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871.
José María González-Jiménez Fernando Gervilla Thomas Kerestedjian Joaquín Antonio Proenza 《Resource Geology》2010,60(4):315-334
The Dobromirtsi Ultramafic Massif, located in the Rhodope Mountains (SE Bulgaria), is a portion of a Paleozoic sub-oceanic mantle affected by polyphase regional metamorphism. This massif contains several small, podiform chromitite bodies which underwent the same metamorphic evolution as their host peridotites. Like other ophiolite chromitites, those found in Dobromirtsi carry abundant platinum-group minerals (PGM) and base-metal minerals (BMM). The PGM consist mainly of Ru-, Os-, and Ir-based PGM (laurite RuS2-erlichmanite OsS2, Os-Ru-Ir alloys, irarsite [IrAsS], Ru-rich pentlandite, and an unknown Ir-sulfide) but minor Rh- and Pd-based PGM (hollingworthite [RhAsS] and a series of unidentified stannides and sulfantimonides) are also present. In contrast, the BMM are dominated by pentlandite (Ni,Fe)9S8, followed by heazlewoodite (Ni3S2), breithauptite (NiSb), maucherite (Ni11As8), godlevskite (Ni7S6), gersdorffite (NiAsS), millerite (NiS), undetermined minerals containing Ni, As and Sb, orcelite (Ni5-XAs2), awaruite (Ni3Fe) and chalcopyrite (CuFeS2). The detailed study of the textural relationships, morphology and composition of the PGM and BMM inclusions indicate the existence of two different PGM-BMM assemblages: (i) a primary or magmatic; and (ii) a secondary related with postmagmatic alteration. The PGM and BMM inclusions in unaltered zones of chromite crystals (mainly laurite-erlichmanite and pentlandite) are considered to be primary magmatic minerals formed under variable temperature (1200–1100°C) and sulfur fugacity (between −2 and −0.5 log fS2). In contrast, PGM and BMM located along altered edges of chromite and serpentinised silicate matrix are considered to be secondary, formed from or re-equilibrated with altering fluids. Secondary PGM and BMM assemblages are considered as result of the combination of reducing and oxidising events related with regional metamorphism. Under low fO2 states, fS2 also drops giving place to the formation of S-poor Ni-rich sulfides and secondary Ru-alloys by desulfurisation of primary S-containing minerals. In contrast, predominance of platinum-group elements and/or base-metal arsenides and sulfarsenides associated with the altered edges of chromite (chromite strongly enriched in Fe2O3) is related with the fixing of remobilised PGE (mainly Ir, Rh and Pd) and base-metals (mainly Ni and Fe) when late oxidising fluids supplied As as well as Sb and Sn. 相似文献
872.
Rita M. F. Fonseca Fernando J. A. S. Barriga Patrícia I. S. T. Conceição 《International Journal of Earth Sciences》2010,99(8):1899-1916
The Southern region of Portugal is subjected to several forms of over-erosion. Most leached products, mainly composed of fine
particles containing nutrients, metals or pesticides, are easily transported by river flows. When these are hindered by a
physical barrier such as a dam, the particulate load accumulates on the bottom of the reservoirs, often leading to a pronounced
decrease of water quality. Bottom sediments from three reservoirs were subjected to grain-size analysis and a study of clay
minerals by X-ray diffraction. Most sediments contain a diverse set of clay minerals, mostly illites, smectites, chlorites
and kaolinites. The nature of the clay minerals reflects the nature of the parent rocks. During the cycles of transport and
temporary deposition, they may undergo significant chemical and physical transformations, which lead to an increase of expandable
properties and therefore, to a higher cationic exchange capacity, determining its important role as vehicles of environmental
pollutants. 相似文献
873.
O.S. Pokrovsky A. Feurtet-Mazel R.E. Martinez S. Morin M. Baudrimont T. Duong M. Coste 《Applied Geochemistry》2010
This study addresses the interaction of Cd with natural biofilms of periphytic diatoms grown during different seasons in metal-contaminated and metal-non-contaminated streams, along a tributary of the Lot River, France. Specifically, it aims to test whether the biofilms from contaminated sites have developed a protective mechanism due to high Cd exposure. Towards this goal, reversible adsorption experiments on untreated biofilms were performed in 0.01 M NaNO3 with a pH ranging from 2 to 8, Cd concentration from 0.5 to 10,000 μg/L and exposure time from 1 to 24 h. Two types of experiments, pH-dependent adsorption edge and constant-pH “Langmuirian”-type isotherms were conducted. Results were adequately modeled using a Linear Programming Model. It was found that the adsorption capacities of natural biofilm consortia with respect to Cd do not depend on season and are not directly linked to the growth environment. The biofilms grown in non-contaminated (4.6 ppb Cd in solid) and contaminated (570 ppb Cd in solid) settings exhibit similar adsorption capacities in the Cd concentration range in solution of 100–10,000 μg/L but quite different capacities at low Cd concentration (0.5–100 μg/L); unexpectedly, the non-contaminated biofilm adsorbs approximately 10 times more Cd than the contaminated one. It is therefore possible that the strong low-abundant ligands (for example, phosphoryl or sulfhydryls) are already metal-saturated on surfaces of biofilm grown in the contaminated site whereas these sites are still available for metal adsorption in samples grown in non-contaminated sites. 相似文献
874.
Jorge Fernndez Vera Markgraf Hctor O. Panarello Miguel Albero Fernando E. Angiolini Susana Valencio Mirta Arriaga 《Geoarchaeology》1991,6(3):251-272
The Barro Negro site (23°S lat., 65°37′W long.) in the Altiplano (Puna) of northwestern Argentina contains a well stratified sequence of remains of Hippidion, the American extinct horse, camelids, and archaeological materials, which is the focus of this study. In addition to establishing a reliable chronology, paleoenvironmental information was obtained based on analyses of pollen and stable isotopes (oxygen and carbon) from bone and marl. The data indicate that Hippidion was present at the site between 12,000 and 10,000 yr B. P., at a time when Altoandean grasslands had expanded to lower elevations. By 10,000 yr B.P., when modern semi-arid sub-puna scrub had replaced the Altoandean grasslands, only camelids (Lama or Vicugna) were present, simultaneous with the first evidence of local human occupation. This suggests that a climatic shift from cool and moist (winter rain regime) to warm and dry (summer rain regime) conditions took place simultaneously with the disappearance of the American horse and the appearance of camelids and man. 相似文献