Skarns containing gold as the predominant, and in some cases the only, economic mineral are classed as gold skarns while precious metal enriched (PME) skarns include any iron or base metal skarn that contains gold, silver or, more rarely, platinum in sufficient quantities to be economically recoverable. More than 350 skarn occurrences are known in British Columbia and at least 130 of these are either gold or PME skarns. Of the 130 occurrrences, 29 deposits have produced 95 tonnes of gold, which represents about 10% of the estimated gold production from skarns worldwide. Fifty-three per cent of the province’s gold production from skarn was won as a primary commodity from gold skarns, while byproduct gold from PME-copper and PME-iron skarns accounted for about 40% and 7% respectively. By contrast, gold skarns have produced less than 2% of the provinces total skarn-derived silver (342 tonnes), virtually all of which was obtained as a byproduct from PME-copper and PME-iron skarns.
Gold production from skarn in British Columbia has been predominantly from two major deposits, the Nickel Plate/Hedley Mascot deposit at Hedley, which is the largest gold skarn known in Canada, and the Phoenix PME-copper skarn deposit in the Greenwood camp (Figure 1). Between 1898 and 1955, the Nickel Plate orebody (now being worked by Corona Corp. as an open pit) produced more than 48 tonnes of gold from 3.6 million tonnes of ore, while the Phoenix deposit produced 30 tonnes of gold, 192 tonnes of silver and 230,050 tonnes of copper from nearly 27 million tonnes of ore. The size and grade of these two deposits make exploration for other gold and PME skarns in the province potentially rewarding.
Distribution of Gold and PME Skarns
Base metal skarns are developed worldwide in a variety of geological environments and hostrock lithologies, but gold and PME skarns are generally more restricted, being mostly concentrated in Phanerozoic mobile belts. Gold skarns are commonly found in the same geological environment as iron skarns and copper skarns of oceanic island arc affinity, and they are generally associated with arc-related, high-to-intermediate-level calcalkaline plutons. There is a worldwide spatial and temporal relationship between gold skarns and the porphyry copper belts, both of which are developed along major destructive plate boundaries. The location of the major gold and PME skarns in British Columbia is shown in Figure 1, and their distribution in relation to the tectonic belts is shown in Figure 2.
The 130 gold and PME skarn occurrences are evenly distributed within each of the four western belts, which include major components of remobilized and acreted island arc assemblages. However, no gold or PME skarns are known in the East Foreland Belt, which is underlain by relatively stable cratonic basement. The gold production from skarn in each belt is highly variable (Figure 3). More than 90% of production was from skarns hosted in carbonate- bearing oceanic island arc or back-arc basin assemblages in the Intermontane and Omineca belts, while only 9% was from the westernmost Insular Belt (Figure 3). The latter belt contains low-potassium immature island arc rocks, calcalkaline continental margin arc intrusions and tholeiitic oceanic flood basalts; these are less favorable for gold skarns than the mature, shoshonitic arc and marginal basin sequences present in the Intermontane and Omineca belts.
There is practically no skarn-derived gold production from the Coast Complex Belt (Figure 3), despite the presence of abundant intrusions and some carbonates; this belt contains very little high-level arc material and is believed to represent either a deeply eroded root zone of an arc or a plutonic zone resulting from plate collision and accretion. The East Foreland Belt has produced no gold (Figure 3), despite containing abundant carbonates and some tin and tungsten skarns.
Although the 130 gold and PME skarn occurrences in the province are distributed through 14 different tectonic terranes (as defined by the Geological Survey of Canada), more than 90% of the gold and 65% of the silver from skarn were derived from deposits in the southern part of the Quesnellia Terrane (Figure 4). Quesnellia is characterized by Late Triassic to Early Jurassic island arc and marginal basin assemblages and coeval intrusions. Rocks of similar age, composition and tectonic setting occur farther north in the Stikinia Terrane (Figure 4), suggesting that this less accessible region has good exploration potential for gold and PME skarns.
The close association between precious metal enrichment in skarn and the volcanic arc assemblages, which in British Columbia are mainly Triassic to Jurassic in age, results in an apparent temporal control to economic gold and PME skarn deposits in the province. More than 96% of gold production has come from Late Triassic arc assemblages and 65% is associated with the slightly younger, arc-related plutons of Early Jurassic age.
Studies in the Hedley gold skarn camp and elsewhere indicate that structural hinge zones, flanks of fore arcs and the rifted margins of back arc basins are favorable sites for gold skarns because the deep-rooted, controlling structures preferentially channel the calcalkaline plutons into reactive calcium carbonate-rich sediments. These favorable fore arc and back arc environments are also often characterized by sedimentary breccias (olistostromes) that comprise a chaotic mass of limestone clasts and boulders.
Characteristics of Gold and PME Skarns
Gold enrichment occurs mostly in calcic skarns, and is exceedingly rare in magnesian skarns. It is most common in deposits having copper or iron-skarn affinities, less common in tungsten, zinc or lead skarns, and virtually non-existant in tin skarns. The gold may be coarse and visible, but usually forms micron-sized particles intimately associated with sulphides, making it difficult to visually distinguish ore from waste.
Gold skarns are associated with intrusions that vary compositionally from granite to gabbro, but are mainly found with rocks of quartz diorite to diorite composition. Like iron skarns, they are commonly related to ocean-crust derived intrusions with low to intermediate initial 87Sr/86Sr ratios, tend to be enriched in arsenic and cobalt, and are sporadically associated with late scapolite alteration.
All the gold and PME skarns in British Columbia are genetically related to intermediate to high-level island arc plutonism of calcalkaline affinity. No alkalic-intrusive-related gold skarn deposits are known in the province, although some gold-bearing porphyry copper deposits associated with high level alkalic plutons contain skarn-like garnet-pyroxene-scapolite alteration assemblages.
The calcalkaline intrusions related to gold and PME skarns vary from large stocks to narrows sills and dykes which may form swarm complexes; many are also porphyritic with phenocrysts of hornblende or plagioclase; the latter often exhibit marked oscillatory and reverse compositional zoning that may indicate the intrusions were derived from hybridized magmas or volatile-rich melts. In many cases, the intrusions are coeval with the late arc volcanism, but some PME-copper and PME-iron skarns on Vancouver and Texada Islands formed when younger plutons intruded considerably older but lithologically favorable hostrocks.
The degree of retrograde alteration overprinting the prograde garnet-pyroxene assemblages varies considerably in gold and PME skarns and cannot be used as an exploration guide. Most gold skarns, like iron skarns, are characterized by low-manganese (less than one weight per cent manganese dioxide) grandite garnets and low manganese (less than four weight per cent) iron-rich hedenbergitic pyroxenes. They tend to be richer in pyroxene relative to garnet; and compared with copper skarns, their pyroxenes are commonly more iron-rich. Garnet-pyroxene compositions and sulphide assemblages indicate that precious metal enrichment in skarn can occur in both oxidized and reduced states, but most end-member gold skarns (such as all those in the Hedley camp) formed in relatively reduced environments and have high pyrrhotite-to-pyrite ratios.
Economic gold mineralization is rarely developed in the endoskarn but is most commonly hosted by pyroxene-rich exoskarn in the outer parts of the alteration envelope. The amount of exoskarn varies from narrow zones fewer than 10 metres wide up to the size of the large envelope that surrounds the Nickel Plate deposit, which is many hundreds of metres thick and has a volume of 0.75 to 1.5 cu. km. The morphology of the exoskarn envelopes varies from stratiform to subcircular to vein-like and sharply discordant.
Gold mineralization in skarns is associated with opaque minerals that were introduced mainly after development of the prograde garnet-pyroxene assemblages. Pyrrhotite, arsenopyrite, pyrite, chalcopyrite, bornite, sphalerite and bismuth and tellurium minerals are the commonest opaques in both gold and PME skarns. Less commonly, cobaltite, scheelite, molybdenite and galena are also present.
There is a highly variable metallic trace element assemblage in gold and PME skarns which may be enriched in arsenic, bismuth, tellurium, copper, zinc, cobalt, tungsten, antimony, lead, nickel and molybdenum.
Many gold skarn deposits show systematic geochemical variations throughout the envelope; at the Nickel Plate deposit, for example, the copper grades increase toward the endoskarn dioritic intrusions. The presence of lead, silver, nickel or (most commonly) bismuth tellurides, together with native bismuth and other bismuth minerals, is highly characteristic of precious metal enrichment in skarn.
Tellurides recorded in gold skarns include hedleyite (bismuth-tellurium), tetradymite (telluric bismuth), altaite (lead telluride) and hessite (silver telluride), while other characteristic minerals include bismuthinite (bismuth trisulphide), wittichninite, breithauptite (nickel antimodide), lollingite (iron arsenide), maldonite (gold, bismuth alloy), cobaltite (cobalt, arsenic sulphide), electrum (gold, silver alloy), gersdorffite (nickel sulfarsenide), vesuvianite, axinite and scapolite.
Since gold is a highly valued metal that generally only reaches concentrations of a few parts per million, gold skarns cannot always be classified adequately using the criteria used to classify iron and base metal skarns. A classification based on the highest value contained metal works well with these deposits. However, fluctuations in the relative prices of gold and base metals can change the status of some deposits such as the Phoenix from a PME base metal skarn to a gold skarn. Plotting copper-to-silver versus copper-to-gold ratios can also be used to broadly differentiate gold, copper and iron skarns. Gold skarns generally have copper-to-gold and copper-to-silver ratios of less than 1,000. By contrast, copper skarns have copper-to-gold ratios between 2,000 and 25,000 and copper-to-silver ratios ranging from 500 to 2,500, while iron skarns are characterized by copper-to-gold ratios ranging from 20,000 to 160,000, and copper-to-silver ratios of 2,500 to 5,000.
Conclusions
Gold and PME skarns are commonly found in the same oceanic island arc and back arc environments as iron and copper skarns, and they are genetically associated with dioritic calcalkaline intrusions derived from oceanic crust. Precious metal enrichment is often accompanied by the presence of arsenic, bismuth and tellurium minerals; some gold skarns are enriched in cobalt and are also associated with late scapolite alteration. Many gold skarns appear to form in relatively reduced conditions, and the gold is probably derived from, and largely carried in, magmatic fluids. The presence of scapolite and chlorine-rich amphiboles in some deposits suggests that chlorine-rich fluids are important for the transportation of gold in the skarn environment.
British Columbia has good potential for the discovery of economic gold and PME skarn deposits. The most prospective regions are where calcalkaline dioritic plutons intrude calcium carbonate-bearing island arc and back arc basin assemblages of the Quesnellia and Stikinia terranes. By contrast, dolomitic sequences, arcs formed along sialic continental margins or within continental crust, or the deeply eroded root zones of magmatic arcs are less attractive environments for gold skarns. The presence of slump carbonate deposits (olistostromes), indicating tectonic instability at the time of sedimentation, may be an additional regional indicator of high gold skarn potential.
The close association between gold skarns and arc sequences in Quesnellia and Stikinia of British Columbia suggests that equivalent packages further south in the Cordillera of the U.S. also have gold skarn potential. Prospective areas include rocks of the Wallowa, Olds Ferry, Rattlesnake Creek, Jackson and Foothills terranes, and of the Inyo Mountains and Mojave Desert areas (Figure 4).
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