"Rock of the Month # 40, posted for October 2004" --- A complex nickel- cobalt- arsenic (Ni-Co-As) ore from the Cobalt mining camp of northeastern Ontario, Canada. This sample (GCW 2424) is from the Frontier mine, Silver Centre. This sample displays a vein rich in the ore minerals safflorite (CoAs₂), nickeline (niccolite, NiAs), and nickel-skutterudite (Ni-bearing CoAs₃). Such ores often contain associated minerals such as breithauptite (NiSb). The wallrock is greenish- black, strongly altered, foliated Huronian sediment. This chloritic, carbonatized host rock is cut by a vein roughly 42 mm wide with complex rosettes of ore minerals in a pinkish-white calcitic matrix. The rosettes, mostly 2-3 mm wide, are composed of a coppery-pink niccolite core, partially surrounded and perhaps replaced by calcitic rinds, with small cubes of nickel-skutterudite, mantled by thick silvery rims, spiny in close-up, of the third and most abundant arsenide, safflorite. Rosette textures are noted in related ores elsewhere, including the bonanza-grade silver ores of the fabled Silver Islet mine in northwestern Lake Superior near Thunder Bay (Kissin, 1992). The Silver Islet ores display rosettes with native silver cores rimmed by pararammelsbergite (orthorhombic NiAs₂), the intergrowths everywhere separated from later sphalerite by dolomite.

GCW sample 2424, a fine specimen supplied by Gil Benoit of Hanmer, Sudbury, Ontario. Digital image, 07 November 2004. No silver minerals were noted in a brief examination, but the abundant arsenides are proof of a remarkably rich ore containing approximately 50 weight percent arsenic, 15 percent cobalt and 5 percent nickel. Modern metallurgical processes would treat this ore to remove the fabulously high grades of cobalt and nickel, important additives in the steel industry, while retaining the potentially toxic arsenic byproduct. New uses for metals have arisen since since these ores were originally mined, including fast gallium arsenide semiconductors for the computer industry.

The wallrock appears to be a Huronian sediment, foliated parallel to the vein walls, with detrital quartz and plagioclase feldspar in a fine-grained matrix of secondary carbonate and sheet silicates plus fine silica. Thin chlorite- calcite veinlets penetrate the host rock from the margins of the ore vein, which are lined by a thin selvage of two varieties of chlorite, ripidolite and penninite.

This sample is an example of the complex spectrum of so-called five-element arsenide ores. These elements are nickel, cobalt, silver, arsenic and bismuth, although the complex chemistry of such ores often includes appreciable amounts of other metals (iron, antimony, uranium, etc). Kissin (1992) describes these deposits and notes their development in continental crust at times of extensional tectonism and rifting.

The source locality for this sample is in South Lorrain township, an area of the Cobalt district which sported a number of significant mines (Keeley, Lorrain, etc), although the township may have provided just 3% of the silver from the prolific mining region (Petruk, 1971; Anderson, 1990). Silver Centre is now one of many `ghost towns' in Ontario which serve as mementoes of a mining past (Brown, 1983). The 1903 discovery of high-grade silver ore in the Cobalt district was the precursor to a prospecting and mining boom of rare scope. Provincial Geologist Willet G. Miller recognized native silver, and later secured the distinctive name of Cobalt for the new town. By 1910 Cobalt had matured as a mining town, and had entered its heyday, 1909-1919. In these boom years 15 mines paid back their full capital investment in net profits: "Nipissing 130 times, Kerr Lake 260 times, and Timiskiming and Hudson Bay, 300 times" (MacDougall, 1946, p.53). Banks and lawyers appeared at Cobalt, a sure sign of wealth and progress: "the legal fraternity was fully represented, as shingles indicated, for in the mining game stakes were high and integrity often correspondingly low" (ibid., p.54). The history, geology and mineralogy of the district have all been well-documented (e.g., MacDougall, 1946; Moore, 1934; Petruk, 1971). Mineralogists soon found that the rich ores, commonly containing percent levels of silver and more, had very complex internal textures, hinted at in the polished face of our specimen (Miller, 1913; Guild, 1917).

The silver ores are distinguished by a calcitic carbonate-rich gangue, the most typical ore minerals being Co-Ni arsenides and sulphides, and native silver (Moore, 1934). Miller's classic description, first published in 1905, covers the early years of the booming mining camp, and provides the first detailed documentation for what would become a cornucopia of mineral species. Examples: native bismuth, smaltite, niccolite, breithauptite and chloanthite, and secondary minerals such as crimson erythrite ("cobalt bloom"). Other local species include loellingite (lollingite), safflorite, and pararammelsbergite.

Halls and Stumpfl (1970) examined ore suites from the area and recognized four main phases of mineralization: arsenide-silver, carbonate-base metal, calcite- pyrite, and calcite- silver sulphosalt. Petruk (1971) and others reviewed the geology and mineralogy of the region at a mature stage in the mining cycle. The deposits are hosted by mid-Proterozoic (Huronian) sediments, late Archean (Keewatin) volcanic rocks and Proterozoic Nipissing diabase, all within or no more than about 700 feet (200 m) from Nipissing diabase. They are arranged about basin and dome structures in the diabase, as veins in zones of intensely fractured rock. The veins vary from roughly 10 cm to 1 km in length, 10 cm to 120 m in vertical extent and from hairline fracture to 1.2 m in thickness (Petruk, 1971).

References

Anderson,P (1990) South Lorrain Township. OGS GDIF 480, 183pp.

Brown,R (1983) Ghost Towns of Ontario, Volume 2. Cannonbooks, Toronto, 175pp.

Guild,FN (1917) A microscopic study of the silver ores and their associated minerals. Econ.Geol. 12, 297-353.

Halls,C and Stumpfl,EF (1970) Geology and ore deposition, Western Kerr Lake Arch, Cobalt, Ontario. In `Mining and Petroleum Geology', Proc.Ninth Commonwealth Min.Met. Congress 1969 (editor Jones,MJ), Vol. 2, 774pp., Inst.Min.Met., London, 241-284.

MacDougall,JB (1946) Two Thousand Miles of Gold. McClelland & Stewart Limited, Toronto, 234pp.

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.

Moore,ES (1934) Genetic relations of silver deposits and Keweenawan diabases in Ontario. Econ.Geol. 29, 725-756.

Petruk,W (1971) General characteristics of the deposits. In `The Silver-Arsenide Deposits of the Cobalt-Gowganda Region, Ontario' (Petruk,W and Jambor,JL compilers), Can.Mineral. 11 Part 1, 429pp., 76-107.