[en] Our study extends the petrographic zonation of the devitrified rhyolitic tuff matrix of the Topopah Spring Member of the Paintbrush Tuff observed in USW G-4 to four other cored holes in the Yucca Mountain area of the Nevada Test Site: UE-25a No. 1, USW G-1, USW G-2, and USW GU-3. The four petrographic zones occur above the basal vitrophyre and in ascending order are the lower nonlithophysal (ln); the lower lithophysal (ll); the middle nonlithophysal (mn); and the upper lithophysal (ul). Drill hole USW G-2, about two miles north of the Yucca Mountain Exploratory Block, differs significantly from the other four cored holes within or near the block; it has essentially one thick microlitic zone, largely lithophysal, above the vitrophyre. Textural attributes (from coarsest to finest) are phenocrysts, lithics, granophyre, amygdules, spherulites, and cryptocrystalline groundmass. Among individual phenocrysts, only quartz shows significant decrease upward. The four petrographic zones agree fairly well with those defined by contacts placed by USGS geologists and, with minor reservations, can be correlated between the four cored holes in the vicinity of the exploration block. The ln zone is characterized by dense welding, upwardly decreasing cryptocrystallinity, common lithics, and quartz phenocrysts. The ll zone is largely spherulitic with 1 to 13% granophyre generally increasing upward and shard texture becoming less distinct upward. The mn zone is similar to the ln zone except for the moderate welding and fewer quartz and lithic fragments present in the mn zone. The ul and ll zones are similar in microscopic texture, but the ul has more amygdules with tridymite rather than cristobalite and can generally be recognized by its ''Swiss cheese'' appearance in core or hand specimens. A series of discriminatory statistical analyses were made with the thin section modal data to test variation in textural type and quartz phenocrysts

[en] The activity of the Department of Geology, Petrology and Geochemistry of the University of Liege, Belgium, is discussed. (author)

[en] A pulsed neutron generator in a well logging instrument is pulsed at a clock frequency of 20 KHz. Inelastic scatter gamma rays are detected during a first time interval coinciding with the neutron source being on and capture gamma rays are measured during a second interval subsequent to the end of each neutron burst. Only a single detected pulse, assuming detection occurs, is transmitted during each of the two detection intervals. Sync pulses are generated in the well logging instrument scaled down to a frequency of 200 Hz for transmission to the earth's surface. At the earth's surface, the scaled-down sync pulses are applied to a phase-locked loop system for regenerating the sync pulses to the same frequency as that of the clock frequency used to pulse the neutron source and to open the detection gates in the borehole instrument. The regenerated sync pulses are used in the surface instrumentation to route the pulses occurring in the inelastic interval into one section of a multichannel analyzer memory and the pulses occurring in the capture interval into another section of the multichannel analyzer. The use of memory address decoders, subtractors and ratio circuits enables both a carbon/oxygen ratio and a silicon/calcium ratio to be struck, substantially independent of the chlorine content of the borehole and formation

[en] The feldspars of many tonalitic plutonic rocks in the coastal regions and West Andean regions are zoned. This leads to the conclusion that they are relatively flat intrusions and to some extent transition rocks in the subvulcanite direction. This is in accordance with the genetic and chronological relationship between plutonites and the surrounding vulcanites of the Basic Igreous Complex (BIC). The composition of representative minerals, e.g. alkali feldspar, plagioclase feldspar, biotite, chlorite, and amphibole has been determined as well as the age of plutonite samples by the K/Ar dating method. (DG)

[en] Results of rapid chemical, spectrographic, scanning electron microscope and neutron activation analyses for U are discussed in relation to metamorphism, recrystallization, replacement textures and mineralization of hybrid rocks collected at mine sites in the Bancroft area. The samples represent the following: 1. gneisses, migmatites and granitoid rocks in and adjacent to the Faraday granite; 2. Cheddar granite, mylonite and surrounding gneisses with sedimentary textures; 3. pegmatite and adjacent high grade metamorphic rocks surrounding Halo mine; 4. selected ore specimens from the Madawaska, Greyhawk, Bicroft and Halo mines, and their relation to petrographic-textural characteristics and ore and accessory mineral associations. Relicts of diverse metasedimentary rocks, prophyroblastic and anatectic textures, impregnations, mylonitiation and alteration account for the mineralogical and petrochemical diversity of the hybrid rocks. With a few exceptions which contain radioactive accessory minerals, most of the metasediments contains less than 4 ppm U and are unlikely sources for the uranium mineralization

[en] Analyses of clay and chalk layers in the Linde salt dome are presented. (BP)

[en] This paper discusses the petromineralogical aspects of uranium mineralization at Deshnur in the Kaladgi-Badami Basin, Belgaum district, Karnataka. The mineralization occurs as veins and veinlets along fractures in Badami arenites of Kerur formation. Alteration features like kaolinisation of feldspars is pronounced in the area. The sediments display deformational features like pulverization, fracturing, elongation and undulose extinction. Boehm twin lamellae are observed in quartz grains. The basement rocks of Deshnur display intense shearing and brecciation. Seven radioactive phases have been identified from petromineralogical studies. They are pitted uraninite, pitchblende, secondary uranium minerals along fracture planes and intergranular spaces, labile uranium present along grain boundaries, adsorbed uranium in kaolinite and substitution in refractory minerals like zircon. The presence of pyrite and pitchblende along fractures and intergranular spaces indicates that the mineralization at Deshnur is hydrothermal epigenetic type. From the mineral assemblage and textural associations, it is inferred that the temperature of mineralization is epi-to mesothermal in the range of 1000 to 3000 deg C and is fracture controlled. From the textural relationships an attempt has been made to understand the paragenetic sequence of the minerals. (author)

[en] A system of marine plateaus occurs in the western equatorial Indian Ocean, forming an arcuate series of wide and shallow banks with small islands in places. The oceanic basins that surround the Seychelles - Amirante region are of various ages and reflect a complex seafloor spreading pattern. The structural analysis of the Seychelle - Amirante - Mascarene region reflects the tectonic evolution of the western equatorial Indian Ocean. It is suggested that due to the seafloor spreading during a tectonic stage, the Seychelles continental block drifted southwestwards to collide with the oceanic crust of the Mascarene Basin, forming an elongated folded structure at first, and then a subduction zone. The morphological similarity, the lithological variability and the different origin of the Seychelles Bank, the Mascarene Plateau and the Amirante Arc emphasizes the significant convergent effects of various plate tectonic processes on the development of marine plateaus

[en] The petrological, geochemical and geochronological studies allow some insight on the genesis and evolution of the Musa Massif. The different facies of the granitic body are cogenetic, although each of these facies presents some peculiarities in its genesis and evolution. These data suggests that the granite magma evolution was complex or, alternatively, that the facies were generated by liquids derived from different sources. A model of magmatic emplacement, genesis and differentiation is proposed and discussed. The granitic facies show a calc-alkaline compositions, exhibiting strong analogies with cordilleran granites or magnetite granites. An age of 1692 +- 11 Ma (Rb/Sr) with IR of 0,70777 +- 0,00023 was obtained for different facies of Massif. A preliminary attempt to individualize geochronology the principal facies was done and showed that there is a coincidence between the ages and the emplacement sequence of these facies of the pluton. (author)

[en] A methodology for computing the gamma-ray source yield from the ground using an airborne NaI(Tl) detector is discussed. The results have been obtained for various thicknesses of the ground of varying densities of material which contribute to the gamma-ray signal. The source yield has also been computed for a given circle of investigation of the ground in terms of the ground clearance of the detector. Curves have been presented for the percentage source yield for different radii of the circles of investigation for different attenuation coefficients of gamma-rays which encompass a wide range of energies (1-4 MeV). Similarly, curves have been plotted for infinite source yield for different flying heights for two extreme densities of ground. The source yield vs. the product of density and depth of ground penetration for various attenuation coefficients of gamma-rays for the extreme densities (1.4 and 2.6 g cm^-3) of the ground have also been plotted. 6 refs.; 8 figs