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Geology topics

G. A. Desborough

Publications and source records attributed to G. A. Desborough.

At least 37 records · Page 2Linked to original sources
USGS1992

A monoclinic, pseudo-orthorhombic Au-Hg mineral of potential economic significance in Pleistocene Snake River alluvial deposits of southeastern Idaho

A mineral with the approximate composition of Au94Hg6 - Au88Hg12 (atomic %) has been identified in Pleistocene Snake River alluvial deposits. The gold-mercury mineral occurs as very small grains or as polycrystalline masses composed of subhedral to nearly euhedral attached crystals. Vibratory cold-polishing techniques with 0.05-??m alumina abrasive for polished sections revealed a porous internal texture for most subhedral crystals after 48-72 hours of treatment. Thus, optical character (isotropic or anisotropic) could not be determined by reflected-light microscopy, and pore-free areas were too small for measurement of reflectance. X-ray-diffraction lines rather than individual reflections (spots), on powder camera X-ray films of unrotated spindles of single grains that morphologically appear to be single crystals, indicate that individual subhedral or euhedral crystals are composed of domains in random orientation. Thus, no material was found suitable for single-crystal X-ray diffraction studies. -from Authors

Canadian Mineralogist↗
USGS1976

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

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. ?? 1976.

Chemical Geology↗
USGS1976

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 206 Pb/ 204 Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the 206 Pb/ 204 Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the 207 Pb/ 204 Pb vs 204 Pb/ 204 Pb 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 87 Sr/ 86 Sr 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.

Geochimica et Cosmochimica Acta↗
USGS1975

The Pikes Peak batholith, Colorado front range, and a model for the origin of the gabbro-anorthosite-syenite-potassic granite suite

This study of the Pikes Peak batholith includes the mineralogy and petrology of quartz syenite at West Creek and of fayalite-bearing and fayalite-free biotite granite near Mount Rosa; major element chemistry of the batholith; comparisons with similar postorogenic, intracratonic, sodic to potassic intrusives; and genesis of the batholith. The batholith is elongate in plan, 50 by 100 km, composite, and generally subalkalic. It was emplaced at shallow depth 1,040 m. y. ago, sharply transects its walls and may have breached its roof. Biotite granite and biotite—hornblende granite are predominant; quartz syenite, fayalite granite and riebeckite granite are present in minor amounts. Fayalite-bearing and fayalite-free quartz syenite, fayalite-biotite granite and riebeckite granite show a well-defined sodic differentiation trend; the less sodic fayalite-free granites exhibit a broader compositional range and no sharp trends. Crystallization was largely at P H2O < P total ; P H2O approached P total only at late stages. Aplite residual to fayalite-free biotite granite in the north formed at about 1,500 bars, or 5 km depth. Feldspar assemblages indicate late stages of crystallization at about 720°C. In the south ilmenite and manganian fayalite indicate f O2 of 10 −17 or 10 −18 bars. Biotite and fayalite compositions and the ‘granite minimum’ imply completion of crystallization at about 700°C and 1,500 bars. Nearby fayalite-free biotite granite crystallized at higher water fugacity. All types of syenite and granite contain 5–6% K 2 O through a range of SiO 2 of 63–76%. Average Na 2 O percentages in quartz syenite are 6.2, fayalite granite 4.2, and fayalite-free granite 3.3 MgO contents are low, 0.03–0.4%; FeO averages 1.9–2.5%. FeO/Fe 2 O 3 ratios are high. Fluorine ranges from 0.3 to 0.6%. The Pikes Peak intrusives are similar in mode of emplacement, composition, and probably genesis to rapakivi intrusives of Finland, the Younger Granites of Nigeria, Cape Ann Granite and Beverly Syenite, Mass., and syenite of Kungnat, Greenland, among others — allowing for different levels of erosion. A suite that includes gabbro or basalt, anorthosite, quartz syenite, fayalite granite, riebeckite granite, and biotite and/or hornblende granites is of worldwide occurrence. A model is proposed in which mantle-derived, convecting alkali olivine basaltic magma first reacts with K 2 O-poor lower crust of granulite facies to produce magma of quartz syenitic composition. The syenitic liquid in turn reacts with granodioritic to granitic intermediate crust of amphibolite facies to produce the predominant fayalite-free biotite and biotite-hornblende granites of the batholith. This reaction of magma and roof involves both partial melting and the reconstitution and precipitation of refractory phases, as Bowen proposed. Intermediate liquids include MgO-depleted and Na 2 O-enriched gabbro, which precipitated anorthosite, and alkali diorite. The heat source is the basaltic magma; the heat required for partial melting of the roof is supplied largely by heats of crystallization of phases that settle out of the liquid — mostly olivine, clinopyroxene and plagioclase.

Colorado↗
USGS1971

Primary distribution of silver and copper in native gold from six deposits in the Western United States

Electron-microprobe analyses and mineragraphic studies of native gold demonstrate considerable variations in the primary intergrain and intragrain distribution of silver. The gold grains have from 1–55 weight percent silver; copper is present in grains from only one locality and ranges from 0.1–0.6 weight percent. Some gold grains have strong zoning of silver whereas others have no detectable zoning. Gold grains from some deposits show remarkable intergrain homogeneity of silver and/or copper content, but others exhibit extreme heterogeneity. We believe that the inhomogeneities and variations in silver content recognized and emphasized here are features of primary deposition. We also recognize low-silver rims with sharp boundaries bordering many of the grains examined but believe these are developed in a relatively oxidizing, low-temperature environment and are not primary lode features. Opaque mineral inclusions of primary origin in gold grains are common in some deposits, scarce in many, and virtually absent from others. These inclusions may be of value in characterizing some gold deposits. For the majority of gold crystals from Copper Basin, Arizona, the lowest silver content observed was in the central portion of each grain and the highest silver content was in the rim. This is believed to be due an increase in the proportion of silver to gold in solution during growth of the crystals. Analysis of sized fractions of 331 gold grains from Pennsylvania Mountain, Colorado, shows no systematic correlation of grain size with silver content. Electron microprobe step-scanning of gold from Alder Gulch, Montana, suggests more than one mineralization event took place. Pyrite and acanthite inclusions less than 0.05 mm in the largest dimension, are present in some grains from this deposit. Inclusions of pyrite, pyrrhotite, chalcopyrite, and an isotropic Co-As-S mineral are present in the low-silver, copper-bearing gold from Ninemile Creek, Montana. The presence of copper and the low silver content in this gold is not typical of the gold-quartz-pyrite association which is common in the Western United States.

Arizona, Colorado, Montana, South Dakota, Washingt↗
USGS1968

Mineralogy of sulfides from certain Hawaiian basalts

Polymineralic sulfide grains, composed principally of Fe sulfide and Fe-Cu sulfide, with magnetite, have been studied mineragraphically and by electron probe, and interpreted in terms of experimental data for the system Fe-Ni-Cu-S. The three main phases are monosulfide solid solution, a Cu-Fe sulfide (solid solution) with composition near cubanite, and Ti-free magnetite. The grains are believed to represent phases unmixed from an immiscible liquid phase in the basalt magma. Compositions of the two main sulfide phases suggest quenching below 700 degrees C. Most of the Ni has been retained in the monosulfide solid solution by rapid quenching.

Hawaii↗