Alicja Lacinska

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.

Microbiological influences on fracture surfaces of intact mudstone and the implications for geological disposal of radioactive waste

Abstract The significance of the potential impacts of microbial activity on the transport properties of host rocks for geological repositories is an area of active research. Most recent work has focused on granitic environments. This paper describes pilot studies investigating changes in transport properties that are produced by microbial activity in sedimentary rock environments in northern Japan. For the first time, these short experiments (39 days maximum) have shown that the denitrifying bacteria, Pseudomonas denitrificans, can survive and thrive when injected into flow-through column experiments containing fractured diatomaceous mudstone and synthetic groundwater under pressurized conditions. Although there were few significant changes in the fluid chemistry, changes in the permeability of the biotic column, which can be explained by the observed biofilm formation, were quantitatively monitored. These same methodologies could also be adapted to obtain information from cores originating from a variety of geological environments including oil reservoirs, aquifers and toxic waste disposal sites to provide an understanding of the impact of microbial activity on the transport of a range of solutes, such as groundwater contaminants and gases (e.g. injected carbon dioxide).

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.

Influence of biofilms on transport of fluids in subsurface granitic environments – some mineralogical and petrographical observations of materials from column experiments

Abstract Landfill and radioactive waste disposal risk assessments focus on contaminant transport and are principally concerned with understanding the movement of gas, water and solutes through engineered barriers and natural groundwater systems. However, microbiological activity can impact on transport processes changing the chemical and physical characteristics of the subsurface environment. Such effects are generally caused by biofilms attached to rock surfaces. This paper will present some mineralogical and petrographical observations of materials extracted at the completion of an experimental column study which examined the influences of biofilm growth on groundwater flow through crushed diorite from the Äspö Hard Rock Underground Research Laboratory, Sweden.

The structure and petrology of the Cnoc nan Cuilean Intrusion, Loch Loyal Syenite Complex, NW Scotland

In NW Scotland, several alkaline intrusive complexes of Silurian age intrude the Caledonian orogenic front. The most northerly is the Loch Loyal Syenite Complex, which is divided into three separate intrusions (Ben Loyal, Beinn Stumanadh and Cnoc nan Cuilean). Mapping of the Cnoc nan Cuilean intrusion shows two main zones: a Mixed Syenite Zone (MZ) and a Massive Leucosyenite Zone (LZ), with a gradational contact. The MZ forms a lopolith, with multiple syenitic lithologies, including early basic melasyenites and later felsic leucosyenites. Leucosyenite melts mixed and mingled with melasyenites, resulting in extreme heterogeneity within the MZ. Continued felsic magmatism resulted in formation of the relatively homogeneous LZ, invading western parts of the MZ and now forming the topographically highest terrane. The identification of pegmatites, microgranitic veins and unusual biotite-magnetite veins demonstrates the intrusions complex petrogenesis. Cross-sections have been used to create a novel 3D GoCad™ model contributing to our understanding of the intrusion. The Loch Loyal Syenite Complex is known to have relatively high concentrations of rare earth elements (REEs), and thus the area has potential economic and strategic value. At Cnoc nan Cuilean, abundant REE-bearing allanite is present within melasyenites of the MZ. Extensive hydrothermal alteration of melasyenites here formed steeply dipping biotite-magnetite veins, most enriched in allanite and other REE-bearing accessories. This study has thus identified the area of greatest importance for further study of REE enrichment processes in the Cnoc nan Cuilean intrusion.

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.

Rare earth element placer deposits and alkaline volcanics: a case study from Aksu Diamas, Çanakli, Turkey

included detail on the igneous facies and simply have attempted to recognise weathered flow tops. Hitherto the Lower Basalt Formation was considered internally consistent despite having a geographical extent of over 3500 km and a variable thickness, which in areas has been recorded up to 531 m. The lack of robust and detailed stratigraphical information for the unexposed parts of the Antrim Plateau presents a significant hindrance to exploration that relies on information about the location and distribution of feeder systems (dykes, sills and plugs). In an attempt to address this lack of stratigraphic information we have undertaken detailed graphical logging of two cores from the Carnduff area, a field section from Browns Bay, Island Magee, as well as comprehensive petrographical analysis. The study focuses on the lowermost 160 m of the basalts from which it is clear that there are complex internal variations. The basalts were formed by a series of tabular and compound flows most of which can be correlated laterally across the area. By identifying and characterising the geometry and morphologies of these flows, it is possible to break the Lower Basalt Formation down into three smaller lower and upper units consisting of tabular basaltic lava flows and the middle unit of much smaller compound flows. Ultimately, the aim is to generate a robust stratigraphic model for the Lower Basalt Formation that can be applied elsewhere on the plateau to correlate key flows at the base of the Antrim Lava Group. By mapping these flows across the area it may be possible to locate mineralogical important feeder dyke systems.

Volcanic ash-fall placers: a new type of REE deposit?

The Çanakli deposit, a part of the Aksu Diamas project in western Turkey, owned by AMR Mineral Metal Inc., has an inferred resource of 494 million tonnes at 0.07% TREO (total rare earth oxide) in unconsolidated sediments at surface. This deposit is a potentially important source of European rare earth element (REE) supply. Alternative supplies of critical minerals are being intensely investigated across the globe, with a focus on by-product or added-value minerals such as REE, for example from alumina processing (Deady et al. 2016) and phosphate production (Emsbo et al. 2015). The deposit is hosted in Quaternary sediments situated in a basin in Mesozoic limestone. REE-bearing heavy minerals occur in lenses and are disseminated in channelised debris flows. There is no obvious source of these minerals within the basin catchment. However, the alkaline Gölcük volcano is located approximately 20 km north-west of the deposit and repeatedly erupted during the Plio-Quaternary (Platevoet et al. 2014). The related ash falls represent a potential source for the REE-bearing minerals. The source of the REE is being investigated by the British Geological Survey through the EU-funded FP7 project EURARE (www. eurare.eu). Scanning electron microscopy (SEM) analysis of heavy mineral concentrates has confirmed the presence of REE-bearing minerals such as allanite and chevkinite. These minerals are also found in samples of pyroclastic material from the Gölcük alkaline volcano. Chemical fingerprinting of both major minerals, such as magnetite and pyroxene, and the minor REE-bearing minerals, using a combination of LA-ICP-MS and quantitative SEM, is used to investigate the potential for Gölcük as the source. If the Çanakli REE deposit resulted solely from the accumulation of air-fall tuff-borne minerals in a topographic depression, without additional concentration processes, then this is a new type of deposit. Improved understanding of the origin of this deposit type would aid identification of additional REE resources in similar settings globally, including those at Monte Vulture, Italy.

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

Alicja M. Lacinska and Michael T. Styles reply: We appreciate the comment by C. R. M. Butt on the publication by Lacinska & Styles (2013) on the silicified serpentinites described from the Hajar Mountains in the United Arab Emirates. This comment is based on his very extensive knowledge of laterites and regoliths from ancient shield areas around the world; the degree to which this knowledge is directly applicable to the rocks formed at the margins of a recently uplifted mountain range, as described in the original paper, is debatable.