Characterisation of gold from Fiji

This is a study of the variation in chemistry and inclusion mineralogy of bedrock and placer gold from Fiji. It forms part of a large project, undertaking gold characterisation from a wide range of geological environments in Ecuador, Zimbabwe, Malaysia and Fiji. The work was carried out under the Overseas Development AdministratiodBritish Geological Survey Technology Development and Research programme (Project R5549) as part of the British Government’s provision of technical assistance to developing countries. For the Fijian component of the project, samples were collected from river gravels, primary ore, and table concentrates. In total thirty-five samples from five localities were examined. Data collected from these samples are represented by over 100 point analyses of gold, identification of associated minerals, and microgeochemical maps of gold-mineral intergrowths. A framework for identifying possible sources of alluvial gold is given. This was achieved by characterising bedrock gold mineralisation from a variety of epithermal and porphyry environments. The environments studied included alkali (the Emperor Gold Mine), high-sulphidation (the Mount Kasi prospect) and telescoped (the Tuvatu prospect) systems. A study of placer gold from the Waimanu alluvial deposit, the only alluvial gold deposit in Fiji, showed two distinct sources of gold: one, a low-silver type, associated with Cu-Fe sulphides, can be related to the nearby Namosi porphyry copper deposit. The other source, a high-silver type with abundant tellurides, indicates an alkali epithermal association, suggesting a source similar in style to the mineralisation observed at Emperor Gold Mine. The identification of two bedrock sources for the Waimanu alluvials clearly shows that there must be a, yet unknown, alkali epithermal (Emperor) source within the Waimanu catchment. This demonstrates the power of alluvial gold characterisation and its role in gold exploration. An important implication of these results is that future exploration within the area should be focussed on locating this “Emperor type” source

Polymer adsorption to titania surfaces studied by adsorption isotherm, rheology and atomic force microscopy

This Ph.D. study investigates the adsorption of monomeric (isostearic acid, ISA) and oligomeric fatty acids (poly(hydroxystearic acid), PHS 1400 and PHS 3500) to nanoparticles of industrial interest in carriers with different solvent properties. It was found that it was necessary to apply a number of methods to study the adsorption and subsequent stabilisation of the particles by the dispersing molecules, namely adsorption isotherm methods, rheological techniques, atomic force microscopy and UV/vis spectroscopy. The particles (surface) studied consist of uncoated titania and a commercially available titania coated with a combination of alumina and silica. The use of gel permeation chromatography (GPC) to study the adsorption behaviour of polydisperse dispersants provided information not only about the amount of the dispersant adsorbed to the surface, but also the preferential adsorption of low molecular weight components. The displacement of large molecules by smaller ones could be monitored in order to gain insight into the types of adsorption mechanisms at work. It was found that small molecules were unable to fully displace their larger counterparts, suggesting that there was more than one adsorption mechanism in effect. This was supported by the adsorption study of one of the fatty acid dispersants which was esterified with methanol to remove the acid functionality. Adsorption at the particle surface still occurred, but at a much reduced rate. The preferential adsorption of the smaller molecules was also found to be largely eliminated. Adsorption isotherms showed Langmuir-like adsorption behaviour of the molecules to the surface, probably through a combination of acid-base interaction and other specific interactions between surfactant molecule and surface. Adsorption of the dispersant molecules at the particle surface was found to vary with solvent properties reflecting the equilibrium which is established between solubility of the dispersant in solution (χ) and that adsorbed on the surface (г) as a result of adsorption affinity (χs). Steric layer thickness δ could be varied by altering dispersion medium (and hence solvency of the stabilising chain) and by altering molecular weight of the surfactant. This had a significant effect upon the bulk properties of the suspended particles measured by rheological methods. For optimised dispersion performance with high solids load at low viscosity whilst maintaining descrete dispersed particles, as determined by assessing the optical properties of the suspension, the stabilising layer required is dependent upon the particle type and size: small particles require small steric layers. It was found that some degree of polydispersity was important for the oligomeric fatty acid dispersants. Although good adsorption characteristics were observed for the monodisperse monomeric dispersant, adsorption appeared to be optimum for the 1400 Mw oligomeric which had a high proportion of low molecular weight components as well as larger molecules present. Removal of the low molecular weight components from PHS 3500 resulted in interactions measured by AFM that appeared to indicate bridging behaviour due to reduced packing efficiency

Rapid Computation of Thermodynamic Properties Over Multidimensional Nonbonded Parameter Spaces using Adaptive Multistate Reweighting

We show how thermodynamic properties of molecular models can be computed over a large, multidimensional parameter space by combining multistate reweighting analysis with a linear basis function approach. This approach reduces the computational cost to estimate thermodynamic properties from molecular simulations for over 130,000 tested parameter combinations from over a thousand CPU years to tens of CPU days. This speed increase is achieved primarily by computing the potential energy as a linear combination of basis functions, computed from either modified simulation code or as the difference of energy between two reference states, which can be done without any simulation code modification. The thermodynamic properties are then estimated with the Multistate Bennett Acceptance Ratio (MBAR) as a function of multiple model parameters without the need to define a priori how the states are connected by a pathway. Instead, we adaptively sample a set of points in parameter space to create mutual configuration space overlap. The existence of regions of poor configuration space overlap are detected by analyzing the eigenvalues of the sampled states' overlap matrix. The configuration space overlap to sampled states is monitored alongside the mean and maximum uncertainty to determine convergence, as neither the uncertainty or the configuration space overlap alone is a sufficient metric of convergence. This adaptive sampling scheme is demonstrated by estimating with high precision the solvation free energies of charged particles of Lennard-Jones plus Coulomb functional form. We also compute entropy, enthalpy, and radial distribution functions of unsampled parameter combinations using only the data from these sampled states and use the free energies estimates to examine the deviation of simulations from the Born approximation to the solvation free energy

A Review of Welding in Space and Related Technologies

Deployment of welding and additive manufacturing (AM) technologies in the space environment has the potential to revolutionize how orbiting platforms are designed, manufactured, and assembled. These technologies offer the option for repair of sustained damage to habitat structures on space missions, as astronauts would be able to manufacture new parts (using welding-derived AM processes suitable for use in the external space environment) and weld cracks. An added benefit is that required repairs can be achieved more economically, as new parts need not be shipped from Earth. With further maturation of in-space welding capabilities, astronauts could operate under given standards and weld damaged structures rather than rely on cargo resupply. This Technical Memorandum (TM) begins by reviewing the available literature relevant to welding in space, focusing on solidification, heat and mass transfer, and fluid flows in microgravity. This survey considers research on the effects of welding in microgravity on a material system. The various in-space welding devices that have been previously designed and tested are examined to determine their capabilities and shortcomings, with a focus on the results of their individual welding experiments. Safety measures are discussed to protect the orbiting International Space Station (ISS) and crew during welding operations. Finally, the state of the art is examined by focusing on current approaches to AM and on-orbit welding that are being developed by several companies in conjunction with NASA

Active geothermal systems with entrained seawater as analogues for low-sulphidation epithermal mineralization

The paradigm for low-sulphidation (LS) volcanic-arc associated mineralization is the active geothermal systems located along the Taupo Volcanic Zone (e.g. Broadlands). However, this analogue is inapt where fluid salinities are consistently in excess of 3.5 wt % NaCl. LS mineralization on Milos (Aegean arc) records high paleofluid-salinities. The δD and δ18O data do not exemplify 18O-shifted meteoric waters—typical of terrestrial geothermal systems. Nor is a submarine origin indicated—stable isotope data show mixing between meteoric, seawater and volcanic-arc gases. Strontium isotope data are comparable to a nearby active seawater-entrained geothermal system. These are features seen in hydrothermal systems associated with emergent volcanoes. For the Milos LS mineralization, high-salinity fluids show it cannot be explained by a Broadlands-type model. The absence of saliferous sequences and significant intrusive rocks preclude these as salinity sources. The similarities between paleo and active systems in terms of salinity, δD–δ18O and strontium isotope systematics strongly suggest that seawater is the main source for Na and Cl. We suggest geothermal systems, containing seawater, associated with emergent volcanoes are an alternative analogue for LS epithermal mineralization. Furthermore, they bridge the gap between submarine, and large-scale terrestrial geothermal systems—the modern analogues for VHMS and epithermal mineralisation in the scheme of intrusion-centered hydrothermal mineralization