Superficial geochemical dispersion around sulphide deposits: Some examples in France

Examination of geochemical exploration data accumulated in France over a period of almost 20 years has led to detailed consideration of the relationship between anomalies and mineral deposits.Geochemical anomalies associated with sulphide mineralizations seem to follow several general rules i.e. Ag and Pb are almost systematically present, As is very frequent, while Cu and Sb may also be present. The behaviour of Zn seems to be controlled by the lithological character of the host rock with anomalies in carbonate media even for occurrences without economic interest and an absence of anomalies in non-carbonate media, even for Zn deposits of considerable size. Moreover, the contents of this element are greatly amplified by contamination from old workings. The abundance (or scarcity) of supergene minerals in the occurrences directly influences the nature of the anomalies. Lead anomalies frequently correspond to the presence of Pb oxides such as pyromorphite while Zn anomalies derive from Zn oxides which are much more common in carbonate media (smithsonite-hydrozincite) than in schistose host rock. High Cu and As contents are most frequently linked to the presence of “ferruginous fragments” (gossans) derived from sulphides. As a general rule, the various geochemical peculiarities or associations encountered in soils are very much similar to those of the oxidized part of the deposits.We are therefore led to assume that, contrary to the usual conceptions, superficial dispersion is basically mechanical and that the anomalies detected near mineralizations are due to the presence in the soils of supergene minerals, gossan microfragments and more or less mineralised rock debris, etc. Mechanical migration of this kind can be explained by the existence in France during the Pleistocene of cold climates of the tundra type with associated permafrost. The thawing in summer of the superficial part of the frozen overburden resulted in massive downslope solifluction favoring mechanical migration over distances in the order of 100 m and contributing to the surface dispersal of the mineralizations. These phenomena are extremely important for geochemical exploration because they allow and justify the use of large grids (200 m × 200 m or even 400 m × 400 m) during soil surveys at least at the initial stage of prospecting on areas of several square kilometers.These conclusions are valid not only for France but for all regions with the same paleoclimate. These form a belt bounded to the north by glacial formations linked to ancient inland ice and to the south by the maximum extension of Pleistocene permafrost, including, in particular, the north of the United States and Central Europe. This paleo-climate belt characterized by traces of solifluction constitutes a well-defined unit in which geochemical prospecting has proved itself extremely effective.     

Mineral-fluid equilibria in the system CaO–MgO–SiO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O–CO<Subscript>2</Subscript>–NaCl and the record of reactive fluid flow in contact metamorphic aureoles

Phase equilibria in the system CaO–MgO–SiO₂–CO₂–H₂O–NaCl are calculated to illustrate phase relations in metacarbonates over a wide-range of P–T–X[H₂O–CO₂–NaCl] conditions. Calculations are performed using the equation of state of Duan et al. (Geochim Cosmochim Acta 59:2869–2882, 1995) for H₂O–CO₂–NaCl fluids and the internally consistent data set of Gottschalk (Eur J Mineral 9:175–223, 1997) for thermodynamic properties of solids. Results are presented in isothermal-isobarical plots showing stable mineral assemblages as a function of fluid composition. It is shown that in contact-metamorphic P–T regimes the presence of very small concentrations of NaCl in the fluid causes almost all decarbonation reactions to proceed within the two fluid solvus of the H₂O–CO₂–NaCl system. Substantial flow of magma-derived fluids into marbles has been documented for many contact aureoles by shifts in stable isotope geochemistry of the host rocks and by the progress of volatile-producing mineral reactions controlled by fluid compositions. Time-integrated fluid fluxes have been estimated by combining fluid advection/dispersion models with the spatial arrangement of mineral reactions and isotopic resetting. All existing models assume that minerals react in the presence of a single phase H₂O–CO₂ fluid and do not allow for the effect that fluid immiscibility has on the flow patterns. It is shown that fluids emanating from calc-alkaline melts that crystallize at shallow depths are brines. Their salinity may vary depending mainly on pressure and fraction of crystallized melt. Infiltration-driven decarbonation reactions in the host rocks inevitably proceed at the boundaries of the two fluid solvus where the produced CO₂ is immiscible and may separate from the brine as a low salinity, low density H₂O–CO₂ fluid. Most parameters of fluid–rock interaction in contact aureoles that are derived from progress of mineral reactions and stable isotope resetting are probably incorrect because fluid phase separation is disregarded.     

2-D SIMULATION OF CHANNEL FLOWS WITH MOVEABLE BED

1 INTRODUCTIONFor many hydraulic engineering problems, the analysis of flow and bed level variations in openchannels is a fundamental prerequisite. forcal methOds fOr alluvial rivers are well develoPednowadays as far as onediInensional descriPtions are concemed. A cOmPrhensive analysis of Ihe wellknown models is Presented by Habersack(l998). HOwever, for a number of Problems such as channelwidening, flow pattem close to sPuds and etc. a more deailed knowledge of the bed level behavio…     

Euv Spectroscopy of the Sunspot Region Noaa 7981 Using Soho – I. Line Emission and Time Dependence

EUV spectra of a medium-size sunspot and its surroundings, NOAA 7981, were obtained on 2 August 1996 with the Coronal Diagnostic Spectrometer (CDS) and the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) on the Solar and Heliospheric Observatory (SOHO). The spectral lines formed in the transition region and corona show considerable structure and large deviations from a uniform spatial distribution over the active region. Enhanced EUV emissions in transition region lines are concentrated in small regions outside the umbra of the sunspot throughout most of the observing sequence. Only during a short, active period do we find an enhanced line emission that reaches into the umbra. Preliminary values for the umbral intensity are given.Marked changes are detected between the spatial distribution of line emission in the transition region and the low corona. The difference is not limited to cool and hot non-flaring loops not being cospatial, but includes differences both regarding the time variability and the orientation and size of the emission features. Whereas both rapid ( 4 and 2 min) and slow ( 10 and 12 h) temporal variations are found in the chromosphere (Hei 584 Ú) and transition region (Ov 629 Ú), the response in the low corona (Mgix 368 Ú) is slow ( 5 h). Furthermore, marked differences between the spatial distributions in the Mgviii 315 Ú, Mgix 368 Ú lines formed in the low corona and the coronal Fexiv 334 Ú, Fexvi 360 Ú lines are detected.     

Liquidus temperatures and phase compositions in the system Qz-Ab-Or at 5 kbar and very low water activities

Liquidus phase relations have been experimentally determined in the systems Qz-Ab-Or-(H₂O), Qz-Ab-(H₂O) and Qz-Or-(H₂O) at H₂O-undersaturated conditions (a _H2O = 0.07) and P = 5 kbar. Starting materials were homogeneous synthetic glasses containing 1 wt% H₂O. The liquidus temperatures were bracketed by crystallization and dissolution experiments. The results of kinetic studies showed that crushed glasses are the best starting materials to overcome undercooling and to minimize the temperature difference between the lowest temperature of complete dissolution (melting) and the highest temperature at which crystallization can be observed. At P = 5 kbar and a _H2O = 0.07, the Qz-Ab eutectic composition is Qz₃₂Ab₆₈ at 1095 °C (±10 °C) and the Qz-Or eutectic is Qz₃₈Or₆₂ at 1030 °C (±10 °C). The minimum temperature of the ternary system Qz-Ab-Or is 990 °C (±10 °C) and the minimum composition is Qz₃₂Ab₃₅‐ Or₃₃. The Qz content of the minimum composition in the system Qz-Ab-Or-H₂O remains constant with changing a _H2O. The normative Or content, however, increases by approximately 10 wt% with decreasing a _H2O from 1 to 0.07. Such an increase has already been observed in the system Qz-Ab-Or-H₂O-CO₂ at high a _H2O and it is concluded that the use of CO₂ to reduce water activities does not influence the composition of the minima in quartz-feldspar systems. The determined liquidus temperature in melts with 1 wt% H₂O is very similar to that obtained in previous nominally “dry” experiments. This discrepancy is interpreted to be due to problems in obtaining absolutely dry conditions. Thus, the hitherto published solidus and liquidus temperatures for “dry” conditions are probably underestimated. Received: 27 March 1997 / Accepted: 1 October 1997     

Formation and ascent of granitic magmas

New results obtained by the investigation of liquidus and solidus phase relationships in the haplogranite system Qz-Ab-Or are used to discuss the evolution of magmas during their ascent in the crust. It is assumed that the magmas are formed at 720°C, 820°C, 920°C and at a depth corresponding to a pressure of 8 kbar. The starting composition of the magma is taken as 50% melt plus 50% quartz and feldspars. In case of a closed system (no heat exchange and no transfer of elements) the melt fraction of magmas, the water activity and the viscosity increase with decreasing pressure. The temperature slightly decreases. At 700°C the viscosity is approximatively 2 orders of magnitude lower than at 900°C. This is related to the higher amount of water in the (H₂O-undersaturated) melt at low T. It is also shown that dehydration melting is only realistic at high T (900°C). At lower temperatures water has to be added from outside to obtain an intrusive magma with approximatively 50% melt.

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Zusammenfassung Neue Ergebnisse, erzielt durch Untersuchungen von Liquidus und Solidus Phasenbeziehungen des Granitsystems Qz-Ab-Or, werden benutzt, um die Entwicklung eines granitoiden Magmas während seines Aufstiegs zu diskutieren. Es wird vorausgesetzt, daß die Magmen bei Temperaturen von 720°C, 820°C und 920°C gebildet werden, sowie in einer Tiefe die einem Druck von 8 kbar entspricht. Die anfängliche Zusammensetzung des Magmas wird mit einem Verhältnis von 50% Schmelze sowie 50% Quarz und Feldspäten angenommen. Im Falle eines geschlossenen Systems (kein Austausch von Wärme und Elementen) steigt die Teilschmelzbildung von Magmen, die Aktivität des Wassers und die Viskosität bei abnehmenden Druck; hierbei sinkt die Temperatur leicht. Bei 700°C ist die Viskosität um ca. 2 Größenordnungen geringer als bei 900°C. Dies wird bedingt durch den höheren Gehalt an Wasser in der (H₂O-untersättigten) Schmelze bei tieferen Temperaturen. Es wird außerdem gezeigt, daß Magmenbildung durch Dehydratation nur bei hohen Temperaturen realistisch ist (900°C). Bei tieferen Temperaturen muß Wasser von außen zugeführt werden um ein intrusives Magma zu erhalten, das ungefähr 50% Schmelze besitzt.

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Résumé L'évolution des magmas granitiques au cours de leur ascension dans la croûte est discutée à la lumière de données nouvelles relatives aux relations entre phases liquides et solides dans le système Q-Ab-Or. On suppose que les magmas se forment à des températures de 720°C, 820°C, 920°C et à une profondeur correspondant à une pression de 8 Kb. On admet pour leur composition initiale un mélange de 50% de liquide et 50% de quartz + feldspaths. Dans le cas d'un système fermé (pas d'échange de chaleur ni de matière), la fraction liquide du magma, l'activité de l'eau et la viscosité augmentent quand la pression diminue; en même temps, la température décroît légèrement. A 700°C, la viscosité est d'environ 2 ordres de grandeur plus basse qu'à 900°C. Cette propriété est en relation avec la teneur en eau plus élevée dans le liquide (sous-saturé en eau) à basse température. On peut également montrer qu'une fusion déshydratante n'est vraisemblable qu'à haute température (900°C). Aux températures plus basses, de l'eau doit être apportée de l'extérieur pour l'obtention d'un magma à 50% de liquide.

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Die Graubündner Erdbeben und Wetterstürze im August 1927

Ohne Zusammenfassung     

Das Vorland der Alpen und die Vorgeschichte der Alpenfaltung

Ohne Zusammenfassung     

Verbreitung und Ausbildung der Siegener Schichten in der Osteifel

Ohne ZusammenfassungVortrag, gehalten auf der Hauptversammlung der Geologischen Vereinigung zu Frankfurt a. M., Januar 1933.     

V. Bücher- und Zeitschriftenschau

Ohne Zusammenfassung