Contrasting mechanisms for crustal sulphur contamination of mafic magma: evidence from dyke and sill complexes from the British Palaeogene Igneous Province

Abstract

The addition of crustal sulphur to magma can trigger sulphide saturation, a process fundamental to the development of some Ni–Cu–PGE deposits. In the British Palaeogene Igneous Province, mafic and ultramafic magmas intrude a thick sedimentary sequence offering opportunities to elucidate mechanisms of magma–crust interaction in a setting with heterogeneous S isotope signatures. We present S-isotopic data from sills and dykes on the Isle of Skye. Sharp contrasts exist between variably light δ 34 S in Jurassic sedimentary sulphide (−35‰ to −10‰) and a local pristine magmatic δ 34 S signature of −2.3 ± 1.5‰. Flat-lying sills have restricted δ 34 S (−5‰ to 0‰) whereas steeply dipping dykes are more variable (−0‰ to −2‰). We suggest that the mechanism by which magma is intruded exerts a fundamental control on the degree of crustal contamination by volatile elements. Turbulent flow within narrow, steep magma conduits, discordant to sediments, and developed by brittle extension or dilation have maximum contamination potential. In contrast, sill-like conduits emplaced concordantly to sediments show little contamination by crustal S. The province is prospective for Ni–Cu–PGE mineralization analogous to the sill-hosted Noril’sk deposit, and Cu/Pd ratios of sills and dykes on Skye indicate that magmas had already reached S-saturation before reaching the present exposure level. Supplementary material: Details of the whole-rock chemical sulphur extraction method; simplified geological maps of the Isle of Skye, a geological cross-section of part of southern Skye, and a stratigraphic log of the Mesoproterozoic and Mesozoic sediments of western Scotland; trace element diagrams of sills and dykes used in this study; and tables for QA/QC of S-isotope results, sample location information, and whole-rock major and trace element results for sill and dyke samples and for Jurassic mudrocks are available at http://www.geolsoc.org.uk/SUP18834 .

Appraisal of Crustal Contamination in Southern Bastar Mafic Dykes in Chattisgarh, India

“…Quebec [19]. Contrasting mechanisms for crustal sulphur contamination of mafic dyke and sill complexes from the British Palaeogene Igneous Province has also been reported [20]. Comparison of Spidergrams of BD1 dykes and Bastar Granitoids (Figure 8) show that both have almost similar trends in LILE and "Nb" but other HFSE show different trends.…”

Distinct sulfur saturation histories within the Palaeogene Magilligan Sill, Northern Ireland: implications for Ni – Cu – platinum group element mineralisation in the North Atlantic Igneous Province

“…Some modern studies of MORB lava sulfides suggest larger ranges (Eckardt 2001, Seal 2006, Labidi et al 2012, but the original values presented by Sakai et al account for seafloor sulfate/sulfide fractionation and are thus more precise and reliable (Lesher 2017). The local pristine magmatic δ 34 S signature of the Scottish BPIP has been found to be −2.3 ± 1.5 ‰ (Hughes et al 2015a) based on the mean composition of the picritic Trotternish Sills (Hughes et al 2015a), which is comparable with that of Icelandic basalts (Torssander 1989) and MORB (Labidi et al 2012).…”

“…It follows that most of the sill magmas have experienced some degree of crustal contamination, as the addition of isotopically light crustal sulfur from the Mesozoic country rocks will shift their primary magmatic sulfide signature (assumed to be -2.3 ‰ for the BPIP; Hughes et al, 2015). Based on work on the Trotternish Sills (Hughes et al 2015a), the proportion of sedimentary sulfur assimilated by the magma (f) can be calculated, using the observed δ 34 S values achieved in the contaminated sill (δ 34 S mix ) using the following equation:…”

“…The region is part of the North Atlantic Igneous Province (NAIP), an area that has been highlighted as potentially prospective for orthomagmatic PGE mineralisation (Andersen et al, 2002). Recent studies on the islands of Skye and Rum have also highlighted extensive contamination of shallow Palaeogene intrusions by crustal S from Mesozoic sediments (Power et al 2003, Hughes et al 2015a, exemplified by light S-isotopic compositions of steeply dipping conduits (e.g. Hughes et al 2015).…”

Sulfide Immiscibility Induced by Wall-Rock Assimilation in a Fault-Guided Basaltic Feeder System, Franklin Large Igneous Province, Victoria Island (Arctic Canada)

“…Dike-like magma conduits are dynamic systems that provide opportunities for magma-crust interaction (Naldrett, 1992;Li et al, 2000;Maier et al, 2001;Mungall, 2007;Ding et al, 2010;Hughes et al, 2015;Lightfoot and Evans-Lamswood, 2015) and are important sites of Fe-Ni-Cu mineralization (Naldrett, 1992;Ripley and Li, 2011). Economic conduit-type Fe-Ni-Cu deposits are typically associated with olivine-rich magmas (Li and Naldrett, 1999), commonly situated close to major crustal faults (Naldrett, 1992;Begg et al, 2010;Song et al, 2012;Lightfoot and Evans-Lamswood, 2015), generally hosted by (or related to) intrusive or extrusive facies that show a depletion in chalcophile elements (Naldrett, 1992), commonly emplaced into (or traverse) sulfur-bearing country rocks (Naldrett, 1992;Li et al, 2009), and may show field and/or geochemical evidence of interaction between magma and country rock Naldrett, 2004;Li et al, 2009;Keays and Lightfoot, 2010;Ganino et al, 2014).…”