THE STABLE AND RADIOGENIC ISOTOPIC ATTRIBUTES OF PRECIOUS-METAL-BEARING POLYMETALLIC VEINS FROM THE COBALT EMBAYMENT, NORTHERN ONTARIO, CANADA: GENETIC AND EXPLORATION IMPLICATIONS

Abstract

The Cobalt Embayment is a large (~30,000 km 2 ) domain of Paleoproterozoic (2.45–2.22 Ga) siliciclastic sedimentary rocks (the Huronian Supergroup) that unconformably overlies Archean basement rocks of the Abitibi Greenstone Belt. The sedimentary rocks were affected by subgreenschist-facies metamorphism and regionally distributed metasomatism related to waning stages of the Penokean Orogeny ( ca . 1.9–1.7 Ga). Precious-metal-bearing polymetallic calcite–quartz vein systems occur throughout the embayment and share many common features, including their morphological characteristics, ore and gangue mineralogy, paragenetic sequence, and spatial association with regional faults rooted in the Archean basement. The polymetallic veins are typically hosted in Nipissing Diabase, a regionally distributed suite of mafic sills and dykes, which intruded the Huronian sedimentary rocks at ca. 2.2 Ga. Integrated carbon, oxygen, and strontium isotopic data for calcite, the major mineral in the gangue, support a common hydrothermal origin for the regionally distributed systems of polymetallic veins, involving mixing of meteoric-water-dominated fluids with basin brines. Local variations in δ 13 C likely reflect the availability of, and degree of hydrothermal fluid interaction with, organic matter in different regions of the embayment. Significant intradeposit variation in δ 34 S values of vein sulfides (pyrite, chalcopyrite, galena) suggests that localized sources of sulfur existed in the Huronian and Archean rocks, with positive values (>0‰) in several of the mineralized vein systems likely reflecting the introduction of sulfur remobilized from the basement as a result of the conversion of pyrite to pyrrhotite in the underlying Archean rocks. Data and observations suggest that metals were transported in oxidized fluids, and precipitation occurred as a result of oxidation–reduction reactions along reactivated faults, which facilitated fluid mixing. Together with Pb isotope data for the vein systems, the isotopic data presented support a model for ore genesis in which the systems of precious-metal-bearing polymetallic calcite–quartz veins formed, and were reactivated, in response to two major episodes of hydrothermal fluid activity that occurred throughout the Cobalt Embayment, at ca. 2.2 and ca. 1.7 Ga. The proposed genetic model identifies the polymetallic veins as one manifestation of hydrothermal fluid activity during these basin-wide events and also recognizes the regional unconformity as a potential site for undiscovered, unconformity-associated hydrothermal polymetallic mineralization.