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

Leake, R.C.; Auld, H.A.; Stone, P.; Johnson, C.E.. 1981 Gold mineralisation at the southern margin of the Loch Doon granitoid complex, south-west Scotland. Institute of Geological Sciences, 46pp. (WF/MR/81/046) (Unpublished)

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Abstract/Summary

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.

Item Type: Publication - Report (UNSPECIFIED)
Programmes: BGS Programmes > Other
Funders/Sponsors: Department of Industry
Additional Information. Not used in RCUK Gateway to Research.: This item has been internally reviewed but not externally peer-reviewed
NORA Subject Terms: Earth Sciences
Date made live: 02 Nov 2010 14:14
URI: http://nora.nerc.ac.uk/id/eprint/11904

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