The geology behind Ontario’s world class metal districts

VANCOUVER — Ontario has a long and lively geological past dating back 3.3 billion years, when the Earth was covered by oceans. During that time, intense volcanism formed the Earth’s crust, giving way to a barren and lifeless landscape blasted by radiation from the sun.

Three billion years ago, plate tectonics moved thin rafts of rocks at a rate much faster than they do today. Over time, the volcanic islands butted together into one large block of land, with most of the action starting around the Red Lake area, a prolific gold-mining camp in northwest Ontario.

As the volcanic terranes collided, balls of magma were injected into the volcanic pile, which cooled into enormous granitic batholiths. During the deformation and magmatism, the rocks were baked and transformed into what’s known as “greenstone belts,” which are excellent hosts for orogenic, high-grade gold deposits, along with volcanogenic massive sulphides (VMS), magmatic nickel-copper, platinum group metals (PGMs) and chromium deposits.

Tectonic models for typical orogenic gold deposits. Credit: After Groves et. al. 1998.

Today, greenstone belts such as the “Abitibi” which spans the Ontario-Quebec border, make up the southern part of a much larger craton in Canada called the “Superior.” The Abitibi is one of the largest greenstone belts in the world, with gold production exceeding both the Kalgoorlie camps in Western Australia, and the Homestake deposits in South Dakota.

Ontario’s portion of the Abitibi hosts world-class gold camps, including Timmins, Kirkland Lake, and Larder Lake. Quebec’s portion of the Abitibi includes the Val-d’Or, Malartic and Rouyn-Noranda camps. Gold production from the Abitibi tops 170 million oz., with mining dating back to 1901.

Geologic provinces and principal metal districts of Ontario. Credit: TNM.

Simplified geology and mineral occurrences of Ontario. Credit: Ontario Ministry of Northern Development and Mines.

Explorers look for gold within structural splays off regional, east-trending fault zones, such as Porcupine-Destor and Larder Lake-Cadillac. These fault zones represent deep crustal sutures between the ancient volcanic chains, and served as a pathway for several pulses of gold-rich fluids that occurred 2.9 and 2.7 billion years ago.

The hydrothermal fluids may have also interacted with a number of VMS deposits entrenched within the ancient volcanic rocks. These deposits developed around submarine volcanic arcs, where metal-rich fluids were exhaled from hydrothermal vents, or “black smokers,” and formed blankets of sulphides on the ancient seafloor.

Ontario’s most famous example of a VMS includes the giant, 2.7-billion-year-old Kidd Creek deposit in the Timmins camp, which yielded 153 million tonnes of copper-zinc ore since 1966.

Other VMS deposits, including chromite, magmatic nickel-copper and PGM deposits, are clustered within Ontario’s “Ring of Fire,” a 2.7-billion-year-old, intrusive suite in the James Bay lowlands of northern Ontario.

The second stage in Ontario’s geological story is captured in its Southern craton. The craton had its start 2.5 billion years ago during a “snowball” period that may have triggered the first appearance of oxygen in the atmosphere.

At the time, vast rains and sheets of ice scraped the tops off of Superior’s huge mountain chains — exposing the deep roots of the greenstone belts — and deposited large packages of sedimentary rocks. Some of the rocks contained uranium, which was mined for over 35 years at Elliot Lake.

More mountain building and an attempted breakup of the continent between 2.2 billion and 570 million years ago formed the rest of the craton. During the first of these attempted breakups, the silver and cobalt deposits of the Cobalt camp were formed.

Around 1.9 billion years ago, a 10 km wide meteorite smashed into the margin of the Superior and Southern cratons, shattering and melting the rocks beneath.

The impact created the world-class Sudbury nickel-copper-PGM camp, where magmatic-sulphide deposits cluster around an oval-shaped bowl measuring 27 by 60 km at surface and 30 km at depth. The deposits are hosted within a unique igneous rock called the “Sudbury Igneous Complex” (SIC), and are also found in dike-like offsets that extend for several kilometres away from the SIC.

Since 1883, the Sudbury district has produced more than 40 billion lb. copper, 44 billion lb. nickel and 62 million oz. gold, silver and PGMs, and remains the most active mining camp in the world today.

Pinned against the Southern craton is Ontario’s slice of the Grenville craton, composed mainly of gneiss (a metamorphosed granite) that finished forming 1 billion years ago after a series of mountain-building events, similar to the modern-day collision between the Indian and Asian continents.

This Himalayan-scale mountain range slowly eroded for 500 million years, before being covered by shallow seas that deposited a blanket of sandstone and limestones on the flank of North America during the Paleozoic Era.

Iron ore, apatite, molybdenum, pyrite, magnesium and uranium have all been mined from the Grenville craton over the past 150 years, including the first significant gold found in Ontario in 1866. The region is currently being explored for graphite, rare earth metals and gold, whereas production includes nepheline-syenite and marble.

The younger Paleozoic rocks underlying the northern and southern ends of the province are known for their industrial minerals such as salt and gypsum, as well as oil and gas. In fact, the first oil extracted in North America was from Oil Springs in 1858, a year before oil was found in Pennsylvania.

A sudden pulse of intense magmatic activity between 155 and 180 million years also led to a number of diamond-bearing kimberlite pipes in the northern reaches of the province, such as those seen at De Beers’ Victor mine.

Ontario is a leading producer of metals, such as platinum, nickel, cobalt, gold, copper, silver and zinc, with 22 precious and base metal mines active in the province in 2015, and one diamond mine.