Pretium explores porphyry potential at Brucejack

VANCOUVER — Warwick Board, chief geologist at Pretium Resources (TSX: PVG; NYSE: PVG), says excitement is building for exploration at the company’s Brucejack project in northwestern B.C., as his team zeroes in on discoveries within a district that compares in metal content to Grasberg, the world’s largest gold mine in Indonesia.

“One of the things I always thought about before I moved to Canada is just how spectacular the rest of the Cordillera is from Chile, Peru and to Arizona,” Board tells The Northern Miner during a phone interview. “At the time it didn’t make sense why we’re not finding the same mineral wealth here in Canada, but now I think, ‘Wow, we’re on the edge of finding another massive mineral province with big potential.’”

The company has explored its 1,030 sq. km Brucejack property with a 20,000-metre drill program this year despite already preparing its Valley of the Kings (VOK) gold deposit on the Brucejack property for commercial production in 2017.

The epithermal, vein-hosted Valley of the Kings deposit boasts proven and probable reserves of 6.9 million oz. gold and 4.6 million oz. silver, within 13.6 million tonnes of 15.7 grams gold per tonne and 11 grams silver per tonne, assuming US$1,100 per oz. gold and US$17 per oz. silver.

“In short, there’s a heck of a lot going on at Brucejack right now,” he says. “And there’s a lot of excitement for an expansion on our brownfields exploration — I’ve never worked on a project that has this much potential.”

In 2015 the company extended mineralization at the VOK deposit by 1 km along strike to the east at the new Flow Dome zone, with results including 8.7 metres of 9.94 grams gold, 2.1 metres of 2,100 grams gold and 0.5 metre of 137 grams gold.

What’s more, Board says, is finding broad, lower-grade gold mineralization in drill holes located 1 to 2 km south and southeast of VOK, with intercepts such as 277 metres of 0.88 gram gold and 15.64 grams silver, including 8.7 metres at 9.94 grams gold and 90.3 grams silver, and 562 metres at 0.53 gram gold, including 3 metres at 6.34 grams gold, 24.4 grams silver and 0.64% copper.

The company reported the metal content improves at depth, with several high-grade gold intercepts being made near the bottom of the hole.

Based on indications from drill core and surface data, Board reckons that Brucejack — and the other gold-rich veins exuding out to surface — may be the golden branches of a much larger porphyry deposit hidden beneath.

“There’s something driving Brucejack, these systems don’t exist in isolation, and there’s certainly a lot of rusted rock out there,” he says.

Brucejack is located 1 km west of Seabridge Gold’s (TSX: SEA; NYSE: SA) Kerr porphyry deposit, which, together with the Sulphurets and Mitchell orebodies, has total proven and probable reserves of 38.2 million oz. gold and 9.9 billion lb. copper, within 2.2 billion tonnes grading 0.55 gram gold and 0.2% copper.

Board says geologists once thought Brucejack might be related to the Kerr porphyry deposit, but more recent insights into the geology are quickly proving otherwise.

“All the vectors are pointing us towards the east and southeast — away from the KSM system — and that has really opened up significant potential for brownfields expansion,” he says. “The porphyry intercepts we’ve made on the property look similar to Kerr in many ways, being copper poor, but we’re probably intercepting a late-mineral phase, and we’ll only know the copper grades until we actually intersect the main mineral phase of the system.”

Board describes the metal that was flushed through the KSM-Brucejack camp as being “comparable” to Freeport-McMoRan’s (NYSE: FCX) Grasberg gold-copper deposit in Indonesia — the world-class deposit that has 39.7 million oz. gold in developed and undeveloped reserves, according to the company’s 2014 annual report.

“The average gold grade at KSM-Brucejack runs 0.65 gram per tonne, and average silver is 3.2 grams per tonne, which is similar to Grasberg,” he says. “The copper grade in the camp is much lower, but the contained metal is quite similar.”

In light of the recent exploration drill results, Board expects the mineral wealth could grow in later drill programs, although he couldn’t yet provide details on how many metres Pretium would drill this year.

“We’ve done a lot of infill drilling and the learnings we’re getting out of that are filtering through to our greenfields exploration, and learnings from greenfields are cycling back to our brownfields, so it’s a great synergy, and we have some ideas going forward,” he adds.

In recent years, Pretium has come to understand the role of Devonian-aged structures in directing porphyry and epithermal deposits into the crust, and the trail of breadcrumbs it has left for B.C. explorers to follow.

Unfortunately, most of these structures today are hidden from view, Board explains, largely overprinted by later deformation events during the Cretaceous, when the margin of western North America collided with the outboard terranes.

“During island arc development there were earthquakes going on, and that created the structural plumbing needed to promote emplacement of metal-rich magmas,” he says. “What we see in the camp is a distinct north–south trend in mineralization, and that highlights where these old structures are. With our geophysics, we’ve made inroads towards placing them in a regional sense, and we’ll be testing a lot of these hypotheses next year.”

The potential for a large porphyry system at Brucejack brings into question whether similar, high-grade, vein-hosted deposits have been overlooked at properties elsewhere in the region, such as Imperial Metals’(TSX: III; US-OTC: IPMLF) Red Chris, or Skeena Resources’ (TSXV: SKE; US-OTC: SKREF) GJ porphyry deposits.

Board says that although that may be the case, there’s a lot of variability in predicting the distribution of deposits.

“Maybe the structural regime did not allow for a plumbing system … after the porphyry was emplaced, or it blew its top and destroyed what epithermal deposit had been there with a violent eruption,” he explains.

Pretium shares have traded within a 52-week range of $5.57 to $9.05 per share, and last closed at $6.89. The company has 144.6 million shares outstanding for a $1-billion market capitalization.