Norman Moles

The use of microchemical analysis of alluvial gold grains in mineral exploration: experiences in Britain and Ireland

Abstract The examination of populations of gold grains by SEM and EPMA generates a microchemical signature based upon the assemblage of opaque mineral inclusions within the gold and the concentration of minor alloying metals. Duplicate samples of alluvial gold from the same locality have yielded the same microchemical signature independent of the field worker who collected the sample, or the date of collection. A study of five separate alluvial localities yielded a microchemical signature consistent with the mineralogy of the adjacent host mineralisation. This result has permitted informed speculation on the styles and complexity of gold mineralisation contributing to the alluvial population of gold grains where the style of source mineralisation is unknown. The technique can yield information about several aspects of the source mineralisation of an alluvial population at an early stage in the exploration process. Distinctive microchemical signatures can be used to evaluate the dispersion of gold either by fluvial or glacial action. The technique can identify gold derived from separate mineralising events either within a region or along a watercourse, thereby allowing speculation on the magnitude of the source mineralisation. It is also possible to establish whether specific types of gold mineralisation of potential economic interest have contributed to an alluvial population. In areas subjected to glaciation, analysis of the microchemical signature of populations of alluvial gold may indicate whether the gold grains are of proximal or distal origin. The volume of information made available through the study of alluvial gold grains has contributed greatly to the understanding of regional gold mineralisation. Interpretation of microchemical signatures of populations of alluvial gold from about 130 localities throughout Great Britain and Ireland has facilitated characterisation of gold in terms of the style of the source mineralisation. This information has augmented that available from about ten bedrock gold localities and has permitted a more complete classification of gold occurrences throughout the region. Quantitative analysis of some minerals within the opaque inclusion assemblage can provide information on the chemistry of the mineralising fluid, which may be related to the capacity for gold transportation. In Devon, South West England, a model for the emplacement of gold mineralisation was developed from interpretation of the microchemical signature of alluvial gold grains. The selenide-rich inclusion suite containing no sulphides suggested that oxidising conditions prevailed during precipitation. The model of transport of gold in solution through the oxidising environment of Permian red beds and its subsequent precipitation in underlying rocks due to reduction in Eh was consistent with the observed distribution of gold in alluvial sediment, residential overburden and drill core. Using this model other targets for gold mineralisation were successfully predicted.

Influence of geology, glacial processes and land use on soil composition and Quaternary landscape evolution in The Burren National Park, Ireland

The Burren in County Clare, western Ireland is a glaciated karstic landscape with a fragmented soil and vegetation cover on a predominantly limestone substrate. A representative area approximately 4 km2 in extent located within The Burren National Park was studied in order to determine the distributions of soil types and relate these to geology, glacial erosion and deposition, post-glacial mass movements and land use. Mineralogical and particle size analyses indicate the presence of two distinct soil types, one shale-bearing, the other granite-bearing. Geochemical analyses confirm this division and also reveal soils of mixed composition and a distinctive subtype of granite-bearing soil that occurs exclusively on drumlin-like landforms. The soils on the drumlins formed by the decalcification of limestone-rich glacial diamicton (till) containing granite and metamorphic rocks derived from County Galway to the north. The occurrence of mineralogically similar though geochemically heterogeneous soils throughout the area suggests that there was once an extensive thin cover of diamicton, which has almost entirely vanished because of pedogenesis and erosion. Soils containing shale fragments occur in a glacial dispersion train extending south from outcrops of a small body of shale-rich limestone breccia. Isolated patches of shale-bearing soil occurring up to 1.2 km from these outcrops may have formed by the weathering of ice-transported boulders. The presence of relatively deep soil cover on outcrops of the shale/limestone unit is attributed to Holocene weathering to form an erosion-resistant clay-rich soil. Tree charcoal, 14C-dated to 3300 BP and buried to a depth of 2 m by mixed granite- and shale-bearing soil, indicates that Bronze Age deforestation triggered mass movement and soil erosion. Denudation processes are still active in the Burren landscape.

Replacement and authigenic mineralogy of metal contaminants in stream and estuarine sediments at Newtownards, Northern Ireland

Abstract Tidal mudflats are locally enriched in heavy metals at the head of Strangford Lough in Northern Ireland, where drainage from the hinterland enters the sea lough via a tidal canal in an urban area. To characterize the metallic contaminants and investigate their provenance, heavy particles separated from stream, canal and estuarine sediments were analysed by electron microprobe and laser Raman microspectroscopic methods. Potential metal sources are mineralization in the catchment area and industrial or domestic pollution. Anthropogenic particles include metallic grains, alloys and compounds of Pb, Zn, Cu, Fe, Cr and Sn. Alteration of metallic particles includes de-zincification of brass in freshwater sediment and replacement of Cu wire by covellite in brackish to marine sediment. Mobility of Cu, Fe and S in canal and estuarine sediments is indicated by the authigenic growth of framboidal Fe sulphide on oxide substrates and of chalcopyrite rims on covellite. Intricate colloform and platy crystalline textures suggest a cyclical deposition of covellite and chalcopyrite under conditions of varying redox and salinity. Lead and Cr mobility in the contaminated estuarine sediment is shown by the authigenic formation on Pb-rich substrates of heterogeneous Pb- and Cr-rich sulphate-phosphate compounds and Pb-oxychlorides.

Radiocarbon dated episode of Bronze Age slope instability in the south-eastern Burren, county Clare

While previously reported evidence from lacustrine sediments has linked Burren soil erosion to anthropogenic forest clearance, there have been no previous accounts of dateable episodes of prehistoric slope instability based on evidence from terrestrial sites. Described here is the occurrence of charcoal buried lo a depth of C. 2m within diamicton at a site on Knockanes hill close to Mullach Mor hill in the south-eastern Burren. An AMS radiocarbon dale places the formation of this charcoal in the Bronze Age. contemporary With exceptionally high rates of soil erosion and forest clearance in a nearby lake catchment. EMS images indicate that the charcoal is of fine grained deciduous wood, most probably hazel (Corylus avellana) but perhaps birch (Betula spp.). Results of analyses of diamictons present in the vicinity of the charcoal support the view that the pattern of occurrence of diamictons in the present day Burren landscape is in pan the product of prehistoric mass movement.

The significance of copper concentrations in natural gold alloy for reconnaissance exploration and understanding gold-depositing hydrothermal systems

Analytical studies of placer (detrital) gold grains, intended to track bedrock sources and styles of mineralization, mainly consider alloy Ag content and inclusion mineralogy to generate ‘microchemical signatures’. Occasionally Hg and Pd help discriminate gold from different sources; Cu infrequently because electron microprobes seldom detect the low concentrations. Reporting of Cu has been largely confined to gold from porphyry–epithermal environments, and this has biased subsequent interpretations of observed Cu-bearing gold alloys. This study focuses on placer gold in the Mourne Mountains, Northern Ireland, where auriferous bedrock remains undiscovered and the complex geology is consistent with either orogenic or intrusion-related mineralization. Over 500 gold grains analysed average 0.17% Cu ranging to >1% Cu and show a wide variation in Ag content. Inclusion mineralogy mainly matches that of orogenic gold elsewhere in the host terrane; however, some signatures are suggestive of zonation within an orogenic hydrothermal mineralizing system, or possibly metamorphic-hydrothermal remobilized gold associated with subsequent intrusive activity. One locality in the study area has placer gold of a distinctive Cu-rich alloy composition containing Cu sulphide inclusions, an association noted elsewhere in gold derived from alkali Cu-Au porphyries. Consideration of Cu-bearing gold alloys worldwide indicates that previously proposed compositional correlations with deposit type are of limited value. We show that Cu contents to at least 0.8% are permissible within orogenic gold – the first time that such compositions have been clearly ascribed to orogenic mineralization. The result is particularly important considering on-going exploration in northern Canada which employs gold grain analysis to help define exploration targets.

Evidence for the Presence of Quaternary Loess-Derived Soils in the Burren Karstic Area, Western Ireland

Quaternary loess-derived soils are found in many mid-latitude countries. Extensive loess deposits have not been described from Ireland except in unpublished reports of possible loessic soils (“stone-free drift”) on karstic terrain in die Burren, Co. Clare. The objective of this study is to verify or discount the presence of loess-derived soils in the Burren. Data on particle size, mineralogy and bulk geochemistry are presented for 14 soil samples from 12 sites, including 6 from areas which unpublished studies had identified (in map form) as covered by stone free drift. Most samples comprise silly clay soils (up to 58% sill) with low carbonate contents and low organic carbon contents. With regard to the carbonate-poor samples, XRD and XRF analyses enabled two groups to be distinguished. Group A soils show characteristics consistent with weathered till, while Group B soils show characteristics consistent with weathered loess. All sites falling within areas mapped as possible loess in previous studies provid...

Abundant sulphate in the Neoproterozoic ocean: implications of constant δ34S of barite in the Aberfeldy SEDEX deposits, Scottish Dalradian

Abstract The timing and extent of ocean oxygenation is controversial. Proterozoic sulphur isotope datasets often show marked fluctuations over small stratigraphic intervals, suggesting that oceanic sulphate concentrations were much lower than modern values. A large accumulation of Neoproterozoic sulphate (>8 million tonnes preserved), as stratiform barite rock, is located in the Grampian Highlands near Aberfeldy. Diagenetic/metamorphic alteration has caused pronounced δ34S variations near bed margins. This aside, barite throughout the deposits shows a narrow range in δ34S, mean 36±1.5‰. We infer that this is representative of contemporaneous seawater sulphate, and that δ34Sseawater was constant during deposition of a stratigraphical thickness >250 m of mostly fine-grained clastic sediments. Uniformity of δ34Sseawater during barite precipitation, even in thick (>10 m) beds and with the co-occurrence of abundant sulphides incorporating bacteriogenically reduced sulphur, implies no limit to availability of seawater sulphate during hydrothermal exhalative events. Our data, combined with previous δ34S research on Dalradian metasediments, suggest a stability, abundance and constancy of ocean sulphate in the Neoproterozoic. This contrasts with isotopic data using trace sulphate in limestones. It appears that, around the time of the Marinoan glaciation (c. 635 Ma), the ocean, although stratified at least locally, comprised a substantial reservoir of sulphate-bearing oxygenated seawater.

A learning resource to support Masters-level training of geologists in professional practice

Abstract Postgraduate-level training in ‘professional practice’ is essential to enable students (a) to understand business management and the role of geologists, civil engineers and environmental scientists in industry, and (b) to advance their skills in report writing, oral presentation, negotiation and financial acumen. This is best achieved through exposure to ‘real world’ case study material. With GEES Subject Centre support we are developing a multimedia learning resource for students to explore the interactions between environmental legislation, corporate strategy, financial constraints, geotechnical engineering and applied geology. Our case study is an investigation of wastewater treatment by means of sub-surface dispersion within a Chalk aquifer in southern England. We have evaluated the course with groups of students on final year undergraduate and Masters level geology courses. Issues include achieving a suitable balance between individual and group work, between scientific understanding and professional skills development, and between specialist knowledge and transferability to other GEES disciplines.

Geochemical and mineralogical controls on the distribution of rare earth elements in the laterites of Serra de Monchique, Portugal

Current European manufacturing relies heavily on imports from China for unprocessed rare earth elements (REEs) and rare earth oxides (REOs). It has been suggested that the EU holds viable reserves of REEs that, with adequate research, could satisfy 10% of EU demand, by the recycling of mine waste from bauxite production (red muds) alone (Deady 2014). Focus has been turned to the potential for similar South China type laterite deposits being exploited in the EU, but limited exploration and understanding of EU laterite formations currently makes them unattractive to investment (Humphries 2013). Following early work by Bernard-Griffiths (BernardGriffiths 1996) into the REE content of the Cretaceous Serra deMonchiqueNepheline-Syenite intrusion, this project examines the clay mineralogy and geochemistry of a selection of pedogenic profiles, logging their variations in mineralogy, bulk chemistry and specific REE content and comparing them to currently producing regions in Southern China. Investigation is under way using a five stage sequential leach-ICP-MS method to identify REE hosting minerals and mineralogical hosting phases, supported byXRDanalysis ofmineralogy and clay structures. Initial results indicate similarities in clay hosted REE distributions through laterite profiles to other researched formations in Madagascar (Berger 2014) and China (Zhou 2013), where specific organic rich (acidic) soils and biogenic profiles of tropical (rainforest) environments were presented as a key factor for REE mobilisation during pedogenesis. Serra de Monchique has a unique climate, with similar seasonal rainfall and cycling, but is not a rainforest, nor in a tropical region. It is therefore hypothesised that classic REE distribution models may also be applied to non-classic environments for this deposit type, supporting the hypothesis of Herrington (Herrington 2007) that China-type REE hosting ionadsorption clay profiles could exist across Europe in novel and previously unresearched environments.

Mechanisms for concentrating critical metals in granite complexes: insight from the Mourne Mountains, Northern Ireland

The critical metals are the raw materials required for emerging technologies. Their production is concentrated in a small number of countries, resulting in a high risk of supply disruption [1]. The overall objective of this project was to utilise and enhance the Tellus data set for the Mourne Mountains so that the potential for polymetallic critical metal-bearing mineralisation could be investigated. We used a multidisciplinary approach that combined deep soil and stream sediment geochemical data from the Tellus Survey with mineral data from a new QEMSCAN® investigation of 55 heavy mineral concentrates (HMCs) from large volume stream sediment samples [2] and eight bedrock locations. For both sample types, the QEMSCAN® generated a statistical analysis of grainsize, mineral abundance and mineral associations. Additional higher resolution element and chemical mapping was used to place the critical metal host minerals into their petrogenetic context. The Mourne Mountains provides a natural laboratory to investigate how economic concentrations of the critical metals can develop in granite complexes. Specifically, we identified four main geochemical patterns in the Tellus data and determined their mineralogical source.Abstract Abstracts from the 37th Annual Winter Meeting of the Geological Society’s Mineral Deposits Studies Group held on the 6–7th January 2014 at the University of Oxford, UK. The meeting was hosted by the Department of Earth Sciences and Keble College. Sessions included: critical metal supply – a global perspective; exploration in difficult times; the mineral industry – challenges in the 21st century; Ore deposit research in progress; and Metallogeny of new jurisdictions.