Michael Styles

Determination of the three-dimensional distributions of precious metals in sulphide minerals by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAMP-ICP-MS )

Precious metals are o f great financial importance and the form of their occurrence will dictate the economics of their recovery from ore. It has been postulated that “invisible” gold in sulphides occurs in solid solution, as particulates, or associated with micro-inclusions of other minerals in the host. This study applied the high spatial resolution (< 20 μm) and high sensitivity (sub-μg g−1 detection limits) of the laser ablation microprobe-inductively coupled plasma-mass spectrometer to identify both the form and concentration of gold in sulphides. Data are presented demonstrating the three-dimensional distribution of gold in minerals from Hemlo, Ontario, Canada, and Penjom Prospect, Kuala Lipis, Malaysia. Gold at μg g−1 ` levels was determined and, through its correlation with other trace elements, identified as either particulates or associated with micro-inclusions.

Polyphase Neoproterozoic orogenesis within the East Africa–Antarctica Orogenic Belt in central and northern Madagascar

Abstract Our recent geological survey of the basement of central and northern Madagascar allowed us to re-evaluate the evolution of this part of the East Africa–Antarctica Orogen (EAAO). Five crustal domains are recognized, characterized by distinctive lithologies and histories of sedimentation, magmatism, deformation and metamorphism, and separated by tectonic and/or unconformable contacts. Four consist largely of Archaean metamorphic rocks (Antongil, Masora and Antananarivo Cratons, Tsaratanana Complex). The fifth (Bemarivo Belt) comprises Proterozoic meta-igneous rocks. The older rocks were intruded by plutonic suites at c. 1000 Ma, 820–760 Ma, 630–595 Ma and 560–520 Ma. The evolution of the four Archaean domains and their boundaries remains contentious, with two end-member interpretations evaluated: (1) all five crustal domains are separate tectonic elements, juxtaposed along Neoproterozoic sutures and (2) the four Archaean domains are segments of an older Archaean craton, which was sutured against the Bemarivo Belt in the Neoproterozoic. Rodinia fragmented during the early Neoproterozoic with intracratonic rifts that sometimes developed into oceanic basins. Subsequent Mid-Neoproterozoic collision of smaller cratonic blocks was followed by renewed extension and magmatism. The global ‘Terminal Pan-African’ event (560–490 Ma) finally stitched together the Mid-Neoproterozoic cratons to form Gondwana.

Pyroxenous greywackes in the Southern Uplands of Scotland and their petrotectonic implications

Detrital clinopyroxenes occur abundantly in several greywacke formations in the Southern Uplands, both as single grains and within andesite lithoclasts. Compositionally, these pyroxenes fall into two main groups. Those from the oldest unit, the early Caradoc Galdenoch Formation, were derived from a simple tholeiitic volcanic arc. In contrast, the pyroxenes in the younger Portpatrick (late Caradoc) and Glenlee (Ashgill) Formations and the Gala Group (early Llandovery), show initial strongly calc-alkaline affinities, followed by a more complex calc-alkaline to transitional (calc-alkaline to tholeiitic) volcanic arc signature. The latter may be due to a mixture of calc-alkaline and tholeiitic components. A presently exposed source for the andesites has not yet been established, although the Ballantrae Complex is a potential candidate for a tholeiitic component.

Near-surface diagenesis of ophiolite-derived conglomerates of the Barzaman Formation, United Arab Emirates: a natural analogue for permanent CO2 sequestration via mineral carbonation of ultramafic rocks

Abstract Carbon capture and storage by mineralization is a potential method for storing anthropogenic CO2 emissions, and is based on the reaction between Mg silicate and CO2 to form Mg carbonate. The conglomerates of the Barzaman Formation exposed in the eastern United Arab Emirates represent an excellent natural analogue of this process. These conglomerates were deposited as a series of alluvial fans along the western margin of the Hajar Mountains, part of the Oman-UAE Ophiolite, and are composed largely of ultramafic and lesser-mafic clasts. The clasts and matrix have been extensively altered to dolomite during diagenetic processes. Analysis and interpretation of rock textures provide evidence for the various factors that influenced the diagenetic processes and shed light on the silicate–carbonate transformation process. All the reactions have taken place in the near-surface environment; the silicate–carbonate conversion reaction is exothermic and occurs spontaneously at near-ambient pressure and temperature, probably no greater than 50 °C. Estimates of the amount of CO2 stored in this way can be obtained from considerations of outcrop area, formation thickness and percentage of dolomite replacement, and show that c. 150 billion tonnes (equivalent to about 4 years of worldwide CO2 emissions at current rates) are stored.

Geochemistry and petrography of phosphorus in urban canal bed sediment

Abstract Urban canal sediment provides an opportunity to investigate the processes controlling P geochemistry in a system dominated by anthropogenic input. To achieve this a combined study was undertaken of pore water geochemistry and petrology of the top 25 cm canal bed sediment from two urban canals in the English West Midland conurbation. Processes in the sediment are dominated by the geochemistry of Fe, P and organic matter. Results indicate that sediment re-suspension by boat traffic has a major influence on pore water chemistry. In addition to the sediment water interface an additional interface at 6–7 cm depth was identified and corresponds to the maximum depth of sediment re-suspension by boat traffic. Phosphorus in the sediment occurs predominantly as vivianite. Low PO4 concentrations occur close to the sediment water interface in summer due to frequent re-suspension and Fe oxidation. High PO4 concentrations occur close to the sediment water interface in winter when boat traffic is infrequent. The greatest potential to exchange phosphate between sediment and water column will therefore occur during infrequent boat traffic regimes.

Silicified serpentinite – a residuum of a Tertiary palaeo-weathering surface in the United Arab Emirates

Mineralogical studies of a silicified serpentinite from the United Arab Emirates throw light on the formative processes. The silicified serpentinite is a residuum of a palaeo-weathering surface that probably developed in a temperate climate with alternating wet and dry periods during middle Eocene to late Miocene times. The rock textures indicate that silicification occurred in a fluid-saturated zone. Silica precipitation is favoured at near-neutral pH. In this study we infer that these pH conditions of the mineralizing fluids could arise in a near-surface mixing zone where acidic meteoric and hyperalkaline groundwater fluids are mingled. This mingling is believed to have resulted from alternating processes of evaporation and precipitation that prevailed during dry and wet seasons, respectively. The silicified serpentinite is composed of > 95% quartz and exhibits a ghost texture of the protolith serpentinite. Preservation of the textures indicates an iso-volumetric grain-by-grain replacement by dissolution of Mg-silicate and simultaneous precipitation of either opal or microquartz as siliceous seeds. These were subsequently overgrown by silica that was probably remobilized from deeply weathered regolith elsewhere.

Chromite and PGE in the Logar Ophiolite Complex, Afghanistan

Abstract The Logar Ophiolite Complex (LOC) is located 30 km south of Kabul, Afghanistan, and extends over approximately 2000 km2. It comprises a lower lherzolitic-dunitic-harzburgitic-gabbro ultramafic-mafic unit that passes upwards into a dolerite dyke complex, basaltic pillow lavas and an uppermost sequence of volcaniclastic- and terrigenous-dominated sedimentary units. The ophiolite represents an obducted remnant of intra-Tethyan basin oceanic crust, thrust onto a platform-style cover component of the Kabul Terrane during the Himalayan orogeny. Platinum-group minerals have been detected for the first time in chromitites and ultramafic units from the LOC. Two distinct types of chromitites and ultramafic lithologies with different origins have also been identified in this study. The first type is a low Cr, PGE-poor chromitite interpreted to have been produced in a mid ocean ridge (MOR) environment. The second type is a high Cr, relatively PGE-rich dunite and peridotite from a boninitic magma produced in a supra-subduction zone (SSZ) setting. Platinum group element (PGE) abundances in these chromitites average 12 ppb and 26 ppb for Pt+Pd+Rh for the dunite and peridotite. Chondrite-normalised PGE patterns have two distinct trends: (a) the MOR rocks have a positive Ru anomaly with a negative Pt anomaly and a generally negative slope; and (b) the SSZ rocks show weak positive Ru and Pt anomalies and a positive slope. It is concluded that the negative sloping pattern is typical of PGE in most ophiolites elsewhere. In contrast, the positively sloping pattern is more unusual and may indicate PGE remobilisation and enrichment.

Synthetic magnetic soils for landmine detector testing

This study aims to move towards a reliable method for synthesizing artificial soils that emulate the effect of real soils on pulse-induction metal detectors. The signals resulting from some mineralized soils can greatly impair the effectiveness of mine detectors. We analysed a sample of one such soil from Cambodia, using resistivity measurement, X-ray diffraction, magnetic susceptibility measurement and electron microscopy. The physical origin of the soil signal was found to be high proportions of small grained magnetite. By mixing a U.K. topsoil with finely crushed magnetite, a synthetic soil with similar magnetic susceptibility to the real Cambodian soil was created. Other problematic soils from around the world were also synthesized. Comparative tests with a pulse induction metal detector showed decay characteristics for the artificial soils to be within 13% of those for the real soil. We achieved close susceptibility matching between artificial and real soils, either for a 2kHz measurement or a pulse-induction measurement. In our experiments it was not possible to match with both measurements at once. We propose that this discrepancy is due to differences in the average properties of magnetic grains between those in real soil and the magnetite concentrate used for the synthesis tests.

An experimental study of the carbonation of serpentinite and partially serpentinised peridotites

In situ sequestration of CO2 in mantle peridotites has been proposed as a method to alleviate the amount of anthropogenic CO2 in the atmosphere. This study presents the results of eight-month long laboratory fluid-rock experiments on representative mantle rocks from the Oman-United Arab Emirates ophiolite to investigate this process. Small core samples (3 cm long) were reacted in wet supercritical CO2 and CO2-saturated brine at 100 bar and 70°C. The extent of carbonate formation, and hence the degree of carbon sequestration, varied greatly depending on rock type, with serpentinite (lizardite-dominated) exhibiting the highest capacity, manifested by the precipitation of magnesite MgCO3 and ferroan magnesite (Mg,Fe)CO3. The carbonate precipitation occurred predominantly on the surface of the core and subordinately within cross-cutting fractures. The extent of the CO2 reactions appeared to be principally controlled by the chemical and mineralogical composition of the rock, as well as the rock texture, with all these factors influencing the extent and rate of mineral dissolution and release of Mg and Fe for subsequent reaction with the CO2. It was calculated that ≈ 0.7 g of CO2 was captured by reacting ≈ 23 g of serpentinite, determined by the mass of magnesite formed. This equates to ≈ 30 kg CO2 per tonne of host rock, equivalent to ≈ 3% carbonation in half a year. However, recycling of carbonate present in veins within the original rock sample could mean that the overall amount is around 2%. The increased reactivity of serpentinite was associated with preferential dissolution of more reactive types of serpentine minerals and brucite, that were mainly present in the cross-cutting veins. The bulk of the serpentinite rock was little affected. This study, using relatively short term experiments, suggests that serpentinite might be a good host rock for CO2 sequestration, although long term experiments might prove that dunite and harzburgite could be an effective in an engineered system of CCSM. Wet scCO2 proved to be chemically aggressive than CO2-saturated brine and its ingress along fractures and grain boundaries resulted in greater host rock dissolution and subsequent carbonate precipitation.