Silver deposits associated with the Proterozoic rocks of the Thunder Bay District, Ontario

Abstract

The silver deposits to the immediate north and west of Lake Superior are divided into three groups. The Mainland veins, the largest group, occur along a zone of normal faults near the western margin of the Proterozoic rocks of the Southern Province. The most economically productive deposits, the Island group, arc in or very near a northeast-trending swarm of gabbro dykes lying immediately offshore the northwestern shore of Lake Superior. The third group occurs near the western margin of the Port Coldwell alkalic complex; these veins are in a shear zone that cuts both Archean metasedimentary rocks and a Proterozoic diabase dyke. The Mainland deposits occur in the Rove shale, immediately below the contact with Logan diabase sills. The veins locally extend upwards into the sills, but the silver-bearing portions, consisting of acanthite and native silver associated with base-metal sulphides, fluorite, barite, quartz, and calcite, are largely bounded by locally silicified shale. The Island veins, typified by the Silver Islet mine, are in fractures perpendicular to the gabbro-dyke host rocks. These veins contain both native silver and acanthite, associated with a Ni–Co sulpharsenide suite and the same mineral assemblage as the Mainland deposits. The veins near Coldwell are rich in sphalerite and galena.Lead-isotope data indicate that the Mainland and Island veins are genetically related and that the Mainland veins formed from an inhomogeneous fluid. Two-stage calculations indicate an early Proterozoic source rock, possibly the Rove shale. The Island veins are more isotopically homogeneous, and their metals may have been derived partially from the gabbro. The Coldwell veins contain lead that is less radiogenic than that of the other two groups and is possibly derived from the adjacent Archean rocks. All three groups of deposits have isotopic compositions that are much less uranogenic and more thorogenic than the nearby Pb–Zn–Ba veins of the Dorion area. Preliminary fluid-inclusion data from the Mainland veins indicate that deposition occurred from a fluid whose temperature varied from approximately 200 °C to more than 400 °C; deposition occurred during boiling induced by adiabatic expansion of the fluid at relatively shallow crustal depths. The Mainland veins developed in the shale (rather than the diabase), as its high fissility, and hence permeability, made it susceptible to intense fracturing by the expanding fluid. Both the Mainland and Island groups were deposited in structures formed dominantly by listric normal faulting during late stages of intracontinental rifting. Heat was supplied by abundant mafic intrusions that formed coincident with rifting. The ore fluid was probably formed as a result of metamorphic dewatering, with metals released to the fluid because of silicate and sulphide recrystallization.

The Port Coldwell veins, northern Ontario: Pb-Zn-Ag deposits in a rift setting

“…Lead isotopic evidence strongly suggests that the veins are of Keweenawan age and related to silver vein deposits of the Thunder Bay area, 250 km to the west (Franklin et al, 1986). The lead data from the Port Coldwell veins group closely with those of the Mainland Belt silver deposits at Thunder Bay; however, they are less radiogenic and lie close to a normal growth curve with a model age of 1,172 Ma.…”

“…The metals would have been scavenged by chloride-rich brines from deeper crustal levels. Evidence for this source of metals is available for the Deadhorse Creek occurrences in four lead isotopic analyses reported by Franklin et al (1986). The analyses lie in a tight array on the lower crust curve in 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb and 208 Pb/ 204 Pb versus 206 Pb/ 204 Pb diagrams of the plumbotectonics II model of Zartman and Doe (1981).…”

“…The suggestion has been made that the Port Coldwell veins are genetically related to silver-bearing vein deposits of the Thunder Bay area, approximately 250 km to the west (Franklin et al, 1986). The Thunder Bay area veins are of the fiveelement (Ni-Co-As-Ag-Bi) type, a deposit type distributed worldwide and of generally controversial origin.…”

Fluid-inclusion evidence of basinal brines in Archean basement, Thunder Bay Pb–Zn–Ba district, Ontario, Canada

“…Anomalous lead isotopic compositions from vein galena suggest a basement source for at least some of the lead (Franklin and Mitchell 1977;Franklin et al 1986) but do not fully constrain the timing of vein formation. The consistent orientation of the structures (Fig.…”

The Cobalt Mining District: Silver Sources, Transport and Deposition

“…Hydrothermal concentration of components from Huronian shales (Kerrich et al, 1986), Archean massive sulfides (Smyk, 1995;Smyk and Watkinson, 1990;Watkinson, 1986;Goodz et al, 1986;Franklin et al, 1986) and Archean volcanoclastic rocks (Boyle and Dass, 1971) is a popular model for the genesis of the Cobalt silver veins (Table 1). These models agree well with the observed spatial associations of the Huronian-Archean unconformity and Nipissing diabase.…”