Epithermal base-metal vein mineralization in the southern uplands of scotland: Nature and origin of the fluids

The sedimentary sequences of the Southern Uplands of Scotland host numerous lead-zinc-copper-silver vein deposits, the genesis of which has never been adequately explained. Fluid inclusion and stable isotope analysis of vein minerals from these deposits indicate that, for the vein stages studied, the mineralizing fluids were low temperature (< 150°C), high salinity (19 to 30 equiv. wt. % NaCl + CaCl₂) modified meteoric waters. A consideration of the availability of such fluids throughout the geological history of the Southern Uplands suggests a Lower Carboniferous (Dinantian) age for the mineralization.     

The diagenetic history of some septarian concretions from the Kimmeridge Clay, England

ABSTRACT
Large septarian concretions from the Kimmeridge Clay, up to 1.2 m in diameter, have centres comprising anhedral calcite microspar passing into margins of radiating fibrous calcite microspar, with a pyrite-rich zone at the transition. Septarian veins formed and were lined with brown calcite synchronously with fibrous matrix growth, with white calcite precipitated in septarian cavities after concretion growth ceased. Septarian veins, filled only with white calcite, formed later, at the same time as the outermost calcite microspar crystals were enlarged.
The concretions were buried in the Late Jurassic to about 130 m, and in the Late Cretaceous to about 550 m, with uplift between. Oxygen isotopes show that the concretion grew throughout the first burial, with septarian veins forming from about 30 m depth onwards. Later septarian veins formed between about 200 and 500 m during the second burial.
Carbon isotopes show that the compact inner matrix grew in the sulphate reduction zone, the end of which is marked by the pyrite-enriched zone. Dissolving shells, and possibly minor methanogenic carbonate, slowly diluted sulphate reduction-zone carbonate during deeper burial. During early concretion growth, Mg and Sr were depleted in the pore water. During later stages of the first burial, Mg, Sr, Mn and Fe all increased, especially after concretion growth ceased. During the second burial, Fe, Mn and Mg decreased as calcite precipitated, implying relatively closed systems for these elements.
Synchronous formation of septarian fractures and fibrous calcite matrix shows that the Kimmeridge Clay became overpressured during the later stages of both burials.     

Primary cracking of kerogens. Experimenting and modelling C1, C2–C5, C6–C15 and C15+ classes of hydrocarbons formed

Mathematical models of hydrocarbon formation can be used to simulate the natural evolution of different types of organic matter and to make an overall calculation of the amounts of oil and/or gas produced during this evolution. However, such models do not provide any information on the composition of the hydrocarbons formed or on how they evolve during catagenesis.From the kinetic standpoint, the composition of the hydrocarbons formed can be considered to result from the effect of “primary cracking” reactions having a direct effect on kerogen during its evolution as well as from the effect of “secondary cracking” acting on the hydrocarbons formed.This report gives experimental results concerning the “primary cracking” of Types II and III kerogens and their modelling. For this, the hydrocarbons produced have been grouped into four classes (C₁, C₂–C₅, C₆–C₁₅ and C₁₅₊). Experimental data corresponding to these different classes were obtained by the pyrolysis of kerogens with temperature programming of 4°C/min with continuous analysis, during heating, of the amount of hydrocarbons corresponding to each of these classes.The kinetic parameters of the model were optimized on the basis of the results obtained. This model represents the first step in the creation of a more sophisticated mathematical model to be capable of simulating the formation of different hydrocarbon classes during the thermal history of sediments. The second step being the adjustment of the kinetic parameters of “secondary cracking”.     

Algorithmic techniques of identifying discriminative ocean areas in oceanology problems

A problem of locating and comprehensively investigating ocean areas that exercise a significant influence over the migration dynamics of fishing ranks among important problems of fisheries oceanology. Among all the environment factors affecting various fishery situations, factors should be chosen that are constantly logged on large water areas for a long time comparable with fishery observation time. As described in a number of works, the most suitable information for these purposes is data on the distribution of surface temperature fields. The problem of locating discriminative areas can then be stated as follows. For a particular fishery situation (e.g., for the period of maximum catch in fishing season), temperature values or thermostructural features should be singled out that are suitable to define the situation, using hydrological charts of surface temperature fields. Correlating the feature with the areas where they were observed, we will obtain discriminative areas. The important problem is then to develop techniques and means for algorithmically identifying areas. This relates to a vast volume of information and complex structure. Logical-mathematical models of locating discriminative areas are being developed at the Pacific Oceanological Institute for some years. These models are to be for forecasting fishery situations in the NW Pacific. Techniques for representing isotherm fields in a computer have been developed. Formal operations of comparing the fields have been proposed which aid in distinguishing significant differences in their structure. A software for analyzing isotherm fields has been developed based on threshold logic models. This approach allows one not only to identify discriminative areas for relatively stable fishery situations but also to trace their dynamics. This has been achieved due to multiple-alternative computations and the presentations of data obtained in the form suitable for their interpretation. Some components of proposed models have been used in the system of short-term forecasting saira and sardine fishery in the South-Kuril area.     

Modelling of cloud formation due to air-sea interactions in an energy-active zone

A mesoscale 3D numerical model is described, with which detailed calculations have been made of turbulence and wind characteristics in the atmospheric boundary layer (ABL), as well as cloud particle size distribution, longwave and solar radiation fluxes and flux divergences, and atmosphere-ocean heat exchange. Based on numerical experiments simulating winter conditions of the Newfoundland energy-active zone of the ocean (EAZO), atmosphere-ocean energy exchange is investigated. It is shown that the basic mechanisms for the EAZO formation involve the following processes: (i) at the hydrological front between cold and warm ocean currents, the fluxes of sensible and latent heat grow significantly; (ii) at this front, in a particular synoptic situation, overcast low-level cloudiness forms, screening solar radiation so that in winter, the radiation budget at the front is reduced, and the radiative flux into the ocean is less than the energy release to the atmosphere; (iii) frequent occurrence of such synoptic situations with cloudiness decreases the oceanic enthalpy and creates negative SST anomalies. The transport of these anomalies by currents to the western coasts of the continents causes anomalies of weather and climate.     

Activity-composition relations in FeTiO3-MnTiO3 ilmenite solid solutions from EMF measurements at 1050–1300 K

The activity of FeTiO₃ in the binary ilmenitepyrophanite (FeTiO₃-MnTiO₃) solid solution has been determined for the temperature range 1050–1300 K by displacement of the reaction: 2 FeTiO₃=2 Fe+2 TiO₂+O₂ in T-O₂ space, using an electrochemical technique with calcia- and yttria-stabilized zirconia solid electrolytes. The activities show small positive deviations from ideality, which, although the data are unusually precise, may be perfectly described by the simple regular solution model, with an interaction parameter, W _Fe-Mn ^ilm , of 2.2±0.3 kJ g-atom^–1, which is independent of temperature. This value, together with the experimental data on the partitioning of Fe and Mn between ilmenite and garnet (Pownceby et al. 1987), gives W _Fe-Mn ^gt =1.2±0.5 kJ g-atom^–1 for the almandine-spessartine (Fe₃Al₂Si₃O₁₂-Mn₃Al₂Si₃O₁₂) solid solution.     

Early Cretaceous uplift and erosion of the northern Appalachian Basin, New York, based on apatite fission track analysis

The thermal history of outcropping Devonian sediments of the northern Appalachian Basin, New York, has been investigated using fission track analysis of detrital apatites from 57 sandstone samples. Based on lengths and apparent age measurements using fission tracks in apatite it is concluded that Lower Devonian sediments presently at the surface in the Catskill region were cooled rapidly from temperatures higher than about 110°C during Early Cretaceous times (120–140 Ma ago). In the western part of New York (Wellsville-Buffalo) data from late Devonian sediments are consistent with cooling at the same time as that identified for the Catskill region but from lower temperatures, in the range of approximately 80–110°C, the maximum temperature these sediments experienced since deposition. For a pre-uplift paleogeothermal gradient of 25–35°C/km, the confined track length data indicates uplift and erosion of 2–3 km for western New York and greater than 3–4 km for the Catskill region, a differential uplift pattern which is consistent with the historical stratigraphic data from the region. This conclusion is at variance with earlier interpretations put forth by others.Rapid broad scale uplift and erosion of the scale identified imply that large volumes of sediment could have been supplied from the northern Appalachian Basin during the Early Cretaceous. This timing for the dominant post-Devonian cooling phase in the basin is not accounted for by recent models of the tectonic evolution of the Appalachian Orogen but is compatible with the change from carbonate to siliciclastic deposition in the Atlantic coastal plain. It is suggested that this style of broad regional uplift without significant deformation is characteristic of a tectonic regime associated with, and subsequent to, continental rifting.Apatite fission track analysis is shown to be a basic tool in providing fundamental limits for thermal history assessment in regional tectonic problems.     

Seismic imaging of deep hydrocarbon reservoirs

This paper examines those aspects of reflection seismology which require special consideration when imaging deeper hydrocarbon reservoirs, including the constraints imposed by vertical resolution, lateral resolution, and velocity analysis. We derive quantitative expressions relating the uncertainties in stacking velocities and in interval velocities derived from stacking velocities to acquisition parameters, as well as expressions for the lateral resolution which can theoretically be achieved for migrated seismic images. This analysis shows that the most significant limitations of seismic imaging at depth involve the finite lateral resolution of the seismic method, and the proper lateral positioning of seismic images. These difficulties are overcome in large measure through the proper migration of a seismic dataset, which becomes more critical as deeper horizons are imaged. If these horizons are suspected of having significant 3-D structure, a strong argument may be made for acquiring a 3-D seismic survey over the prospect. Migration of this dataset will then generate an image of the subsurface with good lateral resolution in both the X and Y directions.     

STATISTICAL TECHNIQUES APPLIED TO BOREHOLE GEOPHYSICAL DATA IN GOLD EXPLORATION1

Gold, as a trace element, is not directly detectable by geophysical borehole logging techniques. Geophysical logging methods have therefore been used to infer the presence of structural features and alteration processes associated with gold. Since these features and processes differ with the style of gold mineralization, a unique relation between geophysical anomalies and gold cannot be established. However, in a particular environment, such an association can be established. We examined geophysical log data (self potential, induced polarization, resistivity, gamma, temperature, and temperature gradient) and drill core from the Barber-Larder property in NE Ontario. The geophysically detectable alteration processes associated with gold were identified as sericitization and pyritization, by means of factor analysis. A linear discriminant function was constructed which allowed the zones of economic mineralization (nominally 0.05 oz/ton or 1.5 ppm) to be identified with a 75% success rate based on geophysical log data alone. This rate was achieved without sophisticated non-linear data transformations.     

Spatial and seasonal distribution of bacterial physiological groups in two reservoirs with different trophic levels

The spatial and seasonal distribution of the viable heterotrophic population and three heterotrophic physiological groups, i.e. ammonifyers, nitrate reducers and sulphate reducers, were studied in two reservoirs in the province of Vizcaya (Spain). Using indicative parameters, both reservoirs were classified as warm monomictics, one hypereutrophic and the other mesotrophic. A direct relation between the size of the bacterial populations and the trophic levels of the reservoirs was observed. Likewise, the distributions of the bacterial populations in the water column are basically different. In the hypereutrophic lake there is a direct relationship between the bacterial distribution and the physical-chemical environment of the reservoir that cannot be observed in the mesotrophic lake.