Formation of Sulphides in the Canadian High Arctic Large Igneous Province; Testing the Influence of Sedimentary Rocks

Sammanfattning: Large Igneous Provinces (LIPs) form during short-lived pulses of extensive magmatic activity. LIPs are known for their ability to affect global climate as well as for their Ni-Cu-PGE ore potential. A key factor that controls the intensity of the climate impact of a LIP and its ore potential is the assimilation of volatile-rich sedimentary host rocks. Magmas of the High Arctic Large Igneous Province (HALIP), exposed in the Arctic, intruded volatile-rich black shales, carbonates and evaporites in the Canadian Arctic Islands, offering a great opportunity for studying magma-sediment interaction. The purpose of this study is to test whether assimilation of sedimentary sulphide can promote sulphide immiscibility in magma and thus aid formation of Ni-Cu-PGE ore bodies. This is done by analysing sulphur isotopes in pyrite grains hosted in a HALIP dolerite sill, which was emplaced into black shale, by using Secondary Ion Mass Spectrometry (SIMS). Four dolerite samples are analysed; two coming from the lower contact margin of the sill, one from 60 cm into the sill and one sample from a basaltic vein at the upper contact margin of the sill. A total of 14 pyrite grains (n = 246 individual SIMS spot analyses) were analysed for their sulphur isotope ratios. The results of the SIMS analyses show that all analysed sulphides have highly negative δ34S values ranging from -19.5 to -5.7‰ (average δ34S = -8.2 ± 0.83‰, 2SD), which therefore differ largely from that of the primitive mantle (0 ± 1.8‰). In order to put our four analysed dolerite samples into a broader context, δ34S data of our sulphides are compared with whole-rock δ34S and δ18O data from Hare Fiord shale and dolerite samples. The δ34S values of the sulphide samples from the sill typically trend toward the negative sulphur isotope composition of the sulphides in the surrounding shale, and the shale surrounding the sill experiences a loss of 32S near the contact of the sill. This indicates that sedimentary light sulphur (32S) has been locally incorporated into the sill by the surrounding shale, resulting in negative δ34S values in the magmatic sulphides. Since sulphide immiscibility in the Hare Fiord sill was triggered by assimilation of sulphur from host rock shale, the igneous rocks of the HALIP may be prospective for Ni-Cu-PGE mineralization, though more studies are needed. Furthermore, our results suggest that incorporation of crustal sulphur increased the volatile budget of HALIP magmas, which therefore could have contributed to a deterioration of the environmental conditions during the emplacement of the HALIP.