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961.
The first occurrence of platinum group minerals (PGM) in a chromite deposit in the Eastern Desert, Egypt 总被引:1,自引:0,他引:1
M. A. Elhaddad 《Mineralium Deposita》1996,31(5):439-445
The platinum-group mineralogy (PGM) of the chromitite from Gebel Lawi, in the southeastern desert has been investigated.
The most abundant base metal sulfides (BMS) associated with the Lawi chromite are pentlandite, millerite and heazlewoodite.
The major platinum-group minerals identified were as follows: laurite (IrOsRu)S2, osmian iridium (OsIr), hollingworthite (RhAsS),
tellurian arsenopalladinite (PdTeSbAs), potarite (PdHg) besides cuprian palladian gold (CuPdAu), a Pd-Sb-Hg and HgTe phases.
Laurite and osmian iridium occur preferentially in chromite. Os-Ir commonly forms composite PGM with laurite. Hollingworthite
and tellurian arsenopalladinite are included within serpentine and, close to the base-metal sulfides, the cuprian palladian
gold shares boundaries with chromite. Potarite together with the Pd-Sb-Hg and HgTe phases are embedded in serpentine. Palladium
is the most abundant PGE in the Gebel Lawi chromite.
A paragenetic sequence of PGM formation is described. Textural evidence indicates that Os-, Ir- and Ru-bearing PGM formed
early and were followed by Rh- and Pd-bearing PGM. The concentration of all five PGE could be magmatic, but much of the PGE
mineralogy except for laurite and osmian iridium in the center of chromite grains, has been modified by subsequent processes.
At later stages, the environment became Te-, Sb-, As- and Hg-rich, which finally led to the formation of low-temperature alteration
minerals.
Received: 24 April 1995 / Accepted: 28 March 1996 相似文献
962.
Polycyclic aromatic hydrocarbons (PAH), important constituents of petroleum products, are known to enter freshwater systems via industrial effluents. In the present study the possible toxicity of automobile effluent presumably containing PAH was demonstrated in the freshwater prawn Macrobrachium lamarrei. The prawns were exposed for 1, 8, 15 and 28 days to two sublethal concentrations (0.75% and 3.75%) of automobile effluent. A decrease was noted in the total protein content in haemolymph and hepatopancreas of prawns exposed for 28 days to both these sublethal concentrations. SDS-PAGE revealed that the effluent caused reduction in high molecular weight proteins (106-90 kD) of the haemolymph. The exposure to automobile effluent resulted in changes in the lamellar structure of the gills and shrinkage of hepatopancreatic cell types. Thus it is imperative to discard such wastes carefully ensuring that there is no entry into aquatic ecosystems.author for correspondence 相似文献
963.
N. V. Sobolev F. V. Kaminsky W. L. Griffin E. S. Yefimova T. T. Win C. G. Ryan A. I. Botkunov 《Lithos》1997,39(3-4):135-157
The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb/Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO2 content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P-T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m2 conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle. 相似文献
964.
965.
Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil 总被引:1,自引:0,他引:1
S. A. Gibson R. N. Thompson R. K. Weska A. P. Dickin O. H. Leonardos 《Contributions to Mineralogy and Petrology》1997,126(3):303-314
High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser 40Ar/39Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (87Sr/86Sr?=?0.704 and 143Nd/144Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C. 相似文献
966.
Sources of Water Supplying Pumpage from Regional Aquifer Systems of the United States 总被引:1,自引:0,他引:1
Richard H. Johnston 《Hydrogeology Journal》1997,5(2):54-63
967.
968.
969.
P. G. Ditmar 《Pure and Applied Geophysics》1997,150(2):269-284
—This numerical study is devoted to distortions of local anomalies which are revealed by 2-D and 3-D cross-borehole seismic tomography based on first arrival travel times. The fact that prolonged subvertical anomalies may disappear in the final velocity model is well known. But distortions are also inherent to images of local inhomogeneities. These distortions are accompanied with false anomalies of the opposite sign located just above and below true ones. I offer a semi-empirical explanation of their existence, proving that they are an inherent consequence of observation geometry incompleteness. To improve the quality of the model obtained with cross-borehole tomography, a 3-D acquisition geometry may be used when additional boreholes are positioned around the target object. Results of 3-D tomographic experiments however may be considerably distorted for another reason nonaccounted bending of seismic rays.¶To generate the travel times, a ray-tracing procedure making use of the graph theory was applied. Tomographic inversion was performed by an algorithm based on the assumption of the unknown function smoothness. 相似文献
970.