An attempt to correlate non-predicted variations of distribution coefficients with mineral grain internal inhomogeneity using a field example studied near Sudbury,Ontario

This study is an attempt to understand anomalous distribution coefficient values in the light of the fact that many metamorphic minerals are internally inhomogeneous. A number of models are proposed to show how the non-sytematic variation of distribution coefficient values may be produced by partial re-equilibration (and/or reaction) of ions between minerals and which samples would give the values most closely corresponding to those expected from the index minerals. It is suggested that the irregularity of the distribution coefficients is due to chemical variations of the bulk chemistry, both from handspecimen to handspecimen and in different parts of a single handspecimen. Samples from a field area containing the Staurolite-Quartz, Kyanite-Muscovite and Sillimanite-K feldspar subfacies of the Almandine Amphibolite facies, located near Sudbury, Canada, were submitted to mineral separation and analysis as well as electronmicroprobe analysis. Despite the fact that much scatter occurs between distribution coefficients due to partial re-equilibration, the values of Mg and Fe⁺² of D _{biotite/Garnet} decrease and Mn and Ca increase recognizably across the area. Generally, the model(s) proposed seem applicable to the field data presented.     

The selective replacement of the aluminum silicates by white mica

From considerations of relativeG-T surfaces inferred from publishedP-T data and the occurrence of replacement textures of Al₂SiO₅ polymorphs in rocks, the relative positions of curves representing the following equation in _K+ —T — pH ispace on substituting Al₂SiO₅ different polymorphs are derived.3 Al₂SiO₅ + 3 SiO₂ (quartz) + 2 K⁺ + 3 H₂O 2 KAl₂[AlSi₃O₁₀](OH)₂ (muscovite)+ 2 H⁺. The curves are different because of the differentG-T values for the polymorphs which, in the field, is borne out by the observation that in a rock containing two or three Al₂SiO₅ polymorphs, in nearly all instances only one polymorph is replaced by white mica. Instances of textural relations showing the interpreted selective replacement of one Al₂SiO₅ polymorph by a white mica in the presence of one (or two) other Al₂SiO₅ polymorph(s) are cited both from the literature and various field examples. The selective replacement of kyanite if sillimanite and/or andalusite is/are present, and of andalusite if only sillimanite is present are interpreted to show that generally during the muscovitization reaction, the field of sillimanite in the above reaction (left hand side) at a particular pH (H⁺ concentration) and is larger in _K+ —T space than that of andalusite which in turn is larger than that of kyanite. Theoretically it is shown that variations to this can exist but the field evidence suggests these only occur under rare geological conditions. Although this is not totally conclusive, the selectiveness of the replacement is interpreted to show that the fluid was buffered with respect to K⁺ and H⁺ on or near the curve of the polymorph showing the lowest stability field until that polymorph is totally consumed, after which the fluid composition moves to the next lowest curve for the remaining polymorph(s) present in the rock. The alteration of more than one polymorph by an apparently simultaneous process of alteration is rare and usually occurs at a low grade of metamorphism. This is interpreted to show that the buffering reaction could not keep pace with the influx of fluid and change the composition of this fluid (in most cases).     

Temperature affects the timing of spawning and migration of North Sea mackerel

Climate change accentuates the need for knowing how temperature impacts the life history and productivity of economically and ecologically important species of fish. We examine the influence of temperature on the timing of the spawning and migrations of North Sea Mackerel using data from larvae CPR surveys, egg surveys and commercial landings from Danish coastal fisheries in the North Sea, Skagerrak, Kattegat and inner Danish waters. The three independent sources of data all show that there is a significant relationship between the timing of spawning and sea surface temperature. Large mackerel are shown to arrive at the feeding areas before and leave later than small mackerel and the sequential appearance of mackerel in each of the feeding areas studied supports the anecdotal evidence for an eastward post-spawning migration. Occasional commercial catches taken in winter in the Sound N, Kattegat and Skagerrak together with catches in the first quarter IBTS survey furthermore indicate some overwintering here. Significant relationships between temperature and North Sea mackerel spawning and migration have not been documented before. The results have implications for mackerel resource management and monitoring. An increase in temperature is likely to affect the timing and magnitude of the growth, recruitment and migration of North Sea mackerel with subsequent impacts on its sustainable exploitation.     

Natural staurolite breakdown reactions at moderate to high pressures

Existing staurolite breakdown reactions are briefly discussed and examples of each of the main types suggested from textural evidence from different well studied areas are analyzed. From the staurolite textural data, it is shown that: the pseudomorphic replacement is generally equal area; the replacement is usually by a single mineral or, where a number occur, one volumetrically greatly predominates; and successive reaction coronas exist around a single reactant grain. The analogy between the textural coronas and “monomineralic zones” at the contact of dissimilar rock types is drawn with the subsequent inference that reaction coronas in some, if not most, cases are out of equilibrium with the rest of the assemblage as well as the reactant mineral. In staurolite reaction textures, different grains in a single thin section normally have: different amounts (ratios) of product minerals, different product minerals, some grains which have reacted while others have not and different grains which have reacted to different degrees. Traverses of essentially total chemical analyses across staurolite grains in two test sections showed that considerable intertextural chemical variation in single thin sections exist which is much greater than the chemical variations within textures. Variations of, from 0.59 to 1.86 weight percent ZnO in different homogeneous staurolite grains occur in a single thin section (Z-5670911) and measurable variations of Si, Al, Fe, Mg and Mn are recorded. In all the textures analyzed both Zn and Al was deficient on the product side of balanced reactions, and particularly Zn can be shown to have been lost from the area of the section. The chemical and textural conclusions show that the limits of equilibrium systems, from which metamorphic reactions can be inferred, must be confined to the boundaries of the pseudomorphic texture, which means that ionic (complex bearing ?) species are involved and these systems must be treated as being open.     

Geochemical and temperature controls on ore mineralization at the Emperor gold mine, Vatukoula, Fiji

Emperor is a large epithermal gold-tellurium deposit which occurs at the margin of the Tavua caldera and is associated with high-level mineralized tuffs, breccias and silicious sinters. The deposit consists of mainly narrow, steep “shear” and shallow dipping “flatmake” mineralized vein structures. Mineralization spanned a period during which different veins and dyke intrusions occurred so that some early structures are offset by later ones. High Au values and high Au/Ag ratios, corresponding to the intersection of shear and flatmake structures, generally correlate with high Ag, Te and Ba values in some structures (Crown Shear) and high Hg in others (166 Flatmake). Other elements commonly anomalous in the high-grade ore zones are As, Mo and Cu. Thallium, Se and Sb are commonly high below ore zones while Hg may be high above (e.g., Crown Shear), in (166 Flatmake) or even below (e.g., Prince of Wales Shear) ore zones.Gold precipitated in a temperature interval of 180–210°C; higher temperatures commonly occur at depth in some flatmakes (166 N, 608) and lower temperatures nearer the surface. In the two steep shears analyzed, higher temperatures correspond to where they are intersected by flatmakes. Temperatures may decrease below such intersections in which case Hg, Sb, etc., commonly increase with depth.Evidence of boiling, characterized by coexisting gas- and liquid-rich fluid inclusions, is minor and spatially sporadic. Quartz and carbonate found in ore containing bonanza concentrations of Au contain few to no gas-rich fluid inclusions. The intensity of wall-rock alteration is unrelated to gold grades.Ore deposition is inferred to have been caused mainly in response to fluid mixing rather than extensive boiling and/or wall-rock alteration, although these processes occurred. The source of the Au, Ag, Te, etc. is inferred to be near a neutral, relatively reduced, bisulphide-rich, ore solution at near 300°C and derived in some way from the shoshonitic volcanics or associated monzonitic intrusions. The second non-ore solution necessary for mixing, is inferred to have been an acid, oxidized solution at 150°C and having a large meteoric component. The result of fluid mixing was ore genesis, and a telluride, minor sulphide and very minor sulphate (barite) and anhydrite ore assemblage.     

Comparing uncertain alternatives for a possible airport island location in the North Sea

This paper presents a methodology for making decisions based on uncertain information through the use of an analytical feasibility study of an airport island in the North Sea as an alternative to the present inland airport, Amsterdam Schiphol, in The Netherlands. The multidisciplinary project, called Flyland, quantified several conflicting aspects, including uncertainties, for each of five alternative locations for the new airport. The methodology combines the Analytic Hierarchy Process and Monte Carlo approaches and allows comparison of the alternatives on the basis of their morphological and ecological effects. The resulting scores rank the alternatives and quantify the distinctions between them by taking uncertainties into account. We found that with respect to morphology and ecology, it is most favourable to keep the Dutch national airport inland. The methodology can easily be extended to include other factors and reflect a wider range of multidisciplinary aspects (costs, accessibility, environmental aspects) of the airport island location.