38 Porphyry

Prospect, Active

Alternative names

38 Zone

Commodities and mineralogy

Main commodities Cu
Other commodities Ag; Au; Mo
Ore minerals chalcopyrite; galena; molybdenite; pyrite; sphalerite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale IL
Quadrangle map, 1:63,360-scale D-7
Latitude 59.7981
Longitude -155.4219
Nearby scientific data Find additional scientific data near this location
Location and accuracy The center of the 38 Porphyry prospect is about 1.6 miles northwest of Sharp Mountain, near the center of section 26, T. 4 S., R. 36 W., Seward Meridian. The location is accurate.

Geologic setting

Geologic description

The 38 Porphyry prospect is a classic copper-gold-molybdenum porphyry deposit on the southeast flank of a granodiorite-tonalite batholith of Late Cretaceous (about 90 Ma) age (Detterman and Reed, 1980).
The 38 Porphyry was discovered in 2002 by Northern Dynasty Minerals Ltd. by drilling an IP geophysical anomaly covered with about 25 meters of glacial gravel (Rebagliati, 2003; Haslinger and others, 2004; Rebagliati and Payne, 2005; Gaunt and others, 2010). The main host for the mineralization is porphyritic hornblende quartz monzonite and granodiorite, with a core of granite to quartz monzonite. The deposit has classic porphyry-style alteration. There is a central zone of fracture-controlled, pervasive potassium feldspar alteration. It is succeeded outward by a large zone of secondary biotite alteration that has the best copper values and about 1.5 percent pyrite. Both the potassium feldspar and biotite alteration are overprinted by phyllic or sericitic alteration. There are several phases of mineralization which generally follows the pattern: 1) quartz with potassium feldspar halos; 2) quartz or quartz-pyrite with chalcopyrite and/or molybdenite; 3) quartz-pyrite with quartz-sericite-pyrite halos, 4) calcite; and 5) clay along fractures. Almost all the copper is in chalcopyrite; bornite and tetrahedrite are rare. The chalcopyrite occurs in three principal ways: 1) as pervasive disseminated grains associated with secondary biotite replacement of hornblende phenocrysts; 2) as coatings with pyrite along fracture surfaces; and 3) in the core or along the margins of quartz, quartz-pyrite, and quartz-pyrite-molybdenite veinlets. Native gold is rarely seen under the microscope but there seems to be a consistent ratio of 1 gram of gold per tonne to 1 percent copper. Molybdenite is widespread but sporadic; it typically is in quartz or quartz-pyrite veinlets. Sphalerite and galena are present in few calcite and quartz veinlets.
Northern Dynasty drilled 17 holes in 2002 and 2003. Some of the notable intercepts were: 37.7 meters with 0.40 percent copper, 0.45 gram of gold per tonne, and 0.02 percent molybdenum; 30.5 meters with 0.41 percent copper and 0.31 gram of gold per tonne; 149 meters with 0.42 percent copper, 0.41 gram of gold per tonne, and 0.01 percent molybdenum.
Geologic map unit (-155.424088424626, 59.7974063176136)
Mineral deposit model Porphyry copper-gold-molybdenum deposit (Cox and Singer, 1986; models 20c or 21a).
Mineral deposit model number 20c, 21a
Age of mineralization Probably Late Cretaceous, approximately 90 Ma, assuming contemporaneity with the nearby Pebble deposit (IL007).
Alteration of deposit The deposit has classic porphyry-style alteration. There is a central zone of fracture-controlled, pervasive potassium feldspar alteration. It is succeeded outward by a large zone of secondary biotite alteration that has the best copper values and about 1.5 percent pyrite. Both the potassium feldspar and biotite alteration are overprinted by phyllic or sericitic alteration.

Production and reserves

Workings or exploration The 38 Porphyry was discovered in 2002 by Northern by Northern Dynasty Minerals Ltd. by drilling an IP geophysical anomaly covered with about 25 meters of glacial gravel. Northern Dynasty drilled 17 holes in 2002 and 2003.
Indication of production None
Reserve estimates None.
Production notes None.

References

References

Bouley, B.A., St. George, P., and Wetherbee, P.K., 1995, Geology and discovery at Pebble Copper, a copper-gold porphyry system in southwest Alaska: Canadian Insitute of Mining, Metallurgy, and Petroleum, Special Volume 46, p. 422-435.
Gaunt, J.D., Rebagliati, C.M., Lang, James, Titley, Eric, Melis, Lawrence, Barratt. Derek, and Hodgson, Stephen, 2010, Technical report on the 2009 program and update on metallurgy, Pebble copper-gold-molybdenum project, Iliamna Lake area, southwestern Alaska, U.S.A: Unpublished Technical Report for Northern Dynasty Minerals Ltd., 195 p. (posted on www.sedar.com, Mar. 18, 2010).
Haslinger, R.J., Payne, J.G., Price, S. and Rebagliati, C.M., 2004, 2003 Summary report on the Pebble Porphyry gold-copper project: Unpublished report for Northern Dynasty Minerals Ltd., 119 p. (posted on www.sedar.com, Jun. 30, 2004).
Rebagliati, C.M., 2003, Summary report on the Pebble copper-gold porphyry project; Unpublished report for Northern Dynasty Minerals Ltd., 94 p. (posted on www.sedar.com, Feb. 13, 2003.)
Rebagliati, C.M., and Payne, J.G. 2005, 2004 summary report on the Pebble gold-copper project: Unpublished report for Northern Dynasty Minerals Ltd., 118 p. (posted on www.sedar.com, Apr. 4, 2005).
Rebagliati, C.M., and Payne, J.G., 2007, 2006 summary report on the Pebble gold-copper-molybdenum project: Unpublished report for Northern Dynasty Minerals Ltd., 119 p. (posted on www.sedar.com, Apr. 2, 2007).
Reporters C.C. Hawley, Hawley Resource Group, Anchorage, Alaska; D.J. Grybeck (Contractor, USGS)
Last report date 2/28/2011