Abstract
Abstract Ni–Cu–PGE (platinum group element) sulphide mineralization is commonly found in magmatic conduit systems. In many cases the trigger for formation of an immiscible sulphide liquid involves assimilation of S-bearing crustal rocks. Conceptually, the fluid dynamics of sulphide liquid droplets within such conduits is essentially a balance between gravitational sinking and upwards entrainment. Thus, crustal contamination signatures may be present in sulphides preserved both up- and down-flow from the point of interaction with the contaminant. We examine a suite of ultramafic volcanic plugs on the Isle of Rum, Scotland, to decipher controls on sulphide accumulation in near-surface magma conduits intruded into a variable sedimentary stratigraphy. The whole-rock compositions of the plugs broadly overlap with the compositions of ultramafic units within the Rum Layered Complex, although subtle differences between each plug highlight their individuality. Interstitial base metal sulphide minerals occur in all ultramafic plugs on Rum. Sulphide minerals have magmatic δ34S (ranging from –1·3 to +2·1‰) and S/Se ratios (mean = 2299), and demonstrate that the conduit magmas were already S-saturated. However, two plugs in NW Rum contain substantially coarser (sometimes net-textured) sulphides with unusually light δ34S (–14·7 to +0·3‰) and elevated S/Se ratios (mean = 4457), not represented by the immediate host-rocks. Based on the Hebrides Basin sedimentary stratigraphy, it is likely that the volcanic con duits would have intruded through a package of Jurassic mudrocks with characteristically light δ34S (–33·8 to –14·7‰). We propose that a secondary crustal S contamination event took place at a level above that currently exposed, and that these sulphides sank back to their present position. Modelling suggests that upon the cessation of active magma transport, sulphide liquids could have sunk back through the conduit over a distance of several hundreds of metres, over a period of a few days. This sulphide ‘withdrawal’ process may be observed in other vertical or steeply inclined magma conduits globally; for example, in the macrodykes of East Greenland. Sulphide liquid sinking within a non-active conduit or during magma ‘suck-back’ may help to explain crustal S-isotopic compositions in magma conduits that appear to lack appropriate lithologies to support this contamination, either locally or deeper in the system.
Highlights
Upper crustal conduits within mafic–ultramafic magmatic systems provide favourable settings for magmatic sulphide genesis, promoted by crustal contamination, resulting in some of the world’s largest orthomagmatic Ni–Cu and platinum group element (PGE) deposits
As our study focuses on the S-isotopic composition of the plugs and trace element compositions of base metal sulphides within the plugs, we did not undertake a comprehensive search for platinum group minerals (PGM) across all plug samples
The peridotite plugs included in this study have wholerock major and trace element and PGE signatures that are subtly distinctive from one another (Supplementary Material Table B) but, broadly, the range of major element compositions overlaps with those recorded for the various portions of the Rum Layered Suite, including peridotite units from the Eastern, Western and Central Layered Series and the M9 picritic dyke (Upton et al, 2002)
Summary
Upper crustal conduits (pipes, dykes and sills) within mafic–ultramafic magmatic systems provide favourable settings for magmatic sulphide genesis, promoted by crustal contamination, resulting in some of the world’s largest orthomagmatic Ni–Cu and platinum group element (PGE) deposits. Sulphide liquid that ponds within conduits can become enriched in chalcophile elements, possibly to economic levels, as a result of interaction with magma continuing to pass through the conduit Sulphides within conduits commonly occur as massive to semi-massive accumulations and are present as rounded globules or droplets of varying sizes; for example, at Norilsk–Talnakh, Russia (Czamanske et al, 1992), the Insizwa Complex, South Africa (Lightfoot et al, 1984), Voisey’s Bay, Canada (Huminicki et al, 2008), a mafic dyke in Uruguay (Prichard et al, 2004) and in East Greenland macrodykes (Holwell et al, 2012). The fluid dynamics involved in this transport, such as settling rate versus entrainment rate, has been the subject of many recent studies based on both empirical and experimental evidence (e.g. de Bremond d’Ars et al, 2001; Mungall & Su, 2005; Godel et al, 2006, 2013; Barnes et al, 2008; Chung & Mungall, 2009; Arndt, 2011)
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