'Institute of Geological Sciences of the National Academy of Sciences of Ukraine'
Abstract
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
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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