North of 60 Mining News - The mining newspaper for Alaska and Canada's North

The best place to find a new mine is in the shadow of a headframe" is an adage that has served geologists well in their search for new deposits of metals that the world needs and desires. This proverb has once again proven its worth at Alaska's largest producing gold mine.

Though Sumitomo Metal Mining Co. Pogo Mine does not boast an iconic headframe towering over its underground operations, new gold-rich zones discovered in the shade of the modern mill and mine facilities may overshadow the Liese gold deposit on which the mine was built.

Dave Larimer, senior mine geologist at Pogo, outlined new discoveries made at the Interior Alaska gold mine during a presentation at the Alaska Miners Association/Society of Mining, Metallurgy and Exploration luncheon held in Anchorage Feb. 8.

"To me, Pogo is the center of the universe. I spent five years down in the trenches of Pogo trying to figure out what is going on," Larimer told a roomful of mining professionals.

This half decade of work at the Interior Alaska mine led Larimer and Pogo's geological team to Deep East, a zone of high-grade gold mineralization that lies on the opposite side of the mill from the ore body currently being mined at Pogo.

"We really want to find out the expanse of this target … It is so close to the mill," Larimer said.

Not going anywhere

Since Pogo went into production in 2006 the feedstock for the mill has come from the Liese zone - three flat-lying, parallel quartz veins that carry high-grade gold. These gold-rich shear zones are dubbed L1, L2 and L3.

In 2010, the mill processed 2,467 metric tons of this ore per day. Out of the 900,585 metric tons milled that year, 383,434 troy ounces of gold was produced at a cost of US$449 per ounce. While the Japan-based operator has yet to release final production numbers, similar gold production from Pogo is expected for 2011. Recovering more than 1,000 ounces per day, Pogo ranks as the largest gold producer in Alaska.

At the end of 2010, the Liese zone had reserves of about 5.6 million metric tons of ore averaging 0.411 ounces of gold per ton, or about 2.3 million ounces of the cherished metal. At current production rates, this would provide ore for the mill until 2017.

Larimer said the Liese zone remains open to the south, and there is potential for additional gold-enriched shear zones at depth.

"A lot of people used to think that Pogo is out of here in 2017," Larimer told the crowd. "We are not as small as originally thought, and we are not going anywhere yet."

The senior geologist's confidence that Pogo is going to be around for awhile is not grounded solely in the expansion potential of Liese, but also in the discovery of related zones nearby.

Feeder zones

The first new gold zone find in the shadow of the mill was made by Pogo geologists while collecting surface samples to the north of the current mine. These rock samples outlined a high-grade gold trend returning assays as high as 1 ounce of gold per metric ton.

"We went back and took a look at some of the condemnation drilling that was done for the mill. We had some vein intercepts," Larimer explained.

In 2010, the company decided to target this discovery with a road-based surface drill program.

"To test this out, we decided to drill off the road right below the mill, and sure enough, we hit a continuous quartz-vein body," Larimer said.

The drilling revealed three nearly vertical north-striking quartz veins in the shadow of the mill. These upright structures have a very close mineralogical correlation to the gold being mined from the Liese zone.

"We call this our North Zone vein-system," the Pogo geologist explained.

Further drilling discovered that at least one of the vertical veins intersects the L1 vein of the Liese zone. The near-identical mineralogy of the North zone veins to the ore currently being mined has led Pogo geologists to hypothesize that these upright structures are the feeders that provided a conduit to deliver the gold-rich fluids to Liese.

Larimer said the North zone veins are typically narrower than those being mined at Liese.

Drilling has outlined an inferred resource of 396,000 ounces of gold in the North zone, and mining of this new vein-system is expected to start shortly.

"I think the engineers are planning to make a little drift and mine it out in the next couple of months," Larimer said.

East Deep

The North zone was not the last or largest of the discoveries made adjacent to the mine infrastructure at Pogo.

The gold-rich veins of the Liese zone end abruptly where they come in contact with a gold-barren body of diorite to the northeast. Two years ago the Pogo exploration team set out to see if it could find additional gold mineralization on the opposite side of the intrusive body.

"That's what we started doing in 2010, we tried to do some road-based drilling just to the north of the diorite," Larimer said.

With early evidence indicating that this area northeast of the diorite hosted steeply-dipping veins reminiscent of the recently discovered North zone, the Pogo exploration team decided to target this prospective area from the underground working of the Liese zone to the south.

"So a year ago we decided to do some underground exploration drilling from what we call our east exploration drift," Larimer said.

The first hole drilled from underground failed to hit the vertical vein Pogo geologists hoped to intersect on the other side of the diorite. The second hole though, cut a remarkable 100 meters of quartz vein and a third hole cut 20 meters of quartz vein. The geological team came to realize that the veins of the East Deep target are not vertical but horizontal bodies similar to those being mined in the Liese zone.

"This is where we are sitting at the beginning of the (2011) surface season," Larimer explained.

Wanting to find out the extent of this discovery that lies adjacent to the mill, Sumitomo put three surface rigs onto expanding this target during the 2011 summer season.

What the drilling has uncovered is a zone containing quartz-vein structures with grades and thicknesses remarkably similar to those being mined in the Liese zone.

The Pogo geological team now believes that the Liese and East Deep zones are actually the same ore body that became separated when the diorite split the two zones as it intruded along the Liese Creek fault about 95 million years ago, or about 12 million years after the zones were mineralized.

Two vein systems, E0.5 and E1, have been intersected in the East Deep zone.

To date, drilling at E1 has outlined an estimated 1.5 million ounces of gold in the inferred and indicated resource categories.

Larimer hypothesizes that E0.5 surfaces at Liese, leaving only scattered gold-veined rubble as an indicator of its existence and E1 is a continuation of L1. If the senior geologist's theory holds true then deeper drilling at East Deep should intersect E2 and E3 vein systems.

Sumitomo is currently driving two exploration drifts through the some 350 meters of diorite that separate Liese from East Deep. From this deeper vantage point drills will be able to test for the hypothesized E2 and E3 veins.

The total extent of the East Deep mineralization is unknown and the deposit remains open to the west and north and at depth.

Finding the full extent and relationship of the gold-rich ore bodies lying in the shadow of the mill is the near-term focus of the Pogo exploration team.

"We have our Liese zone, our North zone and East Deep - and these have to be related together in here somewhere. This is where the bulk of our exploration in the next two years is going," Larimer said.

Author Bio

Shane Lasley, Publisher

Over his more than 16 years of covering mining and mineral exploration, Shane has become renowned for his ability to report on the sector in a way that is technically sound enough to inform industry insiders while being easy to understand by a wider audience.

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