Phenomenal rocks at Pebble

Whether the Northern Dynasty Minerals Ltd. Pebble project in Southwest Alaska is economic to develop is a question for others, but Pebble field geologist Richard Moses is certain of one thing: The rocks at and around the Pebble prospect are the richest he has seen in his long career.

Questions of economic viability rest on a host of educated assumptions regarding factors such as market values of copper, gold and molybdenum over the life of the mine; the cost of transportation and transportation infrastructure to the remote site; and the cost of bringing vast amounts of electric power into the wilderness to serve the mine. The quality of the deposit, however, is a sure thing, Moses told Mining News Oct. 4 during a field visit.

At the company's field sample processing facility across the road from the Iliamna village airport, Moses is the first to inspect the core samples that arrive via helicopter from test drill rigs at the mine site in the hills 20 miles away.

Moses is particularly excited about the series of core samples he saw from the company's core hole 5327 that indicated rich mineralization east of the proposed Pebble mine - 800 meters of 1.45 percent copper equivalent.

"Stop and think about that, that's a half a mile - 2,613 feet - that's a huge interval," Moses told Mining News Oct. 4.

"I've been in the business 30 years and that's the biggest intercept I've ever dreamed of or drilled or ever heard of - it's phenomenal."

Looking east

Moses started on the Pebble prospect in April 2004 after four years working at the Donlin Creek gold project, a 27,000 acre NovaGold Resources Inc. property located in the historic Kuskokwim Gold Belt of Southwestern Alaska. In 2004, Pebble test drill rigs were busy with a program designed to upgrade a substantial portion of the Pebble deposit's mineral resources to measured and indicated classifications, Moses said.

"We had enough drills on site to do that within the timeframe that we had to do it in, so we couldn't do other stuff until that was over," he said.

Moses was anxious to test the company's hunch that there was substantial additional mineralization to the east of the Pebble deposit, he said.

In 2005, he got his chance to find out.

"We've drilled a total of 60,000 feet this year versus 135,000 feet last year, and of that 60,000 a little over a half of it has been in the east zone," Moses said.

"Most of our hole depths over (in the east zone) have been 4,000 feet.

In late 2004 some drilling was done in the east zone, but the depth was limited by the equipment on hand.

Over the winter of 2004-2005, Northern Dynasty lined up drill rigs that could go beyond the 2,200 feet the crew was limited to in 2004, Moses said.

"We drill two sizes of core, the bigger diameter - which is called H - to 2,200 feet," Moses said, adding that the H core is 2 7/8 inches in diameter, while the N core which can reach 4,000 feet is 2 3/8 inches in diameter.

The core samples are sawed in half at Northern Dynasty's Iliamna core sample facility.

One half is kept in storage at Iliamna in 10-foot intervals.

The other half is bagged and shipped to the ALS Chemex prep lab in Fairbanks, and then on to the ALS Canada Ltd. assay lab in Vancouver, British Columbia for analysis, Moses said.

Rich rocks

Based on current prices, the pay rock Northern Dynasty is finding at Pebble is worth about $20 per ton, however based on the long term prices the company is estimating over the life of the mine the rock is worth about $10 per ton, Moses said.

The rocks are rich, but the mine is a costly proposition and the cost of meeting the logistics of working carefully in the remote environment is no exception, Moses said.

"We have to do everything by helicopter which makes our exploration more difficult and expensive," he said.

"Everything's done by helicopter and by shovel and rake; reclamation is all done by shovel and rake." The company is realizing promising results from the exploration, and as the crew moves east, the results are intriguing, Moses said.

"The nature of the deposit makes it really exciting to work on," he said.

"As a geologist you can work a lot of years and you don't see anything even approaching it, so it's very exciting in that aspect.

"Our published grade for the main deposit - depending on what cutoff you have - ranges anywhere from I think 0.55 to 0.8," he said.

"My estimate is the east zone will probably double that grade - double if not more - that's a ballpark on my part, it's not scientific, it's not published." Moses said the company's present drills cannot go any deeper, but test holes have been bottoming at more than 1 percent copper equivalent.

"Sooner or later we're going to have to get a bigger drill out here so we can go deeper because we're open at depth," he said.

"We know it extends at least 3,500 feet below the surface, but we don't know how much farther." While the original Pebble find lies near the surface, the east zone is covered with 1,500 feet or more of barren un-mineralized rock, the company said.

Just because the overburden isn't pay rock, it doesn't mean it has no value, Moses said, adding that the barren rock contains calcite, which can be used to reduce acidity for mining and in tailings.

"It does have some value - not monetary value - but value to build tailings dams and stuff like that, so we log that core much more detailed than we normally would, recording pyrite content and calcium carbonate content," he said.