the H-ranges only very slightly reduced at the coastal station of Kalingapatnam. This coastal effect of 6γ in Z_r at Kalingapatnam gradually decreases inland and probably exists upto Salur (70 km inland from the coast). At Bhadrachalam, both H- and Z-ranges of quiet daily variations are found to be enhanced, possibly due to induction effects from deep-seated conductors in the Godavari rift valley. The enhancement of both H and Z daily ranges at Jeypore in the eastern “ghats” (hills) is attributed to induced electric currents in conductors arising from the orogeny well below the “ghats”.It is difficult to separate the coastal effect from the orogenic effect, both of which seem to taper off and merge at Salur.     

Properties, origin and nomenclature of rodlets of the inertinite maceral group in coals of the central Appalachian basin, U.S.A.

Resin rodlets, sclerenchyma strands and woody splinters, which are collectively called rodlets, were studied by chemical, optical petrographic, and scanning-electron microscopic (SEM) techniques. A study was made of such rodlets from the bituminous coal beds of the central Appalachian basin (Pennsylvanian; Upper Carboniferous) of the United States. Comparisons were made with rodlets from coal beds of the Illinois basin, the Southern Anthracite Field of Pennsylvania, the St. Rose coal field of Nova Scotia, and European and other coal fields. In order to determine their physical and chemical properties, a detailed study was made of the rodlets from the Pomeroy coal bed (high volatile A bituminous coal; Monongahela Formation; Upper Pennsylvanian) of Kanawha County, West Virginia. The origin of the rodlets was determined by a comparative analysis of a medullosan (seed fern) stem from the Herrin (No. 6) coal bed (high volatile C bituminous coal; Carbondale Formation) from Washington County, Illinois. Rodlets are commonly concentrated in fusain or carbominerite layers or lenses in bituminous coal beds of the central Appalachian basin. Most of the rodlets examined in our study were probably derived from medullosan seed ferns. The three types of rodlets are distinguished on the basis of cellularity, morphology and fracture.The resin rodlets studied by us are noncellular and appear to be similar in properties and origin to those found in coal beds of the Middle and Upper Pennsylvanian of the Illinois basin. The resin rodlets extracted from the Pomeroy coal bed exhibit high relief and high reflectance when polished and viewed in reflected light; they are opaque in transmitted light. In cross section, the resin rodlets are oval to round and have diameters ranging from 60 to 450 μm. Many are solid, but some have vesicles, canals or cavities, which are commonly filled with clay, probably kaolinite. Typically, they have distinct fracture patterns (“kerfs”) in longitudinal and cross sections and many are characterized by dense (probably oxidized) rims. The orientation and amounts of void space and mineralization of resin rodlets in coal have resulted in much confusion in their recognition and classification. The resin rodlets are petrographically recognized as sclerotinites of the inertinite maceral group. We here propose that resin rodlets be assigned to the maceral variety of sclerotinites termed “resino-sclerotinite” because of their presumable resinous origin. Other investigators have confused some fusinitized resin rodlets with fungal masses, which have different morphological properties and which probably have different chemical properties. We here propose that such fungal masses be assigned to the maceral variety of sclerotinites termed “fungo-sclerotinite.”The sclerenchyma strands examined in our study are cellular, thick-walled, and crescent-shaped in cross section. They exhibit high reflectance and high relief and belong to semifusinite and fusinite of the inertinite maceral group. Sclerenchyma strands are commonly associated with resin canals in Medullosa and related seed-fern genera, which are common in coal balls of the Illinois basin. We here propose adoption of the maceral varietal terms “sclerenchymo-fusinite” and “sclerenchymo-semifusinite” for these bodies.The woody splinters in the Pomeroy coal bed are cellular and thin-walled and have scattered pits as much as a few microns in diameter. They are dark brown to black in transmitted light and commonly have a lower reflectance than the resino-sclerotinite and sclerenchymo-fusinite of the Pomeroy coal. The woody splinters belong to semifusinite and fusinite of the inertinite maceral group. The maceral varietal terms “xylemo-semifusinite” and “xylemo-fusinite” are here proposed for these bodies.Elemental chemical data for the resin rodlets of the Pomeroy coal bed of the central Appalachian basin indicate that resin rodlets have significantly lower atomic H/C and O/C ratios than do sclerenchyma strands and woody splinters. The lower atomic H/C and O/C ratios of the resin rodlets correlate with the highest reflectance. In the coal ball medullosan seed-fern stem from the Herrin (No. 6) coal bed of the Illinois basin, the reflectances of the resin rodlets, woody splinters and sclerenchyma strands are similar and comparable to those of associated vitrinite in the coal ball stem and in the attached coal. However, resin rodlets and sclerenchyma strands in the attached coal have significantly higher reflectances, similar to those of the Pomeroy coal.     

Lower Eocene carbonate-cemented “chimney” structures (Varna, Bulgaria) — Control of seepage rates on their formation and stable isotopic signature

The genesis of Lower Eocene calcite-cemented columns, “pisoid”-covered structures and horizontal interbeds, clustered in dispersed outcrops in the Pobiti Kamani area (Varna, Bulgaria) is related to fossil processes of hydrocarbon migration. Field observations, petrography and stable isotope geochemistry of the cemented structures and associated early-diagenetic veins, revealed that varying seepage rates of a single, warm hydrocarbon-bearing fluid, probably ascending along active faults, controlled the type of structure formed and its geochemical signature. Slow seepage allowed methane to oxidize within the sediment under ambient seafloor conditions (δ¹⁸O = − 1 ± 0.5‰ V-PDB), explaining columns' depleted δ¹³C ratios of − 43‰. Increasing seepage rates caused methane to emanate into the water column (δ¹³C = − 8‰) and raised precipitation temperatures (δ¹⁸O = − 8‰). Calcite-cemented conduits formed and upward migrating fluids also affected interbed cementation. Even higher-energy fluid flow and temperatures likely controlled the formation of “pisoids”, whereby sediment was whirled up and cemented.     

Surface carbonate and land-derived clastic marine sediments from southern Chile: mineralogical and geochemical investigation

Surface carbonate and land-derived deposits in the sea off southern Chile were investigated for their mineralogical and geochemical composition. The data were related to environmental features and compared with those of similar temperate and polar carbonate deposits from Tasmania, New Zealand, Arctica, and Antarctica. The mineralogy of the siliciclastic fraction is typical of cold areas and is mainly composed of chlorite, mica, quartz, feldspars and amphibole. The CaCO₃ content varies from 30 to 90%; carbonate mineralogy is made up of low-Mg calcite, high-Mg calcite and minor amounts of aragonite. The Ca, Mg, Sr, Fe, and Mn contents of bulk carbonates and some selected skeletal hard parts are comparable to those of carbonates from Tasmania. The elemental composition is mainly related to carbonate mineralogy, skeletal components, and seawater conditions. The δ¹³C and δ¹⁸O values of carbonates are positive, and their field falls between the “seafloor diagenesis” and “upwelling water” trend lines, because the sediments are likely to be in equilibrium with waters of Antarctic origin. The mineralogical, elemental, and isotopic compositions of carbonates from southern Chile show better similarities with the “temperate” carbonates from Tasmania and New Zealand than with the “polar” carbonates from Arctica and Antarctica. Carbonate deposition is allowed by the low terrigenous input, the low SPM concentration and, probably, the upwelling of seawater from Antarctica.     

Controls on pore-fluid concentration of He and Rn and the calculation of He/Rn ages

Controls governing the production of ⁴He and ²²²Rn in the solid phase as well as parameters and processes contributing to their transport into the pore fluid are discussed. ²²²Rn activity is used to quantify the uranium sources for ⁴He and the result is a simplified pore-fluid age equation which is virtually independent of the porosity, the water/rock ratio, the rock density and the uranium concentration, but does require a Th/U estimate. The crucial parameter is the ratio of the release factors for the two species, /GL_Rn//GL_He, which is discussed in terms of three possible release mechanisms: (1) recoil; (2) recoil followed by diffusion; and (3) weathering release of accumulated ⁴He. It was found that /GL_Rn//GL_He can vary over several orders of magnitude, but can be expressed in terms of the effective grain size r, and the diffusive half-length for ²²²Rn decay, r_e.⁴He measurements are used to “date” gases from known gas fields and the agreement with the assumed source-rock age is good. Application of ⁴He/ ²²²Rn measurements to continental freshwater springs indicates that the weathering release of accumulated ⁴He dominates the input and results in a large overestimate of groundwater age. Measurement in the Lardarello geothermal field indicates that the ⁴He/²²²Rn method can indicate relative transport direction. Other possible applications in various geochemical fields are suggested.     

The average molecular weight and shape of the “polymeric acids” found in Black Trona Water from the Green River Basin

Macromolecular organic material, called “polymeric acids”, has been isolated from Black Trona Water by exhaustive dialysis and characterized as the sodium salt in 0.10 M sodium carbonate, pH 10, by several physico-chemical methods. Analysis by gel filtration chromatography on Sepharose-CL 6B indicates that the “polymeric acids” are polydisperse and composed of species of relatively high molecular weight ( 4 × 10⁵, using proteins as standards). With this method, the range of molecular weights appears to be rather narrow. If “polymeric acids” are transferred from sodium carbonate, pH 10, into distilled water, selfassociation occurs and all species elute in the void volume. The weight-average molecular weight determined in 0.10 M sodium carbonate, pH 10, by the light scattering method is 1.7 × 10⁵. Sedimentation velocity analysis at 20°C with the analytical ultracentrifuge gives a value for S_20,w of 5.4 and the shape of the Schlieren patterns suggest a polydisperse sample with a relatively narrow range of sizes. Analysis of the molecular weight distribution by a sedimentation equilibrium method indicates that the range of molecular weights is 8 × 10⁴ to 2.1 × 10⁵. The partial specific volume ( ) of “polymeric acids” is 0.874 ml/g. Viscosity measurements yield a value for [η] of 2.5 ml/g, which indicates that the “polymeric acids” are compact (spherical or ellipsoidal) in shape.     

Magnitudes of great shallow earthquakes from 1953 to 1977

The surface-wavemagnitudes M_s are determined for 30 great shallow earthquakes that occurred during the period from 1953 to 1977. The determination is based on the amplitude and period data from all available station bulletins, and the same procedure as that employed in Gutenberg and Richter's “Seismicity of the Earth” is used. During this period, the Chilean earthquake of 1960 has the largest M_s, 8.5. The surface-wave magnitudes listed in “Earthquake Data Reports” are found to be higher than M_s on the average. By using the same method as that used by Gutenberg, the broad-band body-wave magnitudes m_B are determined for great shallow shocks for the period from 1953 to 1974. m_B is based on the amplitudes of P, PP and S waves which are measured on broadband instruments at periods of about 4–20 s. The 1-s body-wave magnitudes listed in “Bulletin of International Seismological Center” and “Earthquake Data Reports” are found to be much smaller than m_B on the average. Through the examination of Gutenberg and Richter's original worksheets, the relation between m_B and M_sis revised to m_B = 0.65 M_s+ 2.5 which well satisfies the mg and M_sdata for M_sbetween 5.2 and     

Senescence- and death-related alteration of chlorophylls and carotenoids in marine phytoplankton

This report extends previous work ([Louda et al., 1998a] and [Louda et al., 1998b]. Chlorophyll degradation during senescence and death. Organic Geochemistry 29, 1233–1251.) in which we detailed type-I (alteration) and -II (destruction) degradation of chlorophyll with representative fresh water phytoplankton. The present study covers similar experiments with marine phytoplankton, namely, a cyanobacterium (“ANA” Anacystis sp), a coccolithophore (“COC” Coccolithophora sp.), a dinoflagellate (“GYM” Gymnodinium sp.) and two diatoms (“CYC” Cyclotella meneghiniana and “THAL” Thalassiosira sp.). Mg loss (‘pheophytinization') was rapid and continuous in all species under room-oxic conditions and slow or sporadic under anoxic conditions. The proportion of dephytylated pigments (pheophorbides-a, chlorophyllides-a), relative to the phytylated forms (chlorophyll-a, pheophytins-a), increased over the first year under room-oxic conditions and in room-anoxic conditions only in “CYC”. Pheophorbide-a was converted to pyropheophorbide-a within 15 months only in “THAL” and “ANA”, and slightly in “COC”. After 9–15 months of oxic incubation, “COC” was found to contain traces of purpurin-18 phytyl ester. Consideration of carotenoid pigments is also included herein. All fucoxanthin containing species, except “THAL”, exhibited conversion of fucoxanthin to fucoxanthinol in room-oxic conditions. Diadinoxanthin was rapidly de-epoxidized to give diatoxanthin within the first 2–4 weeks. Diatoxanthin then disappeared from all species by 15 months with a concurrent increase in a pigment which we tentatively identify as a cis-zeaxanthin. Incubations of pure cultures are found to be an effective way by which to model the early type-I reactions for both chlorophylls and carotenoids. The influence of oxygen during senescence-death and the onset of early diagenesis is of paramount importance. The absence of oxygen and, by inference, aerobic microbiota, retards the breakdown of these pigments dramatically.     

The Cenomanian/Turonian Boundary Event (CTBE) at Wunstorf, north-west Germany, as reflected by marine palynology

The Cenomanian/Turonian Boundary Event (CTBE) at Wunstorf, north-west Germany, has been analysed palynologically by high resolution sampling to reconstruct changes in relative sea-level and water mass character within photic zone waters. Based on changes in the ratio of terrigenous sporomorphs to marine palynomorphs (t/m index), the distribution of the organic-walled algal taxa as well as of selected dinocyst taxa and groups the section can largely be subdivided into pre-“plenus-bed” and post-“plenus-bed” intervals, reflecting different stages of third-order relative sea-level cycles and/or changes in water mass influence in the photic zone. Accordingly, the pre-“plenus-bed” interval is placed in a transgressive systems tract starting at the “facies change” event (C. guerangeri/M. geslinianum ammonite Zone boundary) with the maximum flooding surface at the top of the “Chondrites II” bed (top of R. cushmani Biozone). A highstand systems tract is suggested from the base of the “plenus-bed” up the base of the “fish-shale” event. Within the “fish-shale” event interval, a transgressive systems tract is suggested to start at the base of the thin, grey-green marly interbed. The Cenomanian/Turonian boundary proper, as defined by the first occurrence of Mytiloides spp., as well as the lowermost Turonian are located within the initial phase of a transgressive systems tract. With respect to water mass characteristics within photic-zone waters, the pre-“plenus-bed” interval is predominantly characterized by warm water masses that changed gradually towards the deposition of the “Chondrites II” bed, where a strong influence of cool and/or salinity-reduced waters is indicated by various palynological proxies. Within the post-“plenus-bed” interval a mixture and/or alternation of warmer and cooler waters is indicated, with the warmer water influence increasing gradually towards and within the Lower Turonian stage. The increased proportions of prasinophytes within the “Chondrites II” bed and parts of the “fish-shale” interval may indicate availability of reduced nitrogen chemospecies, especially ammonium, within photic-zone waters as a function of a vertical expansion of the oceanic O₂-minimum zone.     

The optical features and hydrocarbon-generating model of “barkinite” from Late Permian coals in South China

Leping coal is known for its high content of “barkinite”, which is a unique liptinite maceral apparently found only in the Late Permian coals of South China. “Barkinite” has previously identified as suberinite, but on the basis of further investigations, most coal petrologists conclude that “barkinite” is not suberinite, but a distinct maceral. The term “barkinite” was introduced by (State Bureau of Technical Supervision of the People's Republic of China, 1991, GB 12937-91 (in Chinese)), but it has not been recognized by ICCP and has not been accepted internationally.In this paper, elemental analyses (EA), pyrolysis-gas chromatography, Rock-Eval pyrolysis and optical techniques were used to study the optical features and the hydrocarbon-generating model of “barkinite”. The results show that “barkinite” with imbricate structure usually occurs in single or multiple layers or in a circular form, and no definite border exists between the cell walls and fillings, but there exist clear aperture among the cells.“Barkinite” is characterized by fluorescing in relatively high rank coals. At low maturity of 0.60–0.80%R_o, “barkinite” shows strong bright orange–yellow fluorescence, and the fluorescent colors of different cells are inhomogeneous in one sample. As vitrinite reflectance increases up to 0.90%R_o, “barkinite” also displays strong yellow or yellow–brown fluorescence; and most of “barkinite” lose fluorescence at the maturity of 1.20–1.30%R_o. However, most of suberinite types lose fluorescence at a vitrinite reflectance of 0.50% Ro, or at the stage of high volatile C bituminous coal. In particular, the cell walls of “barkinite” usually show red color, whereas the cell fillings show yellow color under transmitted light. This character is contrary to suberinite.“Barkinite” is also characterized by late generation of large amounts of liquid oil, which is different from the early generation of large amounts of liquid hydrocarbon. In addition, “barkinite” with high hydrocarbon generation potential, high elemental hydrogen, and low carbon content. The pyrolysis products of “barkinite” are dominated by aliphatic compounds, followed by low molecular-weight aromatic compounds (benzene, toluene, xylene and naphthalene), and a few isoprenoids. The pyrolysis hydrocarbons of “barkinite” are mostly composed of light oil (C₆–C₁₄) and wet gas (C₂–C₅), and that heavy oil (C₁₅⁺) and methane (C₁) are the minor hydrocarbon.In addition, suberinite is defined only as suberinized cell walls—it does not include the cell fillings, and the cell lumens were empty or filled by corpocollinites, which do not show any fluorescence. Whereas, “barkinite” not only includes the cell walls, but also includes the cell fillings, and the cell fillings show bright yellow fluorescence.Since the optical features and the hydrocarbon-generating model of “barkinite” are quite different from suberinite. We suggest that “barkinite” is a new type of maceral.     

Solid bitumen reflectance and Rock-Eval Tmax as maturation indices: an example from the “Nordegg Member”, Western Canada Sedimentary Basin

Marine, organic-rich rock units commonly contain little for vitrinite reflectance (VR₀) measurement, the most commoly used method of assessing thermal maturity. This is true of the Lower Jurassic “Nordegg Member”, a type I/II, sulphur-rich source rock from the Western Canada Sedimentary Basin. This study examines the advantages and pitfalls associated with the use of Rock-Eval T_max and solid bitumen reflectance (BR₀) to determined maturity in the “Nordegg”. Vitrinite reflectance data from Cretaceous coals and known coalification gradients in the study area are used to extrapolate VR₀ values for the “Nordegg”.T_max increases non-linearly with respect to both BR₀ and extrapolated VR₀ values. A sharp increase in the reflectaance of both solid bitumen and vitrinite occurs between T_max 440–450°C, and is coincident with a pronounced decrease in Hydrogen Index values and the loss of solid bitumen and telalginite fluorescence over the same narrow T_max interval. This T_max range is interpreted as the main zone of hydrocarbon generation in the “Nordegg”, and corresponds to extrapolated VR₀ values of 0.55–0.85%. The moderate to high sulphur contents in the kerogen played a significant role in determining the boundaries of the “Nordegg” oil window.A linear relationship between BR₀ and extrapolated VR₀, as proposed elsewhere, is not true for the “Nordegg”. BR₀ increases with respect to extrapolated VR₀ according to Jacob's (1985) formula (VR₀=0.618×(BR₀)+0.40) up to VR₀≈0.72% (BR₀≈0.52%). Beyond this point, BR₀ increases sharply relative to extrapolated VR₀, according to the relatioship VR₀ = 0.277 × (BR₀) + 0.57 (R² = 0.91). The break in the BR₀−VR₀ curve at 0.72%VR₀ is thought to signifiy the peak of hydrocarbon generation and represents a previously unrecognized coalification jump in the solid bitumen analogous to the first coalification jump of liptinites.     

Uranium in Phosphorites

The uranium concentration in phosphorites on continents and modern seafloor varies from 0.nto n· 10²ppm (average 75 ppm). The average uranium concentration is 4–48 ppm in Precambrian and Cambrian deposits, 20–90 ppm in Paleozoic and Jurassic deposits, 40–130 ppm in Late Cretaceous–Paleogene deposits, 30–130 ppm in Neogene deposits, and 30–110 ppm in Quaternary (including Holocene) deposits. On the whole, the variation range is almost similar for phosphorites of different ages. The U/P₂O₅ratio in phosphorites ranges from less than unity to 24 · 10^–4(average 3.2 · 10^–4). Major phosphorite deposits of the world with ore reserves of approximately 250 Gt (or 58 Gt P₂O₅) contain up to 19 Mt of uranium. Uranium is present in phosphorites in the tetra- and hexavalent, i.e., U(IV) and U(VI) forms, and their ratio is highly variable. At the early diagenetic stage of the formation of marine phosphorites in a reductive environment, U(VI) diffuses from the near-bottom water into sediments. It is consequently reduced and precipitated as submicroscopic segregations of uranium minerals (mainly uraninite) that are probably absorbed by phosphatic material. During the subsequent reaction between phosphorites and aerated water and the weathering in a subaerial environment, uranium is partly oxidized and lost. The uranium depletion also occurs during catagenesis owing to a more complete crystallization of calcium phosphate and replacement of nonphosphatic components.     

Collection of radon decay products — a uranium exploration technique

The spontaneous deposition of short-lived radon decay products onto solid surfaces (“collectors”) provides the basis for a simple and efficient way of prospecting for uranium. The alpha activity of two of the decay products, ²¹⁸Po and ²¹⁴Po, can be measured by conventional counting techniques following the exposure of a collector to a radon source. Laboratory studies have shown: (a) radon decay products can be collected on a wide variety of materials; (b) the number of radon decay products increases with the collector surface area; (c) a negative charge applied to the collectors enhances the number of decay products collected; (d) the shape of the collectors is relatively unimportant; and (e) reproducibility is about ± 5% of the measured value.Field tests on known uranium anomalies involved suspending collectors for an overnight exposure, in either covered holes or inverted containers buried in soil. Subsequent removal of the collectors was immediately followed by counting the alpha activity. Anomalies were readily detected at three different test sites. Replicate measurements at selected sites both on an hour-to-hour and on a day-to-day basis showed remarkably good agreement. Some anomalies were also outlined using activity measurements from collectors suspended above soil aliquots in sealed containers.Because the half-lives of two of the thoron (²²⁰Rn) decay products are much longer than those of the radon (²²²Rn) decay products, a thoron correction can be applied to the original activity measured from the collectors, thus discriminating against thoron in thorium-rich terrains.The counting of radon decay products deposited onto collectors is an effective method for delineating radon anomalies in uranium exploration. The field techniques are simple to apply, and results are available after very short exposure times (about 18 hours) and after short counting intervals (5 minutes).     

Geochemical exploration for uranium and other metals in tropical and subtropical environments using heavy mineral concentrates

The problems of carriage mechanisms have for long bedevilled the exploration geochemist, and only recently has sequential analysis indicated the relative importance of various mechanisms. A commonly held view is that metals are somehow scavenged by clay minerals. This is demonstrably not so. There is no “plating” process possible. The importance of coatings and heavy mineral species is illustrated with reference to existing literature and other short case studies.A general view of geochemistry in mineral exploration is presented, with particular reference to uranium exploration in the wet tropical environment. Examples are taken from Indonesia and Sri Lanka. These are complemented by results of various short projects in North Queensland, New South Wales and in Timor. A variety of elements has been examined, notably Au, Sn, Ag, As and Cu, Pb, Zn and some of the more interesting behavioural characteristics have been reported.These data indicate that a great enhancement of the anomalies for most of these elements can be expected from heavy mineral concentrates, relative to the convential minus 80 mesh fraction or the like. In fact, U, Sn and Au anomalies may be lost in the sieving process. Many failures of exploration geochemistry may be explained by referring to the role of heavy minerals. Resampling of areas, which have not originally shown anomalies with conventional methods, has produced anomalies using heavy minerals. Hitherto unsuspected gold provinces have been shown to exist, and areas within historically goldbearing regions have been found to be anomalous where no pre-existing workings occurred.Previous areas not showing positive results might well be favourably resampled. The elements from a weathering deposit must go somewhere. It is not the principle which fails, but the failure to select the most suitable medium or the most appropriate or enlightened analytical technique.Some careful field procedures are described and a few practical hints are given, predominantly with reference to broad reconnaissance and early follow-up phases of exploration, (the “super-stratégique” and “stratégique” phases).It is suggested that combinations of techniques will produce a better understanding of elemental dispersal patterns. No single approach can be expected to be adequate considering the number of parameters — some not fully understood — which operate on the sampling medium.     

A study of the kinetic parameters of individual macerals from Upper Permian coals in South China via open-system pyrolysis

A unique Upper Permian coal, Leping coal, is widely distributed in South China. The coal samples studied in the paper were collected from two mines in the Shuicheng coalfield of Guizhou Province, southwest China. The geochemical works including coal petrography, maceral content, Rock–Eval pyrolysis, and kinetic modelling of hydrocarbon-generating have been carried out on whole coal and individual macerals. The higher contents of volatile matter, elemental hydrogen, and tar yield, and the high hydrocarbon generation potential of the Leping coals are attributed to their high content of “barkinite”, a special liptinite maceral.The hydrocarbon generation potential of “barkinite” (S₂=287 mg/g, hydrogen index (HI)=491 mg/g TOC) is greater than that of vitrinite (S₂=180 mg/g, HI=249 mg/g TOC), and much higher than that of fusinite (S₂=24 mg/g, HI=35 mg/g TOC). At the same experimental conditions, “barkinite” has a higher threshold and a narrower “oil window” than those of vitrinite and fusinite, and consequently, can generate more hydrocarbons in higher coalification temperature and shorter geological duration. Data from the activation energy distributions indicate that “barkinite” has a more homogenous chemical structure than that of vitrinite and fusinite. The above-mentioned characteristics are extremely important for exploring hydrocarbon derived from the Leping coals in South China.     

Ductile faulting, dynamic recrystallization and grain-size-sensitive flow of olivine

Twiss (1976) has suggested that the “ductile faulting” events observed by Post (1973) during high temperature creep of dunite are due to a transition from creep by dislocation movement to a diffusion accommodated, grain-boundary sliding mechanism following a reduction in grain size by dynamic recrystallization. Similarly, Goetze (1978) has explained both ductile faulting and water weakening of dunite by transition to a “nonlinear Coble” creep mechanism. However, the fundamental assumption made by Twiss (1976) that the stress exponent, n, reduces to unity during ductile faulting events is questionable. If the stress exponent remains high, (n≥3), then a diffusion-accomodated grain-boundary sliding mechanism is excluded. “Nonlinear Coble” creep would remain a viable alternative; however, this model fails to adequately explain the water weakening phenomenon, and the available data do not constrain us to this model. Assuming that the water-weakening phenomenon can be explained by other models (e.g., Blacic, 1972), it will be shown (by analogy with the behavior of metals) that a third model, also consistent with the available data, also qualitatively explains the observations associated with ductile faulting without appeal to a transition in creep mechanisms. The model is similar to one for metals undergoing deformation by dislocation movement and recovery by dynamic recrystallization, which commonly exhibit behavior virtually identical to that observed in dunite during ductile faulting events without transition to grain-size-sensitive creep mechanisms.