The physical and chemical evolution of low-salinity magmatic fluids at the porphyry to epithermal transition: a thermodynamic study

Fluid-phase relationships and thermodynamic reaction modelling based on published mineral solubility data are used to re-assess the Cu-Au-mineralising fluid processes related to calc-alkaline magmatism. Fluid inclusion microanalyses of porphyry ore samples have shown that vapour-like fluids of low to intermediate salinity and density (~2-10 wt% NaCl eq.; ~0.1-0.3gcm−3) can carry percentage-level concentrations of copper and several ppm gold at high temperature and pressure. In epithermal deposits, aqueous fluids of similar low to intermediate salinity but liquid-like density are ubiquitous and commonly show a magmatic isotope signature. This paper explores the physical evolution of low-salinity to medium-salinity magmatic fluids of variable density, en route from their magmatic source through the porphyry regime to the near-surface epithermal environment, and investigates the chemical conditions required for effective transport of gold and other components from the magmatic to the epithermal domain. Multicomponent reaction modelling guided by observations of alteration zonation and vein overprinting relationships predicts that epithermal gold deposits are formed most efficiently by a specific succession of processes during the evolution of a gradually cooling magmatic-hydrothermal system. (1) The low-salinity to medium-salinity fluid, after separating from the magma and possibly condensing out some hypersaline liquid in the high-temperature porphyry environment, must physically separate from the denser and more viscous liquid, and then cool within the single-phase fluid stability field. By cooling under adequate confining pressure, such a vapour will evolve above the critical curve and contract, without any heterogeneous phase change, to an aqueous liquid of the same salinity. (2) High concentrations of gold, transported as stable Au bisulphide complexes supporting >1ppm Au even at 200°C, can be maintained throughout cooling, provided that the fluid initially carries an excess of H2S over Cu+Fe on a molal scale. This condition is favoured by an initially high sulphide content in a particularly low-salinity magmatic fluid, or by preferential partitioning of sulphur into a low-salinity vapour and partial removal of Fe into a hypersaline liquid at high temperature. (3) Acid neutralisation further optimises gold transport by maximising the concentration of the HS− ligand. This may occur by feldspar destructive alteration along pyrite±chalcopyrite±sulphate veins, in the transition zone between the porphyry and epithermal environments. An alternative acid/base control is the dissolution of calcite in sediments, which may enable long-distance gold transport to Carlin-type deposits, because of the positive feedback between acid neutralisation and permeability generation. The three physical and chemical transport requirements for high-grade epithermal gold mineralisation are suggested to be the common link of epithermal gold deposits to underlying magmatic-hydrothermal systems, including porphyry-Cu-Au deposits. Both mineralisation types are the result of gradual retraction of isotherms around cooling hydrous plutons in similar tectonic and hydrologic environments. As magmatic fluid is generated at increasing depths below the surface the importance of vapour contraction increases, leading to the typical overprinting of potassic, phyllic and advanced argillic alteration and their related ore style

Eclogite Facies Regional Metamorphism of Hydrous Mafic Rocks in the Central Alpine Adula Nappe

The Adula Nappe is a slice of Pre-Mesozoic continental basement affected by Early Alpine (Mesozoic or Lower Tertiary) high-pressure metamorphism. Mineral compositions in mafic rocks containing omphacite + garnet + quartz record a continuous regional trend of increasing recrystalliza tion temperatures and pressures that can be ascribed to this regional high-pressure metamorphic event. P-T estimates derived from mineral compositions grade from about 12 kb and 500 °C or less in the north of the nappe to more than 20 kb/800 °C in the south. The regional P-T trend is associated with a mineralogical transition from assemblages containing additional albite and abundant amphiboles, epidote minerals, and white micas in the north (omphacite-garnet amphibolites) to kyanite eclogites containing smaller amounts of hornblende and zoi.site in the south. Textures and mineral compositional data show that these hydrous and anhydrous silicates associated with omphacite + garnet + quartz arc primary parts of the high-pressure assem blages. Observed phase relations between these primary silicates, theoretical Schreinemakers analysis, and the thermobarometric results, together indicate that the regional transition from omphacite amphibolites to kyanite eclogites can be described by two simplified reactions: alb+epi+hbl=omp+kya+qtz+par (H2O-conserving) (15) par+epi+hbl+qtz=omp+kya+H2O (dehydration) (12) which have the character of isograd reactions. Local variations of water activity (aH2O) as indicated by isofacial mineral assemblages, and the H2O character of the reaction (15), are interpreted to reflect largely H and predominantly fluid-absent high-pressure metamorphism within the northern part of the nappe. The omphacite amphibolites and paragonite eclogites in this area are thought to have formed by H2O reactions from Pre-Mesozoic high-grade amphibolites, i.e. from protoliths of similar bulk H2O-countent. The second ‘isograd' (12) is interpreted to mark the regional transition from largely fluid-absent metamorphism in the north to fluid-present metamorphism in the south, where metamorphic pressures and temperatures in excess of 12-15kb and 500-600°C were sufficient for prograde in-situ dehydration of similar hydrous protoliths to kyanite eclogites. The observation of abundant veins, filled with quartz+kyanite+omphacite, suggests that a free fluid coexisted locally with the kyanite eclogites of the southern Adula Nappe at some time during progressive dehydratio

On the Appropriateness of Contractor-Led Procurement: an investigation of circumstances and consequences

Major clients of the construction industry have been found to organise construction work into fewer, but larger, contracts with more transfer of risk and responsibilities in response to a change from a sellers market to a buyers market, and facing a greater choice of procurement methods than ever before. Main contractors and consultants alike are moving towards multidisciplinary teams offering design and management services, challenging single service consultants or contractors and are in competition with each other over who is leading the process. A consolidation of firms at the upper end of the industry can be witnessed in order to access a wider market and new clients, and at the lower end a specialisation into specific skills or locations takes place, while medium sized firms are increasingly struggling to survive. A general procurement model serves to identify the appropriate procurement approach for construction needs, as neither clients or construction service suppliers represent a homogenous market. Clients demands for a ready purchase of design, procurement and management of construction from a single source have been found to be met most appropriately by contractor-led procurement under most, but not all, circumstances, particularly in respect of higher levels of efficiency, cost certainty and punctuality among other benefits. The consequences faced by a contractor in the leading role of the procurement process are significant, especially in terms of integrating and co-ordinating the entire supply chain to the satisfaction of the client and for anticipated repeat business. This is the chief factor of competitive strength for the struggle of long term survival. A classification model of procurement strategies in respect to parameters of supply risk, strategic importance and frequency of spend offers a tool for the appropriate choice of business relationship with different suppliers. It is to be anticipated that the future will see an intensification of the changes in the processes of construction procurement described and analysed, which may vary in extent from one market to another, but not in direction

Effect of the iron valence in the two types of layers in LiFeO2_2Fe2_2Se2_2

We perform electronic structure calculations for the recently synthesized iron-based superconductor LiFeO$_2$Fe$_2$Se$_2$. In contrast to other iron-based superconductors, this material comprises two different iron atoms in 3$d^5$ and 3$d^6$ configurations. In band theory, both contribute to the low-energy electronic structure. Spin-polarized density functional theory calculations predict an antiferromagnetic metallic ground state with different moments on the two Fe sites. However, several other almost degenerate magnetic configurations exist. Due to their different valences, the two iron atoms behave very differently when local quantum correlations are included through the dynamical mean-field theory. The contributions from the half-filled 3$d^5$ atoms in the LiFeO$_2$ layer are suppressed and the 3$d^6$ states from the FeSe layer restore the standard iron-based superconductor fermiology.Comment: 9 pages, 11 figure

Simultaneous cathodoluminescence hyperspectral imaging and X-ray microanalysis

A facility has been developed to acquire hyperspectral cathodoluminescence (CL) images simultaneously with X-ray composition data. Based around an electron microprobe, the system uses a built-in Cassegrain microscope to efficiently couple emitted light directly into the entrance slit of an optical spectrograph. A cooled array detector allows the parallel acquisition of CL spectra, which are then built up into a multidimensional data-cube containing the full set of spectrally- and spatially-resolved information for later analysis. This setup has the advantage of allowing wavelength-dispersive X-ray (WDX) data to be recorded concurrently, providing a powerful technique for the direct comparison of luminescent and compositional properties of materials. The combination of beam and sample scanning thus allows the correlation of composition and luminescence inhomogeneities on length scales ranging from a few cm to sub-micron

Borderline hospitality: homestays as a commercial hospitality development project in Grahamstown, Eastern Cape

This study started as an anthropological investigation of commercial hospitality from the point of view of the hands-on host. The chosen case study for this investigation was the Kwam eMakana Government Initiated Poverty Alleviation Project which offered homestays in the townships of Grahamstown East since 2004. Homestays are the most intimate form of commercial hospitality, one step removed from non-commercial or social hospitality. Even at the homestay level there is a conceptual conflict between poverty and (Westernized) commercial hospitality, however, Kwam homes are more middle class than poor. Later the investigation revealed the deeper-seated form of poverty of the Kwam participants being (almost) illiterate. Kwam was a development project like many others, in which huge amounts of money were spent in the name of the project but very little of the benefits reached the intended beneficiaries. Thus, as fieldwork ensued, the emphasis of research migrated from an empirical study of homestay hospitality, to actively assist with the struggle of the Kwam hostesses to maintain the project and gain autonomy for themselves. This study was from the outset reflexive, as the host’s point of view could technically only be presented by auto-ethnography. Then the investigation shifted to a form of engaged anthropology far exceeding advocacy as it is usually understood. The presentation of this can be called radical reflexivity, while it is simultaneously an ethnographical account in the sense of anthropology ‘at home’. It also implied, besides ethical concerns, revisiting literary sensibilities, such as the use of a third person narrative for the reflexive account. To conceptualize the development process of both Kwam and the research interventions Bourdieu’s ‘totality of capital’ (in which the strands of economic, symbolic, cultural and social capitals intertwine) proved most useful. By assessing the various capitals the development of the project and the power struggles central to it can be understood. This study confirms that long-term anthropological investigation is best suited to the study of development projects, if not necessary for real development to be effected. Reflexivity and ethnography are complementary methods to reveal truths which under certain research circumstances may have been very difficult or even impossible to research

Moments and central limit theorems for some multivariate Poisson functionals

This paper deals with Poisson processes on an arbitrary measurable space. Using a direct approach, we derive formulae for moments and cumulants of a vector of multiple Wiener-It\^o integrals with respect to the compensated Poisson process. Second, a multivariate central limit theorem is shown for a vector whose components admit a finite chaos expansion of the type of a Poisson U-statistic. The approach is based on recent results of Peccati et al.\ combining Malliavin calculus and Stein's method, and also yields Berry-Esseen type bounds. As applications, moment formulae and central limit theorems for general geometric functionals of intersection processes associated with a stationary Poisson process of $k$-dimensional flats in $\R^d$ are discussed

Laser-ablation ICP-MS analysis of silicate and sulfide melt inclusions in an andesitic complex II: evidence for magma mixing and magma chamber evolution

Laser-ablation microanalysis of a large suite of silicate and sulfide melt inclusions from the deeply eroded, Cu-Au-mineralizing Farallón Negro Volcanic Complex (NW Argentina) shows that most phenocrysts in a given rock sample were not formed in equilibrium with each other. Phenocrysts in the andesitic volcano were brought together in dominantly andesitic—dacitic extrusive and intrusive rocks by intense magma mixing. This hybridization process is not apparent from macroscopic mingling textures, but is clearly recorded by systematically contrasting melt inclusions in different minerals from a given sample. Amphibole (and rare pyroxene) phenocrysts consistently contain inclusions of a mafic melt from which they crystallized before and during magma mixing. Most plagioclase and quartz phenocrysts contain melt inclusions of more felsic composition than the host rock. The endmember components of this mixing process are a rhyodacite magma with a likely crustal component, and a very mafic mantle-derived magma similar in composition to lamprophyre dykes emplaced early in the evolution of the complex. The resulting magmas are dominantly andesitic, in sharp contrast to the prominently bimodal distribution of mafic and felsic melts recorded by the inclusions. These results severely limit the use of mineral assemblages to derive information on the conditions of magma formation. Observed mineral associations are primarily the result of the mixing of partially crystallized magmas. The most mafic melt is trapped only in amphibole, suggesting pressures exceeding 350MPa, temperatures of around 1,000°C and water contents in excess on 6wt%. Upon mixing, amphibole crystallized with plagioclase from andesitic magma in the source region of porphyry intrusions at 250MPa, 950°C and water contents of 5.5wt%. During ascent of the extrusive magmas, pyroxene and plagioclase crystallized together, as a result of magma degassing at low pressures (150MPa). Protracted extrusive activity built a large stratovolcano over the total lifetime of the magmatic complex (>3m.y.). The mixing process probably triggered eruptions as a result of volatile exsolutio