Electrochemical technique for exploration of base metal sulphides

A method based on the principles of electrochemistry has been applied for the first time with indigenous equipment in the search for base-metal sulphides, following a field exploration technique known as ‘CHIM — 10’, which is in use in the U.S.S.R. for the last few years. The technique utilizes the electro-positive property of the metallic ions. When a large section of the ground is artificially electrolyzed by the introduction of a direct current, the metallic ions within the moist soil profile close to the mineralized zone tend to move towards the energizing cathodes of a powerful widely spaced electrode array. A laboratory model experiment was conducted first which showed encouraging results. Subsequently, a prototype field unit was fabricated and employed to examine its effectiveness in the field on a soil profile over the base-metal sulphide zones of Aladahalli area, Hassan District, Karnataka, where the geology of the deposit is well known.The results obtained during the field trials were quite good and in full conformity with the known geology. However, detailed evaluation of those data after applying necessary corrections for the different variable parameters of the system is yet to be done. Work is in progress to refine the technique with a view to standardizing and developing its use for qualitative and quantitative assessment of a concealed base-metal deposit in a given area, specially where conventional geochemical exploration methods are not suitable.The paper deals with the details of the technique and discusses the results obtained during the trial run in the field.     

Snow measurement techniques for land-surface-atmosphere exchange studies in boreal landscapes

Summary  Snow has been studied widely in hydrology for many decades whereas recent meteorological interest in snow is caused by increased emphasis on high latitudes and wintertime in climate-change research as well as by the need to improve weather-forecast models during these conditions. Ground-based measurements of snow properties are needed both to improve understanding of surface-atmosphere exchange processes and to provide ground truth to new remote-sensing algorithms. This justifies a review of techniques to measure snow in combination with establishment of criteria for the suitability of the methods for process studies. This review assesses the state-of-art in ground-based snow-measurement techniques in the end of the 1990s in view of their accuracy, time resolution, possibility to automate, practicality and suitability in different terrain. Methods for snow-pack water equivalent, depth, density, growth, quality, liquid-water content and water leaving the snow pack are reviewed. Synoptic snow measurements in Fennoscandian countries are widely varying and there is no single standard on which process-related studies can build. A long-term, continuous monitoring of mass and energy properties of a snow cover requires a combination of point-measurement techniques. Areally representative values of snow properties can be achieved through a combination of automatically collected point data with repeated manual, areally covering measurements, remote-sensing data and digital elevation models, preferably in a GIS framework. Received August 27, 1999     

An Aerosol Model of the Marine and Coastal Atmospheric Surface Layer

We present a microphysical model for the surface layer marine and coastal atmospheric aerosols that is based on long-term observations of size distributions for 0.01–100 µm particles. The fundamental feature of the model is a parameterization of amplitudes and widths for aerosol modes of the aerosol size distribution function (ASDF) as functions of fetch and wind speed. The shape of ASDF and its dependence on meteorological parameters, height above sea level (H), fetch (X), wind speed (U) and relative humidity (RH), are investigated. At present, the model covers the ranges H = 0–25 m, U?=?3–18 km s^?1, X?≤?120 km and RH?=?40–98%.

The latest version of the Marine Aerosol Extinction Profiles model (MaexPro) is described and applied to the computation and analysis of the spectral profiles of aerosol extinction coefficients α(λ) in the wavelength band λ?=?0.2–12 µm. MaexPro is based on the aforementioned aerosol model assuming spherically shaped aerosol particles and the well-known Mie theory.

The spectral profiles of α(λ) calculated by MaexPro are in good agreement with observational data and the numerical results obtained from the Navy Aerosol Model (NAM) and the Advanced Navy Aerosol Model (ANAM). Moreover, MaexPro was found to be an accurate and reliable tool for investigating the optical properties of atmospheric aerosols.     

The Removal of Dissolved Metals by Hydroxysulphate Precipitates during Oxidation and Neutralization of Acid Mine Waters, Iberian Pyrite Belt

This study examines the removal of dissolved metals during the oxidation and neutralization of five acid mine drainage (AMD) waters from La Zarza, Lomero, Esperanza, Corta Atalaya and Poderosa mines (Iberian Pyrite Belt, Huelva, Spain). These waters were selected to cover the spectrum of pH (2.2–3.5) and chemical composition (e.g., 319–2,103 mg/L Fe; 2.85–33.3 g/L SO₄⁼) of the IPB mine waters. The experiments were conducted in the laboratory to simulate the geochemical evolution previously recognized in the field. This evolution includes two stages: (1) oxidation of dissolved Fe(II) followed by hydrolysis and precipitation of Fe(III), and (2) progressive pH increase during mixing with fresh waters. Fe(III) precipitates at pH < 3.5 (stages 1 and 2) in the form of schwertmannite, whereas Al precipitates during stage 2 at pH 5.0 in the form of several hydroxysulphates of variable composition (hydrobasaluminite, basaluminite, aluminite). During these stages, trace elements are totally or partially sorbed and/or coprecipitated at different rates depending basically on pH, as well as on the activity of the SO₄⁼ anion (which determines the speciation of metals). The general trend for the metals which are chiefly present as aqueous free cations (Pb²⁺, Zn²⁺, Cu²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺) is a progressive sorption at increasing pH. On the other hand, As and V (mainly present as anionic species) are completely scavenged during the oxidation stage at pH < 3.5. In waters with high activities (> 10⁻¹) of the SO ₄⁼ ion, some elements like Al, Zn, Cd, Pb and U can also form anionic bisulphate complexes and be significantly sorbed at pH < 5. The removal rates at pH 7.0 range from around 100% for As, V, Cu and U, and 60–80% for Pb, to less than 20% for Zn, Co, Ni and Mn. These processes of metal removal represent a significant mechanism of natural attenuation in the IPB.     

Origin and evolution of the Steamboat Springs siliceous sinter deposit, Nevada, U.S.A.

Siliceous hot spring deposits from Steamboat Springs, Nevada, U.S.A., record a complex interplay of multiple, changing, primary environmental conditions, fluid overprinting and diagenesis. Consequently these deposits reflect dynamic geologic and geothermal processes. Two surface sinters were examined—the high terrace, and the distal apron-slope, as well as 13.11 m (43 ft) of core material from drill hole SNLG 87-29. The high terrace sinter consists of vitreous and massive-mottled silica horizons, while the distal deposit and core comprise dominantly porous, indurated fragmental sinters. Collectively, the three sinter deposits archive a complete sequence of silica phase diagenetic minerals from opal-A to quartz. X-ray powder diffraction analyses and infrared spectroscopy of the sinters indicate that the distal apron-slope consists of opal-A and opal-A/CT mineralogy; the core yielded opal-A/CT and opal-CT with minor opal-A; and the high terrace constitutes opal-C, moganite, and quartz. Mineralogical maturation of the deposit produced alternating nano–micro–nano-sized silica particle changes. Based on filament diameters of microbial fossils preserved within the sinter, discharging thermal outflows fluctuated between low-temperatures (< 35 °C, coarse filaments) and mid-temperatures ( 35–60 °C, fine filaments). Despite transformation to quartz, primary coarse and fine filaments were preserved in the high terrace sinter. AMS ¹⁴C dating of pollen from three horizons within core SNLG 87-29, from depths of 8.13 to 8.21 m (26′8″ to 26′11″), 10.13 to 10.21 m (33′3″ to 33′6″), and 14.81 to 14.88 m (48′7″ to 48′10″), yielded dates of 8684 ± 64 years, 11,493 ± 70 years and 6283 ±60 years, respectively. In the upper section of the core, the stratigraphically out-of-sequence age likely reflects physical mixing of younger sinter with quartzose sinter fragments derived from the high terrace. Within single horizons, mineralogical and morphological components of the sinter matrix were spatially patchy. Overall, the deposit was modified by sub-surface flow of alkali-chloride thermal fluids depositing a second generation of silica, and periodically, by acidic steam condensate formed during periods when the water table was low. Local faulting produced considerable fracturing of the sinter. Hence, the Steamboat Springs sinter experienced a complex history of primary and secondary hydrothermal, geologic and diagenetic events, and their inter-relationships and effects are locked within the physical, chemical and biological signatures of the deposit.     

Geochemistry and Crustal Evolution of Volcano-sedimentary Successions and Orthogneisses in the São Gabriel Block, Southernmost Brazil — Relics of Neoproterozoic Magmatic Arcs

Precambrian metaplutonic rocks of the São Gabriel block in southernmost Brazil comprise juvenile Neoproterozoic calc-alkaline gneisses (Cambaí Complex). The connection with associated (ultra-)mafic metavolcanic and metasedimentary rocks (Palma Group) is not well established. The whole complex was deformed during the Brasiliano orogenic cycle. Both metasedimentary and metavolcanic rocks as well as metaplutonic rocks of the Cambaí Complex have been sampled for geochemical analyses in order to get constraints on the tectonic setting of these rocks and to establish a tectonic model for the São Gabriel block and its role during the assembly of West-Gondwana. The major element compositions of the igneous rocks (Palma Group and Cambaí Complex) indicate a subalkaline character; most orthogneisses have a calc-alkaline chemistry; many metavolcanic rocks of the Palma Group show signatures of low-K tholeiitic volcanic arc basalts. Trace element data, especially Ti, Zr, Y, Nb, of most igneous samples from both the lower Palma Group and the Cambaí Complex indicate origin at plate margins, i.e., in a subduction zone environment. This is corroborated by relative enrichment in LREE, low contents of Nb and other high field strength elements and enrichment in LILE like Rb, Ba, and Th. The data indicate the possible existence of two suites, an oceanic island arc and a continental arc or active continental margin. However, some ultramafic samples of the lower Palma Group in the western São Gabriel block indicate the existence of another volcanic suite with intra-plate character which possibly represents relics of oceanic island basalts (OIB). Trace element data indicate contributions from andesitic to mixed felsic and basic arc sources for the metasedimentary rocks. The patterns of chondrite- and N-MORB-normalized spider diagrams resemble the patterns of the igneous rocks, i.e., LILE and LREE enrichment and HFS depletion. The geochemical signatures of most igneous and metasedimentary samples and their low (⁸⁷Sr/⁸⁶Sr)_t ratios suggest only minor contribution of old continental crust.A geotectonic model for the São Gabriel block comprises east-ward subduction and following accretion of an intra-oceanic island arc to the eastern border of the Rio de la Plata Craton at ca. 880 Ma, and westward subduction beneath the newly formed active continental margin between ca. 750 and 700 Ma. The São Gabriel block represents relics of an early Brasiliano oceanic basin between the Rio de la Plata and Kalahari Cratons. This ocean to the east of the Rio de la Plata Craton might be traced to the north and could possibly be linked with Neoproterozoic juvenile oceanic crust in the western Brasília belt (Goiás magmatic arc).     

Kinetic study of the kaolinite-mullite reaction sequence. Part I: Kaolinite dehydroxylation

The decomposition reaction of kaolinite has been investigated as a function of the defectivity of the starting material and the temperature of reaction. Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation, both under a constant heating rate (heating rates from 10 to 100° C/min) and in isothermal conditions (in the temperature range 500 to 700° C). The apparent activation energy of the dehydroxylation process is different for kaolinites exhibiting a different degree of stacking fault density. The results of the analysis of the kinetic data indicate that the starting reaction mechanism is controlled by diffusion in the kaolinite particle. The diffusion process is dependent on the defective nature of both kaolinite and metakaolinite. At high temperatures, and at higher heating rates, the reaction mechanism changes and the resistance in the boundary layer outside the crystallites becomes the rate-limiting factor, and nucleation begins within the reacting particle. During the final stage of the dehydroxylation process the reaction is limited by heat or mass transfer, and this might be interpreted by the limited diffusion between the unreacted kaolinite domains and the metakaolinite matrix.     

Structural and petrological evidence for the continuation of the Isfahan fault system across the Urumieh-Dokhtar zone of central Iran

The Isfahan fault system, a north-trending, dextral strike-slip fault across the Sanandaj-Sirjan zone, represents the boundary between the northwestern and the southeastern parts of the Sanandaj-Sirjan zone and it terminates in the north at the southern boundary of the Urumieh-Dokhtar zone. This paper focuses on the continuation of the Isfahan fault system across the Urumieh-Dokhtar zone north of Isfahan city. The Urumieh-Dokhtar magmatic assemblage belt is located along the active margin of the Iranian plate and the Arabian plate. The Karkas fault strikes nearly north-south, has a length of about 40 km, a normal component of movement, and it truncates the Urumieh-Dokhtar zone. Due to the location of this fault and the mechanism similar to the Isfahan fault system, the Karkas fault can be considered a continuation of the Isfahan fault system that has been displaced dextrally by the southwestern bordering faults of the Urumieh-Dokhtar zone. The unique juxtaposition association of the Silurian volcanic rocks in the Urumieh-Dokhtar zone, near the Karkas fault, provides an important evidence regarding the major role of this fault in the geological evolution of the region. The Silurian volcanic rocks outcrop in two districts of the study area and generally are composed of basalts. The alkaline basalt composition is determined from mineralogy and immobile elements geochemistry. The geotectonic setting diagrams classified the Silurian volcanic rocks as the within plate basalts. Thus, an intracontinental rifting under extensional tectonic regime can be inferred as the setting that controlled formation of these volcanic rocks. They were created by an alkaline to transitional magmas generated due to low partial melting at depth. The alkaline basalts were most likely derived from an asthenosphere-dominated mantle source due to extension and partial melting. The north trending extensional faults affected thinned overlying continental lithosphere in the Paleozoic era, facilitating magma penetration and eruption.     

Scandium,chromium, iron,cobalt and nickel in some basic and ultrabasic rocks from west Norway

Sixteen eclogites and ultrabasic rock samples from west Norway have been analyzed for Sc, Cr, Fe, Co and Ni by instrumental neutron-activation analysis.In gneiss-eclogites, Sc and Co give values from about 30 to 60 ppm and Cr and Ni from 180 to 700 ppm. An eclogite surrounded by a peridotite body shows nearly the same content of Sc and Co as do the gneiss-eclogites, but up to 20 times more Cr. Ni is only slightly more enriched in this particular eclogite than in the others.The ultrabasites have only a few ppm Sc, Co concentrates around 100 ppm while the Cr and Ni values are mainly found between 2 and 3,000 ppm.Fe shows an average value of nearly 10%.