Exploration for volcanogenic sulphide mineralisation at Benglog, north Wales

Exploration for volcanogenic sulphide mineralisation around Benglog is one of three investigations designed to assess the metallogenic potential of the Ordovician Aran Volcanic Group. Detailed geological mapping in the Benglog area enabled an interpretation of the volcanic environment, critical to such an assessment, to be made. The eruptive rocks are acid and basic in composition; the acid rocks are mostly ash-flow tuffs derived from outside the area, whereas the basic rocks have a local derivation. They are all interbedded with dark grey or black silty mudstone and were probably erupted in a submarine environment. Contemporaneous dolerite sills were intruded into wet sediment. This environment was suitable for volcanogenic exhalative sulphide deposits to form and indications of a metallogenic horizon were found at the top of the Y Fron Formation in the form of abundant pyrite, minor pyrrhotite and minor base metal enrichment. Soil samples, analysed for copper, lead and zinc, were collected and geophysical surveys were carried out along eleven east-west trending traverse lines 300 m apart across the volcanic succession. Indications were found of minor vein mineralisation at dolerite intrusion margins and locally along faults. Very high chargeability and low resistivity anomalies over mudstones did not spatially coincide with geochemical anomalies in soil, but the secondary redistribution of metals in soils and variable thickness of overburden precluded confident interpretation of the source of many soil anomalies. Geochemical drainage data, in conjunction with rock analyses, show strong barium enrichment in mudstones which could be volcanogenic in origin but related to two separate eruptive episodes. The findings of the survey were inconclusive. An environment suitable for the formation of volcanogenic exhalative sulphide deposits was established, but the geochemical and geophysical surveys located only minor vein mineralisation and tenuous indications of other styles of mineralisation. Recommendations are made for further work

Molecular dynamics in arbitrary geometries : parallel evaluation of pair forces

A new algorithm for calculating intermolecular pair forces in molecular dynamics (MD) simulations on a distributed parallel computer is presented. The arbitrary interacting cells algorithm (AICA) is designed to operate on geometrical domains defined by an unstructured, arbitrary polyhedral mesh that has been spatially decomposed into irregular portions for parallelisation. It is intended for nano scale fluid mechanics simulation by MD in complex geometries, and to provide the MD component of a hybrid MD/continuum simulation. The spatial relationship of the cells of the mesh is calculated at the start of the simulation and only the molecules contained in cells that have part of their surface closer than the cut-off radius of the intermolecular pair potential are required to interact. AICA has been implemented in the open source C++ code OpenFOAM, and its accuracy has been indirectly verified against a published MD code. The same system simulated in serial and in parallel on 12 and 32 processors gives the same results. Performance tests show that there is an optimal number of cells in a mesh for maximum speed of calculating intermolecular forces, and that having a large number of empty cells in the mesh does not add a significant computational overhead

Non-perturbative effective field theory for two-leg antiferromagnetic spin ladders

We study the long wavelength limit of a spin 1/2 Heisenberg antiferromagnetic two-leg ladder, treating the interchain coupling in a non-perturbative way. We perform a mean field analysis and then include exactly the fluctuations. This allows for a discussion of the phase diagram of the system and provides an effective field theory for the low energy excitations. The coset fermionic Lagrangian obtained corresponds to a perturbed SU(4)_1/U(1) Conformal Field Theory (CFT). This effective theory is naturally embedded in a SU(2)_2 x Z_2 CFT, where perturbations are easily identified in terms of conformal operators in the two sectors. Crossed and zig-zag ladders are also discussed using the same approach.Comment: 14 pages LaTeX, 5 PostScript figures included using epsfig.sty; minor corrections and a few references adde

Transition Spectra for a BCS Superconductor with Multiple Gaps: Model Calculations for MgB_2

We analyze the qualitative features in the transition spectra of a model superconductor with multiple energy gaps, using a simple extension of the Mattis-Bardeen expression for probes with case I and case II coherence factors. At temperature T = 0, the far infrared absorption edge is, as expected, determined by the smallest gap. However, the large thermal background may mask this edge at finite temperatures and instead the secondary absorption edges found at Delta_i+Delta_j may become most prominent. At finite T, if certain interband matrix elements are large, there may also be absorption peaks at the gap difference frequencies | Delta_i-Delta_j | . We discuss the effect of sample quality on the measured spectra and the possible relation of these predictions to the recent infrared absorption measurement on MgB_2

A Model for the Elasticity of Compressed Emulsions

We present a new model to describe the unusual elastic properties of compressed emulsions. The response of a single droplet under compression is investigated numerically for different Wigner-Seitz cells. The response is softer than harmonic, and depends on the coordination number of the droplet. Using these results, we propose a new effective inter-droplet potential which is used to determine the elastic response of a monodisperse collection of disordered droplets as a function of volume fraction. Our results are in excellent agreement with recent experiments. This suggests that anharmonicity, together with disorder, are responsible for the quasi-linear increase of $G$ and $\Pi$ observed at $\varphi_c$.Comment: RevTeX with psfig-included figures and a galley macr

Lattice Dynamics and the High Pressure Equation of State of Au

Elastic constants and zone-boundary phonon frequencies of gold are calculated by total energy electronic structure methods to twofold compression. A generalized force constant model is used to interpolate throughout the Brillouin zone and evaluate moments of the phonon distribution. The moments are used to calculate the volume dependence of the Gruneisen parameter in the fcc solid. Using these results with ultrasonic and shock data, we formulate the complete free energy for solid Au. This free energy is given as a set of closed form expressions, which are valid to compressions of at least V/V_0 = 0.65 and temperatures up to melting. Beyond this density, the Hugoniot enters the solid-liquid mixed phase region. Effects of shock melting on the Hugoniot are discussed within an approximate model. We compare with proposed standards for the equation of state to pressures of ~200 GPa. Our result for the room temperature isotherm is in very good agreement with an earlier standard of Heinz and Jeanloz.Comment: 13 pages, 8 figures. Accepted by Phys. Rev.

Algorithm for numerical integration of the rigid-body equations of motion

A new algorithm for numerical integration of the rigid-body equations of motion is proposed. The algorithm uses the leapfrog scheme and the quantities involved are angular velocities and orientational variables which can be expressed in terms of either principal axes or quaternions. Due to specific features of the algorithm, orthonormality and unit norms of the orientational variables are integrals of motion, despite an approximate character of the produced trajectories. It is shown that the method presented appears to be the most efficient among all known algorithms of such a kind.Comment: 4 pages, 1 figur