Geology of sulphide-facies iron-formations and associated rocks in the lower Steel River-Little Steel Lake area, Terrace Bay, Ontario

Abstract

The Lower Steel River - Little Steel Lake area is located about 25 km east of Terrace Bay, Ontario. Seventeen iron-formations were investigated within this metamorphosed Archean volcanic and sedimentary terrain, which represents a portion of the Abitibi - Wawa Subprovince of the Superior Structural Province. Sulphide-facies iron-formations are the dominant chemical sedimentary rocks in the Schreiber - Terrace Bay area, and represent deposition during quiescent periods in clastic accumulation and in volcanic activity. The iron-formations commonly mark contacts between the volcanic and sedimentary rocks. They are interbedded with carbonaceous slates at the base of sedimentary successions and overlain by DE turbidites. This suggests chemical sedimentation on a subaqueous basinal plain slightly distal to an outer fan. Sulphide-facies iron-formations consist of a mixture of chemical and clastic components, and while highly variable, the iron-formations in the study area contain three commonly recognizable units: 1) pyritic-carbonaceous slate; 2) disseminated, massive, laminated and nodular pyrite; and 3) chert or siliceous sedimentary rock. Sedimentary textures and structures combined with trace element abundances and geochemical evidence support the mixing of clastic and chemical components, and a hydrothermal source for the iron. Exhalative discharge probably occurred episodically as moderate- to low-temperature solutions percolated through near-surface pillowed volcanics and, once vented, deposited a blanket of chemical sediment. Vent-proximal deposits include massive and layered pyrite; the domal and colloform varieties support the existence of organic mats. The presence of carbon is interpreted as a relic of the organic activity. Pyritic-carbonaceous slates consisting of alternating chemical and clastic components represent vent-distal deposits. Radial structures within pyrite nodules provide evidence of diagenetic transformation and tectonic deformation. Structural evidence suggests that the study area was affected by a complex folding event related to one deformational episode, probably preceded by syn-sedimentary slumping in some areas. The competency differences between sedimentary and volcanic rocks, combined with the fine-grained nature of the sedimentary rocks (DE turbidites and iron-formation ) at major volcanic-sedimentary contacts, focussed deformation and dike emplacement in the contact areas.