Trace Element Variations in Olivine from the Eastern Deeps Intrusion at Voisey's Bay, Labrador, as a Monitor of Assimilation and Sulfide Saturation Processes

Abstract

Ni-Cu-Co sulfide mineralization in the Mesoproterozoic Voisey’s Bay intrusion is spatially and genetically associated with magmatic breccia zones within an extensive dike system and, in particular, with the entry point of this system into a larger intrusion (Eastern Deeps intrusion). The massive sulfide- and breccia-bearing inner basal margin of this upper intrusion is enveloped by weakly mineralized variable-textured troctolite that progressively decreases in sulfide content, contains fewer paragneiss fragments, and eventually grades into a largely sulfide-barren, homogeneous plagioclase and olivine (meso)cumulate, termed normal troctolite, toward the top of the intrusion. Olivine from the troctolite lithologies, the mineralized basal breccia sequence, and the uppermost olivine gabbro in the Eastern Deeps intrusion was analyzed with secondary ion mass spectrometry (SIMS). We present multitrace element data (including petrogenetically important elements V, Cr, Mn, Co, Ni, Zn, Sr, and Y), which indicate that four petrological processes important to ore formation are recorded by the compositional diversity of olivine. The heterogeneous Ni-Co-Fo contents of olivine in the Eastern Deeps intrusion (Co/Ni ratios in olivine gabbro: 0.18–1.1, normal troctolite: 0.07–0.26, variable-textured troctolite: 0.11–0.29, basal breccia sequence: 0.20–0.34) are due to variable modification of original compositions by reequilibration with trapped silicate and sulfide liquids. The abrupt changes of Fe (Fo79–Fo66) and Ni (~2,500 to ~80 ppm) contents and some trace elements (up to 1.5x more Sr, up to 3x more Y than average normal troctolite olivine) in olivine from intervals in the barren upper normal troctolite indicate that olivine crystallized from separate pulses of magma with variable degrees of differentiation. The severe Ni depletion of one troctolite melt batch (~10 ppm Ni vs. 290 ppm Ni for average normal troctolite magma) is likely a result of sulfide saturation and segregation, either in a deeper staging chamber, or in the conduit, prior or contemporary to olivine precipitation and emplacement. The gradational increase of Fe (from Fo80 to Fo62), Mn (from ~4,000 to ~10,000 ppm), and Zn (from ~400 to ~620 ppm) in olivine from the weakly mineralized and contaminated lower part of the variable-textured troctolite downward into the massive sulfide-bearing and strongly contaminated basal breccia sequence is consistent with olivine fractionation from an increasingly country rock-contaminated, sulfide-saturated, silicate magma. Olivine here crystallized prior to or contemporaneously with the segregation of an immiscible sulfide liquid. Olivine grains that are enclosed by sulfide display homogeneous forsterite contents (~Fo62) with pronounced intragrain trace element zonation (Mn>Ni>Co>Zn). Diffusion profiles (Mn-Ni) indicate a relatively short immersion time (<<1,000 years) of these olivine grains in sulfide liquid, which likely resulted in the partial retention of Ni in the olivine. Olivine in the vicinity of massive sulfides in the Eastern Deeps intrusion (from variable-textured troctolite to basal breccia sequence) has a distinct minor and trace element signature (V-Cr-Mn-Co-Ni-Zn) compared to olivine from the barren troctolite and olivine gabbro. A multiple regression analysis using these data yields an index that describes the relative vertical proximity of olivine to massive sulfides. In conjunction with the stratigraphic context of the host lithology, this information could potentially be used as a vector toward massive sulfide mineralization in the Eastern Deeps intrusion and also support Ni exploration in other olivine-bearing mafic intrusions with similar style of sulfide mineralization.