Mineralogy, geochemistry, and paragenesis of the Eastern Metals serpentinite-associated Ni-Cu-Zn deposit, Quebec Appalachians

Abstract

The Eastern Metals deposit, located in the southern Quebec Appalachians, occurs at the contact of a slice of ultramafic rock enclosed within ophiolitic melange sediments of Cambro-Ordovician age that are slightly metamorphosed to lower greenschist facies. Disseminated to massive sulfides are hosted by intensely carbonatized and silicified rocks at the sheared contacts between serpentinite and graphitic slate. The deposit comprises two mineralized zones, the Ni-Zn North zone and the Cu-Ni-Zn-Co-Au South zone. Textural and structural evidence suggests that the deposit formed in two main phases: early alteration and mineralization in serpentinite; and late main-stage Cu-Ni mineralization.Hydrothermal alteration by CO 2 -, Ca-, S-, and As-rich fluids during a relatively late stage of serpentinization took place as ophiolite slices were obducted during the Taconic orogeny. Serpentinite was initially transformed into talc-carbonate schist, and then, in a progressively more oxidizing and lower temperature environment, into listwaenite (quartz-carbonate rock), and finally birbirite (pyrite-bearing siliceous rock). All hydrothermally altered rocks are rich in Cr, Ni, and Co. The presence of relict chromite and ghosts of typical serpentinite net texture confirms the ultramafic nature of the protolith. The isotopic compositions of carbonate (delta 13 C = -2.4 to -7.0ppm) suggest a juvenile source of carbon, whereas sulfur isotopes from pyrite (delta 34 S = 7 to 19ppm) indicate interaction between seawater or meteoric water from sedimentary country rock.Late main-stage copper and nickel sulfides replace early pyrite. The sulfide assemblage in the North zone, comprising pyrite, violarite-polydymite, and millerite, is compatible with maximum temperatures of formation of 356 degrees and 282 degrees C. Massive sulfides in the South zone are composed principally of pyrite and chalcopyrite. Cubanite exsolutions in chalcopyrite indicate a minimum temperature of formation of approximately 250 degrees C. Gold, present in the South zone only, exhibits good positive correlations with lead, arsenic, antimony, and mercury. Metal zonation in the Eastern Metals deposit developed in response to changes in the redox potential in the host-rock environment.This deposit provides a unique opportunity to study a polymetallic serpentinite-associated deposit which has not been significantly overprinted by supergene alteration, intense metamorphism, or tectonic fabrics that occur in other examples of this deposit class such as Bou Azzer in Morocco or Outokumpu in Finland.