38 Porphyry prospect, Bristol Bay Mining District, Lake and Peninsula Borough, Alaska, USAi

Regional Level Types
38 Porphyry prospect	Prospect
Bristol Bay Mining District	Mining District
Lake and Peninsula Borough	Borough
Alaska	State
USA	Country

Location: The 38 Porphyry prospect is in the upper Koktuli River drainage at an elevation of about 1,000 feet; it is about 1.5 miles NNW of the peak of Sharp Mountain. The site is in the E½SW¼ sec. 26, T4S, R36W, Seward Meridian. The location is accurate for the approximate center of the prospect.

Geology: The 38 Porphyry prospect is a porphyry copper-gold (-molybdenum?) deposit on the southeast flank of a granodiorite-tonalite batholith of Late Cretaceous (about 90 Ma) age (Detterman and Reed, 1980).

The deposit is in the south part of an approximately 90-square-kilometer (about 34.8-square-mile) area of sulfide mineralization delineated by an IP/chargeability anomaly (Northern Dynasty Minerals, 2003).

Pebble Copper (IL007) and a copper-gold skarn (IL002) are in the same anomalous area.

The 38 Porphyry deposit was discovered in 2002 and is still (2003) incompletely explored.

It is probably associated with porphyritic to equigranular granodiorite phases of a granodiorite batholith which crops out north of the 38 Porphyry deposit and intrudes Cretaceous or Jurassic flysch.

Assuming ore mineralogy similar to that at the Pebble Copper deposit (IL007), the 38 Porphyry deposit contains chalcopyrite and pyrite as the most abundant sulfides, and probably contains molybdenite.

Potassic alteration, mainly development of secondary potassium feldspar and biotite, is characteristic of the mineralized rocks in the area.

The 38 Porphyry deposit is aligned east-northeast and its north-south outcrop width is about 2,000 feet. The deposit is delineated mainly on the basis of 13 holes drilled by Northern Dynasty Minerals, Ltd., in the 2002 season.

Ten of the holes intersected porphyry copper-type mineralization.

They indicate that the deposit is at least a mile long and is open at both ends.

Two drill holes intersected substantial zones of mineralization.

Drill hole 36 intersected 524 feet of rock that contained 0.33 gram of gold per tonne and 0.32 percent copper; the intercept included 124 feet that contained 0.4 percent copper.

Drill hole 44 intersected 530 feet of mineralized rock that contained 0.25 gram of gold per tonne and 0.3 percent copper; a 110-foot interval contained 0.4 percent copper.

The intrusive hostrocks are potassic, similar to the hosts of other copper-gold porphyry deposits (Muller and Groves, 1995).

Bouley and others (1995) have shown that the rocks were both initially high in potassium and were further enriched in potassium during alteration.

A sample of the granodiorite (Bouley and others, 1995, sample 12) plots just below the shoshonite field in a K20-SiO2 diagram.

Pre-mineralization biotite pyroxenite in the area plots in the ultrapotassic shoshonite field (Bouley and others, 1995, analyses 10-11).

Workings: This deposit was discovered in 2002 by Northern Dynasty Minerals, Ltd., during a reevaluation of the Pebble Copper deposit and exploration of a major IP anomaly. Thirteen holes were drilled in 2002; results include: Intercept in total feet Au in gm/tonne, Cu in percent Drill hole 36 524 0.33 0.32 Including 124 0.45 0.40 Drill hole 44 530 0.25 0.30 Including 110 0.36 0.40

Age: Probably Late Cretaceous (approximately 90 Ma), assuming contemporaneity with the Pebble Copper deposit (IL007).

Alteration: Potassic.

Reserves: The 38 Porphyry deposit is a copper porphyry resource of more than 500 million tonnes (Northern Dynastry Minerals, 2003).

Commodities (Major) - Cu; (Minor) - Ag, Au, Mo?
Development Status: None
Deposit Model: Probably porphyry copper-gold or possibly porphyry copper-molybdenum deposit (Cu/Mo)

Other Databases

Link to USGS - Alaska:	IL004

Other Regions, Features and Areas containing this locality

References