GEOLOGY 101 — Skarn deposits, Part 2

Different igneous intrusions tend to put different metals into a skarn. More mafic intrusions, such as diorites and gabbros from island arc settings, typically produce iron, copper or gold skarns. Intermediate-to-felsic intrusions form tungsten, zinc-lead, iron, copper and molybdenum skarns. The most evolved granitic intrusions, which are typically post-tectonic, produce tin, molybdenum and lead-zinc skarns.

Skarn deposits are part of the spectrum of hydrothermal and magmatic-style mineral deposits. They can be interpreted as the transitional phase between porphyry-style deposits and granophile-style deposits. Copper skarns are typically associated with deeper levels in porphyry-copper deposit systems.

Most economic skarn deposits occur around intrusive bodies of Mesozoic age or younger. In Canada, with the exception of the Mines Gaspe copper skarn in Quebec and small iron skarns in Ontario, economic skarn deposits are exclusive to British Columbia and the Yukon.

Gold skarns occur in pyroxene-rich portions of calcic skarns and are characteristically part of a porphyry system surrounding an intermediate intrusive rock, such as diorite or granodiorite. They have low metal-to-gold ratios, as well as enriched arsenic, bismuth, tellurium and silver.

Gold tends to be emplaced at distance from the intrusion, in the company of sulphides, such as arsenopyrite and pyrrhotite. Throughout the world, gold skarns are said to contain an average of 4.6 million tonnes grading 10.6 grams gold per tonne. For example, the Hedley deposit in British Columbia is reported to have contained 8.4 million tonnes at 7.3 grams gold.

Copper skarns are linked to porphyry copper intrusive systems. Economic mineralization develops close to the intrusion and consists primarily of iron-rich garnet. The main ore mineral is chalcopyrite, and the skarn will usually be zoned, with pyrite and chalcopyrite grading outward to a more chalcopyrite-rich fringe.

Examples of such deposits in Canada include Mines Gaspe, with 67 million tonnes grading 1.45% copper, and Copper Mountain in British Columbia, with 216 million tonnes of 0.4% copper (including porphyry mineralization).

Tungsten skarns tend to form around coarse-grained intrusions, typically quartz monzonites with associated pegmatites. The association with deeper-seated intrusive rocks suggests that these skarns form at a higher temperature than other types of skarn. The ore mineral is scheelite, which is associated with pyrite, pyrrhotite, chalcopyrite and molybdenite in exoskarn — that is, in skarn formed in the bedded rock outside the intrusion itself. The best Canadian examples are the Mactung deposit in the Yukon, with 32 million tonnes grading 0.92% tungsten trioxide, and the Cantung deposit in the Northwest Territories, with 9 million tonnes of 1.42% tungsten trioxide.

Lead-zinc mineralization in skarns is generally distant from the intrusion and develops along lithological contacts or fault-fractures in the calcareous rocks in zones of greater permeability. Silver is a common component of the ores.

Canadian examples include Sa Dena Hes in the Yukon, with 4.9 million tonnes of 12.7% zinc, 4% lead and 6 oz. silver per tonne, and Bluebell in British Columbia, with 4.8 million tonnes grading 6.3% zinc, 5.2% lead and 45 grams silver. Leadville in Colorado contained 23.8 million tonnes grading 3% zinc, 4.2% lead and 320 grams silver.

Iron skarns are the largest of all skarns, and the ore mineral is magnetite. Intrusions rich in iron tend to form calcic iron skarns; those lower in iron tend to form magnesian iron skarns with magnesium-rich silicate minerals. Garnet and pyroxene are common in these types of skarn, which, in many cases, will be composed of magnetite with lesser amounts of silicate. The Tasu deposit in British Columbia produced 21 million tonnes grading 40% iron, whereas the Marmora deposit in Ontario produced 1.1 million tonnes of 66% iron. The Sarbai deposit of Siberia is reported to contain 725 million tonnes grading 45.6% iron.

Exploration for skarns should begin in carbonate rocks intruded by water-bearing magmas. Regional geochemical surveys are also useful. Since the skarn zonation halo can be considerably larger than the metallic ore deposit, mapping of the zonation can be used to focus exploration. The high magnetite or pyrrhotite content of some skarns, particularly iron skarns, makes magnetic surveys useful in identifying and outlining these deposits, and disseminated sulphides can often be detected using induced polarization.

— The author is a professor of geology at Memorial University in St. John’s, Nfld.