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

By M.J. Brown, R.C. Leake, M.E. Parker and N.J. Fortey

Abstract

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

Topics: Earth Sciences
Publisher: Institute of Geological Sciences
Year: 1979
OAI identifier: oai:nora.nerc.ac.uk:11871

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