Modelling Regional Geochemistry and As-Bi-Co-Cu-Fe-Ni Mineralisation Using G-BASE in the Lake District, UK

Lake District which define the principal anomalies in considerably more detail than previous widely-spaced observations. The physical properties of the Shap and Skiddaw granites have been analysed from geophysical logs recorded in 300 m deep heat-flow boreholes. New density determinations have been made on outcrop samples from over 350 localities in the western and central Lake District. Samples have been classified in terms of their lithology and lithostratigraphy, and representative in-situ densities have been calculated for the principal formations. The gravity and aeromagnetic data have been interpreted, using a combination of modelling and image processing, in order to study the form and evolution of the Lake District granite batholith and structures within the Skiddaw and Borrowdale Volcanic groups. The modelling studies indicate that Lake District batholith may comprise up to nine separate deep-seated components, and there may be a further five high level intrusions. The Eskdale/Wasdale Granite forms a major component in the western Lake District, and the Shap and Skiddaw granites form separate, steep-sided intrusions on the south-eastern and north-eastern margins of the batholith respectively. Prominent residual gravity anomalies, which coincide approximately with the Scafell, Haweswater and Ulpha synclines of the Borrowdale Volcanic Group, are also tentatively interpreted in terms of separate batholith components, but alternative interpretations in terms of thickened BVG sequences are possible. Further separate components are postulated along the northern side of the batholith and beneath the Haweswater Complex. The Ennerdale Granophyre and Threlkeld Microgranite are modelled as high-level intrusions, in line with previous interpretations, and it is posssible that the Eskdale Granodiorite is also in this category. There is good evidence for a high-level granitic intrusion beneath the Crummock aureole and some evidence for a similar intrusion near Coniston. On a broader scale, the modelling indicates that long wavelength magnetic anomalies are best interpreted in terms of a 'magnetic basement' which represent either a thick layer of pre-Skiddaw Group (magnetic) sedimentary rocks, magnetic crystalline basement, or a combination of both. This 'basement' reaches nearest to the surface in the southern Lake District, deepens northwards beneath the batholith, and approaches nearer to the surface again along the northern margin. The image processing of the potential field data has revealed three important ENE-trending geophysical lineaments across the Lake District (the Crummock, Ullswater and Southern Borrowdales lineaments). Several prominent, but less extensive, NE-trending lineaments are also visible across the central and western parts of the area. The ENE-trending set appear to divide distinctive tracts within the Skiddaw Group and it seems likely that at least some of the lineaments represent fundamental fractures within the underlying basement which were initiated prior to the Borrowdale volcanism and which influenced the subsequent structural development of the Borrowdale Volcanic Group and the intrusive form of the batholith. It is possible that vertical movement influenced by the pre-existing NE- and ENE-trending lineaments may have initiated the Scafell, Haweswater and Ulpha synclines, and associated anticlines, in the Ordovician, leading to a thicker accumulation of BVG in the synclines and/or the subsequent emplacement of late Ordovician (or early Silurian?) components of the batholith beneath them. Alternatively, it is possible that each was initiated as a volcano-tectonic sag over a separate component of an evolving Ordovician batholith, the position of the batholith components themselves being influenced by earlier structural trends. The geothermal characteristics of Caledonian-age granites in the Lake District and Eastern Highlands of Scotland have been studied (in collaboration with other workers). The study has led to a re-examination of the relationship between heat flow (q0) and heat production (A0) for granites and basement rocks in the UK. The data form four separate clusters on the q0-A0 plot; three corresponding to granite batholiths in SW England, northern England and the Eastern Highlands of Scotland, and a fourth to the basement rocks of central England and Wales. A single linear correlation between q0 and A0 is no longer tenable, and an explanation of the data is proposed in terms of the crustal structure and thermo-tectonic setting of each area. In the case of the granite batholiths the data reflect the contrasting depth extent and radioelement - depth functions of the intrusions. These parameters in turn are related to the magmatic evolution and emplacement history of each batholith and the nature of the crust into which they were emplaced.

Stream sediment geochemistry is a useful tool to derive geochemical insights into local geological units within stream sediment source areas. This has significant applicability within the field of mineral exploration where understanding regional geochemistry is fundamental to successful prospection and can facilitate the identification of critical metal deposits. This can help diversify the supply chain of critical metals, as well as tackle the deficit, especially for cobalt (Co). Cobalt is a growing component in many industrial processes but is mostly required for powering Li-Co batteries in Plug-in Hybrid Electric Vehicles (PHEV)1. Demand for Co is growing exponentially in order to meet future carbon-neutral technological demand as part of joint UK-European initiatives towards a more environmentally sustainable society.  The UK Geochemical Baseline Survey of the Environment (G-BASE) dataset is used to demonstrate that this technique provides a useful tool for isolating potential ‘Critical Minerals’2 in host rocks across the UK Lake District, with priority targeting towards Co-bearing ores. We reduced the dimensionality of the G-BASE stream sediment data to create geochemical maps that identify a combination of volcanic, sedimentary, and plutonic lithologies lining up geological boundaries from established 50k scale geological maps of the area. This was conducted through a combined statistical and mapping approach within QGIS and ioGAS. The resultant lithogeochemical map of the region highlights the average geochemistry for each major lithological group with varying degrees of resolution. This technique also allows for the identification of average ore metal concentrations (Ag, As, Bi, Co, Cu, Mo, Ni, Sn, Zn) for the Skiddaw Group and the Borrowdale Volcanic Group, two established host groups for As-Co-Cu-Ni mineralisation. Average concentrations of Co in the Skiddaw have been modelled to be 63.26 ppm, and in the Borrowdale volcanics to be 26.86 ppm. These values, combined with As, Cu, and Ni modelled concentrations, and other publicly available exploration-related data (structural maps, underlying batholith topography, mining history, mineral occurrences etc.) allowed us to identify 10 prospective areas of interest for possible As-Co-Cu-Ni mineralisation across these two lithological groups. Fieldwork was then undertaken to investigate several of these identified areas in order to establish the success of the model targeting tools. Ore metal-bearing minerals, mostly Cu-Fe-As phases, were identified both disseminated in local shales and andesites, and in hydrothermal quartz-chlorite veins at six sites investigated thus far. Characterisation of these minerals and host rocks is still in progress, making use of SEM-EDS and XRF analytics. We demonstrate this workflow has strong applicability within critical metal exploration and should be applied in other, more prospective regions across the globe. The only pre-requisite to the mapping is the availability of stream sediment databases with sufficient resolution across target areas. 1 Dehaine et al. 2021. BATCircle Project Report 04.2 Resilience for the Future: The UK’s critical minerals strategy, policy paper, 22nd July 2022.

Stream sediment geochemistry is a useful tool to analyse the geochemistry of the local geology within the source catchment area. This has significant applicability within the field of mineral exploration where understanding regional lithological geochemistry and how this is reflected in stream sediment geochemistry is needed, facilitating the identification of critical metal deposits. Successful identification of these deposits is essential to help tackle the deficit of these metals supply chains, especially for cobalt. This is in order to meet future carbon-neutral technological demand as part of global initiatives towards a more environmentally sustainable society.We make use of the UK Geochemical Baseline Survey of the Environment (G-BASE) dataset to demonstrate this stream sediment geochemical data has the potential to be used as a useful tool for isolating potential critical metals in host rocks across the UK Lake District. We reduced the dimensionality of the G-BASE stream sediment data, creating geochemical maps that identify a combination of volcanic, sedimentary, and plutonic lithologies lining up geological boundaries from established 50 k scale geological maps of the area. This was conducted through a combined statistical and mapping approach within QGIS and ioGAS.Furthermore, we derived average ore metal concentrations (Ag, As, Bi, Co, Cu, Mo, Ni, Sn, Zn) for the Skiddaw Group and the Borrowdale Volcanic Group, two established host lithologies for As-Co-Cu-Ni mineralisation. Average concentrations of Co in the Skiddaw have been modelled to be ~63 ppm, and ~ 28 ppm in the Borrowdale volcanics. These values, combined with As, Cu, and Ni modelled concentrations, and other available exploration-related data (structural maps, underlying batholith topography, mining history etc.) have allowed us to identify 10 prospective areas of interest for possible As-Co-Cu-Ni mineralisation. This workflow has strong applicability within critical metal exploration in the UK and other, prospective regions across the globe.

The Lake District in the northwest of England contains dramatic scenery associated with the deeply dissected Cumbrian Mountains. The Lake District National Park is the most widely visited rural area in the British Isles, with tourists attracted by outdoor activities including a network of mountain footpaths. Lakes including Coniston Water, Buttermere, Derwent Water, Ullswater, and Windermere are widely known. Scafell Pike is the highest summit in England. Human settlement in the Lake District is traced to Neolithic times. The mining heritage dates from the Romans who constructed transport routes for exporting lead and silver. Mining of copper and graphite was of particular importance during the Elizabethan times, and the Victorian period saw development of extensive copper and lead workings. The high quality, dark blue-grey slate from the Lake District roofs famous buildings in London. The combination of mountainous landscapes and tranquil valleys has inspired a long association with English literature and artists, and the region includes cultural centres associated with literary greats such as William Wordsworth and John Ruskin. The geomorphology of the Lake District is dominated by the ice-sculptured Cumbrian Mountains. Deeply-incised valleys radiate outward from the mountainous core. Valleys contain finger-lakes which developed as the Late Pleistocene ice sheets and glaciers retreated. Scree slopes and rocky summits (pikes) formed due to ice and frost erosion. Hanging valleys with small Alpine lakes and fast-flowing streams are an integral part of the topography. The geological setting is dominated by an inlier of Lower Palaeozoic strata, one of a number of disjointed terrains collectively known as the British Caledonides. Each of the Lower Palaeozoic inliers was subjected to the Caledonide Orogeny. The orogeny peaked in the Early Devonian. The oldest component of the Lake District inlier is the Skiddaw Group, a thick sequence of Lower Ordovician mudstones and shales. These rocks are associated with the rounded massifs in the northern part of the region. The mudstones and shales are unconformably overlain by the Borrowdale Volcanic Group, a succession of mostly sub-aerial calc-alkaline lavas, ashes, and volcaniclastics associated with a Mid-Ordovician volcanic island arc. The Borrowdale volcanics constitute the rugged mountains in the core of the inlier. Some of the volcanic ashes have a prominent cleavage and form high-quality slate deposits. The Windermere Supergroup, which is associated with the gently rolling hills that characterize the southern part of the Lake District, marks a marine transgression in the Upper Ordovician and Silurian. Thick sequences of limestone and shale were deposited in shallow seas rich in marine fossils. The regional unconformity separating the Borrowdale Volcanics from the Coniston Limestone is demarcated by a marked change in the topography. Two groups of intrusive igneous rocks occur in the Lake District inlier. Small bodies of Mid-Ordovician granite and gabbro are associated with many of the ore deposits. The buoyancy of a low-density, Devonian-age, granitic batholith located at depth is in part responsible for uplifting the mountainous core of the Lake District.

SUMMARY The Borrowdale Volcanic Group (BVG) in the SW of the English Lake District (SWLD) consists predominantly of subaerial pyroclastic rocks that host syn-volcanic intrusions ranging in composition from basalt to dacite. The petrology and geochemistry of the sills verify a broad compatibility with the BVG in the central Lake District. The geochemistry indicates a marginal continental arc setting, a petrogenesis involving c. 15% partial melting of a lherzolite mantle source and subsequent fractionation of olivine, ortho- and clinopyroxene, plagioclase and Fe–Ti oxide. Three groups of basaltic sills are recognized in the SWLD, one of which bears a close resemblance, geochemically, to the lower BVG (Birker Fell Formation) basaltic lavas and appears to have been emplaced early in the SWLD succession. The other two groups, which may be cogenetic, appear to be some what geochemically distinct from the first, with trace element and rare-earth characteristics suggesting a source slightly more enriched in incompatible elements. It is suggested that the fault-dominated tectonic setting in the Millom Park area of the SWLD may account for the least-evolved magmas being allowed to migrate directly to a high structural level, accounting for a higher proportion of basic lithologies in the upper BVG succession of the SWLD than is found in other parts of the Lake District.

Compact “streaky” rocks, from the Borrowdale Volcanic Series, English Lake District, are diagnosed in part as welded tuffs. Original glass is completely replaced, but similarity to welded tuffs elsewhere is shown by the dacitic-rhyodacitic composition, lack of bedding, and development of columnar jointing. The deposition of welded tuffs, as nuée ardentes, or incandescent tuff flows, is discussed.

SUMMARY During Late Ordovician (Caradoc) time, the English Lake District was the focus for one of the most intense episodes of magmatism seen during the geological history of the British Isles. There, two thick subaerial volcanic successions aggraded within opposing half-graben. In the northern half-graben, the Eycott Volcanic Group (EVG) comprises basaltic, andesitic and dacitic lavas and sills with subordinate pyroclastic rocks having geochemical affinities that are transitional between medium-K, continental-margin tholeiitic and calc-alkaline suites. In the southern half-graben, in the central Lake District, the Borrowdale Volcanic Group (BVG) comprises more than 6 km of basaltic to rhyolitic lavas, sills and pyroclastic rocks of medium- to high-K calc-alkaline continental margin type. Initial BVG eruptions were phreatomagmatic, but once conduits were opened, the volcanism became dominated by andesite lava effusion from clusters of low-profile volcanoes. The EVG compares to this early phase of BVG volcanism. A switch then occurred to paroxysmal eruptions, which produced many widespread sheets of densely welded silicic ignimbrite. This phase saw the development of major silicic, piecemeal calderas at Scafell and Haweswater. One of the later units, the Lincomb Tarns Formation, which is 150–800 m thick, is the most voluminous ignimbrite preserved within the Lake District. Other large-magnitude silicic eruptions are recorded within depocentres that lay along the current southern margin of the BVG outcrop. During intervals between the major silicic eruptions, andesitic tephra deposits were reworked and re-sedimented by mass-flow and fluvial processes, and were deposited within fluvio-lacustrine basins developed through regional extensional faulting and caldera collapse. Throughout, substantial volumes of magma were intruded contemporaneously into the volcanic piles. Towards the end of the volcanic episode, the granitic Lake District batholith was emplaced beneath the succession. Subsidence during cooling of this mass allowed renewed marine sedimentation across the region.

Garnets separated from andesites and dacites of the calc-alkaline Borrowdale Volcanic Group are highly enriched in heavy rare earth elements (HREE) relative to their host rocks (D Gt Yb from 9 in andesite to 37 in dacite). Sm/Nd ratios and D Gt LREE show a wide range, probably resulting from small quantities of an REE-rich impurity in the separates. The four garnet-host rock pairs analysed yield Sm-Nd ages identical within analytical error of a weighted mean of 457 ± 4 Ma. The Borrowdale. Group overlies upper Llanvirn sediments and is unconformably overlain by mid-Caradoc. The age of 457 Ma is considered to be the best estimate available for the age of the Llandeilo, as the samples analysed are from the lower part of the Group. The age suggests that a Lower Palaeozoic timescale intermediate between those of Gale et al . (1979) and McKerrow et al . (1980) might be the most appropriate.

Between 1989 and 1997 United Kingdom Nirex Limited (Nirex) studied in detail the geology and hydrogeology of a rock volume near Sellafield in Cumbria, NW England. The aim of the study was to determine the suitability, or otherwise, of the site as the location for a deep repository for intermediate-level and certain low-level radioactive wastes. An important factor in determining site suitability was the nature of groundwater flow in the potential repository host rock, the Borrowdale Volcanic Group. In the host rock, interpretation of borehole core, wireline logs and hydrogeological pumping test data indicated that groundwater flow was predominantly through a limited subset of discontinuities, mainly fractures, parts of which form networks of connected channels. Within this overall understanding of the nature of groundwater flow, there is a wide range of possible geometrical descriptions for the flow channels. Determination of one or more appropriate conceptualizations of the flow system must be soundly based on site characterization data as a prerequisite for any numerical modelling study. In the first part of this paper, details of the site characterization studies that were used to identify the location, orientation and mineralogical characteristics of discontinuities, and in particular the set of discontinuities referred to as Potential Flowing Features (PFFs), are provided. These features have either demonstrable present-day open porosity, or display evidence of geologically recent groundwater flow as part of the evolution of the current groundwater system. It is inferred that the PFFs observed in boreholes correspond to flowing features and that the borehole data can be used to infer the distribution and characteristics of flowing features that are present in the unobserved rock mass. On this basis, knowledge of the distribution, orientation and permeability associated with the PFFs provided the framework for developing conceptual models for groundwater flow. Numerical models were constructed to represent the flow. It is not computationally practicable to undertake regional scale groundwater flow and transport calculations in which small-scale variability is explicitly represented. Therefore upscaled effective parameters need to be derived as a precursor to running large-scale numerical model simulations. A summary of the Nirex upscaling procedure applied to the flowing feature network in the Borrowdale Volcanic Group is provided.

In the English Lake District lithostratigraphical classification has not kept pace with either the international and national codes or the code of the Geological Society (Holland et al. 1978). Probelms arise, firstly, from acceptance by several of those working in the area, of terms used in by‐gone days, such as ‘Borrowdale Volcanic Series’, and secondly, and more importantly, because individuals have specialized on certain parts of the sequence (the Skiddaw Slates, the Coniston Limestone, etc.), and have erected classifications for these rocks without reference to other parts of the succession. These classifications are examined and modified in such a way that the lithostratigraphical subdivisions are consistent for the whole of the Lower Palaeozoic sequence of the Lake District, and also relate to the international code as it is applied in other regions. The basic Lake District stratigraphical terminology of the last century is, however, retained in all its essentials.

SUMMARY The Lower Palaeozoic rocks of the Lake District record the rifting of the microcontinental terrane of Avalonia from Gondwana early in the Ordovician, its drift northwards across the Iapetus Ocean and collision with Laurentia and Baltica in the Silurian, and its Acadian deformation in Early Devonian times. Three discrete episodes of magmatism occurred during these events. In late Ordovician (Caradoc) times large volumes of magma were generated in response to subduction of Iapetus oceanic crust beneath Avalonia. Large-scale eruptions formed the basalt–andesite–rhyolite volcanic fields that are now preserved as the Eycott and Borrowdale volcanic groups. Large bodies of mainly granitic magma were emplaced beneath the Borrowdale Volcanic Group as part of the largely Ordovician Lake District batholith; these include, for example, the Eskdale and Ennerdale intrusions. Less than 10 Ma later, a brief episode of silicic volcanism is recorded within the marine sedimentary strata of the lowest part of the overlying Windermere Supergroup. A period of about 40 million years of apparent quiescence then ensued before further granitic masses, the Shap and Skiddaw plutons were emplaced around the margins of the batholith in Early Devonian times. All of these magmatic episodes were accompanied by minor intrusions. The Eycott and Borrowdale volcanic groups are rare examples in the geological record of the products of subaerial volcanism. Preservation is entirely due to subsidence keeping pace with the emplacement of new material. Subsidence resulted both from extensional tectonic processes and, locally, during the movement

Current models for Cu–Fe–As (Devonian), Pb–Zn (Carboniferous) and barite (Upper Carhoniferous/Permian) vein mineralization in the Lake District invoke a variety of sources for fluids and metals including the Skiddaw Group sediments, the Borrowdale Volcanic Group, the Lake District batholith, meteoric water and Carboniferous seawater. This study isotopically characterizes the sulphur reservoirs within the Lower Palaeozoic sedimentary and igneous sequences and Late Caledonian intrusives, and assesses their importance as sources of sulphur for the vein mineralization. The Ordovician Skiddaw Group is a reservoir of isotopically heavy sulphur with a range of δ 34 S from +11‰ to +29‰ [& x~ (1 σ ) = 18.53 ±6.7‰ ( n = 10)]. Four analyses of Borrowdale Volcanic Group rocks yielded values of +1‰ to +15‰. Such positive values are thought to be the result of closed system bacteriogenic reduction of Ordovician seawater sulphate in the case of the Skiddaw Group, and mixing of mantle-derived magmatic sulphur with Ordovician seawater sulphate for the Borrowdale Volcanic rocks. The sulphides in the vein systems hosted by the Skiddaw Group in the Vale of Newlands formed from fluids with δ 34 S H 2 S signatures ranging from +16.7‰, to +22.5‰: partially homogenized Skiddaw Group sulphur is considered to be the source. Mineralization at Coniston, hosted by the Borrowdale Volcanic Group, shows similar sulphur isotopic characteristics, and Skiddaw Group at depth is considered to be the predominant source of sulphur with a possible minor input from the wallrock volcanic rocks at the site of deposition. Although the Lake District batholith provided a heat source for the Cu–Fe–As vein mineralization, the granite did not supply any sulphur to those systems. However, magmatic sulphur was an important constituent in mineralization associated with the Shap and Skiddaw granites. The δ 34 S values for barite mineralization at Force Crag, Greenside, Skiddaw and Shap ranges from +13‰ to +18‰, and Carboniferous seawater provides the most reasonable source for sulphate.

The magnetic fabrics of some slates from the borrowdale volcanic group in the English Lake District and their correlations with strains

Late Ordovician volcanic rocks of the English Lake District typically have a magnetic remanence dominated by a single characteristic component. Previous investigations have interpreted this remanence as both of primary (pre-folding) and secondary origin. Palaeomagnetic field tests have been conducted on (a) andesite blocks from an autobrecciated lava top, (b) andesite blocks in mass-flow breccias, and (c) fault-blocks tilted during Ordovician caldera collapse to establish the time of remanence acquisition. All three tests show that the lavas retain a magnetization acquired during initial cooling: magnetizations of the breccias are coherent within clasts and random between clasts, whilst magnetizations of the tilted fault blocks converge with better that 95% confidence when corrected for the effects of caldera collapse. In contrast, the volcaniclastic sedimentary and pyroclastic rocks possess an Ordovician secondary remanence acquired after strata had been tilted by volcano-tectonic subsidence. A distributed sample of 65 andesite and basalt sheets through the Borrowdale Volcanic Group has a mean remanence direction D/I=341.9/−48.9° (α95=4.0°) yielding a positive fold test and a palaeomagnetic pole at 12.7°E, 4.3°S (dp/dm=3.5/5.3°). A progressive steepening of the palaeofield direction is recorded during emplacement of the Borrowdale Volcanic Group (∼I=−39° to I=−51°) which continued into the interval of volcanotectonic overprinting (I=−62°); the equivalent motion of Eastern Avalonia is ∼20° into higher southerly latitudes.Both the Eycott and Borrowdale volcanic groups exhibit uniform normal polarity throughout. Correlation with the geomagnetic time scale for the Ordovician restores the broad correlation between the two groups by constraining their emplacement and partial overprinting to a single long normal polarity chron occupying the Nemagraptus gracilis and earlier part of the Diplograptus multidens biozones (late Llandeilo and early Caradoc). All the volcanism, therefore, occurred within a period of no more than ∼5 Ma. The palaeomagnetic evidence confirms that the Borrowdale Volcanic Group was affected by both syn-volcanic deformation (caldera collapse) and regional compressive deformation prior to deposition of the (late Ordovician–Silurian) Windermere Supergroup. The succession of primary and secondary Ordovician palaeomagnetic poles from the Lake District inlier defines an anticlockwise apparent polar wander (APW) loop with the apex correlating with ‘soft’ closure of the Iapetus Ocean and late Ordovician deformation. The APW paths from Avalonia and Baltica converge at this point as subduction ceased and the arc subsided beneath the sea after mid-Caradoc times.

Magnetic fabric implications for the relationships between deformation mode and grain growth in slates from the Borrowdale Volcanic Group in the English Lake District