Abstract
The North Atlantic Igneous Province (NAIP) is a large igneous province (LIP) that includes a series of lava suites erupted from the earliest manifestations of the (proto)-Icelandic plume, through continental rifting and ultimate ocean opening. The lavas of one of these sub-provinces, the British Palaeogene Igneous Province (BPIP), were some of the first lavas to be erupted in the NAIP and overlie a thick crustal basement and sedimentary succession with abundant S-rich mudrocks. We present the first platinum-group element (PGE) and Au analyses of BPIP flood basalts from the main lava fields of the Isle of Mull and Morvern and the Isle of Skye, in addition to a suite of shallow crustal dolerite volcanic plugs on Mull, and other minor lavas suites. BPIP lavas display both S-saturated and S-undersaturated trends which, coupled with elevated PGE abundances (>MORB), suggest that the BPIP is one of the most prospective areas of the NAIP to host Ni–Cu–PGE–(Au) mineralisation in conduit systems. Platinum-group element, Au and chalcophile element abundances in lavas from West and East Greenland, and Iceland, are directly comparable to BPIP lavas, but the relative abundances of Pt and Pd vary systematically between lavas suites of different ages. The oldest lavas (BPIP and West Greenland) have a broadly chondritic Pt/Pd ratio (~1.9). Lavas from East Greenland have a lower Pt/Pd ratio (~0.8) and the youngest lavas from Iceland have the lowest Pt/Pd ratio of the NAIP (~0.4). Hence, Pt/Pd ratio of otherwise equivalent flood basalt lavas varies temporally across the NAIP and appears to be coincident with the changing geodynamic environment of the (proto)-Icelandic plume through time. We assess the possible causes for such systematic Pt/Pd variation in light of mantle plume and lithospheric controls, and suggest that this reflects a change in the availability of lithospheric mantle Pt-rich sulphides for entrainment in ascending plume magmas. Hence the precious metal systematics and potential prospectivity of a LIP may be affected by contamination of plume-derived magmas by subcontinental lithospheric mantle at the margins of cratons that have been enriched by Palaeoproterozoic orogenesis.
Highlights
Some of the world's most important orthomagmatic Ni–Cu–platinum-group element (PGE) sulphide deposits reside in continental flood basalt provinces, or large igneous provinces (LIPs)—for example the Norilsk–Talnakh conduit-hosted mineralisation in the Siberian Traps (e.g., Arndt, 2011)
Many factors contribute to the formation of such deposits, not the least the timing of S-saturation and segregation of immiscible sulphide liquids, triggered by various processes including contamination of Abbreviations: NAIP, North Atlantic Igneous Province; BPIP, British Palaeogene Igneous Province; PGE, platinum-group elements; REE, rare earth elements; NAC, North Atlantic Craton; monosulphide solid solution (MSS), monosulphide solution; SDRS, Seaward Dipping Reflector Series; LIP, large igneous province
If a silicate magma achieves S-saturation and exsolves an immiscible sulphide liquid, Cu partitions strongly into the sulphide (Dsul/sil ~ 1000), leaving the remaining silicate magma depleted in Cu (e.g., Andersen et al, 2002). We identify this feature in most BPIP lavas with Mg# b 60, as well as minor Icelandic and East Greenlandic SDRS lavas
Summary
Some of the world's most important orthomagmatic Ni–Cu–PGE sulphide deposits reside in continental flood basalt provinces, or large igneous provinces (LIPs)—for example the Norilsk–Talnakh conduit-hosted mineralisation in the Siberian Traps (e.g., Arndt, 2011). Aside from S-saturation, the initial concentration of PGE, Ni and Cu in mantle-derived magmas is a key factor in the probability of their forming orthomagmatic mineralisation (e.g., Naldrett, 2004). Worldwide, such mineralisation appears to be correlated with the occurrence of mantle plume events impinging on thick cratonic (Archaean) lithospheric mantle (e.g., Groves and Bierlein, 2007; Maier and Groves, 2011 and references therein). Impingement of the proto-Icelandic mantle plume under the continental lithosphere of Greenland, Canada and Europe (i.e., beneath the North Atlantic Craton; NAC) generated the NAIP. The various methods used to assess the centre of the plume (e.g., locus of highly magnesian lavas) suggest that the Palaeogene proto-Icelandic plume lay beneath central Greenland in the Palaeogene and Early Eocene, and gradually tracked SE–ESE beneath what is the Greenland–Iceland ridge (delineated as a zone of thickened oceanic crust) to its present day position beneath Iceland at the Mid-Atlantic Ridge
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