Banded iron formation-hosted gold deposits are attractive targets thanks to their potential for large, district-scale gold mining.
Although large low-sulfidation epithermal gold and sediment-hosted Carlin-type deposits get the most attention from today’s gold mining news, banded iron formation-hosted gold deposits rank highly amongst the world’s most attractive exploration targets.
Across the world, gold exploration companies are searching for this deposit type to unlock the next significant gold deposit.
What are banded iron formation-hosted gold deposits?
Banded iron formations (BIFs) are found in cratonic greenstone belts, and make approximately 60 percent of global iron reserves. The name suggests this deposit type is mainly iron, but BIFs are known to host other minerals, including gold.
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While not as common as other greenstone-hosted orogenic gold deposits, gold-bearing BIFs occur in Archean to Proterozoic-age cratonic greenstone belts all around the world. The gold is usually found in cross-cutting quartz veins and structures in complex terranes where the host strata has been folded and deformed. There is some debate amongst the geological community as to whether the gold mineralization is a part of the original BIF deposits or if it was introduced via another source; however, research suggests that black shales typically found in these deposits might have played a role.
Where in the world can we find BIF-hosted gold deposits?
The best examples of BIF-hosted gold deposits are found in Canada, the United States, Western Australia, Brazil, Zimbabwe and India.
Located in the eastern margin of the Wyoming Archean craton in South Dakota, the Homestake mine was North America’s largest and deepest gold mine prior to its closure in 2002. During its 25 year mine life, Homestake reportedly produced 43.9 million ounces of gold and was the second largest gold producer in the United States after the Carlin district in Nevada.
One of Canada’s largest producing gold mines is a BIF-hosted gold deposit located in the province of Ontario. Newmont Goldcorp’s (TSX:NGT,NYSE:NEM) 4,000 tonne per day operation at Musselwhite mine went into commercial production in 1997 and since then has produced more than 4 million ounces of gold with gold recoveries of 96 percent. Today, Musselwhite has proven and probable gold reserves of 1.85 million gold ounces, measured and indicated resources of 310,000 gold ounces as well as inferred resources of 1.17 million gold ounces.
Musselwhite is not the only BIF-hosted gold deposit in Ontario. The province is also home to several other such deposits including Detour Lake, Madsen Red Lake, Pickle Crow and Dona Lake. Much of Ontario lies in the mineral-rich Canadian Shield, an exposed portion of the North American craton, which covers much of northern and central Canada. BIF-hosted gold deposits are one of the main targets for gold mining in the Canadian Shield, as evidenced by the former 3 million ounce Lupin mine in the Northwest Territories as well as Agnico Eagle’s (NYSE:AEM,TSX:AEM) producing Meadowbank and Meliadine mines in Nunavut.
Why BIF-hosted gold deposits?
What makes BIF-hosted gold deposits such attractive exploration targets? These deposits can be remarkably large with sizeable gold production. Some estimates show the size of these deposits can range from 1 million to 5 million tonnes with grades of 6 to 17 grams per tonne (g/t) gold.
“Gold in banded iron formations make excellent exploration targets because of their scalability and potential for long mine lives,” Locke Goldsmith, Chief Geologist and Exploration Manager for Guyana Goldstrike (TSXV:GYA,OTC Pink:GYNAF,FSE:1ZT), told the Investing News Network.
The district-scale potential for BIF-hosted gold deposits is another attribute that places this deposit type on the radar of both juniors explorers and major producers. “Like VMS deposits, they are often found in clusters. This characteristic is highly attractive to major gold companies looking for new deposits that can be developed into mines with longevity,” said Goldsmith, a member of the Society of Economic Geologists and the American Institute of Mining and Metallurgical Engineers.
Who is exploring for BIF-hosted gold deposits?
In Canada, G2 Goldfields (TSXV:GTWO) has been conducting drill work on the Sandy Lake project, located approximately 180 kilometers from the Musselwhite mine. The primary target is a 60 kilometer long BIF belt that runs east to west across the property. In July 2019, Metals Creek Resources (TSXV:MEK) acquired the formerly producing Dona Lake underground mine and surrounding property from Newmont Goldcorp. Gold mineralization on the project is hosted within three BIFs with the middle iron formation hosting the Dona Lake mine, which produced 246,500 ounces of gold at a grade of 7.44 g/t from 1989 to 1994. The property has not been explored by modern methods and Metals Creek is planning airborne geophysical surveys to help identify new targets.
Like the Canadian Shield of North America, the Guyana Shield in South America has the potential to host this style of gold deposits. The Guyana Shield encompasses Guyana, Suriname, French Guyana, Venezuela and a portion of Colombia. Although it remains largely underexplored, the shield is well-known for its gold and the democratic country of Guyana’s established mining act has helped to establish a strong Canadian public mining company presence. Guyana Goldfields (TSX:GUY) and Troy Resources (ASX:TRY) each operate producing gold mines in the region. Barrick Gold (TSX:ABX) has also expressed an interest in pursuing opportunities in the Guyana Shield by investing in early-stage exploration projects.
Mineralization at Guyana Goldstrike’s Marudi gold project in Guyana’s portion of the Guyana Shield is related to the BIF-hosted gold deposits that occur in Canada and other greenstone belts around the world. According to Guyana Goldstrike’s geological team, the Marudi property has similar stratigraphy and mineralization to the Lupin, Musselwhite and Homestake mines.
The Mazoa Hill zone at Marudi has a mineral resource estimate that includes indicated resources of 259,100 gold ounces and inferred resources of 86,200 gold ounces. North of Mazoa Hill, Guyana Goldstrike recently identified and mapped several weathered outcrops of magnetic quartzite-metachert over a length of 1,100 meters. Twenty-five grab samples from four outcrops confirmed the presence of gold mineralization with assay returns of up to 1.04 parts per million gold. Trenching in the Toucan Ridge area of the property in 2018 returned assays up to 23.5 g/t. A new zone geologically distinguishable from Toucan was discovered in 2019 during trenching that included 3.6 meters grading 4.28 g/t gold and 4.2 meters grading 3.8 g/t gold.
Banded iron formation-hosted gold deposits have the potential to become large tonnage mines with extended mining lives. Due to the appeal of these deposits, gold exploration companies are targeting gold-rich greenstone belts found in cratons around the world.
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