Lithogeochemical patterns related to sedex mineralization, Sudbury Basin, Canada

Abstract

Distal sediments stratigraphically equivalent to sedex Zn-Cu-Pb deposits at the base of the Proterozoic Onwatin argillite formation, Sudbury Basin, are depleted in S and enriched in Mn, occurring as kutnohorite, manganosiderite and spessartine, and in Ba in the form of the K-Ba-feldspar, hyalophane. Fluid inclusion data indicate deposition of the distal sediments at ∼ 125°C and of the sedex carbonate gangue at ∼ 170°C. The overlying Onwatin argillites, deposited at ∼ 60°C, are characterized by a distinctive pattern of S C org ≈ 0.36 indicative of diagenetic S, alternating with S C org ⪢ 0.36 , probably reflecting episodic expulsion of S-bearing fluids into the water column during the waning stages of hydrothermal activity. Similar to the S/C org pattern are patterns of rhythmically alternating low and high Zn, Pb, As, Re, Ni and Pd values in the Onwatin argillite. Coinciding with the onset of hydrothermal activity, δ 34S-values in pyrite decrease abruptly from ∼ + 7‰ at the top of the Onaping Formation to + 3‰ at the base of the overlying Onwatin argillites. δ 13C-values in the carbonate gangue of the sedex deposits, −8.9 to −2.4‰, lie in or close to the range for magmatic carbon, in contrast to values of −18.7 to −14.9‰ in carbonate derived by oxidation of organic carbon in the overlying and underlying rocks. Low δ 34S-values in the sedex deposits and basal Onwatin rocks and enrichment of the Onwatin argillite in S, Ni, Re and Pd suggest derivation of these metals, at least part of the S and possibly Cu, Zn, Pb from the magmatic Ni-Cu ores and/or associated Sudbury Igneous Complex which probably also provided the energy to drive the convective-exhalative system responsible for the sedex mineralization. The model implies potential for further sedex deposits along the 110 km of the Onwatin-Onaping contact.