Whitehorse Copper Belt

The Whitehorse Copper Belt is a ~30 km belt of copper-gold ore deposits and mineral occurrences that showcases fascinating skarn mineralogy.

Overview

The year was 1897. The news of gold discoveries in the Klondike had led to a stream of prospective miners traveling north to Dawson City. The route for many of those travelers took them through the new town of Whitehorse, the transportation hub from which they could board a steamwheeler and continue north. Some of these miners prospected in the Whitehorse area, and copper discoveries were soon made. On July 6, 1898, Jack McIntyre staked the first claim west of Whitehorse, known as Copper King. By 1899, the district, now known as the Whitehorse Copper Belt, had been extensively prospected and most eventual deposits had been staked. When the first ore was shipped from the Copper King claim in 1900, an 82-year-long history of copper mining within the belt began, a history which profoundly impacted the evolution of the City of Whitehorse and the territory as whole.

This field trip explores four areas within the Whitehorse Copper Belt: Arctic Chief, Whitehorse Copper, Best Chance, and the McIntyre Creek drainage. Each area has different components focusing on both geology and mining history. Before you begin exploring, we encourage you to scroll down to learn about copper, copper minerals, and the geologic processes that formed the skarn-type deposits in the belt. When you are ready, jump into the field trip!

There are 28 historic mines and copper showings in the Whitehorse Copper Belt, many of which we do not explore in this experience, but our goal is to give you the background you need to do some exploring on your own. At the end of this section is an Additional Resources section with links to help you do this!

This field trip falls within the traditional territories of the Kwanlin Dun First Nation and the Ta'an Kwach'an Council.

Geologic History

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Late Triassic Limestone Formation

In the latest Triassic period (~200 million years ago), a volcanic arc, or chain of volcanoes, extended off the west coast of North America. This chain of volcanoes formed from the subduction of a tectonic plate under an adjacent tectonic plate, similar to the setting of modern-day Japan. On the edge of this volcanic arc was a shallow marine environment with an abundance of life. Evidence of this environment can be found in limestone, a carbonate rock built from the remains of skeletal and shell material, coral, and other biogenic materials. In the Whitehorse area, this limestone is known as the Hancock member of the Aksala Formation. These rocks formed at a latitude far to the south of where Whitehorse lies today, ~30° N latitude (the current latitude of northern Mexico).

Fossiliferous limestone with mollusc and sponge fragments. Click on the numbered circles to learn about identified fossils. Note: for best results, view in full-screen by hitting the double-arrow button in the bottom right. Hit the escape (Esc) key to exit full-screen.

Jurassic Clastic Sedimentary Rock Formation

As time progressed into the Jurassic period (~200 - 145 million years ago), the volcanic arc became inactive and additional sediment layers including sand, silt, and clay, were deposited into a narrow marine basin called the Whitehorse Trough. These sediment layers were buried, compacted, cemented together, and became lithified into the clastic sedimentary rocks sandstone, siltstone, and shale. These sedimentary rocks formed on top of the original carbonate rocks (limestone) that formed in the late Triassic.

Basin Infilling and Closure

Time progressed from the Jurassic into the early Cretaceous Period (~145 - 115 million years ago), with continued deposition of sediments in the Whitehorse Trough. These sediments were derived from erosion associated with rapid uplift and mountain-building in the geologic terranes on either side of the basin. As these layers of sediment continue to be deposited, they underwent burial, compaction, cementation, and lithification into more layers of clastic sedimentary rocks. Eventually, the subduction between the tectonic plates ceased, and the Whitehorse Trough basin closed. These sedimentary rocks became part of the North American continent and are now a record of the presence of this shallow marine basin.

Cretaceous Volcanism

In the end of the early Cretaceous period (~115 - 100 million years ago), subduction of oceanic crust under the North American continent created a new volcanic arc. Volcanic rocks erupted onto the surface, fed by large subsurface magma chambers located kilometers below. Much of this magma never reached the surface; instead, it slowly crystallized to form intrusive igneous rocks like granites and granodiorites. In the Whitehorse area, the collection of intrusive rocks that crystallized in the subsurface over a ~15 million year period is known as the Whitehorse batholith.

This a fine-grained (or "aphanitic") andesite that is representative of igneous rocks that crystallize at the surface or the shallow subsurface, where temperatures are low and cooling rates are fast. The larger crystals (phenocrysts) seen in this hand sample are plagioclase, whereas the remainder of the crystals in this sample are not visible without the aid of magnification.

For comparison, this is a coarse-grained (or "phaneritic") granodiorite, an intrusive igneous rock that crystallized slowly many kilometres below the surface. Compared to the andesite hand sample above, the individual mineral crystals in this sample are visible without magnification.

Skarn Formation

The magma that formed the granite and granodiorite contained water, and as the magma came closer to the surface it depressurized and minerals began to crystallize. This resulted in the water separating out of the magma as a hydrothermal fluid. This fluid was hot (between 450-600°C), slightly acidic, and carried in solution many economic metals that weren't easily incorporated into the minerals that were crystallizing from the magma.

This metal-bearing hydrothermal fluid moved upwards and outwards from the pluton where it came in contact with the limestone that formed nearly 85 million years before. The interaction with the limestone decreased the acidity of the hydrothermal fluid, and reduced its ability to carry metals like copper, iron, molybdenum, and gold. As it moved away from the magma body, the temperature of the fluid decreased, which also reduced the amount of metals that could be dissolved in the fluid (their "solubility").

Upon contact, the hot acidic fluid partially dissolved the original limestone. The combination of heat, elements from the limestone (calcium, magnesium, carbon), and elements from the fluid (iron, silicon, aluminum, copper, etc.), led to the creation of an entirely new rock type called skarn. The skarn formation was concentrated at the boundary between the limestone and the crystallizing magma body.

Move on to the next section to learn more about skarn and the minerals within it.

What is Skarn?

The word skarn comes from an old Swedish mining term. It was originally used to describe a type of silicate gangue, or waste rock, associated with iron-ore bearing sulfide deposits formed in limestone. Skarns are typically characterized by the presence of calc-silicate minerals. These minerals contain the elements calcium (Ca) and silicon (Si), but are also typically enriched in iron (Fe), magnesium (Mg), manganese (Mn) and aluminum (Al). The specific type of skarn that forms, and the minerals within it, depend on the chemistry of the magmatic fluid, but also on the composition of the original carbonate rock. The two most common carbonate rocks are limestone (composed mainly of calcite, CaCO₃) or dolostone (more magnesium-rich, composed in part by the mineral dolomite, CaMg(CO₃)₂).

Check out the 3D models, images and accompanying descriptions below to learn more about skarn mineralization found in the Whitehorse Copper Belt.

Calcic (or calc-silicate) skarn in the Whitehorse Copper Belt typically consists of two main minerals: green diopside (CaMgSi₂O₆), a type of pyroxene; and red andradite garnet (Ca₃Fe₂Si₃O₁₂). Most of this skarn hand sample is the mineral garnet. Other common minerals include:

epidote
- Ca(Al₂Fe³⁺)(Si₂O₇)(SiO₄)O(OH)
actinolite
- Ca₂(Mg_4.5-2.5Fe_0.5-2.5)Si₈O₂₂OH₂
wollastonite
- CaSiO₃
tremolite
- CaMgSi₈O₂(OH)₂
calcite
- CaCO₃

With the exception of calcite, which is the predominant mineral in the original limestone, all of these minerals are calc-silicate minerals - they all have calcium (Ca) and silicon (Si) in their mineral formulas.

The second type of skarn in the Whitehorse Copper Belt is known as iron (or oxide) skarn. It is composed primarily of the minerals magnetite (Fe₃O₄) and lizardite (Mg₃(Si₂O₅)(OH)₄), a member of the serpentine group of minerals. True to its name the magnetite is strongly magnetic!

While both calc-silicate and oxide skarns host economic mineralization, the oxide skarn typically has higher grades (measured in weight percent, or wt. %) of both copper and gold than does the calc-silicate skarn.

The main copper-bearing ore mineral in the Whitehorse Copper Belt is chalcopyrite (CuFeS₂). It is ~ 35 wt. % copper, and its bright yellow colour is diagnostic. Chalcopyrite will often tarnish to a slightly greenish hue, which native gold will not.

Another important copper sulfide mineral in the Whitehorse Copper Belt is bornite (Cu₅FeS₄). Bornite has a distinct dark purple colour in hand sample, and it has ~63 wt. % copper. That is nearly twice the percentage of copper than what is found in chalcopyrite! The presence of bornite in the Whitehorse Copper Belt deposits greatly increased the copper grades of the mineral concentrates sent to market.

The sulfide mineral pyrite (FeS₂), or "fools gold", is common in the Whitehorse Copper Belt. While pyrite does contain iron (Fe), it is not an economic source of the metal thus it is commonly treated as a waste mineral, known as gangue.

The bright blue mineral azurite (Cu₃(CO₃)₂(OH)₂) is a hydrated copper carbonate mineral that can be described as a secondary copper mineral in the Whitehorse Copper Belt. Primary copper sulfide minerals like chalcopyrite and bornite can be chemically weathered into secondary copper minerals upon interaction with groundwater.

The bright green mineral malachite (Cu₂(CO₃)(OH)₂) is closely related to azurite and is also a secondary copper mineral. You will commonly see it on exposed surfaces and joint planes where there has been enhanced fluid flow.

The secondary copper mineral chrysocolla ((Cu,Al)₂H₂Si₂O₅(OH)₄•nH₂O) can be found across the Whitehorse Copper Belt. This hydrated sheet silicate mineral has a diagnostic blueish-green colour and smooth, almost polished surfaces. It often appears as glassy botryoidal (rounded masses), or bubbly crusts on surfaces.

The presence of chrysocolla varies greatly between different deposits in the Whitehorse Copper Belt. In some, like the Keewenaw deposit at the southern end of the belt, chrysocolla made up a considerable portion of the Cu-oxide mineralization.

The oxide mineral magnetite (Fe₃O₄) is the major mineral constituent in iron skarns. While it was treated as a gangue (waste) mineral during historic mining in the Whitehorse Copper Belt, magnetite has a wide range of industrial uses, from steel production to pigmentation for paints and makeup. The magnetite that was originally discarded as waste in the Whitehorse Copper Belt tailings complex may be a valuable resource for potential extraction in the future.

Interactive Geology Map

Click and drag the sliding tab (the white circle with two black arrows) to view a map of the Whitehorse Copper Belt area, with bedrock geology (left) and satellite imagery (right). Major deposits located along the Whitehorse Copper Belt are indicated with black markers; click on a marker to identify the deposit.

The blue, purple, and green units on the geological map are sedimentary rocks that are all members of the Upper Triassic Aksala Formation, and the red map unit indicates intrusive igneous rocks of the Cretaceous Whitehorse batholith.

The dashed and solid lines represent strike-slip faults that locally offset units within the Aksala Formation. The formation of these faults appears to post-date the intrusion of the Whitehorse batholith, as faults locally offset parts of the batholith margins.

Note: For simplicity, this map hides rock units not within the Aksala Formation or Whitehorse batholith. Check out the Yukon bedrock geology map on GeoYukon for a more complete picture!

Virtual Experience Field Sites

Now that you know what skarn is, how it forms, and the geologic history of the area, it's time to see some outcrops! Click on one of the virtual field trips on the left side of the page to see a brief description of that field trip. Then, click to navigate to your chosen trip and a new window will open allowing you to start your virtual experience.

Best Chance

The Best Chance deposit has great exposure of all the major rock types involved in the formation of the Whitehorse Copper Belt. This is an ideal place to begin learning more about skarn mineralization!

McIntyre Creek

The McIntyre Creek drainage is where copper was first discovered in the area, and where the first underground mines were developed. In this field trip, you will also learn about the region's glacial history and its impact on local landscapes.

Arctic Chief

The Arctic Chief site includes two open pits with lots of interesting geology to explore: faults, folds, dykes, and skarn of course!