First Find of Platinum in Cosmogenic Spherules of Ferromanganese Crusts (Fedorov Guyot,Magellan Seamounts,Pacific Ocean)

Doklady Earth Sciences - More than 250 magnetic spherules 40–200 µm in size were mechanically separated from a ferromanganese crust sample of Fedorov Guyot in the Magellan Seamounts. The...     

Catagenesis and burial metamorphism of terrigenous and volcanic sequences (section recovered by the En-Yakhinskaya parametric borehole SG-7)

Secondary alterations of Cretaceous, Jurassic, and Triassic terrigenous complexes recovered by borehole SG-7 were studied from the depth of 3620 m to 6920 m (roof of basalts). Down to the depth of ∼6770 m, the section shows a gradual intensification of catagenetic alterations of sandy rocks: the formation of pressure dissolution textures and regeneration of clastic quartz. Intensification of the transformation of clay minerals is not observed in this mineral. Variations in the contents of illite, hydromicas, mixed-layer minerals, smectite, chlorite, and kaolinite at different depths of the recovered section are related to changes in the provenance during the accumulation of sedimentary complexes. The Middle Triassic coarse-grained sandy rocks (suprabasalt sequence) are more intensely transformed: they are marked by microstylolitic textures of pressure dissolution, recrystallization blastesis of clastic quartz grains, and newly formed zoisite. The composition of clay minerals is also characterized by variation: micas are represented by sericite with ΔD = 0; Fe-chlorite and kaolinite are noted. These features suggest the absence of linear positive correlation of T and P with the subsidence depth of sedimentary complexes. Intense heating (up to 200–300°C) of the Middle Triassic suprabasalt rocks is likely caused by trap activity (intense effusion of basalt lavas). Investigation of secondary minerals in basalts recovered by borehole SG-7 revealed that the grade of their transformation matches the medium-temperature subfacies of the greenschist facies.     

Metal and metalloid leaching from tailings into streamwater and sediments in the old Ag–Pb–Zn Terramonte mine,northern Portugal

Tailings deposited over the Castanheira, a stream which flows through the old Ag–Pb–Zn Terramonte mine area, showed a great potential environmental risk due to sulphide weathering, facilitated by the tailings–water interaction. The high concentrations of Al, Fe, Pb and Zn in the tailings are associated with the exchangeable, reducible and sulphide fractions and suggest sphalerite and pyrite occurrences. Oxidation of pyrite is responsible for the low pH values (3.38–4.89) of the tailings. The water from the Castanheira stream is not suitable for human consumption due to high concentrations of SO₄ ^2?, Mn, Al, Cd, Ni, and Pb. The lowest concentrations of metals and metalloids were detected in downstream stretches of the Castanheira. However, As, Fe and Zn in deeper sediments tend to increase downstream. Significant concentrations of trivalent forms of arsenic were detected in water samples. In downstream stretches of the Castanheira, some free ions (Fe²⁺, Mn²⁺ and Zn²⁺) also predominate and the water is saturated with ferrihydrite, goethite, hematite, lepidocrosite and magnetite.     

Mineralogical criteria for the diamond potential of Upper Triassic placers on the northeastern margin of the Siberian Platform

Representative sampling of a diamond-bearing basal horizon in the Carnian Stage (Upper Triassic) on the northeastern margin of the Siberian Platform revealed a wide spectrum of indicator minerals, first of all, garnets, whose compositions are the same as in the inclusions in the regional diamonds. Of special interest are garnets of potential eclogite paragenesis with an abnormally high impurity of MnO (0.5–3.2 wt.%), which was earlier detected in more than 20% of garnets present as inclusions in diamonds of northern Quaternary placers and recommended as a new mineralogical criterion for diamond presence. Subcalcic Cr-pyropes of dunite–harzburgite paragenesis were also found in variable amounts, from 0.7 to 3.9 rel.%, in the sample of 973 grains of pyropes of lherzolite and websterite parageneses. Three grains contain 11.9, 12.6, and 16 wt.% Cr₂O₃, which corresponds to the presence of 30–34% of Mg–Cr-knorringite component. Such pyropes have been revealed for the first time in the study region. Cr-spinels are a mixture of compositions typical of kimberlites and the regional alkali-ultrabasic rocks. All studied samples contain picroilmenites with a variable content of Cr₂O₃ impurity. Since Mg–Fe–Ca-garnets with Mg# < 35 can be partly hosted in metamorphic rocks of the Anabar Shield, the elevated content of Na₂O impurity (> 0.09 wt.%) was also taken into account. The different contents of indicator minerals in the samples might be due to the variable composition of the diamond orebodies. The Carnian placers call for new systematic sampling. Special attention should be given to estimation of the composition of garnets of presumably eclogite paragenesis with elevated contents of TiO₂, MnO, CaO, and Na₂O and to search for perovskite and Nb-containing rutile. These minerals, together with zircons, are of interest for determining the U–Pb isotopic age of probable diamond orebodies—kimberlites.     

A new assessment of the Noril’sk type deposits origin

The origin of the Noril’sk and Talnakh sulfide PGE-Cu-Ni deposits that are associated with Triassic basaltic traps of Siberia is considered. It has been shown that ore elements of these deposits (with the probable exception of iron) are of a crustal origin rather than a mantle one. They entered the basalts owing to the remobilization (recycling) of ore elements from Early Proterozoic sediments and rocks that are presented in the basement of the Siberian Craton. The criteria for prospecting for analogous deposits are given.     

Tracing the source of metals in Geogas from metal deposits in a loess-covered region北大核心CSCD

In order to test the distribution characteristics of metallogenic elements and the application effectiveness of the traee elements and REE for tracing the source of Geogas anomaly, the distribution characteristics of metallogenetic elements .trace elements and REE in different media of the Jiaolongzhang Pb-Zn deposit underneath the loess have been studied in this paper. Results indicate that the distribution characteristics of Geogas anomalous elements over the concealed orebodies are coincided with those of the enriched elements of ores, while the distribution characteristics of trace elements and REE in background Geogas samples are distinctly different from those of other media, indicating that the source of background Geogas materials is influenced by various factors( geological bodies, lab environment and blank reagent). The similar distribution characteristics of the trace elements in the Geogas samples of anomalous areas and in the ores suggested that Geogas anomaly is affected by concealed orebodies in depth. The difference between REE patterns of the Geogas samples of the anomalous area and those of other solid media might be caused by inadequate analytical precision and low collecting efficiency of the instrument. It is believed that the REE tracing method can provide the information on the source of Geogas anomaly if the analytical precision and collecting efficiency of REE could be constantly increased.     

Age and origin of rocks of the Daldyn Group of the Anabar Shield: Evidence from U-Pb zircon dating,Sm-Nd and Lu-Hf isotope systematics

The Hf-Nd isotope systematics was used to determine the genesis of zircons from granulites of the Daldyn Group of the Anabar Shield. Obtained age of magmatic crystallization for biotite–hypersthene crystalline schists and garnet amphibolites agree with position of zircons within terrestrial array. Magmatic genesis of plagiogranite neosome under granulite conditions was established for leucocratic plagiogneisses.     

Dokumente zur Ära Heinrich von Dechen (1800–1898) – Seine geologischen Übersichtskarten

Ohne Zusammenfassung Received: 4 January 1999