"Rock of the Month # 126, posted for December 2011" ---
Native silver (Ag) from the famed Silver Islet silver mine, located on a shoal off the east shore of the Sibley peninsula, in the cold waters of Lake Superior, in the Thunder Bay region of northwest Ontario. The silver mineralization was discovered in 1868. The story of the mine has been told many times: the setting might be described as romantic (on a late-summer day) to rugged, or worse! The mighty lake and the local climate conspired to be challenging, the watery setting and periodic storms reminiscent of the cliff-top mines of Cornwall, far to the east. A fine sample provided by Dr J. Doug Scott in 1986 provided a pair of excellent polished sections. The hand samples displayed arborescent native silver and coarse greenish sphalerite, with minor galena in a vein infilled by pinkish-white calcitic gangue. The very fine-grained, black host rock had a "graphitic" appearance. The paragenesis is clearly complex, with sphalerite, galena, native silver, cobaltite and carbonate. The wallrock of this sample (965) is highly altered ophitic diabase with relict turbid plagioclase, late veinlets of carbonate ± carbonaceous matter, secondary silica, chlorite and rutile (Wilson, 1987).
Gold may have been the prime incentive for many European explorers of the New World, but silver was also a big draw. Silver is a precious metal, especially at current prices in excess of the historical (1980) high of US$30/ounce, but not nearly as rare in nature as gold or the platinum group elements. The elemental abundance of Ag in the upper lithosphere is about 0.1 ppm, e.g., 0.12 ppm in gabbro and diabase (Boyle, 1968). Then as now, the saleability of the metal sufficed for the adventurous. As summarized by Smith (1986, pp.36-55), Silver Islet began its brief period of fame and fortune as a tiny islet, no more than 8 feet (2.4 m) above the waters of Lake Superior. In the summer of 1868, prospectors found a rich vein of galena and silver on the tiny rock, and in short order blasted and hacked 1,336 pounds of ore to ship to Montreal, where it averaged an astonishing assay of 2,087 ounces per ton silver (606 kg averaging 7.16 wt.% silver!). While the discovery in a sense was easy, the subsequent brief but fabulously productive mining of the exposed rock, 1869-1884, posed an epic struggle against the elements, as told in detail by Barr (1988). These were colourful, hard-driving times, with the hard work and danger shouldered by powerful Norwegian and Cornish ("Cousin Jack") miners (Newell, 1985; Barr, 1988; Horton, 1989). Popular histories and travelogues of the north shore of Superior often mention the historic hamlet of Silver Islet, small yet still very much alive near the southern tip of the east shore of the Sibley peninsula, and the remarkable mining history just offshore (e.g., Wilkins, 1997; Chisholm et al., 1999; Burtch and Pollett, 2006). The deposit is also inevitably referenced in regional geological reviews and travel guides (e.g., Sutcliffe, 1991; Pye, 1997).
The silver mineralization:
The vein on Silver Islet proved to be of unparalled richness and the only local bonanza, but fortunately related veins were found on the mainland to the north, and these ores have been documented for well over a century. The Silver Islet deposit is the prime example of one of three sets of veins along the north shore of Superior, the Island veins (Franklin et al., 1986). As noted by Ingall (1889), silver dendrites at Silver Islet always occurred associated with graphite, the sulphides themselves were generally very poor in Ag, and combustible gases occurred in the workings in association with CaCl2-rich hydrothermal fluid in large vugs in a chloritic altered "diorite" host rock. It was said that silver never occurs without graphite, but that graphite may be found without silver. The presence of reduced carbon in the Archean, at Silver Islet ("graphite") and at Sudbury "anthraxolite" even led Archibald Blue of the Ontario Bureau of Mines to speculate on the likelihood of finding diamonds in the north of Ontario, a reality that was not to transpire for almost another hundred years (Blue, 1900). The carbon is no doubt derived from host rocks, shaly Animikie sediments, which are intruded by diabase dykes. The dykes are locally faulted, brecciated and cemented by pink carbonate, quartz and a wide range of ore minerals (Tanton, 1931, pp.92-104 and 1:36,000 scale map of Thunder Cape; Bruce, 1969).
Inevitably, once the longer-lived Cobalt mining camp was flourishing, the silver ores of the two areas would be compared (e.g., Miller, 1913; Guild, 1917). Some of the intergrown minerals in the ore caused confusion, leading to short-lived terms such as "animikite", later shown to be a mixture of native silver, galena and niccolite (Horvath, 2003, p.260). The list of verifiable minerals is nonetheless extensive (e.g., Wilson, 1986; Barr, 1988). Such complex veins are often referred to as five-element ores (containing nickel, cobalt, arsenic, silver and bismuth, e.g., Kissin, 1992). Silver Islet is an example, but there are many more in the Cobalt-Gowganda district of eastern Ontario and in the Erzgebirge of Germany. On the nearby mainland to the north, there is evidence that carbon in vein calcite is derived from carbon in the Proterozoic host sediments (Kissin and Sherlock, 1989).
The contacts of Keweenawan diabase dykes or sills with the
carbonaceous shale, particularly where the hard, iron-rich diabase was
fractured by faults, would be a good structural and chemical trap for
The veins of the region are thought to have formed by mineralization
of dilatant faults in early stages of Keweenawan rifting, with
hydrothermal fluids derived from connate water and possibly
Proterozoic seawater heated to 400-450°C at depths of up to 10
Barr,E (1988) Silver Islet, Striking it Rich in Lake Superior. Natural Heritage/Natural History Inc., Toronto, 159pp.
Blue,A (1900) Are there diamonds in Ontario? Ontario Bureau of Mines Rep. 9, 119-124.
Boyle,RW (1968) The geochemistry of silver and its deposits, with notes on geochemical prospecting for the element. GSC Bull. 160, 264pp. plus 2 map sheets.
Bruce,EL (1969) The mineral deposits of Thunder Bay district, Ontario. In `Ore Deposits as Related to Structural Features' (Newhouse,WH editor), Hafner Publishing Company, New York, 280pp., 101-106.
Burtch,P and Pollett,FC (2006) Trans Canada Ecotours: Ontario 1. Pigeon River to Sault Ste. Marie. Canadian Geographic, 140pp.
Chisholm,B, Gutsche,A and Floren,R (1999) Superior: Under the Shadow of the Gods. Lynx Images Inc., Toronto, 2nd edition, 282+40pp.
Franklin,JM, Kissin,SA, Smyk,MC and Scott,SD (1986) Silver deposits associated with the Proterozoic rocks of the Thunder Bay district, Ontario. In `Silver Vein Deposits', a symposium held at the GAC/MAC London meeting in 1984, publ. in Can.J.Earth Sci. 23 no.10, pp.1460-1640, 1576-1591.
Guild,FN (1917) A microscopic study of the silver ores and their associated minerals. Econ.Geol. 12, 297-353.
Horton,RJ (1989) The mining and geologic history of the Silver Islet mine, and a conceptual ore genesis model for the deposit. Proc. Institute on Lake Superior Geology 35, Duluth, MN, Part 1, Abstracts, 104pp., 29-31.
Horvath,L (2003) Mineral Species Discovered in Canada and Species Named After Canadians. Can.Mineral. Spec.Publ. 6, 374pp.
Ingall,ED (1889) Report on mines and mining on Lake Superior. GSC Annual Report 3 part 2, 27-40.
Kissin,SA (1989) Genesis of silver vein deposits on the north shore of Lake Superior, Thunder Bay district, Ontario. GSA Abs.w.Progs. 21 no.6, Annual Meeting (St. Louis), 130.
Kissin,SA (1992) Five-element (Ni-Co-As-Ag-Bi) veins. Geoscience Canada 19, 113-124.
Kissin,SA and Sherlock,RL (1989) The genesis of silver vein deposits in the Thunder Bay area, northwestern Ontario. OGS Misc.Pap. 143, 237pp., 33-41.
Miller,WG (1913) The cobalt-nickel arsenides and silver deposits of Temiskaming (Cobalt and adjacent areas). OBM Ann.Rep. 19 part 2, 4th edition, 279pp.
Newell,D (1985) Silver mining in the Thunder Bay district, 1865-1885. Thunder Bay Historical Museum Society, Papers and Records 13, 28-45.
Pye,EG (1997) Roadside geology of Ontario: North Shore of Lake Superior. OGS Rock On Series 2, 164pp.
Smith,P (1986) Harvest from the Rock. Macmillan of Canada, Toronto, 346pp.
Sutcliffe,RH (1991) Proterozoic geology of the Lake Superior area. In `Geology of Ontario' (Thurston,PC, Williams,HR, Sutcliffe,RH and Stott,GM editors), OGS Spec.Vol. 4, part 1, 709pp., 626-658.
Tanton,TL (1931) Fort William and Port Arthur, and Thunder Cape map-areas, Thunder Bay District, Ontario. GSC Memoir 167, 222pp. plus 3 maps.
Wilkins,C (1997) Breakfast at the Hoito and Other Adventures in the Boreal Heartland. Natural Heritage Books, Toronto, 182pp.
Wilson,GC (1987) Untitled (andesite from the summit of the Pico de Orizaba, Mexico: PGE ore from the Merensky Reef, Bushveld complex, South Africa: and bonanza-grade silver ore from Silver Islet, Ontario). Unpublished technical report, 18pp.
Wilson,WE (1986) Famous mineral localities: the Silver Islet mine, Ontario. Mineral.Record 17 no.1, 49-60.
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