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November 22-28, 1999
Skarn deposits, Part 1
By Derek Wilton

Skarn deposits are significant sources of tungsten, copper, iron, gold, molybdenum, lead, zinc and tin, as well as being minor sources of silver and bismuth. Industrial minerals, including wollastonite, mica, talc and graphite, are also produced from skarn deposits.

Based on the elements that are present, skarns can be divided into seven categories: iron, tungsten, copper, zinc-lead, molybdenum, gold and tin.

Mineral deposits containing skarn typically form at or near the contact between predominantly carbonate-rich rocks (limestone or dolomite) and an igneous intrusive body, or in carbonate veins along faults or fractures.

They form when hot magmatic fluids from the intrusion react with the host carbonate-rich rock, producing calcium, iron, manganese and magnesium silicates (also known as calc-silicates). This process is called metasomatism, meaning that new minerals grow in the host rocks when chemically active pore fluids are introduced into it from an external source. This new growth causes only minor textural or structural disturbances in the original rock.

The new minerals are typically coarse-grained crystals that grow over or replace the fine-grained or massive host rock. The calc-silicate minerals include garnet (calcium-rich grossularite and andradite to magnesium-rich pyrope), pyroxene (diopside to hedenbergite), epidote, olivine (forsterite to fayalite), wollastonite, amphibole (actinolite-tremolite to hornblende) and scapolite.

Garnet and pyroxene are the predominant minerals in most skarns, but not all other minerals develop in every skarn. The mineralogy of the skarn depends on factors including the composition of both the intrusive and carbonate rocks; the structural or relative permeable nature of the host rocks; and the level of intrusion.

In order for skarns to form, host rocks must be permeable so that metasomatic fluids can flow into and through them. If the host rock is impermeable to fluids, the build-up of heat from the cooling intrusion will cause thermal metamorphism, which bakes the rocks and leads to the formation of hornfels (fine-grained rock in which new minerals are created by thermal metamorphism of existing mineralogy).

Although hornfels is typically fine-grained, it can be overgrown by such coarse-grained minerals as andalusite or cordierite. Because of the differences in the permeability of the host rock, carbonates develop skarns, while impermeable calcareous shales develop hornfels.

Skarns are classified as either calcic, if they formed in a limestone, or magnesian, if they formed in a dolomitic host rock. Silicate skarns form when intrusives come into contact with calcium-rich silicate rocks, such as amphibolite. Endoskarn is skarn that develops in the intrusive, whereas exoskarn develops in the surrounding carbonate-rich rocks. Endoskarn is igneous rock-hosted; exoskarn, sedimentary rock-hosted.

Typically, skarns are zoned, their mineralogy changing with distance from the intrusion. Closer to the intrusion, garnet is more abundant than pyroxene. Farther from the intrusion, pyroxene becomes more abundant before grading into unaltered carbonate host rocks.

There are also subtle changes in the chemical compositions of the minerals, particularly in the iron-to-manganese ratio in pyroxene. Closer to the intrusive, pyroxene is iron-rich; farther away, it becomes manganese-rich. Garnets in copper and other skarns change in colour from dark-brown nearest the intrusive to yellow at greater distances.

Skarns form in three stages: First, country rock is heated by an intrusive magma, resulting in thermal metamorphism of the rock into hornfels. Dissolved metals are then deposited during a water saturation phase that follows crystallization in the magma. This process is similar to that of the boiling phases that form porphyry deposits. This vapour-fluid phase infiltrates permeable country rock, causing metasomatism, and leads to skarn formation. Metal deposition (typically as sulphides) takes place in the later, cooling stages of the metasomatic event. Finally, retrograde alteration occurs in the cooling of the system. This alteration develops through circulation of ground waters from country rock.

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

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November 29- December 5, 1999
Skarn deposits, Part 2
By Derek Wilton

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 (such as porphyry coppers) 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, whereas the Fortitude mine in Nevada contained 11 million tonnes grading 5.3 grams gold.

Copper skarns are the largest and most common type of mineralized skarn and 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). The Santa Rita deposit in New Mexico is reported to contain more than 100 million tonnes of 0.9% copper.

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. The Shizhuyan deposit in China is reported to contain 112 million tonnes of 0.33% 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. Typically, the silicate minerals in the skarn -- garnet, pyroxene, olivine, and amphibole -- are rich in manganese.

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 skarn 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.

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July 3-9, 2000
Besshi-type VMS deposits (Part I)
By Derek Wilton

These deposits often contain cobalt, which can be of economic grade, and some have economically recoverable precious metals.

Volcanogenic massive sulphide (VMS) deposits of the Besshi type are named after deposits on the southern Japanese island of Shikoku. These deposits generally contain lower base metal concentrations than other volcanogenic massive sulphides and are viewed as low-grade copper deposits; the zinc grade is typically too low to be mined economically. These deposits do, however, often contain cobalt, which can be of economic grade, and some have economically recoverable precious metals. Metal zoning is usually poorly developed in Besshi deposits.

The sulphide mineralization consists predominantly of iron sulphides (pyrite and/or pyrrhotite), with lesser chalcopyrite; sphalerite may or may not be present. Unlike other VMS styles, however, Besshi sulphides can contain a highly varied and complex mineralogy, including magnetite, arsenopyrite, galena, bornite, tetrahedrite-tennantite, cobaltite, stannite and molybdenite. Quartz, carbonate, albite, sericite, chlorite, amphibole and tourmaline can be found as gangue minerals in the deposits. Unlike the Kuroko-type VMS deposits, there is typically no barite present in Besshi deposits, though other chemical sedimentary rocks (carbonates or iron-oxide beds) may be associated.

The Besshi massive sulphide deposits are thin, stratiform and tabular. The massive sulphides may be finely or coarsely layered, or massive; the sulphide lenses are usually just several metres thick but can extend for several kilometres. The deposits are typically deformed and metamorphosed, and highly deformed ones are almost linear in shape. Deformation can obscure the deposit's feeder systems, but one deposit widely classified as a Besshi type, namely the Windy Craggy deposit in British Columbia, has a well-defined feeder/stockwork of extensively chlorite-quartz altered wall rock cut by sulphide veins. Crosscutting the massive sulphide horizons, there may be veins of recrystallized pyrite and/or chalcopyrite, opened and filled when the horizons were deformed.

A chlorite alteration halo has developed in the country rock surrounding the sulphide horizons, which may be a relic of pre-deformational alteration. Other alteration minerals that may show up in the host rocks of Besshi deposits are quartz, carbonate, pyrite, sericite and graphite.

The sulphide horizons generally occur in thick sequences of marine sedimentary rocks, ranging from black shale to arkose to greywacke. The clastic hosts themselves are generally finely laminated sedimentary rocks that resemble turbidites (sediments deposited on the ocean floor through seafloor slumping). There can also be volcanic tuffaceous interlayers. The clastic sediments are typically graphitic.

There are usually no felsic volcanic rocks present, though thin layers of basalt are often present in the sequence of sediments. The basalts have a tholeiitic composition (with pyroxenes, plagioclase feldspar and olivine, which have a high iron content relative to their sodium and potassium content).

The host rocks to these deposits can be metamorphosed such that the sedimentary rocks have become schists, quartzites, metacherts and/or pelites, and the basalts can be amphibolites.

Their host rocks, mineralogy and chemistry place Besshi deposits along a continuum between the copper-zinc massive sulphides and the sedimentary-exhalative deposits. Although Besshi deposits are modeled as VMS-types, with the massive sulphides forming from the exhalation of hydrothermal fluids on to the seafloor, there is some debate as to the mechanism of deposition. The deposits have been characterized as the products of: seafloor accumulation in the form of sulphide chimneys and the like (that is, "black smokers"); hydrothermal brine pools that formed on the seafloor after exhalation; and replacement of clastic sedimentary rocks by sulphur-bearing hydrothermal fluids that flowed upwards in a convection cell system but did not actually exhale on the seafloor.

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

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July 10-16, 2000
Besshi-type VMS deposits (Part II)

Besshi-type volcanogenic massive sulphide (VMS) deposits range in size from under a million to 300 million tonnes and grade between 0.64% and 3.3% copper. The Besshi deposits themselves contain 30 million tonnes of 2.5% copper and 0.3% zinc, plus 7 grams silver and 0.2 gram gold per tonne.

Examples of Besshi deposits in Canada include Windy Craggy, in northwestern British Columbia, which is said to contain between 210 and 320 million tonnes of ore grading 1.66% copper, 0.09% cobalt, 3.5 grams silver and 0.2 gram gold, and Soucy, in the northern Quebec's Labrador Trough, which contains 4.3 million tonnes of 1.4% copper, 1.09% zinc, 19 grams silver and 2 grams gold.

Windy Craggy is by far the world's largest known Besshi-style deposit. Some authors suggest that the Britannia mine in southern British Columbia, which contained 48 million tonnes grading 1.9% copper, 0.65% zinc, 6.86 grams silver and 0.69 gram gold, might also be a Besshi deposit.

In the Proterozoic continental margin of the Appalachian Orogenic belt of the U.S., the Ducktown massive sulphide in Tennessee contained 163 million tonnes with an estimated 1% copper and 0.9% zinc, whereas the Gossan Lead deposits in Virginia had 20 million tonnes of 0.5% copper and 1.5% zinc.

Besshi deposits occur in Proterozoic to Mesozoic rocks, while the age of most deposits is late Proterozoic to early Paleozoic (1.4 billion to 400 million years). It has been suggested that modern-day sediment-covered examples of Besshi mineralization are forming in the Guaymas Basin in the Gulf of Mexico, the Middle Valley of the Juan de Fuca Ridge (off Vancouver Island) and the Red Sea.

These deposits have low base metal grades and consequently high sulphur contents. Many deposits -- Gossan Lead, for example -- were actually mined for their sulphur and not for their base metals. The high sulphur contents can present environmental problems for the mining and refining of ore.

The deposits appear to have formed in a variety of tectonic environments, from oceanic crust to early-forming rift basins in continental plates. The host rocks are thick, terrigenous clastic sedimentary sequences with lesser tholeiitic mafic magmatism. It has been suggested that the size of the deposit reflects the volume of mafic volcanic rocks in the ore-forming system; the more mafic volcanic rocks present in the basinal stratigraphy, the more copper there may be in the exhalative sulphide body. The deposits would have relative copper, zinc, silver and cobalt enrichments that could be delineated by regional lake and stream-sediment geochemical surveys.

Geochemical definition of tholeiitic basaltic magmatism in a thick sequence of clastic sedimentary rocks, or their metamorphosed equivalents, would be a useful method of regional exploration. It is a reflection of their deformational states that Besshi deposits, unlike other VMS deposit-types, do not commonly exhibit extensive feeder alteration systems in the footwall to the massive sulphide bodies. Alteration is usually a broad enveloping chloritic halo, which may be reflected by a relative magnesium enrichment in country rock. Lithogeochemical surveys of cobalt versus nickel distributions might also be useful, given that the Besshi sulphides have a distinctive cobalt-nickel ratio greater than one.

Although the deposits are composed of metallic sulphides, the abundant graphite in the sedimentary rocks (or their metamorphic equivalents) around these deposits would make it difficult to carry out airborne electromagnetic and magnetic surveys. Ground geophysical surveys using induced polarization and electromagnetics may aid in the delineation of the massive sulphide horizons.

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

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