Multi-element variations in olivine as geochemical signatures of Ni-Cu sulfide mineralization in mafic magma systems-examples from Voisey's Bay and Pants Lake intrusions, Labrador, Canada

摘要

Olivine from the olivine gabbro to troctolite intrusions at Voisey's Bay and at Pants Lake, Labrador, was analyzed for multiple elements (Ca, Sc, Mg, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, and Zr) with secondary ion mass spectrometry (SIMS). Both intrusions have similar lithologies and petrographic characteristics and are approximately coeval (1.34 and 1.32 Ga, respectively) members of the Mesoproterozoic Nain Plutonic Suite. The Voisey's Bay intrusion hosts a producing economic Ni-Cu sulfide deposit, whereas the Pants Lake intrusion displays evidence of Ni-Cu sulfide mineralization, but, to date, a viable ore deposit has not been discovered. Olivine from both barren and mineralized lithologies was analyzed to assess the potential of the olivine composition for providing a record of silicate melt evolution and sulfide saturation related to formation of a massive sulfide deposit. Two detailed transects were sampled, using five diamond drill holes that laterally approach the basal massive sulfide in the Eastern Deeps portion of the Voisey's Bay intrusion from the barren central part. Olivine displays distinct trace element distributions that vary coherently with host lithology and proximity to sulfide mineralization. In particular, olivine shows an increase in Fe (similar to Fo80 to similar to Fo60), Mn (similar to 2500 to 5000 ppm), and Zn (similar to 280 to 700 ppm), generally coupled with a decrease in Ni (similar to 1600 to similar to 900 ppm), Co (similar to 270 to similar to 190 ppm), and Cr (similar to 110 to 45 ppm), from barren troctolite (normal troctolite-NT) and weakly mineralized troctolite (variable-textured troctolite-VTT) towards the heavily mineralized, brecciated basal succession (basal breccia sequence-BBS). The enrichment in Fe-Mn-Zn is most pronounced in samples that laterally approach, but do not directly intersect, the massive sulfide deposit at the base of the intrusion, particularly in samples from the lower variable-textured troctolite and the basal breccia. Olivine from gabbro lithologies within the basal gabbro (BG) and upper gabbro (UG) of the Pants Lake intrusion is fairly homogeneous (similar to Fo60, similar to 4300 ppm Mn, similar to 460 ppm Zn, similar to 340 ppm Ni, similar to 50 ppm Cr), with a more evolved composition (similar to 2100 ppm Ca, similar to 20 ppm Sc, similar to 300 ppm Ti, similar to 4 ppm Y) than the average olivine of the Voisey's Bay intrusion (similar to 570 ppm Ca, similar to 6 ppm Sc, similar to 65 ppm Ti, similar to 0.3 ppm Y). Pants Lake olivine also commonly lacks the mutually Mn-Zn-rich signature of olivine from the basal breccia at Voisey's Bay that is characteristic of contamination of the parent magma by country rock gneiss, and there reflects a close proximity to massive sulfides. In conjunction with petrographic observation and the stratigraphic context, a multiple-element (V-Cr-Mn-Fe-Co-Ni-Zn) regression can be applied to calculate a lateral proximity of olivine to massive sulfide mineralization in the Eastern Deeps part of the Voisey's Bay intrusion, over a distance exceeding 150 m. The compositional variations in olivine from the economic Voisey's Bay intrusion (bimodal and primitive) and the mainly barren Pants Lake intrusion (homogeneous and evolved) provide potential as a regional-scale mineralogical fertility indicator for mafic intrusions with comparable types of sulfide mineralization.