Biogeochemistry, a prospecting tool in the search for mercury mineralization

Summary of biogeochemical prospecting for mercury mineralization in the Pinchi Fault is given. One thousand two hundred and eight plant samples were collected and analyzed for mercury. In mineralized areas the typical mercury content of dried plants was not less than 0.2 and 0.4 μg/g and even concentrations between 0.6 and 1.6 μg/g were observed. In nonmineralized zones at least 90% of the plants contained no more than 0.15 μg/g of mercury. Analytical and sampling procedures are described.     

The granite problem as exposed in the southern Snake Range,Nevada

A geochemically and mineralogically diverse group of granitoids is present within an area of 900 km² in the southern Snake Range of eastern Nevada. The granitoids exposed range in age from Jurassic through Cretaceous to Oligocene and include two calcic intrusions, two different types of two-mica granites, and aplites. The younger intrusions appear to have been emplaced at progressively more shallow depths. All of these granitoid types are represented elsewhere in the eastern Great Basin, but the southern Snake Range is distinguished by the grouping of all these types within a relatively small area. The Jurassic calcic pluton of the Snake Creek-Williams Canyon area displays large and systematic chemical and mineralogical zonation over a horizontal distance of five km. Although major element variations in the pluton compare closely with Daly's average andesite-dacite-rhyolite over an SiO₂ range of 63 to 76 percent, trace element (Rb, Sr, Ba) variations show that the zonation is the result of in situ fractional crystallization, with the formation of relatively mafic cumulates on at least one wall of the magma chamber. Models of trace element and isotopic data indicate that relatively little assimilation took place at the level of crystallization. Nonetheless, an initial ⁸⁷Sr/⁸⁶Sr value of 0.7071 and δ ¹⁸O values of 10.2 to 12.2 permil suggest a lower crustal magma that was contaminated by upper crustal clastic sedimentary rocks before crystallization. The involvement of mantle-derived magmas in its genesis is difficult to rule out. Two other Jurassic plutons show isotopic and chemical similarities to the Snake Creek-Williams Canyon pluton. Cretaceous granites from eastern Nevada that contain phenocrystic muscovite are strongly peraluminous, and have high initial Sr-isotope ratios and other features characteristic of S-type granitoids. They were probably derived from Proterozoic metasediments and granite gneisses that comprise the middle crust of this region. Another group of granitoids (including the Tertiary aplites) show chemical, mineralogic, and isotopic characteristics intermediate between the first two groups and may have been derived by contamination of magmas from the lower crust by the midcrustal metasediments.     

A gradual transition to plate tectonics on Earth between 3.2 to 2.7 billion years ago

Zircon crystals precipitated from granitoid magmas contain a robust record of the age and chemistry of continental magmatism spanning some 4.375 Ga of Earth history, a record that charts initiation of plate tectonics. However, constraining when exactly plate tectonics began to dominate crustal growth processes is challenging as the geochemical signatures of individual rocks may reflect local subduction processes rather than global plate tectonics. Here we apply counting statistics to a global database of coupled U–Pb and Hf isotope analyses on magmatic zircon grains from continental igneous and sedimentary rocks to quantify changes in the compositions of their source rocks. The analysis reveals a globally significant change in the sources of granitoid magmas between 3.2 and 2.7 Ga. These secular changes in zircon chemistry are driven by a coupling of the deep (depleted mantle) and shallow (crustal) Earth reservoirs, consistent with a geodynamic regime dominated by Wilson cycle style plate tectonics.     

Hydrogeochemistry of two river main floodplain lakes,Hessia, FR Germany

Two lakes, Lake Gänseweiher (LGW) and Lake Fechenheimer Weiher (LFW) occupy groundwater filled abandoned gravel pits which serve as fishing grounds and are part of recreational areas, respectively.Both lakes are cut into a near-surface groundwater aquifer, which is located in the pleistocene River Main gravel terrace t₆. They are sealed at the base by miocene clays and above by overlying flood deposits of the River Main. The latter provide protection from pollutants derived from wet and dry deposition of atmospheric contaminants.Groundwater chemistry determines the pool of dissolved constituents in lake water. The temporal and spatial distribution patterns of chemical compounds in lake water are brought about by endo-biogenic processes, of which the indicators are nitrate-nitrogen and dissolved silica. The latter illustrates an instantaneous response to pulses of diatom blooms.Both lakes are eutrophic (polytrophic) according to the classification guidelines presented in literature.     

Compositional variations and reflectance of the common platinum-group minerals

Summary A detailed study of the compositional variations of 22 different platinum-group minerals, of Os-Ir-Ru- and Pt-Fe alloys, Pt-Pd- and Ru-Os-Ir sulfides, RhIr-Pt sulfarsenides and Pd-Pt tellurides from various deposits resulted in verification of extensive solid solution series and element substitution within different mineral groups. On the basis of representative new microprobe analyses and related spectral reflectance curves of the platinum-group minerals investigated significant correlations between chemical composition and reflectance are discussed.

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Chemische Variation und Reflexion der wichtigsten Minerale der Platingruppe

Zusammenfassung Die Untersuchung der chemischen Variation bei 22 Platinmineralen aus der Gruppe der Os-Ir-Ru- und Pt-Fe-Legierungen, Pt-Pd- und Ru-Os-Ir-Sulfide, Rh-Ir-Pt-Sulfarsenide und Pd-Pt-Telluride ergibt signifikante Mischkristallbildung und Element-Substitutionen innerhalb der unterschiedlichen Mineralgruppen. Anhand neuer Mikrosondenanalysen und der zugehörigen spektralen Reflexionskurven der betreffenden Platinminerale werden die Korrelationen zwischen Chemismus und Reflexion diskutiert.

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With 17 Figures

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Contribution to the Ore Mineralogy Symposium (IMA/COM) at the 14th General Meeting of the International Mineralogical Association, at Stanford, California, in July, 1986.     

Die tektonische Evolution der Erde von Pangäa zur Gegenwart ein plattentektonisches Modell

Zusammenfassung Das hier vorgestellte Modell basiert auf der Annahme, daß sich unter großen Landmassen, wie Pangäa zum Beispiel, in tektonischen Ruheperioden Wärme aus dem Erdinneren anstaut. Infolgedessen entwickelte sich vom Perm bis zur Kreide ein weites Konvektionstumorsystem; Pangäa zersplitterte und die kontinentalen Platten bewegten sich vom afrikanischen Zentrum weg. Die ozeanischen Rücken des Atlantiks und Indiks folgten den abwandernden Platten wie sich »öffnende Ringe«. Panthalassa, der Eo-Pazifik, wurde von allen Seiten überdriftet. Es muß einen Gegenstrom vom Pazifik im Mantel geben, welcher für die Auffüllung der zwischen den Pangäabruchstücken entstehenden ozeanischen Räume mit Mantelmaterial sorgt. Auch die ozeanischen Platten des Pazifiks bewegen sich vom zentralen »Darwin-Rise« weg. Der ostpazifische Rükken folgte der Bewegung und bildet heute einen ausgedehnten ostwärts gekrümmten Bogen. In den ozeanischen »Außenbögen« bildeten sich infolge der Dehnung Querrifts. Die Transformstörungen sind in beiden Systemen radial angeordnet. Die Terrains an Nordamerikas Westküste können nur östlich eines ostpazifischen Rückens aus ihrer ursprünglichen Position im zentralen Pazifik herausgewandert sein, also synchron mit dem sich öffnenden Pazifik. Die Kontinente bewegen sich möglicherweise solange von ihrer ursprünglichen Position weg, bis erneut eine große Landmasse zusammengedriftet ist. Unterhalb einer solchen »Neogäa« könnte sich wieder ein Konvektionstumor infolge von Wärmestau entfalten. Das findet vielleicht in Intervallen von einigen hundert Millionen Jahren statt und könnte die WILSON-Zyklen der Erdgeschichte erklären.

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The tectonic evolution of the earth from Pangea to the present a plate tectonic model

The model proposed here is based on the assumption that beneath giant landmasses (e.g. Pangea) heat from the inner earth is stored up and accumulated during periods of tectonic inactivity. Consequently below Pangea a huge convection bulge system developed from the Permian to the Cretaceous; Pangea split up and the continental plates moved away from the African centre. The oceanic ridges of the Atlantic and the Indic also followed the movement of the withdrawing continents like »opening rings«. The oceanic ridges always maintained their position in the middle of the spreading oceans above unidirectional flows in the upper mantle. Panthalassa which surrounded Pangea, was over-drifted from all sides. Since the »expansion« of Pangea is continuing even today, there must be a counter current of mantle material from the Pacific area, compensating the gaps between the fragments of Pangea. Consequently at the subduction zones a suction should exist, which pulls the Pacific plates under the advancing plates of the former Pangean continent. In the centre of Panthalassa another bulge from the upper mantle developed simultaneously with the bulge under Pangea. The Pacific oceanic plates moved away outwards from this central »Darwin rise«. The Eastpacific ridge also followed this movement eastwards and forms today a wide, ringlike arc. In the outer arcs of the Pangean and Pacific spreading ocean systems transverse ridges developed as a result of the extension of the older oceanic crust. The transform faults are radial structures in both »expanding« systems. The hotspot spurs of the Hawaii and Polynesian islands can be explained as the result of material derived from an independent slowly ESE moving deeper part of the mantle. The terrains on North Americas West cost moved away from their original position in the central Pacific ocean synchronously with the opening ocean on the east side of the advancing East Pacific ridge.The energy which drives the whole system is residual plus radioactive heat. Kinetic movements compensate the heat surplus of the earth. The continental plates of the Pangean system are moving away from their original position until a new giant landmass is formed by collision. Below such a stationary »Neogea« a heat bulge can develop again. This may take place perhaps in intervals of hundred of million years, explaining the WILSON-cycles in earth history.

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Résumé Le modèle présenté ici est basé sur l'hypothèse qu'endessous des grandes masses continentales — comme la Pangée p.ex. - il s'accumule, pendant les périodes de calme tectonique, de la chaleur d'origine interne. En conséquence, depuis le Permien jusqu'au Crétacé, un vaste système de convection s'est développé; la Pangée s'est morcelée et les plaques continentales se sont éloignées du centre africain. Les dorsales océaniques circum-africaines ont suivi le mouvement de ces plaques à la manière d'»anneaux concentriques«. La Panthalassa, précurseur du Pacifique, a été chevauchée de tous les côtés. Il doit s'être établi, dans le manteau, à partir du Pacifique, un contre-courant qui compense l'ouverture des océans en formation entre les fragments de la Pangée. Le Pacifique a donné lieu, lui aussi, à une expansion centrifuge; la dorsale est-pacifique a suivi le mouvement et forme aujourd'hui un arc bombé vers l'est. Dans les arcs océaniques extérieurs, l'expansion a provoqué la formation de dorsales transverses. Les failles transformantes montrent, dans les deux systèmes, des dispositions radiales. Les terrains de la côte W de l'Amérique du N ne peuvent provenir que d'une région située à l'Est de la dorsale est-pacifique dans sa position d'origine. Les continents s'éloigment et forme aujourd'hui un arc bombé vers l'est. Dans les arcs océaniques extérieurs, l'expansion a provoqué la forSous une telle »Néogée«, une nouvelle cellule de convection pourrait ensuite se développer. Ces phénomènes pourraient se dérouler dans un intervalle de quelques centaines de millions d'années, expliquant ainsi les cycles de Wilson dans l'histoire de la Terre.

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