Platinum group element signatures in the North Atlantic Igneous Province: Implications for mantle controls on metal budgets during continental breakup

摘要

The North Atlantic Igneous Province (NAIP) is a large igneous province (LIP) that includes a series ofudlava suites erupted from the earliest manifestations of the (proto)-Icelandic plume, throughudcontinental and ultimately oceanic rifting. The lavas of one of these sub-provinces, the BritishudPalaeogene Igneous Province (BPIP), were some of the first lavas to be erupted in the NAIP andudoverlie a thick crustal basement and sedimentary succession with abundant S-rich mudrocks. Weudpresent the first platinum-group element (PGE) and Au analyses of BPIP flood basalts from the mainudlava fields of the Isle of Mull and Morvern and the Isle of Skye, in addition to a suite of shallowudcrustal dolerite volcanic plugs on Mull, and other minor lavas suites. BPIP lavas display both Ssaturatedudand S-undersaturated trends which, coupled with elevated PGE abundances (> MORB),udsuggest that the BPIP is one of the most prospective areas of the NAIP to host Ni-Cu-PGE-(Au)udmineralisation in conduit systems. Platinum-group element, Au and chalcophile element abundancesudin lavas from West and East Greenland, and Iceland, are directly comparable to BPIP lavas, but theudrelative abundances of Pt and Pd vary systematically between lavas suites of different ages. Theudoldest lavas (BPIP and West Greenland) have a broadly chondritic Pt/Pd ratio (~ 1.9). Lavas from EastudGreenland have a lower Pt/Pd ratio (~ 0.8) and the youngest lavas from Iceland have the lowestudPt/Pd ratio of the NAIP (~ 0.4). Hence, Pt/Pd ratio of otherwise equivalent flood basalt lavas variesudtemporally across the NAIP and appears to be coincident with the changing geodynamicudenvironment of the (proto)-Icelandic plume through time. We assess the possible causes for suchudsystematic Pt/Pd variation in light of mantle plume and lithospheric controls, and suggest that thisudreflects a change in the availability of lithospheric mantle Pt-rich sulphides for entrainment inudascending plume magmas. Hence the precious metal systematics and potential prospectivity of a LIPudmay be affected by contamination of plume-derived magmas by subcontinental lithospheric mantleudat the margins of cratons that has been enriched by Palaeoproterzoic orogenesis.