By Tim Bradner
Alaska Journal of Commerce

If growing estimates hold true, the name Pebble may become more and more of a misnomer for a mine project in Southwest Alaska.

Northern Dynasty Minerals Ltd. released new mineral resource estimates on its Pebble copper-gold project Feb. 20 that increase the inferred resource in an eastern ore zone by 90 percent.

The new report, based on the drilling of 132 core holes in the eastern deposit, estimates that at a grade cutoff of 0.6 percent “copper-equivalent,” Pebble East could contain 3.4 billion tons of ore with 42.6 billion pounds of copper, 39.6 million ounces of gold and 2.7 billion pounds of molybdenum.

Prior to the new drilling results, Northern Dynasty had estimated that Pebble East contained 1.8 million tons of ore at the 0.6 percent grade cutoff.

In addition to the Pebble East resources, Northern Dynasty has discovered significant minerals in the nearby Pebble West deposit. Pebble West has about 4.1 million tons of ore identified at a 0.3 percent cutoff and about 1.3 million tons identified at a 0.6 percent cutoff grade.

A grade cut-off of 0.6 percent copper-equivalent for Pebble East and 0.3 percent for Pebble West means that the estimates do not include minerals in ores at grades lower than 0.6 or 0.3 percent. The lower cutoff grade is used for Pebble West because the ore body is at shallow depths and would be mined by low-cost open-pit mining methods, which means ores of lower grade can be recovered. The term copper-equivalent combines the value of the copper, gold and molybdenum as a reference of the value of an ore containing several metals.

“Pebble is the largest known copper and gold resource in North America, and is poised to become the largest known copper-gold porphyry deposit in the world, pending the outcome of the 2007 drilling program,” stated a release issued from Truth About Pebble, an Alaska citizens group supporting the mine project.

Dick Cattanach, co-chair of the approximately 300-member group, said Truth About Pebble will commission an economic impact study of a mine at Pebble to illustrate its effect on the Alaska economy.

Continued exploration core drilling at Pebble is expected to add to estimated resources at the deposit. The company restarted its 2007 drilling in February with four rigs and said it will have nine rigs working by mid-year.

In its Feb. 20 press release, Northern Dynasty said the company's 2006 drilling program expanded the north-south length of the eastern ore zone to more than 7,000 feet, with grades of mineralization exceeding 1 percent copper equivalent over much of the distance. The mineralization is about 1,000 feet below surface, and Northern Dynasty is investigating an underground mine at Pebble East. The Pebble West deposit, in contract, is virtually at the surface and would be mined by an open-pit method.

The company said the northern and southern boundaries of the Pebble East ore body have not yet been determined. Northern Dynasty also indicated that the presence of bornite mineralization in core samples in the northern and southern sections of the ore body indicate that more higher-grade ore may be present in possible extensions to the north and south.

Tim Bradner can be reached at tim.bradner@alaskajournal.com.