Platinum-group element mineralisation in the Unst ophiolite, Shetland

The ophiolitic basic and ultrabasic rocks of the island of Unst, Shetland comprise a sequence of harzburgites, dunites, clinopyroxene-rich cumulates, and gabbro, within tectonic blocks that have been thrust over a migmatite complex during the Laxer Palaeozoic. Concentrations of chromite are found in the harzburgite and dunite, and to a small extent in the pyroxene cumulate rocks. They occur as disseminations, sometimes forming millimetre scale layers, and as more massive schlieren and pods of chromitite. Five alteration or hydrothermal events have been recognised in the ultrabasic rocks. These comprise early pervasive serpentinisation, later fracture controlled serpentinisation, veining and pervasive carbonation, minor late serpentine veining and talc-carbonate alteration controlled by fault zones. Exploration for platinum group element (PGE) mineralisation uas carried out using a combination of drainage, overburden and rock sampling. Analyses of PGE were obtained by fire assay followed by either neutron activation analysis or flameless atomic absorption spectrometry, and up to 20 other elements Here determined by X-ray fluoresence analysis. Panned concentrate samples were taken from 73 drainage sites distributed throughout the complex. Ir, the only PGE determined in all samples, showed a greater concentration in samples derived from the harzburgite unit than those from other units. Lox amplitude anomalies are present in three discrete areas in the harzburgite but the maximum level of 210 ppb Ir is associated with a sample derived from a prominent N-S zone of faulting and hydrothermal activity markedly discordant to the regional trend of layering in the harzburgite and dunite. This discordant zone, which extends for at least 7 km, is also marked by samples containing enrichments in Fe, Co, Ni, Cu and As. The highest Cr levels are associated with an area in the north of the harzburgite with no previous history of chromite working but where many locally derived pieces of chromitite float have been discovered. Relatively high Cr levels are also associated nith the area of dunite containing the greatest concentration of visible chromite and old norkings. A technique of collecting panned heavy mineral concentrates from overburden samples was adopted as a reconnaissance exploration technique after orientation sampling in the harzburgite unit at Cliff, an area with high PGE levels in chromitite and associated dunite. Systematic sampling in the Cliff area outlined a zone of coincident Pd, Pt and Rh enrichment near to but separate from the chromite workings knorrn to be enriched in PGE. In contrast the distribution of Ru was entirely different with scattered lon amplitude anomalous zones and a maximum anomaly 300m from the chromite-rich zone. Reconnaissance lines were sampled at other locations within the harzburgite, dunite and cumulate units. Lore amplitude Pd and Pt anomalies were detected xithin the dunite unit, especially in 'a traverse across the trace of the prominent N-S fault zone at Helliers Uater, adjacent to the outcrop of the cumulate unit. In general the overburden data suggest some association between PGE enrichment and enhanced levels of Ni relative to typical silicate levels apparent when expressed as the ratio Ni/MgC. Rock samples Here collected from all parts of the complex, including most of the main chromitite workings. Very high levels of all PGE occur in samples of chromitite, chromite-rich dunite and dunite from the Cliff area, with a strong positive intercorrelation between all PGE. The proportions of the various PGE are very similar to those present in deposits in major layered basic/ultrabasic complexes like Bushveld and Stillwater, irith strong relative enrichment in Pd and Pt. These PGE proportions are completely different from the Ru-Ir-0s dominant assemblage typical of ophiolitic rocks. Associated with high levels of PGE are enrichments in Ni, Cu, As, Sb and Te. There is no correlation rrith Cr and some samples of chromitite from the Cliff area contain only background levels of PGE. High to moderate levels of PGE with the same proportions of elements as the Cliff samples also occur in samples of chromitite and serpentinised dunite from the dunite unit and in samples of pyroxenite from the cumulate unit, In contrast PGE-rich samples of chromitite from the harzburgite unit near Harold's Grave have entirely different proportions of PGE with Ru and Ir in greatest abundance. This PGE distribution is similar to that in some background samples of harzburgite and closely resembles the pattern found in typical ophiolites. The PGE in the Harold's Grave samples do not exhibit the Ni enhancement noted in the Cliff PGE mineralisation. In samples from the Cliff area the platinum-group minerals (PGM) sperrylite, stibiopalladinite, hollingnorthite, laurite and possibly irarsite have been identified, mostly as grains less than 10 microns in size. In chromite-rich rocks these minerals occur Rithin chlorite haloes around chromite, in the blackened altered rims of chromite grains and in interstitial Ni-rich serpentine/carbonate intergroxths in association with pentlandite, orcellite and other Ni sulphides and arsenides, sometimes spatially related to chlorite-carbonate-magnetite veins. They also occur as fine grains Rithin magnetite rims around chromite and in magnetite or carbonate veins in dunite. The Ni sulphide/arsenide assemblage associated Rith the PGH is characteristic of serpentinisation at temperatures less than 500'C, Rell belox the range of magmatic conditions. A hydrothermal origin for the PGE mineralisation is proposed, probably related to the second phase of serpentinisation. This involved the redistribution of Ni accompanied by the introduction of As, Sb and Te probably with a StrUCtUral Control. Pre-existing concentrations of chromite may have acted as a precipitation barrier causing rich PGH deposition in the alteration haloes around chromite grains. Continuous borehole or trench sections through mineralised zones are required to assess the economic significance of the PGE mineralisation. Nevertheless the high levels of PGE attained and the evidence of xidespread occurrence of the Cliff-type PGE enrichment are favourable indications. The PGE enrichments found in the cumulate complex are of potential interest as they may originally have been of magmatic origin. Larger tonnage targets may therefore be present in this unit compared Rith the likely size of structurally-controlled mineralisation elswhere in the complex

A geochemical drainage survey of the Fleet granitic complex and its environs

A regional geochemical drainage reconnaissance programme was undertaken over 900 km2 of south-west Scotland centred on the Fleet granitic complex. Rocks of Ordovician and Silurian age outcrop over the area, into which have been intruded the Fleet and Loch Doon plutons. Multi-element analysis of stream sediments and heavy mineral concentrates shows a number of patterns of trace element distribution related to different lithologies and to mineralisation. Broad scale patterns exhibited by some elements denote compositional variations within the Lower Palaeozoic sediments and within the Fleet and Loch Doon plutons. On thebasis of the drainage data the sedimentary rocks have been divided into eight distinct geochemical units each characterised by different element distribution patterns. The Fleet and Loch Doon plutons have been sub-divided on the same basis. Follow-up investigations of drainage anomalies led to the discovery of both structure-controlled and disseminated base metal mineralisation in the Penkiln drainage basin within the salthernaureole of the Loch Doon granite. The distribution of Cu, F’b and Zn to the south and south-west of the Fleet granite suggests a zonation of vein minerahsation, with Cu prominent adjacent to the granite contact and F’b and Zn having a wider dispersion away from the granite. Other anomalies delineate a mineralised lineament that follows the regional strike of the Lower Palaeozoic sediments, southeast of the Fleet granite

A reconnaissance geochemical drainage survey of the Criffel-Dalbeattie granodiorite complex and its environs

Regional geochemical reconnaissance by stream sediments and panned concentrates was undertaken over an area of 850 km 2 of south-west Scotland. Rocks of Ordovician to Permian age are exposed within the area but the major part is occupied by the Criffel-Dalbeattie granodiorite complex. Multi-element analysis of the samples collected demonstrates patterns of trace element distribution related to the geology and Broad-scale patterns in the distribution of some elements reflect compositional anumber of mineralisation. variations in both the Lower Palaeozoic turbidite sequence and the Criffel- Dalbeattie granodiorite complex, both of which may be sub-divided into specific units on the basis of the geochemical data. The follow-up of copper anomalies in drainage samples from the Black Stockarton Moor area led to the discovery of porphyry-style copper mineralisation within an arcuate belt at least 5 km by 1 km within the Black Stockarton Moor subvolcanic complex and of related disseminated copper mineralisation at Screel Burn. The area to the west of the Criffel-Dalbeattie plutonic complex is also characterised by relatively high boron levels in stream sediments reflecting the widespread occurrence of tourmaline both in association with and peripheral to the copper mineralisation. Vein mineralisation, usually containing baryte in addition to base.metals, is also identifiable from the drainage survey at the eastern margin of the Criffel-Dalbeattie granodiorite; in association with the Lower Carboniferous rocks along the Solway coast; and within the Lower Palaeozoic turbidites in the west of the area

Fractal Reconnection in Solar and Stellar Environments

Recent space based observations of the Sun revealed that magnetic reconnection is ubiquitous in the solar atmosphere, ranging from small scale reconnection (observed as nanoflares) to large scale one (observed as long duration flares or giant arcades). Often the magnetic reconnection events are associated with mass ejections or jets, which seem to be closely related to multiple plasmoid ejections from fractal current sheet. The bursty radio and hard X-ray emissions from flares also suggest the fractal reconnection and associated particle acceleration. We shall discuss recent observations and theories related to the plasmoid-induced-reconnection and the fractal reconnection in solar flares, and their implication to reconnection physics and particle acceleration. Recent findings of many superflares on solar type stars that has extended the applicability of the fractal reconnection model of solar flares to much a wider parameter space suitable for stellar flares are also discussed.Comment: Invited chapter to appear in "Magnetic Reconnection: Concepts and Applications", Springer-Verlag, W. D. Gonzalez and E. N. Parker, eds. (2016), 33 pages, 18 figure

The Origin, Early Evolution and Predictability of Solar Eruptions

Coronal mass ejections (CMEs) were discovered in the early 1970s when space-borne coronagraphs revealed that eruptions of plasma are ejected from the Sun. Today, it is known that the Sun produces eruptive flares, filament eruptions, coronal mass ejections and failed eruptions; all thought to be due to a release of energy stored in the coronal magnetic field during its drastic reconfiguration. This review discusses the observations and physical mechanisms behind this eruptive activity, with a view to making an assessment of the current capability of forecasting these events for space weather risk and impact mitigation. Whilst a wealth of observations exist, and detailed models have been developed, there still exists a need to draw these approaches together. In particular more realistic models are encouraged in order to asses the full range of complexity of the solar atmosphere and the criteria for which an eruption is formed. From the observational side, a more detailed understanding of the role of photospheric flows and reconnection is needed in order to identify the evolutionary path that ultimately means a magnetic structure will erupt

Base metal mineralisation associated with Ordovician shales in south-west Scotland

Several narrow, lenticular belts of black cherty mudstone and siltstone (the Moffat Shales), alternating with thick greywacke sequences, strike north-east to south-west across Galloway with uniformly steep dip. In the Penkiln Burn area, 13 km NNE of Newton Stewart, one such belt is hornfelsed and considerably broadened near the southwest margin of the Loch Doon granitic pluton. Base metal anomalies in drainage and overburden are spatially associated with the broadened section of the shale belt, which is host to weakly disseminated and epigenetic Pb-Zn-Cu mineralisation. Within the Moffat Shale sequence highly siliceous mudstone and siltstone are interbedded with chert, greywacke and possibly thin tuffaceous horizons. The broadening is structurally controlled, caused by the interference of early structures with a major reciined fold plunging to the south-east. Several phases of faulting and minor intrusion have been recognised, and the abundance of dykes is an unusual geological feature of the area. Lead is particularly enriched in drainage samples, reaching approximately 1% in pan concentrates collected close to a mineralised gossan-like zone. The main leadbearing mineral identified in the anomalous concentrates, and the in situ gossan material, is the secondary lead phosphate plumbogummite. Overburden sampling proved anomalous metal values extending for 2.3 km along strike and 500 m across strike. Lead again shows the greatest enrichment, with values ranging up to about 0.5 % , in soil close to the gossan. Zinc and copper give a weaker response in both overburden and drainage, but drilling showed that zinc, in the form of disseminated sphalerite, has a greater incidence at depth than was suggested by the surface anomalies. Three varieties of mineralisation have been recognised. The earliest consists of fine disseminations, chiefly of sphalerite and pyrite, in the hornfelsed sediments. It is characterised by zinc levels between 500 and 1000 ppm over several metres of drill core; lead levels rarely exceed 300 ppm. The second phase of mineralisation occurs in thin quartz veinlets, which in this case contain accessory sphalerite, galena and pyrite. Where the veining is intense, lead concentrations reach 7000 ppm and those of zinc 1500 ppm, but these values persist over only a few tens of cm of core. Finally, a low-temperature mineral assemblage in which plumbogummite is dominant is associated with the altered margins of dykes and gossanlike zones occupying a north-south fault system. Lead levels in the dyke margins range up to 1.5 % in zones generally less than 50 cm thick, but 4.5 %I Pb has been recorded in one specimen from the exposed gossan. Fine stratiform pyrite iaminae in mudstone interbedded with chert containing disseminated pyrite and sphalerite suggests that at leas: some of the early mineralisation is synsedimentary. Later mineralisation phases are, however, structurally controlled and the origin of the majority of the base metal mineralisation remains problematical. The unusual abundance of minor intrusions in the mineralised zone is strong circumstantial evidence for an igneous source. Full details of the soil geochemical surveys and the geophysical surveys are available for inspection at the Keyworth office of BGS

A mineral reconnaissance survey of the Llandrindod Wells/Builth Wells Ordovician inlier, Powys

A reconnaissance drainage geochemical survey was carried out at a density of about 2 samples per km2 of the Llanclrindod Wells/ Builth Wells Ordovician inlier in Powys. The rocks of the area comprise a sequence of mudstones and shales ranging in age from the Llan\.irn to the Caradoc containing volcanic horizons and a lrolcanic complex of acid/intermediate composition overlain in the south by a further volcanic sequence of more basic composition. An area of about 65 km2 was sampled and from each site a wet screened minus BSI 100 mesh (< 150 micrometres) fraction and panned heavy mineral concentratc samples were obtained. Sie\rcd sediment and concentratc samples were analysed for B, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Zr, MO, Ag, Sn, Ba and Pb and Ca, Ti, Mn, Fe. Ni, Cu, Zn, Sr, Zr, MO, Sn, Sb,Ba, Ce, Pb and U respecti\.ely. Several Pb and Zn anomalies were apparent many of which appeared to be associated with the outcrop of the main volcanic unit of the area and 4 of these were confirmed by further grab stream sediment and bank sampling. Drainage sampling was supplemented by 27 reconnaissance soil tra\rerscs mostl) across the main outcrops of the \.olcanic- rocks which tencled to form the most ele\rated ground. The soil samples were sie\.ed to give a minus 85 BSI mesh (< 180 micrometres) sample for analysis for Cu, Pb, Zn, Ni and either Mn and Fe or Ag and Co. Samples collected from an art‘s of‘old lcaci workings in the west of the inlier failed to pro\ridc c\icience of further vein mineralisation beyond the limits of the workings. Elswhere the biggest concentration of‘ Pb and Zn anomalies and the greatest amplitude of anonlaly (2000 ppm Pb) wcrc located over the northern outcrop of the the main unit of tuffaceous volcanic rocks, particularly in the \.icinity of the farm Pen Rhiw Frank. A clctailccl geological, geochemical and geophysical survey of the Pen Rhiw Frank area was carried out using photogeological interpretation, further soil sampling and a combination of IP/resistivity, VLF, SP and magnetic surveys. Geological interpretation indicated that the area was divided into fault blocks and the geophysical work re\realecl a zone‘ of high resistivity with roughly coincident low amplitude chargeability maxima and a VLF crosso\. t‘r o\c‘r part of the zone. There was also a general association of this zone with lead in soil anomalies and the presence of significant amounts of weathered pyrite in the limited outcrops. The lead in soil anomalies when contoured in the light of the photogeological interpretation can by sc‘cbn as broadly following the local strike of the rocks. Four holes were drilled to in\restiage the source ot the> geochemical anomalies and to investiage any association bt~tn~ecn this and the obser\red geophysical anonlalies. Thy holes wcrc sited to test the down dip extension of‘ apparently stratabound surface soil anomalies. l’uff~accous rocks of andcsitic composition with a horizon of clacitic. tuffs and minor lrcsicular andcsitic 1aL.a wt‘rc intcrsccted in the holes. Mineralisation occurs in the for-m of secondary lc~ad minerals within a poorly rcco\rcred soft clay-rich section 5.8 m thick near the top of ant‘ of the holes. 1~~1 lc\.els up to 0.52 %I o\‘t‘r 3.4 m were found in the zone which is located at the interface between a dacitic tuff unit chcn~icnlly distinct from the rest of the \.olcanic scqucrlcXX and an andesitic la\,a probably of linlitcd c\;- tent .‘I’hc (h-ill holes suggest that thcl dip of the strata is around 5 dcgrccs rather than the 30 degrees d~cluc~d from th t,xisting gc,ological map of the arca and so the holes wcr(‘ not sitccl at locations which wc’rc’ likcsly to iI1tc‘rsc‘c.t any stratabound mineralisation at depth below the zone of surface Lveathcring. A further consequence of the flat dip of strata is that by accidents of topography extensive overburden anomalies could be produced from relatively minor statabound base-metal enrichments on the hill tops

Gold mineralisation at the southern margin of the Loch Doon granitoid complex, south-west Scotland

Following the identification of gold in a panned concentrate sample from the diverted headwaters of Glenhead Burn, south-east of Loch Trool in Galloway, Scotland,and the discovery of nativegold with arsenopyrite in a quartz vein upstream of this site, a programme of geochemical exploration of the margin and aureole of the Loch Doon plutonic complex in the drainage basin was initiated. Since rock samples showed a highly significant positive correlation between gold and arsenic levels a soil grid was sampled in the contact zone and analysed for arsenic, together with copper, lead and zinc. Several arsenic anomalies with levels exceeding 1000 ppm were found within the pluton and its aureole and seven shallow boreholes were drilled to test their source. Geophysical surveys using magnetic, Slingram EM, VLF and IP methods were carried out but none showed anomalies which correlated with zones of high arsenic in soil. In the area around the headwaters of Glenhead Burn, the Loch Doon plutonic complex is intrusive into a sequence of graded turbiditesof probable Caradocian age. In the northern part of the area, steeply-dipping greywacke horizons young consistently to the north while in the south there is evidence of several tight upright folds. South of the area there is a large-scale reclined fold developed in the siliceous mudstones, shales and cherts of a “black shale belt”, though a major strike fault may separate the two sequences. Swarms of concordant minor intrusive rocks of quartz monzonite and granodiorite. which predate the pluton, have been encountered in its aureole. Major differences in chemistry exist between these minor intrusions and the composition of the margin of the plutonic complex. A pervasive phase of metasomatism has affected the sedimentary rocks throughout the area, producing veins and lenses of fine granular quartz accompanied by actinolite, salitic pyroxene, fine magnetite and pyrrhotite, and occasionally carbonate, chalcopyrite, sphalerite, pyrite, sphene, clinozoisite, epidote and apatite. These are surrounded by greener envelopes rich in actinolite which grade into normal hornfelsed sediment. Close to the pluton, clinopyroxene is more conspicuous and betterformed in both altered and unaltered hornfelsed rock. Chemical data indicate that two types of metasomatically altered zones occur, one enriched in calcium and some manganese and strontium, and the other enriched in silicon. The minor igneous rocks are unaffected by the metasomatism but show contact metamorphism due to the pluton ,. Two phases of gold- bearing, arsenic-rich mineralisation have been recognised. The earlier comprises disseminations of pyrrhotite, arsenopyrite and pyrite in the 1 margins of’ monzonitic minor intrusions and disseminations of arsenopyrite in the adjacent metasediments. This mineralisation occurs in zones up to at least 18 m thick with arsenic levels reaching 3000 ppm and gold 0.16 ppm in samples of around one metre of core. It is probable that the majority of soil arsenic anomalies, particularly the lensoid variety trending parallel to the strike, originate from this type of mineralisation. Superimposed upon this are a series of discordant quartz veins and stringers trending roughly south and cutting all rock types. These may be richly mineralised with arsenopyrite and some pyrite and may contain minute grains of native gold. Surrounding the veins is a prominent alteration envelope of sericitic material, commonly with conspicuous disseminated arsenopyrite. Individual veins range up to 30 cm thick but thicker stockwork zones also exist. Arsenic levels in 200-300 gsamples of veined material reach over 3.5oJ, and gold assays up to 8.8 ppm have been obtained. A separate minor phase of sphalerite and galcna mineralisation also occurs within the area, usually in association with carbonate veinlets

Porphyry style copper mineralisation at Black Stockarton Moor, south-west Scotland

Reconnaissance soil sampling within the catchment of streams containing anomalous levels of copper in drainage samples led to the discovery of disseminated copper mineralisation in the Black Stockarton Moor area of Galloway, Scotland. Analysis of aeromagnetic data and of a gravity survey of the area, previously mapped as an irregular complex of dykes, suggested that the region was underlain by laminar bodies of granodiorite. Geological mapping r evealed the presence of a major Caledonian multiphase subvolcanic complex intruding LOvler Palaeozoic turbidi tes, to the west of the Criffe! granodioritic plutonic complex. An induced polarisation survey delineated an arcuate anomaly about 6 km long and the resul t s of a simultaneous geochemical soil survey showed a zone with anomalous levels of copper (> 140 ppm to 5500 ppm) in the southern part of the area to be essentially paralle l to the IP anomaly but partially displaced to the east. A series of three deep angled drill holes and nine shallow holes sited on geochemical and geophysical anomalies confirmed the widespread presence of both veinlet and disseminated pyrite and copper mineralisation of the porphyry type. The Black Stockarton Moor subvolcanic complex is a composite of minor intrusive rocks, the ear l iest phase of which, comprising porphyri te dyke swarms, granodiorite sheet intrusions, small granodiori te stocks, breccia pipes, vent agglomerates with plugs of basic rock and a few basic dykes, predates the adjacent multiphase Crlffel plutonic complex. The second phase of subvo lcanic activity postdates the plutonic rocks and comprises intense en-echelon s igmoidal swarms of porphyrite dykes sharply discordant to the earliest phase rocks. A minor third phase consists of linear porphyrite dykes closely associated with fault - ing. Chemical analysis and mineralogical examination of the borehole material indicates that regular zonation can be observed in the style and intensity of both mineralisation and hydrothermal alteration. This zonation is regular fr om west to east across the IP and soil anomalies. A propylitic alteration zone with the development of chlorite. epidote and minor sericite in igneous rocks and of calcite , quartz, jasperoid, chlorite, amphibole, epidote and albite in sedimentary rocks occurs to the wes t . Within this zone hematite gradually gives way to increasing amounts of pyrite from west t o east. The propylitic zone passes into a sericitic alteration zone where sedimentary rocks are frequently bleached and igneous rocks pink or orange-coloured containing secondary quartz, chlorite and muscovite. Pyrite is most conspicuous within rocks of the outer sericite zone, the outcrop of which coincides roughly with the axis of the IP anomaly. Further east pyrite decreases but chalcopyrite and bornite with some chalcocite become relatively conspicuous and copper levels are the highest attained (in the 400 ppm to 1100 ppm range). save for isolated highly br·ecciated sections. Chemical zonation shows relative enrichment in Mn, Zn, As and Pb in the outer propyli tic zone, Ba in the serid tic 1 zone and Cu in the inner sericitic zone while As, Sb and Au are markedly concentrated with Cu and Mo in isolated brecciated section