Magmatic Cu-Ni-PGE-Au sulfide mineralisation in alkaline igneous systems: An example from the Sron Garbh intrusion, Tyndrum, Scotland

Graham, S.D., Holwell, D.A., McDonald, I., Jenkin, G.R.T., Hill, N.J., Boyce, A.J. , Smith, J. and Sangster, C. (2017) Magmatic Cu-Ni-PGE-Au sulfide mineralisation in alkaline igneous systems: An example from the Sron Garbh intrusion, Tyndrum, Scotland. Ore Geology Reviews, 80, pp. 961-984. (doi: 10.1016/j.oregeorev.2016.08.031)

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Magmatic sulfide deposits typically occur in ultramafic-mafic systems, however, mineralisation can occur in more intermediate and alkaline magmas. Sron Garbh is an appinite-diorite intrusion emplaced into Dalradian metasediments in the Tyndrum area of Scotland that hosts magmatic Cu-Ni-PGE-Au sulfide mineralisation in the appinitic portion. It is thus an example of magmatic sulfide mineralisation hosted by alkaline rocks, and is the most significantly mineralised appinitic intrusion known in the British Isles. The intrusion is irregularly shaped, with an appinite rim, comprising amphibole cumulates classed as vogesites. The central portion of the intrusion is comprised of unmineralised, but pyrite-bearing, diorites. Both appinites and diorites have similar trace element geochemistry that suggests the diorite is a more fractionated differentiate of the appinite from a common source that can be classed with the high Ba-Sr intrusions of the Scottish Caledonides. Mineralisation is present as a disseminated, primary chalcopyrite-pyrite-PGM assemblage and a blebby, pyrite-chalcopyrite assemblage with significant Co-As-rich pyrite. Both assemblages contain minor millerite and Ni-Co-As-sulfides. The mineralisation is Cu-, PPGE-, and Au-rich and IPGE-poor and the platinum group mineral assemblage is overwhelmingly dominated by Pd minerals; however, the bulk rock Pt/Pd ratio is around 0.8. Laser ablation analysis of the sulfides reveals that pyrite and the Ni-Co-sulfides are the primary host for Pt, which is present in solid solution in concentrations of up to 22 ppm in pyrite. Good correlations between all base and precious metals indicate very little hydrothermal remobilisation of metals despite some evidence of secondary pyrite and PGM. Sulfur isotope data indicate some crustal S in the magmatic sulfide assemblages. The source of this is unlikely to have been the local quartzites, but S-rich Dalradian sediments present at depth. The generation of magmatic Cu-Ni-PGE-Au mineralisation at Sron Garbh can be attributed to post-collisional slab drop off that allowed hydrous, low-degree partial melting to take place that produced a Cu-PPGE-Au-enriched melt, which ascended through the crust, assimilating crustal S from the Dalradian sediments. The presence of a number of PGE-enriched sulfide occurrences in appinitic intrusions across the Scottish Caledonides indicates that the region contains certain features that make it more prospective than other alkaline provinces worldwide, which may be linked the post-Caledonian slab drop off event. We propose that the incongruent melting of pre-existing magmatic sulfides or ‘refertilised’ mantle in low-degree partial melts can produce characteristically fractionated, Cu-PPGE-Au-semi metal bearing, hydrous, alkali melts, which, if they undergo sulfide saturation, have the potential to produce alkaline-hosted magmatic sulfide deposits.

Item Type:Articles
Additional Information:Scotgold Resources Ltd. is acknowledged for their support of SDG's MGeol project work and for allowing sampling of core and logistical field support and for granting permission to publish this work. Part of this work was funded by NERC through SoS Minerals catalyst grant NE/L002191/1 awarded to the University of Leicester, and by a NERC SoS Consortium grant NE/M010848/1 “TeaSe: tellurium and selenium cycling and supply” awarded to the University of Leicester. The S-isotope analyses were carried out at SUERC under NERC Isotope Facilities grant IP-1317-0512 awarded to GRTJ and NJH. NJH was funded by NERC Open CASE studentship NE/H017755/1 in conjunction with Scotgold. AJB is funded by NERC support of the Isotope Community Support Facility at East Kilbride, Scotland.
Glasgow Author(s) Enlighten ID:Boyce, Professor Adrian
Authors: Graham, S.D., Holwell, D.A., McDonald, I., Jenkin, G.R.T., Hill, N.J., Boyce, A.J., Smith, J., and Sangster, C.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Ore Geology Reviews
ISSN (Online):1872-7360
Published Online:27 August 2016
Copyright Holders:Copyright © 2017 The Authors
First Published:First published in Ore Geology Reviews 80:961-984
Publisher Policy:Reproduced under a Creative Commons License

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