Native silver in hydrothermal vein rock from the Proterozoic of Ontario, Canada. (cut surface; ~6.0 centimeters along the long axis)
This remarkable rock is from a long-abandoned silver mine in Lake Superior. The bright whitish spots are native silver (Ag). The matrix consists mostly of light-colored calcite (CaCO3) and a dark-colored cobalt-nickel arsenide (?) mineral.
The mine was located at Silver Islet, a tiny island in Lake Superior's Thunder Bay Mining District. Subsurface mining occurred in the 1870s and 1880s. Flooding and storm damage were frequent problems. The mine was abandoned in 1884 when the entire subsurface mining complex flooded with lake water.
Silver specimens from the Silver Islet Mine are rare. This example was retrieved from the underwater spoils piles by a diver using a metal detector in the 1970s or 1980s.
Silver at the site occurs in hydrothermal veins. Confirmed minerals present include native silver, acanthite, chalcopyrite, galena, marcasite, sphalerite, tetrahedrite, cobaltite, domeykite, niccolite, fluorite, quartz, pyrolusite, calcite, dolomite, rhodochrosite, barite, annabergite, and erythrite (list from Franklin et al., 1986).
Isotopic dating of rocks in the Thunder Bay Mining District indicates that the silver-bearing vein rocks at Silver Islet are younger than 1.097 Ga and possibly less than 1.045 Ga. Copper-silver mineralization in the Keweenaw Peninsula on the southern side of Lake Superior occurred at 1.05 to 1.06 Ga (late Mesoproterozoic). It's possible that this is also the general timing of hydrothermal vein emplacement in the Lake Superior area.
Locality: underwater mine dump, Silver Islet Mine, Thunder Bay Mining District, northwestern Lake Superior, Ontario, Canada (48° 19' 18.08" North latitude, 88° 48' 43.24" West longitude)
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Some info. from:
Franklin et al. (1986) - Silver deposits associated with the Proterozoic rocks of the Thunder Bay District, Ontario. Canadian Journal of Earth Sciences 23: 1576-1591.
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See:
www.flickr.com/photos/89902546@N02/8172989578
and
www.flickr.com/photos/28842972@N00/21463842448
and
www.flickr.com/photos/tbaymuseum/16148140630
and
www.flickr.com/photos/31856336@N03/7120109813
and
en.wikipedia.org/wiki/Silver_Islet
