Islands and oil — locality and structure

This paper analyses the meeting between the international petroleum industry and two local communities. The building of an oil terminal in a local community is both a major event, and a generator of processes on the local level. Three different angles is used to enlighten these processes; categorised as as place, people and mind. The place analysis consentrates on the effects on population and workforce development, the people analysis look at the institutional and planning processes, and the mind analysis the effects on the feeling of local identification. The paper demonstrates that local factors are of crucial importance even for the international petroleum industry.     

Interpretation of the Conolly contour diagram of fault-related veins

The Conolly diagram is a section or plan on which the distances from the vein to an arbitrary reference plane are contoured. This graphical technique makes it possible to depict the shape of the vein in a form which resembles topographic contour maps. The Conolly diagram can be used to analyse the relation between oreshoot development in fault-related veins, the attitude of the irregular fault plane and slip direction of the fault.A new graphical discrimination method has been developed by which an apparent-slip direction b is determined in the Conolly diagram by comparing the variations in attitude of the vein with the variations in thickness of the corresponding fissure opening. Theoretically, ore shoots in dextral faults are located where the acute angle between the apparent-slip direction b and the fault surface is smaller than zero and in sinistral faults where is greater than zero. This method of interpretation is especially useful when analysing obliqueslip fault systems, which are difficult to study because of the complexities of the three-dimensional problem.     

Self-Organized Criticality Applied to Natural Hazards

The concept of self-organizedcriticality evolved from studies of three simplecellular-automata models: the sand-pile, slider-block,and forest-fire models. In each case, there is asteady input and the loss is associated with afractal (power-law) distribution of avalanches. Each of the three models can be associated with animportant natural hazard: the sand-pile model withlandslides, the slider-block model with earthquakes,and the forest-fire model with forest fires. We showthat each of the three natural hazards havefrequency-size statistics that are well approximatedby power-law distributions. The model behaviorsuggests that the recurrence interval for a severeevent can be estimated by extrapolating the observedfrequency-size distribution of small and mediumevents. For example, the recurrence interval for amagnitude seven earthquake can be obtained directlyfrom the observed frequency of occurrence of magnitudefour earthquakes. This concept leads to thedefinition of a seismic intensity factor. Both globaland regional maps of this seismic intensity factor aregiven. In addition, the behavior of the modelssuggests that the risk of occurrence of large eventscan be substantially reduced if small events areencouraged. For example, if small forest fires areallowed to burn, the risk of a large forest fire issubstantially reduced.     

Foundation analysis of an arch dam. Comparison of two modelling techniques: No tension and jointed rock material

Summary The paper describes the application of a recently developed joint element to the analysis of a jointed rock foundation of an arch dam. The most relevant joint system and the concrete-rock interfaces were simulated by means of joint elements. Parameters for the constitutive behaviour of the joints were derived from large scale in situ shear tests. The analysis showed the suitability of the jointed model to predict not only stresses and deformations but also the shape of the failure surface al ultimate conditions. This allows an easy determination of conventional safety factors. A parallel no tension analysis incorporating a cohesion threshold was also carried out. The comparison of both approaches indicates that the no tension hypothesis is a crude approximation of the rock response to dam loads and may lead to unsafe estimates of stability and stress conditions.The analysis performed also showed the relevance of initial stress conditions inside the rock mass in controlling the overall foundation response.     

Use of the lattice parameter b 0 of dioctahedral illite/muscovite for the characterization of P/T gradients of incipient metamorphism

The P/T gradient or baric type of low-grade metamorphic terranes can be characterized on the basis of cumulative frequency curves of the lattice parameter b ₀ of dioctahedral micas (Sassi and Scolari 1974), which essentially reflects celadonite content and increases with pressure. The method is here applied to illite/muscovite of sub-greenschist (incipient metamorphism) and low-greenschist facies terranes from the marginal zones of the Swedish Caledonides, Swiss Alps and Venezuelan Andes, where the grade of metamorphism was determined on the basis of illite crystallinity.Complications in the determinations of b ₀ arising from (a) shifts in the spacing of the apparent 060 diffraction peaks of the illites due to the presence of inter-stratified expandable layers, and (b) persistence of more crystalline clastic micas are pointed out.In contrast to the earlier-reported decrease of b ₀ with progressive metamorphism at higher grades, b ₀ tends to increase with grade during incipient metamorphism: it is markedly lower in the diagenetic and low-grade anchimetamorphic grades in the Cambro-Silurian of the Caledonides of Jämtland, western Sweden (zone A), and the Lower Tertiary of the Helvetic zone of the Swiss Alps, than in the higher grade anchizone and epizone of the former terrane.Mean b ₀ values for the intermediate and highest (epizone) grades of the Swedish Caledonides are respectively 9.030 and 9.037 Å compared to the mean value of 9.035 Å for the medium-high pressure, Barrovian-type, terrane of Otago, N.Z. Mean b ₀ of 9.005 Å for the mainly epizonal slates of the Venezuelan Andes lies between those for the low-pressure terranes of Bosost ( 8.992 Å), and the low-medium pressure terrane of northern New Hampshire (9.010 Å). These baric estimates agree well with those for the metamorphic facies series at higher grade produced by the same events.Cumulative b ₀ curves are concluded to be useful for the characterization of P/T gradients of incipient metamorphism, particularly in the higher grade part of the anchizone and the epizone.     

Clinopyroxenes from space-and time-associated “within-plate dominant” and “subduction-related” Variscan basic rocks (Tazekka,Morocco)

A study has been made of clinopyroxene phenocrysts from within-plate dominant and subduction-related basic lavas, which are within the same volcanic pile of the Lower Namurian volcanic complex of Tazekka (Eastern Morocco). Twenty-one clinopyroxene phenocrysts (113 probe analyses in total) have been analysed from two thin sections of the within-plate lava sample, and 17 clinopyroxene phenocrysts (82 probe analyses in total) have been analysed from two thin sections of the subduction-related lava sample. Petrographic and microprobe data on clinopyroxene phenocrysts from the two lavas indicate specific textural and chemical features according to the source, namely: relatively low pressure and high temperature crystallisation conditions for the sector-zoned crystals from the basalt of the first group, although such a sector-zoned growth in clinopyroxenes is usually considered as evidence for a continental within-plate affinity of host-rocks; intermediate pressure and relatively low temperature crystallisation conditions for homogeneous crystals from the basalt of the second group. Thus distinct P-T conditions for equilibrium crystallisation of the pyroxenes in each host-magma are proposed. Clinopyroxene affinities observed in specific discriminative diagrams are in agreement with trace element geochemistry of the host-lavas, i.e. continental within-plate features for clinopyroxenes of the group 1 host-rock and subduction-related features for clinopyroxenes of the group 2 host-rock, which often exhibit extreme parameters highlighting the problem of petrogenesis of these basic lavas in an unusual continental crust subduction-related environment.     

Möglichkeiten und Grenzen des Base Friction Konzepts

ZusammenfassungMöglichkeiten und Grenzen des Base Friction Konzepts Der vorliegende Beitrag befaßt sich mit einer von Goodman eingeführten neuen Technologie für zweidimensionale Modellversuche in der Felsmechanik. Bei diesen Modellen sollen die im Prototyp durch die Gravitation erzeugten Körperkräfte durch Reibungskräfte simuliert werden, die dieselbe Richtung wie die Erdbeschleunigung besitzen müssen und die entstehen, wenn das zweidimensionale Modell in einer horizontalen Ebene aufgebaut und zwischen dem Modell und der horizontalen Basis eine entsprechende Relativbewegung erzwungen wird.Nach einem kurzen Überblick über die einschlägige Literatur wird anhand eines Stabilitätsdiagramms für einen Einzelkörper an der Schiefen Ebene untersucht, ob die im geklüfteten Fels auftretenden Bruchmechanismen Gleiten, Kippen und Kippen bei gleichzeitigem Gleiten im Base Friction Modell exakt modellierbar sind.Aus dieser Untersuchung ergibt sich, daß in den Bereichen Kippen und Gleiten bezüglich der Kinetik mit Schwerkraftmodellen übereinstimmende Ergebnisse zu erzielen sind. Im Bereich Kippen bei gleichzeitigem Gleiten treten dagegen wesentliche Abweichungen auf, weil die Simulation der Schwerkraft durch Reibungskräfte nicht vollständig gelingt. Fälle, bei denen Aussagen über Geschwindigkeits-und Beschleunigungsverhältnisse im Prototyp gemacht werden sollen, können aus demselben Grund mit einem Base Friction Modell nicht untersucht werden.Nach einer ausführlichen Kritik wird dann der Schluß gezogen, daß das Base Friction Modell zwar als Entwurfshilfsmittel ungeeignet ist, daß es aber als Methode für die Grundlagenforschung in der Felsmechanik mit Aussicht auf Erfolg eingesetzt werden kann.

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SummaryPossibilities and Limitations of the Base Friction Model The paper deals with a new technology for two-dimensional model tests in rock mechanics introduced by Goodman. In this model gravity is simulated by base friction forces. Their direction is to be that of gravity. The two-dimensional model is placed on a horizontal base. A movement of this base relative to the model produce frictional forces which model gravity.First a single model element placed on an inclined plane is analysed resulting in a stability diagram. The purpose of the subsequent analysis is to assess whether the mechanisms of failure being investigated, i. e. sliding, toppling, and toppling combined with sliding, are adequately reproduced by the base friction model.The specific mechanisms toppling and sliding produce kineticly valid results. Considerable errors are to be expected for toppling combined with sliding, because of the specific base friction feature. Mechanisms including velocity and acceleration cannot be modeled by base friction.Finally the limitations of the base friction model as a design tool and instrument for rock mechanics research are discussed.

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RésuméPossibilités et limites du base friction model Le présent article traite d'une nouvelle technologie des essais à deux dimensions, introduite par Goodman, sur la mécanique des roches. Par cette technologie les forces en prototype, produites par la gravitation, doivent être remplacées par des forces de frottement. Leur direction doit être la même que celle de la gravitation. Le modèle à deux dimensions est posé sur un plan horizontal. Le mouvement relatif du modèle contre la base horizontale produit des forces de frottement, qui modèlent la gravitation.A l'aide d'un diagramme de stabilité pour un élément isolé sur un plan incliné, on fait des analyses pour savoir si les états de mouvement aparaissant à la rupture de la roche, c'est-à-dire le fait de glisser, basculer, et basculer en même temps que glisser, peuvent être modelés exactement avec le base friction model.Les mécanismes glisser et basculer produisent des résultats qui sont cinétiquement réels. Des fautes remarquables doivent être attendues pour le fait de basculer en même temps que glisser à cause de la mécanique spécifique du modèle. A modeler des vitesses et des accélérations le modèle n'est pas propre.En conclusion on expose une critique détaillée pour savoir jusqu'à quel point cette méthode peut servir comme moyen de dimensionnement dans la pratique et comme instrument pour la recherche en mécanique des roches.

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Mit 9 Abbildungen     

Mineralized hydrothermal solution cavities in the Co-As Aït Ahmane mine (Bou Azzer,Morocco)

In the F53 Aït Ahmane Cobalt-deposit (Bou Azzer, Anti-Atlas, Morocco) a new type of contact mineralization is described. It is carbonate hosted but displays similar depositional sequence and fluid inclusion patterns to the well-known vein mineralization from the same mine. Field studies suggest that such a contact mineralization formed early by infilling solution cavities in formerly calcified serpentinite.     

Laterites of Sri Lanka — A reconnaissance study

Laterites occur extensively in the lowlands of Sri Lanka whereas these are observed as localized outcrops in the uplands and the highlands. In lateritis weathering profiles, Al-rich zones are found between and beneath harder Fe-rich layers. The processes of lateritisation and bauxitisation have produced residual products enriched in both Fe₂O₃ and Al₂O₃. In the lowlands and uplands the lateritisation processes have prevailed and the bauxitisation trend is more pronounced in the highlands.

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Résumé Les gisements latéritiques s' observent largement dans les lowlands du Sri Lanka. Mais dans les uplands et highlands les affleurements latéritiques sont rares. Aux profils d'altération les zones riches en Al se trouvent entre ou sous les zones plus dures mais riches en Fe. Les deux processus de latéritisation et bauxitisation ont donné lieu aux produits résiduels riches en Fe₂O₃ ainsi qu'en Al₂O₃. Il a été observé que le processus de latéritisation se produit plus souvent dans les lowlands et uplands tandis que la bauxitisation est plus repandue aux highlands.

     

An objective replacement method for censored geochemical data

Geochemical data are commonly censored, that is, concentrations for some samples are reported as less than or greater than some value. Censored data hampers statistical analysis because certain computational techniques used in statistical analysis require a complete set of uncensored data. We show that the simple substitution method for creating an uncensored dataset, e.g., replacement by3/4 times the detection limit, has serious flaws, and we present an objective method to determine the replacement value. Our basic premise is that the replacement value should equal the mean of the actual values represented by the qualified data. We adapt the maximum likelihood approach (Cohen, 1961) to estimate this mean. This method reproduces the mean and skewness as well or better than a simple substitution method using3/4 of the lower detection limit or3/4 of the upper detection limit. For a small proportion of less than substitutions, a simple-substitution replacement factor of 0.55 is preferable to3/4; for a small proportion of greater than substitutions, a simple-substitution replacement factor of 1.7 is preferable to4/3, provided the resulting replacement value does not exceed 100%. For more than 10% replacement, a mean empirical factor may be used. However, empirically determined simple-substitution replacement factors usually vary among different data sets and are less reliable with more replacements. Therefore, a maximum likelihood method is superior in general. Theoretical and empirical analyses show that true replacement factors for less thans decrease in magnitude with more replacements and larger standard deviation; those for greater thans increase in magnitude with more replacements and larger standard deviation. In contrast to any simple substitution method, the maximum likelihood method reproduces these variations. Using the maximum likelihood method for replacing less thans in our sample data set, correlation coefficients were reasonably accurately estimated in 90% of the cases for as much as 40% replacement and in 60% of the cases for 80% replacement. These results suggest that censored data can be utilized more than is commonly realized.     

Central place preference by social geographic groups with reference to the tributary area of Shibuya,Southwest Tokyo

Conclusions Central place preference by social geographic groups is in the Tokyo area less determined by social status than by age and life cycles differentiated by sex.In case of the tributary area of Shibuya a simple classification into young people (male and female bachelors) and married people (husbands and housewives) indicates essential differences in central place preferences between these groups.The bachelors, particularly the young men, are most important for central place orientations in that they show distinctive spatial behavioural patterns in terms of complexity, mobility and intensity of distance-movements. The young people being employed — today joculously called dokushin kizoku (bachelors' aristocracy) because of their salary raised remarkably as well as of their extensive concumer demands and high buying power — are representative for the present-day economically independent young generation. The bachelors judge urban center attractiveness on the basis of shopping facilities and also in terms of atmosphere — meeting friends, strolling through town and amusing themselves. The young people are very much courted by commercial enterprises and are regarded as special trend setters having great influence on the image and vitality of Tokyo's inner urban centers.Most attractive for the young people are the ring centers. In the southwestern suburban areas Shibuya predominates; however generally speaking it is Shinjuku, the western and leading (ring)-center, which draws the young people most strongly. Special centers such as Harajuku, Aoyama and Roppongi are also preferred by this group, often in combination with a ring-center visit. Towards the inner city core (Ginza-Nihonbashi) central place preferences chiefly of the young male bachelors are declining gradually.Compared with young people the married people prefer the ring centers only slightly before the Ginza area, which for shopping is primarily visited by housewives, for going out mainly by husbands. Particularly in the inner city core near the main office districts a typical feature for Tokyo of central place preference is evident. For the salaryman — the most representative business type in the Tokyo area — going out is identical with not going home. This phenomenon depends on the place of work in two aspects. First, spatially in terms of a close connection between office and inner city amusement/shopping centers and additionally distance-movements towards a ring center nearest of the place of work or residence. Second, socially — social activities after working hours mainly carried out with colleagues result in a behavioural pattern, in which a clear differentiation between company and private life appears to be difficult. For not going home there are many reasons — among others the constricted living conditions of Japanese homes as Westerners believe — however, other factors play a much more important role: (i) Mainly the extremely long commuting distances, which allow the salaryman a real leaving home to go out only on Sundays or holidays. Other reasons for this typically Japanese feature closely related to inner city tavern and bar visits, may be seen in (ii) the traditionally very limited participation of Japanese housewives to their husbands' out-door social activities; (iii) a system of charging and crediting expenses to one's personal bank account or the company account; (iv) the personal atmosphere of taverns and bars providing the best chances for communication/relaxation and for overcoming stress and frustrations experienced in the office.Shopping is particularly for housewives the most important motivation for visiting urban centers, in this respect two Japanese characteristics should be mentioned: First, fashion and being well-groomed are, as far as the mass of people particularly the young women is concerned, more highly esteemed than in other urban societies. As a consequence good clothes are much in demand. Purchases of these kinds of articles are most often made in department stores and cause the longest distance-movements. Second, in rank of central goods next to fashion and quality clothes follow standard gifts such as seibo and chugen. The purchase of these articles is apart from the unique custom of giving gifts twice a year typically Japanese also in so far as it contradicts the least-effort criterion in central place theory. Though these patterns mostly bought by housewives or middle-aged and older people are substantially more of daily or periodical than episodical demand, they are related to surprisingly long distance-movements because of usually being bought for image reasons in leading department stores located in higher-ranking more distant urban centers.Typical behaviour for all groups visiting higher-order urban centers is an often used combination of shopping and taking part in out-door activities such as meeting friends/acquaintances, strolling around, going to the cinema or other show performances, eating, drinking and chatting together. This is due to the outstanding facilities available not only in retail-trade but also in the gastronomic, entertainment and amusement sectors.Taken all together, complexity and intensity of central place preferences in Tokyo reflect the higher and different attractiveness of Japanese urban centers compared with that of other urban societies. The urban centers are highly esteemed because of their excellent facilities as well as of their vitality, crowds and activity. These latter characteristics are summarized by the word nigiyaka meaning vivacity, which is most typical for Japanese urban centers. The inner city centers of Tokyo can be regarded in the true sense of this term as urban foci compensating for diverse problems such as environmental pollution or lack of inner urban green areas, making life in the most densely populated area of the world worthwhile.     

Röntgenuntersuchungen zum Leverrierit-Problem

Zusammenfassung Eine kritische Durchsicht der neueren Literatur über Leverrierit ergibt, daß es sich in allen Fällen um illitisches Material handelt. Röntgenographische Untersuchungen an fein- und grobblättrigem Material von Dobrilugk zeigten, daß dieser Leverrierit eine orientierte Verwachsung eines dioktaedrischen Illitesmit einem 14 -Mineral darstellt. Der Illit gehört zum Typ 2 M und zeigt eindimensionale Fehlordnung. Das 14 -Mineral scheint kaolinitischer Natur zu sein. Als ebenfalls diokaedrischer 2 M-Illit erwies sich ein Leverrierit aus dem Tiefsee-Schlamm des Tyrrhenischen Meeres. Die Bezeichnung Leverrierit als eigenständiger Mineralname sollte vermieden werden.     

Subsolidus phase relations in the system Zr-Fe-Ti-O in equilibrium with metallic iron. Implications for lunar petrology

An experimental study of the system Zr-Fe-Ti-O in equilibrium with metallic iron has been conducted at atmospheric pressure and in the temperature range 950–1300° C, with the purpose of modelling sub-solidus relations between Zr-bearing Fe-Ti oxide phases in lunar rocks. The phase relations are governed by the coexistence of the Fe-Ti oxides with ZrO₂ at temperatures below 1247° C, but with the new cubic ZFT phase at higher temperatures. ZFT, approximately Zr Fe Ti O₅, is probably related to the so-called stabilized cubic zirconia and could represent a synthetic equivalent of some lunar Zr-Fe-Ti rich oxide minerals.The zirconium contents in all Fe-, Ti and Fe-Ti-oxide phases increase with increasing temperature. At each temperature, Zr is incorporated preferentially in the Ti-oxide (up to 11 wt% ZrO₂), followed by ferropseudobrookite (up to 6 wt% ZrO₂), ilmenite (max. 4 wt%) and wüstite (max. 2.5 wt%). Ulvöspinel coexisting with ilmenite (+Fe⁰+ ZrO₂ and/or ZFT) always contains less than 1.2 wt% ZrO₂, whereas the same mineral in assemblage with wüstite-ZrO₂-Fe⁰ displays even higher ZrO₂ contents than ilmenite (above 4 wt%). Considering that the values determined here in the synthetic parageneses represent saturation concentrations at high temperatures in a simple model system, the ZrO₂ concentrations in the corresponding lunar minerals should generally be significantly lower. This is realized, except for the so-called Cr-Zr-Ca armalcolites which display ZrO₂ contents in the range 3.5–7 wt% and are thus probably related rather to Zr-rich oxide minerals than to armalcolite itself.     

Non-recognition of continental tholeiites using the Ti-Y-Zr diagram

Continental tholeiites cannot be identified using the Ti-Y-Zr diagram. Two-hundred and thirteen analyses from 16 localities representing 7 continental tholeiitic provinces all plot outside the within-plate basalt field of this diagram.Analyses representing about 50% of the population plot in the ocean-floor basalt field, the rest plots in the calc-alkali basalt field. Therefore problems might arise when attempts are made to classify ancient tectonomagmatic environments using the Ti-Y-Zr diagram.     

Artificial intelligence methods in geological forecasting

The objective of applications of expert systems to geological forecasting is to use heuristic knowledge of experienced geologists, as well as their intuition and facility for drawing the right forecast conclusions based on incomplete data. At the same time, considerable experience is accumulated in geological forecasting with formal mathematical models that generalize the setting of some forecast problem classes. Thus, the bottom-up heuristic approach and the more formal top-down methods are aspiring to the same aim, so they supplement and strengthen each other. A forecast expert system called GENESIS that allows both approaches to be used has been produced. The heuristic part of GENESIS is the empty expert system, which consists of the knowledge definition language, destined for an expert geologist, the knowledge base used for forecast models storing, then forming knowledge base facilities, relational database for storing user's data and forecast specimen, components that organize the certain inference and its explanation.     

Criteres d'analyse géochimique des gisements de fluorine basés sur l'étude de la distribution des lanthanides — application au gite de Maine (71—Cordesse,France)

59 fluorite samples have been analysed by neutron activation for La, Ce, Sm, Eu, Tb, Dy, Yb and Lu. Two properties of the partition coefficient of the R. E. E. between fluorite and poorly complexing solutions (L. Marchand, 1976) have been used for interpretations: (R. E. E.) CaF₂/(R. E. E.)_solution are always very high so that CaF₂ records with an important magnification and without major distortion in the R. E. E. pattern from the mother solution; the partition coefficients are higher for the heavy earths than for the lighter ones. Consequently, in finite reservoirs where CaF₂ precipitates, the trend for solutions will be a gradual enrichment in ceric earths. Interpretations at different scales of the Maine hydrothermal deposit (D. Joseph, 1974) permit to emphasize: — a constant geochemical signal at the scale of a growth band of homogenous colour; — a possible characterization of the ore-forming solutions dynamic (infinite vs finite reservoirs) by comparison of successive growth bands in a same sample; — distinct source rocks for the formation blanche and formation verte.     

Synthesis and some properties of iron- and vanadium-bearing kyanites, (Al,Fe3+)2SiO5and (Al,V3+)2SiO5

Iron- and vanadium-bearing kyanites have been synthesized at 900 and 1100° C/20 kb in a piston-cylinder apparatus using Mn₂O₃/Mn₃O₄- and MnO/Mn-mixtures, respectively, as oxygen buffers. Solid solubility on the pseudobinary section Al₂SiO₅-Fe₂SiO₅(-V₂SiO₅) of the system Al₂O₃-Fe₂O₃(V₂O₃)-SiO₂ extends up to 6.5 mole% (14mole %) of the theoretical end member FeSiO₅(V₂SiO₅) at 900°C/20 kb. For bulk compositions with higher Fe₂SiO₅ (V₂SiO₅) contents the corundum type phases M₂O₃(M = Fe³⁺, V³⁺) are found to coexist with the Fe³⁺(V³⁺)-saturated kyanite solid solution plus quartz. The extent of solid solubility on the join Al₂SiO₅-Fe₂SiO₅ at 1 100°C was not found to be significantly higher than at 900° C. Microprobe analyses of iron bearing kyanites gave no significant indication of ternary solid solubility in these mixed crystals. Lattice constants a ₀, b ₀, c ₀, and V₀ of the kyanite solid solutions increase with increasing Fe₂SiO₅- and V₂SiO₅-contents proportionally to the ionic radii of Fe³⁺ and V³⁺, respectively, the triclinic angles ,, remain constant. Iron kyanites are light yellowish-green, vanadium kyanites are light green. Iron kyanites, (Al_1.87 Fe _0.13 ³⁺ )SiO₅, were obtained as crystals up to 700 m in length.     

Sulphide neomagmas and highly metamorphosed sulphide deposits

Orebodies that have undergone high grade regional metamorphism sometimes exhibit features which appear to be metamorphic counterparts of certain magmatic-hydrothermal mineralization. These include ore bearing metapegmatites, metahydrothermal veins and metadeuteric veins and vugs. These mineral facies are completely contained within the metamorphosed orebody the ore minerals of which invariably exhibit textures resulting from annealing recrystallization. Intrusive relationships between metapegmatite, metahydrothermal veins, etc., and the orebody are evident. — Experimental work on certain sulphide systems indicates that partial melting may occur at temperatures around 700 °C, i.e., at the sillimanite grade of metamorphism. Thus a sulphide neomagma may develop along the lines of anatexis. — These aspects are considered with respect to the lead—zinc orebody at Broken Hill, New South Wales.

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Zusammenfassung Erzkörper, die hochgradiger Regionalmetamorphose unterworfen waren, zeigen manchmal Charakterzüge, welche denjenigen gewisser magmatisch-hydrothermaler Vererzungen gleichgestellt werden können. Diese Beispiele umfassen erzführende Metapegmatite, metahydrothermale Adern und metadeuterische Adern und Drusen. Diese Mineralfolgen sind beschränkt auf den metamorphosierten Erzkörper, dessen Erzmineralien dann jeweils Erzgefüge zeigen, die durch Rekristallisation entstanden sind. Intrusive Beziehungen zwischen den Metapegmatiten, den metahydrothermalen Adern usw. und dem Erzkörper sind offensichtlich. — Experimentelle Untersuchungen gewisser Sulfidsysteme deuten darauf hin, daß partielles Schmelzen bei Temperaturen um 700 °C möglich ist; also in dem Sillimanitbereich der Metamorphose. In dieser Weise ist es möglich, daß ein Sulfidneomagma sich in der Art einer Anatexis entwickeln kann. — Diese Probleme werden in bezug auf den Blei-Zink-Erzkörper in Broken Hill, New South Wales, Australien, diskutiert.

     

Reflected light examination of Marcasite and löllingite

Summary The reflectance spectra for the isotypic orthorhombic minerals marcasite, FeS₂ and löllingite, FeAs₂ have been measured in air and in immersion oil between 400 and 800 nm, and the indices of refraction and absorption indices have been calculated. Marcasite shows a practically isotropic behaviour of the optical constants, löllingite is characterized by a uniaxial optical behaviour with maximum absorption occurring perpendicular to the relatively short Fe-Fe distances.

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Auflichtoptische Untersuchungen von Markasit und Lollingit

Zusammenfassung Die Reflexionsspektren der isotypen rhombischen Minerale Markasit FeS₂ and Löllingit FeAs₂ wurden in Luft and in Immersionsöl zwischen 400 and 800 nm gemessen und die Brechungsindices and Absorptionskonstanten berechnet. Markasit zeigt ein praktisch isotropes Verhalten der optischen Konstanten, Löllingit ist durch einachsiges optisches Verhalten charakterisiert, wobei maximale Absorption senkrecht zu den relativ kurzen Fe-Fe Abständen auftritt.

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Dedicated to Prof Dr. J. Zemann on the occasion of his 70th birthday     

The metasomatic development of zoned ultrabasic bodies in Unst,Shetland

Read (1934b) described the mineralogy and regional setting of spectacularly zoned ultrabasic bodies occurring in Unst, Shetland. Chemical and phase compositional data are presented here for a selection of samples taken from each of the several monomineralic zones.The phase sequence and elemental distribution are discussed in some detail since both are complicated. It is shown that virtually all distributional features may be explained on the basis of a diffusion model: transport most probably having occurred via aqueous solutions in pore space.The present configurations represent frozen reaction intermediates. Controls upon the nature and extent of reaction are analysed. Certain features may be explained on the basis of local equilibrium thermodynamics. These arguments, however, are inadequate. Fairly simple kinetic considerations lead to a much more satisfactory analysis.Some of the problems associated with concepts of element mobility are discussed briefly.