66 research outputs found
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
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