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

By R.C. Leake, H.A. Auld, P. Stone and C.E. Johnson

Abstract

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

Topics: Earth Sciences
Publisher: Institute of Geological Sciences
Year: 1981
OAI identifier: oai:nora.nerc.ac.uk:11904

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