"Rock of the Month # 234, posted for December 2020" ---

Skarns and their economic importance

Let's start with some definitions. Skarn is a term of Swedish origin, referring to rock formed by changes to the host rock ("protolith") when it is cut by a body of magma. The intruding magma heats the host rock, effecting contact metamorphism. Fluids associated with the magma alter the mineralogy of the host rocks, the process of metasomatism. Due to their widespread occurrence and their chemical fragility, the most common host rocks are limestone (which forms calcic skarns) and dolostone (which gives rise to magnesian skarns). However, other host rocks such as clastic sediments (e.g., siltstone and sandstone) and volcanic rocks (such as basalt) can also be altered to form skarns. Lastly, the principal alteration lies beyond the intrusion, so-called exoskarn. But the magmatically derived fluids can also stream back into fractures, formed in the intrusion as it cools, giving rise to zones of endoskarn.

Skarns are especially abundant in younger (Phanerozoic, and indeed Mesozoic-Cenozoic) tectonic assemblages of accreted terranes on the margins of continents, as is the case in British Columbia, on the margin of the North American craton, the largest component of which is the Canadian Shield. In a recent review, Chang et al. (2019) note that skarns are one of the most common types of mineral deposit in China. Their review of 386 skarn deposits estimates that these contain some 87% of Chinese tin resources, as well as much tungsten (71%), copper (32%), zinc and lead (25%), molybdenum (17%), gold (11%), silver (10%) and high-grade iron ore (9%). This abundance of skarns is due to a tectonic evolution that juxtaposes abundant hydrous magmas and carbonate host rocks.

Industrial minerals may also be recovered from skarn deposits, such as garnet from the Mount Riordan garnet skarn in the Hedley area of southern British Columbia (Ray et al., 1992). This is a garnet deposit with drill-indicated metric reserves of 40 MT grading an average 78 volume percent garnet (sulphides and other metallic minerals, which in this case would be impurities, are rare). The garnet forms zoned crystals, with isotropic andradite-rich cores (Ca-Fe garnet) and relatively grossular-rich (Al-Ca garnet), birefringent rims (similat to the Crown Point garnets).

The garnet skarn at Rossland

This sample was collected in 1988, as part of the mapping, sampling and description of an old gold mine property on the south side of the Rossland gold camp. Local host rocks include basalt flows, lesser andesites and siltstones. Mineralization is plausibly related to a Mesozoic (Cretaceous) quartz monzonite intrusion, not to Mesozoic and Tertiary alkalic dykes (pulaskite and lamprophyre) in the area. However, Fyles et al. (1973) found that the oldest pluton, the Rossland monzonite, is >90 Ma old, and that all other plutonic, dyke and volcanic rocks in the area are 52-46 Ma, this younger, Cenozoic period including episodes of Mo and Cu-Au mineralization (see also Little et al., 1982).

The sampling program lent itself to further scientific research, summarized in Wilson et al. (1990). In a related petrographic study, about 50% of studied samples were skarns, with key minerals such as pyrrhotite, garnet, clinopyroxene and magnetite. This is quite typical of many skarns, composed of minerals (silicates, sulphides, oxides) rich in elements such as Mg and Ca, Fe and Al, many of them calc-silicates such as species of pyroxene, garnet and epidote. The opaque ore minerals include arsenopyrite, pyrrhotite, chalcopyrite, marcasite and magnetite, and traces of native gold. It was found that ppm levels of Au occur in arsenopyrite, but the mean Au value in 38 analyses of pyrrhotite is only 32 ppb. Arsenopyrite and minor native Au largely account for gold in whole-rock assays. The Crown Point mineralization includes proximal Au-sulphide skarns, distal skarns enriched in Ag and base metals, and arsenical brecciated siltstones. In the South Belt area around the Crown Point mine, the dominant lithology is identified as basaltic tuff east of Tiger Creek, and as argillaceous siltstone to the west of the creek (Hoy and Andrew, 1991).

Historical Context

The Rossland camp (Fyles, 1984; Fyles et al., 1973) was a significant gold producer from 1894 to 1928. In the Crown Point Mine (Drysdale, 1915, pp.161-163), the "ore shoot is a typical Rossland type, occurring as it does along the border of a diorite porphyrite tongue intrusive into augite porphyrite". The ore is cut by a 30-40-foot-thick syenite (pulaskite) dyke which has K-feldspar phenocrysts, a pronounced chilled margin, and flow texture. The ore at this small, short-lived mine is dominated by a massive pyrrhotite body with some chalcopyrite: the average grade of 11 cars of ore shipped in 1905 included 0.5 oz/T (17.1 ppm) Au, 0.3 oz/T (10.3 ppm) Ag, 0.6% Cu and 14.3% S. Drysdale described the Rossland monzonite as a chonolith, "an irregularly shaped intrusive mass bounded in some places by flatly dipping contacts and elsewhere by steeply dipping contacts" (ibid., p.29). In its heyday the camp boasted many mines, e.g., in the North belt there were the Centre Star, War Eagle, Iron Horse, Iron Colt, Monte Christo, the LeRoi group, Evening Star, Jumbo and I.X.L., and in the South belt the Bluebird, Crown Point, Mayflower and Phoenix.

Gold production from the Canadian Cordillera to the end of 1978 totalled 35 million oz, 47% of which has been produced by placer mining, both in the Klondike (Yukon) and in the Cariboo. The balance is derived from lode (80%) and base metal (20%) mines (Barr, 1980). >75% of the lode Au produced to 1978 was from 5 camps, which in declining order of production are Bralorne-Pioneer (Bridge River camp), Rossland, Premier, Hedley and Cariboo (Barkerville). In this context, "lode gold" really refers to hard-rock (in the ground, not in river gravels) deposits, mined primarily for gold (not base metals such as copper), of assorted geometry, not just gold physically in a "lode" (vein) form. This would include skarn-hosted gold, even though gold may occur there both in veinlets and also in the surrounding altered host rock, the skarn.

Thus Rossland in its heyday, around the start of the 20th century, was a major gold producer. The LeRoi No.2 mill in Rossland (photo in Convey et al., 1957, p.25) was the first flotation mill in Canada. Gold mineralization in the region is NOT all in skarn (though gold skarn is very important at Hedley and elsewhere), and includes vein ores additional to skarn.

Fig. 2: Sawn and polished slab of garnet skarn. Close-up of garnet-rich area. Sample (minus tip seen in Fig. 1, broken off) is 476.17 grams, 10x11x1.7 cm, magnetic susceptibility 1.6x10^-3 SI units. The garnet crystals are typically equant but with poorly-developed crystal habit, 3-10 mm in diameter.