Petrology of the St. Stephen Intrusion and the genesis of related nickel-copper sulfide deposits

Abstract

The St. Stephen intrusion is a tholeiitic mafic-ultramafic intrusion of Devonian age occurring in the Gander terrane of the Appalachian orogen. The intrusion is dominated by olivine-plagioclase cumulates, olivine cumulates, and plagioclase cumulates. Olivine ranges in composition from Fo 61 to Fo 85 and plagioclase from An 43 to An 89 . Variations in the compositions of clinopyroxene and orthopyroxene are within Wo (sub 42-47) En (sub 37-50) Fs (sub 6-18) and Wo (sub 3-3) En (sub 50-71) Fs (sub 26-47) respectively. The Cr/Cr + Al and Mg/Mg + Fe (super +2) ratios of chromites range from 0.25 to 0.75 and 0.01 to 0.65, respectively. This appears to have resulted from the changes in the composition of the melt from which chromites were crystallized and reaction of the early formed cumulus chromites with the intercumulus melt. The observed crystallization sequence olivine (+ chromite), olivine + plagioclase, olivine + plagioclase + clinopyroxene, plagioclase + clinopyroxene + orthopyroxene (+ olivine), plagioclase + orthopyroxene (+ clinopyroxene), plagioclase + clinopyroxene appears to have resulted from simple fractional crystallization of a common tholeiitic parent magma. The composition of the parent magma seems to have an affinity to the mid-ocean ridge basalts.The intrusion contains numerous zones of significant Ni-Cu sulfide mineralization. The principal sulfide minerals are those typical of magmatic assemblages: pyrrhotite, pentlandite, and chalcopyrite. Ni and Cu concentrations reach 2.40 and 3.90 percent, respectively. Platinum-group element concentrations are generally low, with maximum abundances in massive sulfides, for example, not exceeding 62 ppb. Attainment of sulfur saturation in the St. Stephen magma during the early stages of the differentiation corresponds to the appearance of plagioclase on the liquidus following crystallization of about 10 percent of the olivine. The nickel-depleted nature of olivines at this interval can be explained by fractional crystallization of olivine and sulfide in the proportion of 50:1. An earlier segregation of sulfide from the parental magma seems to be responsible for the very low levels of platinum-group elements in the interstitial sulfides associated with the olivine cumulates. Preferential assimilation of the sulfide-rich portions of the country-rock metasediments and attainment of sulfide-silicate equilibrium on a local scale are believed to have played important roles in the formation of large pyrrhotite-rich sulfide bodies.