A quarter-century after the Olympic Dam discovery in Australia left geologists struggling to classify the giant oddity they’d found, a new and better-informed wave of exploration for Olympic Dam-style deposits is gathering momentum.
The latest swell advanced largely unrecognized until late last year, when Australian-listed Minotaur Resources announced an intersection of 107 metres grading 1.94% copper and 0.65 gram gold in hematite breccias on the Prominent Hill prospect, a large gravity anomaly centred about 125 km northwest of Olympic Dam.
The news was enough to propel the penny stock to more than A$2 on the Australian Stock Exchange and revive the market’s interest in exploration for iron-rich copper-gold deposits, which tend to be big and relatively high-grade.
This time around, geologists will not be groping in the dark. Thanks to years of research, they now know that Olympic Dam is not unique but part of a class of deposits, including Kiruna in Sweden and the Great Bear magmatic zone in Canada, that share common characteristics, such as high iron oxide content and intense alteration.
Armed with this broader perspective, today’s prospectors have a better chance of building on the success of some of their immediate predecessors, who discovered Candelaria in Chile and Ernest Henry in Australia in the last decade.
“The attraction is obvious,” says Michael Porter, editor of Hydrothermal Iron Oxide Copper-Gold & Related Deposits, a recent report published by the Australian Mineral Foundation. “The prize is both large and high-grade and has not previously been the subject of concentrated exploration activity.”
Just how large? In Brazil, the Salobo deposit weighs in at a hefty 789 million tonnes grading 0.96% copper and 0.52 gram gold per tonne, whereas Chile’s Candelaria contains 366 million tonnes at 1.08% copper and 0.26 gram gold. The almighty Olympic Dam tips the scales at 2 billion tonnes grading 1.6% copper, 0.6 gram gold and 0.04% uranium oxide, making it the sixth-largest copper deposit and the largest uranium resource in the world.
Because these multi-metallic giants share an enrichment in iron oxides, they are classified as iron-oxide/copper/gold (IOCG) deposits. They form mainly in continental arc and intracratonic tectonic settings, are related to significant magmatic events, and are post-Archean and predominantly Proterozoic in age. They tend to be localized along structures that splay off major crustal-scale faults and are associated with magnetite-rich systems characterized by sodic alteration at depth (Candelaria, for example) grading to hematite-rich systems with sericitic alteration and silicification at shallower levels (such as Olympic Dam itself).
Their high iron content makes IOCG deposits good candidates for gravity surveys, which measure differences in rock density. At the Minotaur property in Australia, for instance, several years of exploration, including magnetic surveys and drilling, highlighted areas of intense alteration and yielded some mineralization. But it was the three-dimensional modelling of gravity data using proprietary software provided by Minotaur’s partner, BHP Billiton, that pinpointed the highly mineralized system at Prominent Hill and led to the recent discovery.
But despite recent advances, geologists still lack a comprehensive model to distinguish between iron-rich but otherwise barren rock and potentially economic IOCG systems.
And the very characteristic that makes these deposits so attractive to mining companies, metal diversity, can pose metallurgical challenges. As a result, some otherwise economic deposits, including Salobo, Brazil’s largest copper deposit, may remain in the ground.
“The metallurgy is all over the map for these types of deposits,” says Laurie Curtis, president of
Intrepid is one of a handful of juniors that have teamed up with BHP Billiton to explore for IOCG deposits. Curtis credits BHP Billiton with spearheading the latest wave of exploration for this style of mineralization by throwing its financial and technical weight behind more nimble junior counterparts.
Following what a spokesman describes as an “opportunity-driven” exploration strategy for IOCG targets, the major is backing juniors such as Intrepid,
In exchange for a chance to participate in a discovery, BHP Billiton takes an equity interest in the junior and may even grant access to proprietary technology used to pinpoint IOCG deposits, including the Falcon airborne gravity system. The strategy has obviously paid off for Billiton at Prominent Hill.
Nico and Sue Dianne
Other juniors are searching outside the Billiton fold. For example,
But the Mazenod Lake deposits could soon be joined by other examples if exploration in the rest of Canada bears fruit. Currently, most of the attention is focused on Proterozoic Huronian and Keeweenawan rocks of the Circum-Superior region of northern Ontario, where Intrepid, Western Keltic,
The area is a target for IOCG deposits because these rocks are rift-related sequences associated with copper mineralization and intense alteration that lie on the margins of the Superior craton, says Michael Hailstone, district geologist in Sault Ste. Marie for the Ontario Ministry of Northern Development and Mines. The rocks contain several structurally controlled copper, polymetallic and precious metal occurrences associated with albatization, hematization and soda metasomatism in veins and breccias.
Intrepid has optioned the 64-claim Batchawana Bay property from BHP Billiton and has formed a partnership with Falconbridge covering a 20,000-sq.-km corridor in the Circum-Superior region. Amerigo is earning a 55% interest in the Island Copper property, northeast of Sault Ste. Marie, from Falconbridge, and Eastmain can earn a 100% interest in the nearby Plummer Additional project from Phelps Dodge. Last year,
Kazan project
Just south of Baker Lake, N.W.T. (where Olympic Dam’s discoverer,
In Newfoundland,
Other Canadian juniors are searching for IOCG deposits
, but on foreign soil. In Australia,
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