European Social Fund Project No: 011019SW1

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PART 1: SAMPLE COLLECTION AND PROCESSING INTRODUCTION This research was initiated in October 2001 as a PhD project, undertaken by Kirsty Reid, under the supervision of Professor Peter Scott at Camborne School of Mines and funded by a grant from the European Social Fund (with additional funding from the British Geological Survey). The original specification of the project was to establish the geochemical background, or baseline, within the Devonian and Carboniferous metasediments of Cornwall, for a number of elements typically viewed as pollutants within the environment (such as arsenic and chromium). The potential discovery of high natural background levels of these elements, typically thought of as by-products of mining and metal processing, would require a reinvestigation of how we define and deal with metal ‘pollution’ and its effect on the environment. Sample collection and the construction of an Access database began in November 2001, initially concentrating on the area around Wadebridge in north Cornwall, within what was then termed the Trevone Basin (Andrews et al., 1998; Isaac et al., 1998; see Figure 1).

Figure 1. A map of the sedimentary basins of Cornwall. C-PFZ, Cardinham-Portnadler Fault Zone; C-CFZ, Cambeak-Cawsand Fault Zone; SPL, Start-Perranporth Line; PBF, Plymouth Bay Fault; RFZ, Rusey Fault Zone. After Isaac et al., 1998.

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Further samples were collected from west Cornwall and the Roseland Peninsula, and were processed before being shipped to Canada for analysis (see details below). By November 2002 a total of 104 samples had been collected and processed; the results (Scott et al., 2003) being published in December 2003. At this point Kirsty Reid decided to leave CSM and the PhD project was put on hold until it was redefined as a post-doctoral research project and the author took the work over in March 2003. In the intervening period details were emerging of a radical reinterpretation of the Devonian/Carboniferous paleogeography and sedimentation history of the basins across Cornwall and Devon by Brian Leveridge of the British Geological Survey. Dr Leveridge allowed the project to use his (as yet unpublished) map of the sedimentary basins (see Figure 2) and one of the first tasks undertaken on restarting the project was to revise the database to take account of the new nomenclature.

Figure 2. A map of the sedimentary basins of Cornwall and Devon; after B. Leveridge (currently unpublished, used with permission).

The Trevone basin has been abandoned in favour of the Looe Basin and the (redefined) South Devon Basin, with the Tavy Basin occupying the area from north Cornwall to southern Dartmoor formerly ascribed to the South Devon Basin. This new map expands on the nomenclature used in the mapping of the Plymouth Sheet by the BGS and the subsequent memoir (Leveridge et al., 2002) and has been used throughout the remainder of this project.

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Sampling began again in March 2003 and was tailored to focus on specific areas to achieve as widespread a coverage as possible across Cornwall. The majority of the metasediments sampled range in age from Lower Devonian to the end of the Devonian (Tucker et al., 1998) although sedimentation in some areas (see Figure 3) is thought to have extended into the Lower Carboniferous (Leveridge et al., 2002). Metasediments of Carboniferous age (Gradstein & Ogg, 1996) occupy the Culm Basin; within Cornwall these rocks outcrop from Tintagel to Stratton in the north east, around Launceston and, as a klippe (Isaac & Thomas, 1998) overlying Devonian strata of the Tavy Basin, around St Mellion in south east Cornwall. Due to timing constraints it was only the latter area that was sampled and therefore rocks of Carboniferous age make up only a small proportion of the entire sample set. The metasediments sampled during this program were deposited in a series of ensialic basins in the form of complex grabens and half-grabens (Shail, pers. comm. 2004; Shail, 1992; Andrews et al., 1998; Isaac et al., 1998; Isaac & Thomas, 1998; Leveridge et al., 2002; Leveridge et al., 1990) and comprise a series of mudstones, siltstones, sandstones, grits, greywackes and matrix-supported breccias (very coarse debris flows). The original sedimentary sequences have been subject to complex folding, faulting and thrusting with contemporaneous low-grade regional metamorphism during the Late Carboniferous – Early Permian Variscan Orogeny (Alexander, 1997; Alexander & Shail, 1995, 1996; Shail & Alexander, 1997, 2000; Andrews et al., 1988, 1998; Warr et al., 1991; Primmer, 1985; Goode & Taylor, 1988; Leveridge et al., 1990). They were also subjected to localised contact metamorphism, metasomatism and mineralisation during the emplacement of the Cornubian granite batholith (Floyd et al., 1983, 1993; LeBoutillier, 2003). In the west, within the Gramscatho basin, the Grampound Formation consists of coarse sands and grits, locally carrying basic lithic fragments and detrital chromite. The Porthtowan Formation comprises a thick sequence of interbedded slates and sandstones (thought to be a distal deep-water turbidite fan system; Leveridge et al., 1990). The overlying Mylor Slate Formation comprises a thick sequence of slates with subordinate siltstones, sandstones and contemporaneous basic igneous rocks, interpreted as deep water basinal muds with occasional distal turbidite fans (Leveridge et al., 1990; Goode & Taylor, 1988). Towards the top of the sequence the slates pass into the matrix-supported breccias of the Porthleven Breccia Member which carries a mixture of Portscatho Formation, Mylor Slate and exotic clasts; it is thought to represent debris flow flysch derived from the advancing Carrick Nappe as it advanced across the Gramscatho Basin at the end of the Devonian (Leveridge et al., 1990). Within the Carrick Nappe, the Portscatho Formation comprises alternating grey to greenish-grey sandstones and slates with sporadic thin siltstone beds (Leveridge et al., 1990). The structurally overlying Veryan Nappe consists of the Pendower Formation (manganiferous slates with subordinate sandstones, limestones and cherts), the Carne Formation (sandstones, greywackes and siltstones with subordinate slatey mudstones) and the Roseland Breccia Formation (grey slatey mudstones with matrix-supported conglomerate/breccia containing olistoliths of exotic igneous, metamorphic and sedimentary rock types), this last again representing mass debris flows from an advancing nappe pile (Shail, 1992). The Dodman Nappe consists of the Dodman Formation, which comprises a series of grey sandstone/siltstone turbidites interbedded with grey mudstones. Within the Looe Basin sedimentation appears to have started in the Early Devonian. The formations present within this basin have various names depending on which geological sheet is being viewed; a problem that is more acute with the formations of the South Devon and Tavy Basins. Figure 4 shows the overlap in nomenclature between the various geological maps that cover Cornwall.

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Figure 3. The chronostratigraphy of the sedimentary basins across Cornwall (after Leveridge et al., 1990; Goode & Taylor, 1988; Leveridge et al., 2002; Timescale after Gradstein & Ogg, 1996; Tucker et al., 1998)

PBr Porthleven Breccia Member LLSM Lower Long Sands Member (Bov) Sme St Mellion Formation MrSl Mylor Slate Formation ULSM Upper Long Sands Member (Bov) Bre Brendon Formation Ptn Porthtowan Formation Bov Bovisand Formation NCh Newton Chert Formation Gmp Grampound Formation StG Staddon Formation Pto Portscatho Formation PyL Plymouth Limestone Formation Pdr Pendower Formation FarM Faraday Road Member (PyL) Cne Carne Formation PrkM Prince Rock Member (PyL) RBr Roseland Breccia Formation Slt Saltash Formation Dm Dodman Formation SGTM St Germans Tuff Member (Slt) Polg Polglase Formation WrG Wearde Sandstone Member (Slt) Hln Hallane Formation Tpt Torpoint Formation Wtd Whitsand Bay Formation Tvy Tavy Formation Bin Bin Down Formation Btn Burraton Formation

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Figure 4. The overlap in formation names (dashed lines) between different 1:50,000 geological sheets.

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For the purposes of this report and the corresponding graphic data relating to each basin, the formation names as quoted in the Plymouth Memoir (Leveridge et al., 2002) have been used, where applicable. The Whitsand Bay Formation consists of cycles of grey to purple slatey mudstone, interbedded with grey/green sandstones and grey quartzites in beds up to 30 cm thick. The overlying Bovisand Formation (also known as the Meadfoot Beds or simply Middle Devonian Slates) consists mainly of grey slatey mudstone with subordinate thinly bedded sandstone and quartzite beds Leveridge et al., 2002). The Staddon Formation consists largely of thickly bedded and massive medium to coarse-grained fluviatile sandstones. In south east Cornwall these sands are often deep red and are interbedded with red mudstone-supported conglomerates. No rocks of the Plymouth High were sampled during this project, but extensive sampling in south east Cornwall across the South Devon Basin was completed and additional samples were also donated by the BGS. The Saltash Formation consists of dark-grey to grey slatey mudstones with subordinate beds of siltstone and sandstone. The slate has been extensively quarried in the past for building stone and roofing slate. Within the Saltash Formation, the Wearde Sandstone Member consists of fine to coarse-grained fluvial sandstones (Leveridge et al., 2002). Sediments of the Torpoint Formation are known to occur in both the South Devon Basin and the Tavy Basin (where they are most fully developed). These consist of interbedded brownish-purple and red-purple slatey mudstones with subordinate siltstones and fine-grained sandstones. Within the Tavy Basin the Tavy Formation comprises lustrous grey to grey-green slatey mudstones (bordering on phyllites locally) with rare sandstone beds (Leveridge et al., 2002). The overlying Burraton Formation consists of Black slatey mudstone with sporadic coarse-grained sandstone beds. A single sample of this formation was obtained from the west bank of the River Tamar near Horsebridge. The metasediments of the Culm Basin were obtained (the majority by way of the BGS) from a thrust klippe in south east Cornwall. The Brendon Formation consists of dark-grey slatey mudstones with subordinate siltstones and coarse sandstones, while the St Mellion Formation is dominated by coarse-grained sandstones and turbidites, with minor mudstones. No samples of the overlying Newton Chert Formation were obtained. SAMPLING PROCEDURE & RATIONALE The original aim of the project was to gather as many samples as possible, with the greatest geographical and formational spread, and to conduct an analysis of these samples to establish geochemical baseline values for a number of potentially ‘polluting’ elements, such as arsenic, nickel and chromium. It rapidly became apparent that with the analysis package of 41 elements offered by Acme Laboratories in Vancouver (see below) that this could be extended to allow baselines to ascertained for rock-forming elements and also rare, noble and base metals. The data gathered would also have the potential to be used for a variety of applications far in excess of the original specifications. Sampling was undertaken across Cornwall between March and October 2003, resulting in the collection of a total of 468 metasediment samples (see Figure 5) and 20 samples of basic igneous rocks associated with the various basins, giving a total of 488 samples. Of these, 50 were donated, as powders, by the BGS and approximately 100 chip samples were donated by Dr Robin Shail from his PhD sample set.

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Figure 5. A map showing the sample locations for the metasediments analysed in this project. Due to exposure constraints any sampling exercise was likely to be biased towards coastal exposures, as is the case in this project, but extensive use of first and second edition 6” O.S. maps to identify former quarrying sites and the utilisation of existing road cuttings, temporary exposures, working quarries and former mine sites also allowed a large number of inland samples to be collected. Material was collected from as many formations with as wide a range of lithologies as possible. Samples collected represent clean rock, free of any visible effects of weathering or mineralisation. Material showing any evidence of veining (including metamorphic ‘sweat’ quartz veins) was disregarded. Some samples took considerable time to collect due to stripping away weathered material from the surface before rock of sufficient quality could be extracted. Samples ranged from 2-5 kg in weight (averaging 5 kg); their characteristics and field associations were logged and the sample location fixed using a Garmin Summit GPS to give a 10 figure grid

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reference. The samples were placed in numbered durable plastic sample bags and transported back to CSM. SAMPLE PREPARATION AND PROCESSING Samples were initially washed and dried (if necessary), before being broken down by hand to < 10 cm size. They were then fed through a jaw crusher to give a < 8 mm grit sample. Approximately 100 grammes of this material was placed into a tema barrel for milling with the remainder labelled, bagged and sealed for future reference. The first 104 samples were milled using a chrome steel barrel, but this was changed to a tungsten carbide barrel on re-starting the project as it was felt that this was necessary to ensure the integrity of the chromium values. On changing the barrels there was a noticeable drop in chromium values, but a corresponding sharp rise in tungsten that reached in excess of 100 times the true value (when milling quartzites and siliceous mudstones, which abraded the barrel), which were later discovered when cross-checks were made re-running the high samples with a chrome barrel. So large were the errors that it was felt necessary to discount all the tungsten analyses and remove this element from the results. Samples were milled for an average of 5 minutes (soft mudstones for less, hard sandstones for more). Between runs the barrel was cleaned with diatomaceous sand. The powder sample was split with 50 grammes going into a labelled bag for storage and future reference; the remaining 50 grammes was placed into a labelled sealed envelope ready for shipping to the laboratory for analysis. Not all of this material was needed for the analysis, but the excess was present as an insurance for standard re-runs (every 10 samples), accidental loss of material or queries regarding the results. Pulps were held for three months at the laboratory, prior to disposal. The samples were sent to Acme Laboratories in Vancouver, Canada; where they were treated using Acme’s Group 1EX 41 element package. 0.25 grammes of each sample was dissolved using a four-acid digest (HCl-HF-HClO4-HNO3), the residue from which was further treated with aqua regia (HCl-HNO3-H2O). The sample solutions were then aspirated into an ICP mass spectrometer for analysis. In addition to the regular standards and re-runs used by Acme, two standards were also sent with each sample batch; these included rigorously checked (CSM XRF data) samples of Delabole Slate and Grampound Grit, in addition to a reference sample (SDO-01) of Ohio Shale (Kane et al., 1990), sourced from the USGS. Crosschecks of samples were undertaken at CSM using a Philips PW1400 XRF and some samples were also checked with results gained from the XRF at Keele University (Shail, 1992) that ran a set of samples used in the project. DATA HANDLING At the outset of the project the main databank was a Microsoft Access 2000 database, which held all the locational, geological and analytical data pertaining to each sample. Extracts were made from this master sheet for use with other programs. In March 2003 this set up was changed and the master data sheets were held as Microsoft Excel 2000 spreadsheets; one holding the analytical data and one holding the grid references, descriptions and lithological data. Using this format allowed rapid data extracts to be made and greatly improved

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the interface with other programs, which could import this format directly. It also allowed rapid incorporation of analytical data from Acme Labs to be made as this was supplied in both Excel CSV (comma separated values) and hard copy forms. While the data was held in Excel format, the majority of the statistical handling was done using Statsoft Statistica V 5.5 and 6.1. This program has a far greater range of functions and controls, in addition to a vastly improved graphics capability. Some further statistical work was done using Rockware Rockworks 2002, particularly for standard deviation analysis and log histogram/ternary graph plotting and contouring. Geographic, analytical and statistical data were displayed using Clark Labs Idrisi 32 V2, utilising a digitised map of Cornwall and the Cornubian Batholith supplied by Dr David Watkins of CSM. Graphic manipulation of the resulting maps, to achieve a publication-quality result, was done using Corel Draw V 11 and 12, and Adobe Photoshop V 7. DATA REFINING AND THE DEFINITION OF A GEOCHEMICAL BASELINE International Geological Correlation Programme (IGCP) 259 is working towards the establishment of global geochemical baselines (Darnley et al., 1995; Darnley, 1997). This association of national geological surveys has, since the early 1990’s, been conducting a series of sampling programmes across the World and has been working on the integration of existing datasets into a single coherent global database. Although the term ‘Geochemical Baseline’ is much used, it is very difficult to define. The data collected by IGCP 259 has a rigorous set of collection, analytical and processing guidelines, but as much of the data from the developed world has already been collected, the existing data has to be ‘normalised’ against the database as a whole (Darnley et al., 1995) and so no ‘hard and fast’ definition of exactly what a geochemical baseline is, exists. In terms of the lithogeochemical project, any existing definition would be irrelevant as IGCP 259 collects its data from unconsolidated surface sediments (Darnley et al., 1995; Darnley, 1997) and uses a series of collection grids, sample densities (very low) and analytical techniques that are either of no value, or are beyond the scope of this project. In its raw form the data collected during this project shows a remarkable consistency, both within, and across, the various sedimentary basins. However, a number of high ‘spikes’ were identified, particularly with regard to the base metals. These ‘spikes’ appear to be related to disseminated mineralisation and are most likely to represent metasomatic haloes around known lode systems and/or may be associated with fluid transport through heavily faulted rock units. The majority of these occurrences occur within known mining areas or those on the margins, or in close proximity to, the margins of granite plutons. The effect of these ‘spikes’ (often over 100 times the average value), consisting of a small (<1% to ~2%) proportion of very high elemental values was to drive up the mean value well out of proportion to the number of samples involved. It was therefore decided to remove all values above three positive and negative standard deviations and to use the mean of the remaining total sample set for each individual element as the geochemical baseline value for that element. The values obtained by this operation were made available at the Ussher Society Conference in Taunton in December 2003; they are reproduced below in Table 1.

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Mo_ppm Cu_ppm Pb_ppm Zn_ppm Ag_ppm AVER 0.51 32 19 106 0.1 MEDIAN 0.4 26.2 13.6 98 0.1 MODE 0.2 22.2 13.7 97 0.1 ST DEV 0.5 24 23 50 0.1 Ni_ppm Co_ppm Mn_ppm Fe_% As_ppm AVER 53 24 871 4.74 23 MEDIAN 53.9 23 670.5 4.82 14 MODE 29.9 24 665 4.72 7 ST DEV 21 10 702 1.39 25 U_ppm Th_ppm Sr_ppm Cd_ppm Sb_ppm AVER 2 8 88 0.13 1.5 MEDIAN 2.3 8.1 73 0.1 0.9 MODE 2.3 7.6 52 0.1 0.5 ST DEV 0.64 3 57 0.12 2 Bi_ppm V_ppm Ca_% P_% La_ppm AVER 0.27 124 0.44 0.05 24 MEDIAN 0.2 130 0.15 0.056 23.55 MODE 0.1 147 0.04 0.056 20.3 ST DEV 0.21 40 0.86 0.03 10 Cr_ppm Mg_% Ba_ppm Ti_% Al_% AVER 97 1.28 434 0.35 7.97 MEDIAN 98.5 1.23 438 0.32 7.92 MODE 124 1.32 463 0.27 6.19 ST DEV 33 0.52 175 0.14 2.01 Na_% K_% W_ppm Zr_ppm Ce_ppm AVER 0.99 2.49 37 90 52 MEDIAN 0.64 2.65 26.1 93.6 51 MODE 0.47 2.19 0.6 113.3 48 ST DEV 0.88 1.08 37 27 20 Sn_ppm Y_ppm Nb_ppm Ta_ppm Be_ppm AVER 4 12 8 0.59 2 MEDIAN 2.8 10.6 7.1 0.5 2 MODE 2.9 9.5 5.7 0.5 3 ST DEV 6 6 4 0.24 1 Sc_ppm Li_ppm S_% Rb_ppm Hf_ppm AVER 13 109 0.09 111 3 MEDIAN 13 99.9 0.05 102.5 3.1 MODE 11 53.2 0.05 61 3.5 ST DEV 4 59 0.08 55 0.85

Table 1. Geochemical baseline figures for 40 elements derived from 468 samples of metasedimentary rock from across

Cornwall.

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The nature of dealing with large datasets invariably means that in any statistical treatment given to the dataset as a whole the results will produce values that are subject to change when the dataset is broken down into its component parts (in this case by basin, formation or lithology); in many cases these changes will be fairly minor, but we are already aware of particular formations that are preferentially enriched in certain elements and their individual ‘baseline levels’ will show a marked deviation, for those particular elements, from the values quoted overleaf. It should also be borne in mind that the values given here are for the current baseline levels and are unlikely to reflect the original element budgets of the metasediments (particularly with regard to the trace elements), which have, to varying degrees, been affected by elemental remobilisation during diagenesis, metamorphism and subsequent, post-granite emplacement, mineralisation. It is impossible to gauge with any accuracy, but it is certain that the metasedimentary cover rocks were a source of metals and other elements that were drawn into the mineralising system by hydrothermal convection. We are able to see areas within the metasediments that were affected by the positive redistribution of particular metals; further work may also allow us to see which areas have been depleted and if there is any pattern to that depletion. The values quoted overleaf give a ‘broad brush’ picture of the geochemistry of the metasediments, but there are areas where these values are far exceeded. This is in part due to overprinting by mineralisation, but is also a function of sediment provenance. Some elements, such as As, Ni and Cr give apparent cause for concern as they exceed part of the soil action guidelines across large sections of the study area; however, it has to be borne in mind that these are bedrock values and not soil values. The movement of elements from bedrock to soil varies according to element chemistry and physiochemical conditions at the soil/bedrock interface. The bio-availability of any given element is often species-dependent (e.g. arsenic) and is heavily influenced by Eh/pH conditions and the presence of organic compounds at any given location; therefore the quoted values need to be viewed accordingly. A point at this juncture has to be made regarding data integrity. While much of the data gained during this project appears satisfactory, there are a number of known problems, with the sample processing and with the analytical techniques used. The use of chrome steel and tungsten carbide barrels has introduced contaminants (Cr, Mo and Co with the Cr/Fe barrel and W with the WC barrel) into the samples at the milling stage, that would have been negated had an agate mill been used. In terms of the chrome barrel this contamination has not been too drastic, although as chromium is of prime importance as a ‘potentially harmful’ element, any contamination is not desirable; the tungsten barrel however, has raised the average W content of each sample from 2-3 ppm to ~25 ppm, with some quartzites, sandstones and siliceous mudstones recording values in excess of 200 ppm. Later crosschecks showed the true values to be in the 2-3 ppm range. Such contamination has invalidated the majority of the tungsten results (apart from the first 104 samples). As far as the sample analysis is concerned, there were known problems with elements such as arsenic being lost by sublimation (Scott et al., 2003) during analysis (therefore recording much lower than actual values) and elements such as zirconium being under-reported due to the incomplete digestion of resistate phases. Further problems were encountered when looking at the XRF crosschecks at CSM due to the limits of the machinery and the unreliability of some of the standards used. An analysis of the results from Acme, CSM and Keele by Dr Robin Shail (pers. comm. 2003) has revealed the following:- Good correlation: Al, Ba, Ca, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb, Rb, Sr, V, Zn

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Poor: Ce, Nb, Ti, W, Y, Zr (probably controlled largely by resistant heavy minerals plus opaque oxides - but could also be in phyllosilicates) Unclear: Ag, As, Au, Be, Bi, Cd, Hf, La, Li, Mo, Sb, Sc, Sn, Ta, Th, U [this group need a little more work to refine. Some will probably be very close to detection limit (Au, Ag etc); others will be controlled by heavy minerals (La, Sn, Ta, Th, U)]. The major rock-forming oxides show very good correlations, while many of the base metals, rare earths and some transition metals show poor correlation. This is in part due to the presence of resistate minerals such as the spinel group, with the potential for substitution by numerous ions (Deer et al.’ 1992), zircon, monazite, ilmenite, sphene and allanite. It is likely that incomplete digestion of these phases is in part responsible for the discrepancies seen in the comparative data. Another factor to be borne in mind when analysing the data and drawing comparisons between the various sedimentary basins is the relative sample density within each basin and/or formation. Out of a total of 468 metasediment samples 279 are from the Gramscatho Basin, 74 are from the Looe Basin, 67 from the South Devon Basin, 35 from the Tavy Basin and 13 from the Culm Basin.

PART 2: FURTHER RESULTS INTRODUCTION As well as furnishing geochemical baselines for 39 elements (Au was discounted due to samples being below the detection limit; W due to contamination) the data collected has highlighted some of the geochemical variations between certain formations (a factor that may prove useful in geochemical mapping exercises) and given insights into the mineralogy and provenance of some of the metasediments. MINERAL/ELEMENTAL INDICATORS Elemental characteristics can be used as indicators to highlight some formations. In some cases this is due, in large part, to the presence of particular mineral assemblages that themselves reflect a distinct provenance; in other cases the mineralogy is less distinct and requires further examination. In the first group a prime example is the Grampound Formation, within the Gramscatho Basin. This formation outcrops in a WNW-trending belt running inland from the coast near Pentewan (see Figure 6). This formation consists of a series of sandstones and coarse grits (Shail, 1992) with subordinate siltstones and mudstones. The sandstones are arkoses carrying a variety of lithic fragments, including basaltic clasts, as well as detrital chromite and amphibole. The elemental maps for chromium (Figure 6), sodium and magnesium all show considerable enrichment, above the mean, for these elements. The high chromium cannot be explained by the amount of detrital chromite alone and it is likely that Cr is present within the basic lithic fragments, within the detrital amphibole and within the chlorite present between the sand grains. The high magnesium reflects the high basic lithic content as well as being present in the chlorite matrix and possibly as detrital spinel. SEM examination of this material (sample KR-02-048) confirmed the presence of plagioclase feldspar, quartz, lithic fragments, amphibole and chlorite. Minor components included:- fairly common, well-rounded, zircon grains (to 100 µm +), which show well-developed compositional

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zoning; occasional sphene (CaTiO3; to 100 µm), locked in quartz, and apatite (to 20 µm). Rare thorianite (ThSiO4) was also found, as was ilmenite (to 100 µm), some of which was slightly manganiferous, and monazite (to 5 µm), locked within feldspar crystals.

Figure 6. An element map of Chromium distribution. The +2SD - +3SD high values seen just below the St Austell Granite, trending WNW, mark the strike of the Grampound Formation. The best examples of the second group are the Pendower Formation from the Gramscatho Basin and the Polglase Formation from the Looe Basin. The Pendower Formation outcrops in a belt crossing the Roseland ENE from Pendower Beach. Largely comprising a series of black mudstones, with subordinate sands and silts, this formation (relative to the other formations of the Gramscatho Basin) is markedly depleted in a range of elements, including Ag, As, Ba, Bi, Ce, Hf, K, La, Nb, Th, Rb, Sb, U and Zr, while being markedly enriched in Mn (and to a lesser extent in Mg and Na). The high Mn content (see Figure 7), together with the presence of cherts within this formation suggests a SEDEX origin for these sediments, but this study did not see the corresponding

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enrichment in metals such as Zn, Ni and Cu seen by Shail (1992), however the marked depletion in some elements within these sediments (see Figure 8), does not extend to those metals, which show a distribution comparable to other formations within the basin (copper is slightly elevated).

Figure 7. Box & whisker plot showing the high Mn values encountered with the Pendower Formation, relative to the other units comprising the Gramscatho Basin. No samples of this material were investigated by SEM, so the exact mineralogy of the sediment remains unclear. Further work is needed to discover if the manganese is locked within the lattice of clay minerals and chlorite, or whether it is present as discrete filaments of pyrolusite (MnO2) or some other Mn-oxide species, or a combination of the two. The low values for several metals and species typical of heavy detrital minerals (suggesting a distal, low-energy regime with infrequent debris flows) is distinctive, but only of limited value with regard to mapping. Of far more value, potentially, is the marked enrichment in As (see Figure 9), Li, Cd, Mn and Sb (and to a lesser extent Bi, Cu and Be) displayed by the Polglase Formation within the Looe Basin. This formation has been mapped on the Mevagissey Sheet, striking WNW from the coast near Pentewan, but has not been traced beyond the sheet boundary, although it is suspected that the formation may cross the Cornish Peninsula to be exposed on the north coast, within the area around Perranporth, which is due to be remapped as part of the Newquay Sheet. The chemical characteristics of these featureless black mudstones may therefore be of real value in distinguishing them from similar lithologies exposed along the coastline.

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Figure 8. Box & whisker plot showing the low Ba values encountered with the Pendower Formation, relative to the other units comprising the Gramscatho Basin.

Figure 9. Box & whisker plot showing the high As values encountered with the Polglase Formation, relative to the other units comprising the Looe Basin.

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MINERALOGY The distribution of many elements, when broken down by lithology, is grainsize-dependent (see Figure 10), with the highest concentrations seen in the clay-size (mudstone) fraction (although there are notable exceptions, which seem to relate to particular detrital mineral concentrations in some lithological units), indicating incorporation of the majority of elements analysed, preferentially within the lattices of clay mineral species and chlorite species within the mudstones, as opposed to detrital species within the sandstones (Baker, 1962). This at least would be the assumption for the distributions seen in the various graphs, but SEM examination of mudstones and sandstones has shown that these assumptions can be incorrect.

Figure 10. Box & whisker plot showing As values encountered within the various lithologies that make up the five basins in the dataset. Note the increasing concentration with decreasing grainsize seen in all cases except the Culm Basin. The examination of two mudstones from the Carne Formation has revealed a complex mineralogy with a large number of detrital species, which occur as grains down to the order of 1 µm. Sample KR-02-020, from Menaver Point on the Lizard, is a black mudstone with thin intercalated silt laminae. Consisting largely of sub-angular quartz grains (~30 – 40 µm in size) in a finer quartz and chlorite matrix, the rock carries an extensive suite of anhedral detrital clasts including zircon, magnetite, apatite, monazite (all to 30 µm in size), chalcopyrite (to 20 µm), sphalerite, ilmenite, pyrite and rutile (all to 10 µm). In some instance both pyrite and rutile show both angular and

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rounded (framboidal) to vein-like forms, suggesting that some of these occurrences are diagenetic in origin. In addition to the above species, both anhedral amphibole (in places degrading to chlorite) and cerium-rich allanite [Ca(Ce,La)2(Al,Fe)3(SiO4)3OH] occur in clasts up to 50 µm across, typically within the chlorite matrix, pointing towards a basic to syenitic (Deer et al., 1992) source for some of this material. Sample KR-02-017, from Carreg-a-Pilez on the Lizard, is similar in lithology to the previously described sample. Within its quartz/Ti-rich chlorite matrix anhedral rutile (10 – 30 µm) is fairly common (some occurrences appear framboidal, though the majority appear detrital); other species present include monazite (angular to rounded, 1 – 5 µm), zircon (5 – 15 µm, sub-angular to subhedral), sphalerite (angular, to 20 µm), albite (to 50 µm) and pyrite, occurring as both angular grains (to 20 µm) and framboidal masses (5 – 6 µm in size). In comparison, sample KR-02-029, a gritty iron-stained sandstone from the Portscatho Formation of Porthcurnick beach, near Falmouth, carries a slightly more restricted assemblage. Consisting largely of sub-rounded quartz grains (300 µm to 1 mm in size), some of which are rutilated, the rock also carries common subhedral to sub-rounded albite and perthite feldspars (to 100 µm). In addition zircon (as well rounded grains to 50 µm and as lithic fragment intergrowths with quartz), apatite (anhedral, to 50µm), rutile (anhedral, to 20 µm) and pyrite (anhedral, to 20 µm) are also present; much of the pyrite is breaking down to haematite, which carries small amounts of Al and Mg. This sample also contained a single chromite grain, 80 µm across, which had been disaggregated into a series of closely associated shards. ZAF analysis showed it to be carrying small traces of Mg and Mn. Contrary to expectations, the mudstones examined showed that a significant amount of the Ce, Zr, Ti and Mg present within the rock is as detrital minerals (to very small grain sizes), rather than being present solely within clay (and related) species. The origin of the metal sulphides is unknown; while some of the pyrite (and rutile) appears sedimentary or diagenetic in origin, the vast majority of Cu, Zn and Fe sulphides appear detrital. The association with ilmenite, chromite, zircon and monazite (McLennan, 1989) suggests as possible mixed basic/intermediate/acid igneous (continental) source, possibly with the presence of nearby black smoker (SEDEX) deposits undergoing submarine erosion. Many of the binary graphs (bracketed by lithology) produced from the data show a distinct division into lithological groups (see Figure 11); it would appear that this bracketing of results is in many cases grainsize dependent (with larger concentrations of smaller grains in finer sediments) and not simply due to clays within mudstones acting as accumulators of metal species. Some relationships do reflect mineralogical variations between lithologies; the Na and K (see Figure 12) graph shows Na preferentially occurring within sandstones (as detrital plagioclase), while K occurs at its highest concentrations in mudstones (as clays and phyllosilicates). Other relationships are more obscure. There is a strong correlation between Cr, Ni and V (see Figure 13) and between Cr and Ni and Cr and V. The most likely focal point for these elements is within clay species (neither Ni or V are known to substitute within chromite; Deer et al., 1992), nickel in particular has a strong affinity with clays. However, when the spatial distribution of these elements is examined, there is strong partitioning between chromium and nickel on the one hand (occurs most prominently within the Grampound Formation, reflecting a strong basic input), and

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Figure 11. Binary plot. The higher concentration of Al in mudstones reflects increasing clay mineral content. The corresponding rise in Zr may reflect increasing amounts of detrital zircon, rather than higher levels within the clays.

Figure 12. Binary plot. The higher concentration of K in mudstones reflects increasing clay mineral content. The higher Na levels in sandstones reflect the plagioclase content of the arkosic sediments.

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Figure 13. Ternary plots. Cr-Ni-V for sandstones, siltstones and mudstones. The Cr-V relationship appears most important for sandstones and siltstones, while Ni-V becomes more important in mudstones, perhaps reflecting a closer association within clay species. Vanadium, which gives a strong positive anomaly in sediments of the South Devon Basin (see Figure 14). The forms in which vanadium is occurring remain obscure, no traces of primary V-bearing species, or detectable concentrations in other minerals have been found during this project and it appears most likely that most vanadium is held within clays, but this does not account for some of the high V values found in some sandstones, in theses cases vanadium must be present in some of the silicate phases present, or within lithic fragments.

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Figure 14. Vanadium in metasediments. Note the positive anomaly, running to the south of Bodmin Moor, within the sediments of the South Devon Basin. SUGGESTIONS FOR FURTHER WORK The amount of data produced by this project has far outstripped the time available to analyse it fully; it has also raised more questions than it has answered. While we have a geochemical baseline for 39 elements across Cornwall (that will be subject to further refining and re-analysis with new equipment), the mineralogical relationships behind those values remain to be fully understood. Further work (including detailed SEM analysis of a large number of samples) on the mineralogy of the metasediments will allow a more detailed provenance to be established and some of the mineralogical characteristics may then, along with the gross geochemical signatures (such as those seen in the Polglase Formation, etc), provide useful techniques to distinguish between particular formations, especially where outcrop is poor and small temporary exposures (trial pits) are used to elucidate the local geology. Such ‘geochemical mapping’ techniques may be of great importance, particularly where discriminating between sections containing sediments with few, or no, distinguishing features.

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In addition to the geochemical baselines established for the metasediments, a further set of programmes should be established to examine the soils above many of the exposures used in this study, and also the local plant life; thus it would be possible to track the 39 elements from bedrock, through the soil and into the biosphere, examining which elements are lost or accumulated in the process. Such a body of data would have direct application across a range of sphere and bring geology, botany, biology, chemistry and environmental together to present a truly holistic study of the environment in Cornwall. REFERENCES Alexander, A.C. 1997. Late- to Post-Variscan deformation in south Cornwall. Unpublished PhD thesis, University of Exeter. Alexander, A.C. & Shail, R.K. 1995. Late Variscan structures on the coast between Perranporth and St Ives, South Cornwall. Proceedings of the Ussher Society, 8, 398-404. Alexander, A.C. & Shail, R.K. 1996. Late- to post-Variscan structures on the coast between Penzance and Pentewan, South Cornwall. Proceedings of the Ussher Society, 9, 72-78. Andrews, J.R., Barker, A.J. & Pamplin, C.F. 1988. A reappraisal of the facing confrontation in north Cornwall: fold- or thrust-dominated tectonics? Journal of the Geological Society, London, 145, 777-788. Andrews, J.R., Isaac, K.P., Selwood, E.B., Shail, R.K. & Thomas, J.M. 1998. Variscan structure and regional metamorphism. In: Selwood, E.B., Durrance, E.M. & Bristow, C.M. (eds) The geology of Cornwall, University of Exeter Press, 82-119. Baker, G. 1962. Detrital heavy minerals in natural accumulations. Australasian Institute of Mining and Metallurgy, Monograph Series No: 1, 146 p.p. Deer, W.A., Howie, R.A. and Zussman, J. 1992. An introduction to the rock forming minerals (second edition), Longman, Harlow, 696 pp. Darnley, A.G. 1997. A global geochemical reference network: The foundation for geochemical baselines. Journal of Geochemical Exploration, 60, 1-5. Darnley, A.G., Björklund, A., Bølviken, B., Gustavsson, N., Koval, P.V., Plant, Jane A., Steenfelt, A., Tauchid, M., and Xuejing, Xie. 1995. A Global Geochemical Database for Environmental and Resource Management. Recommendations for International Geochemical Mapping, Earth Science Report 19. UNESCO Publishing, Paris. Floyd, P.A., Exley, C.S. & Stone, M. 1983. Variscan magmatism in SW England - discussion and synthesis. In: Hancock, P.L. (ed) The Variscan fold belt in the British Isles, Adam Hilger Ltd, Bristol, 178-185. Floyd, P.A., Exley, C.S. & Styles, M.T. 1993. Igneous rocks of south-west England. Chapman & Hall, London.

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Goode, A.J.J, and Taylor, R.T. 1988. Geology of the country around Penzance. Memoir of the British Geological Survey, Sheets 351 and 358 (England and Wales). Gradstein, F.M. and Ogg, J. 1996. A Phanerozoic time scale. Episodes, 19, 3-4. Isaac, K.P., Selwood, E.B. & Shail, R.K. 1998. Devonian. In: Selwood, E.B., Durrance, E.M. & Bristow, C.M. (eds) The geology of Cornwall, University of Exeter Press, 31-64. Isaac, K.P & Thomas, J.M. 1998. Carboniferous. In: Selwood, E.B., Durrance, E.M. & Bristow, C.M. (eds) The geology of Cornwall, University of Exeter Press, 65-81. Kane, J.S., Arbogast, B.F., and Leventhal, J.S. 1990. Characterization of Devonian Ohio Shale SDO-1 as a USGS geochemical reference sample. Geostandards Newsletter, 14, 169-196. LeBoutillier, N.G. 2003. The tectonics of Variscan magmatism and mineralisation in South-West England. Unpublished PhD thesis, University of Exeter. Leveridge, B.E., Holder, M.T. and Goode, A.J.J. 1990. Geology of the country around Falmouth. Memoir of the British Geological Survey, Sheet 352 (England and Wales). Leveridge, B.E., Holder, M.T., Goode, A.J.J., Scrivener, R.C., Jones, N.S. and Merriman, R.J. 2002. Geology of the Plymouth and south-east Cornwall area. Memoir of the British Geological Survey, Sheet 348 (England and Wales). McLennan, S.M. 1989. Rare earth elements in sedimentary rocks: Influence of provenance and sedimentary processes. In: Lipin, B.R. and McKay, G.A. (eds) Geochemistry and mineralogy of rare earth elements, Reviews in Mineralogy, Vol 21, Mineralogical Society of America. Primmer, T.J. 1985. A transition from diagenesis to greenschist facies within a major Variscan fold/thrust complex in south-west England. Mineralogical Magazine, 49, 365-374. Scott, P.W., Reid, K.S., Shail, R.K. and Scrivener, R.C. 2003. Baseline geochemistry of Devonian low-grade metasedimentary rocks in Cornwall: Preliminary data and environmental significance. Geoscience in south-west England, 10, 424-429. Selwood, E.B., Thomas, J.M., Williams, B.J., Clayton, R.E., Durning, B., Smith, O. and Warr, L.N. 1998. Geology of the country around Trevose Head and Camelford. Memoir of the British Geological Survey, Sheets 335 and 336 (England and Wales). Shail, R.K. 1992. Provenance of the Gramscatho Group, south Cornwall. Unpublished PhD thesis, University of Keele. Shail, R.K. & Alexander, A.C. 1997. Late Carboniferous to Triassic reactivation of Variscan basement in the western English Channel: evidence from onshore exposures in south Cornwall. Journal of the Geological Society, 154, 163-168. Shail, R.K. & Alexander, A.C. 2000. Did it fall or was it pushed? - The fate of the Variscan Orogen in SW England. In: Abstracts, 2000 Ussher Society Conference, Torquay, January 2000.

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Tucker, R.D., Bradley, D.C., Version Straeton, C.A. et al. 1998. New U-Pb zircon ages and the duration and division of Devonian time. Earth and Planetary Science Letters, 158, 175-186. Warr, L.N., Primmer, T.J. & Robinson, D. 1991. Variscan very low-grade metamorphism in southwest England: a distathermal and thrust-related origin. Journal of metamorphic geology, 9, 751-764.