Twin Peaks

Prospect, Active

Alternative names

Napoleon

Commodities and mineralogy

Main commodities Ag; Au; Pd; Pt
Ore minerals gold; hematite; pyrite
Gangue minerals carbonate; chlorite; epidote; k-feldspar; quartz; sericite

Geographic location

Quadrangle map, 1:250,000-scale EA
Quadrangle map, 1:63,360-scale A-2
Latitude 64.1698
Longitude -141.5601
Nearby scientific data Find additional scientific data near this location
Location and accuracy This prospect is about 2.2 miles east of peak 3750, just northeast of the head of Napoleon Creek. It is about 0.5 mile north of the center of section 31,T. 28 N., R. 20 E. This prospect is often combined with the nearby Main Zone, Saddle Zone, and Trench 24 sites (EA115) and with the Burnt Ridge (EA168) prospects under the name Napoleon or Napoleon Block/claims.

Geologic setting

Geologic description

The Twin Peaks prospect is about 2 miles east of ARDF site EA115. Although it is a discrete mineralized area, it is along the same regional structure and shares the same characteristics and origin. The Twin Peaks prospect, like those to the west, is in the Napoleon pluton which is roughly equidimensional and about 4 miles wide. The pluton is mainly weakly to strongly foliated hornblende quartz monzonite. Hornblende from this pluton gives a Jurassic 40Ar/39Ar plateau age of 186.5 +/- 1.3 Ma (Layer and others, 2002). The pluton contains small bodies of biotite clinopyroxenite and coarse-grained hornblende gabbro, which form inclusions, dikes, and/or marginal phases (Werdon and others, 2001). Clinopyroxenite bodies within the Napoleon pluton contain as much as 25 parts per billion (ppb) platinum and 6 ppb palladium (Werdon and others, 2000). Biotite clinopyroxenite dikes, which are compositionally and texturally similar to those in the northeastern Eagle quadrangle, give K-Ar ages of 184-185 Ma (Newberry and others, 1996). The Napoleon pluton and the inclusions/dikes of clinopyroxenite and hornblende gabbro are cut by minor, granite aplite-pegmatite dikes of unknown age. The Napoleon pluton intrudes amphibolite-facies amphibolite, quartzite, paragneiss, marble, orthogneiss, and minor schist (Werdon and others, 2001). The Napoleon pluton and surrounding country rocks are cut and offset by high-angle faults.
Airborne magnetic and resistivity surveys were flown in the Fortymile Mining district in 1998 (Alaska Division of Geological and Geophysical Surveys and others, 1999). In 1998 and 1999, Kennecott Exploration did detailed soil- and rock sampling, and airborne and ground geophysical surveys which led to the discovery of the Twin Peaks mineralization (Jones and Olson, 1999; Eden, 2010). In 1999, Kennecott drilled 16 holes on similar mineralization to the west (EA115 and EA New G1002) but dug only one trench at Twin Peaks. Encouraged by strong geochemical anomalies, Teck Resources drilled one hole at Twin Peaks in 2001. A large block of claims was staked over the property in 2007 by Millrock Resources Inc., the current owners of the property (Millrock, 2011) and they have done considerable rock and soil sampling in the area.
While not as well known as the mineralization at site EA115 about two miles to the west, the Twin Peaks mineralization is almost certainly controlled by a major east-west trending shear zone that has been traced along strike for at least 4 miles (Eden, 2010). As best exposed in the Main Zone propsect (EA115), this shear zone is at least 150 feet wide. Along this shear zone, the Napoleon pluton is cut by several discrete swarms of narrow quartz veins, including at this prospect. These veins typically strike northwest or north-northeast and dip steeply south. These veins are up to 12 inches thick, and consist of mainly of quartz +/- pyrite +/- hematite +/- epidote +/- carbonate +/- rare visible gold. Vein selvages commonly contain K-feldspar, some of which is partly replaced by sericite. Sericite-altered K-feldspar from the selvage of a gold + pyrite + quartz vein gave a Cretaceous 40Ar/39Ar isochron age of 127.8 +/- 1.6 Ma (Layer and others, 2002); this is approximately 60 Ma younger than the Napoleon pluton and is tentatively interpreted to reflect the time of mineralization. Quartz vein samples contained up to 15 ounces of gold per ton and rarely less than 0.2 ounce of gold per ton. The best intercept in the 2001 Teck drilling at Twin Peaks was 7.6 meters that contained 2.69 grams of gold per tonne.
Jones and Olson (1999) classified the prospect as a porphyry gold deposit. More recently, Eden (2010) classified it as a plutonic related gold-quartz vein deposit as defined by Lefebure and Hart (2005).
Geologic map unit (-141.562388160716, 64.1694782079582)
Mineral deposit model Plutonic related gold-quartz vein deposit (Lefebure and Hart, 2005).
Age of mineralization Sericite-altered K-feldspar from the selvage of a gold- quartz vein gave a Cretaceous 40Ar/39Ar isochron age of 127.8 +/- 1.6 Ma (Layer and others, 2002); this is tentatively interpreted to reflect the time of mineralization.
Alteration of deposit K-feldspar adjacent to quartz veins is partly altered to sericite.

Production and reserves

Workings or exploration In 1998 and 1999, Kennecott carried out detailed soil- and rock sampling, and airborne and ground geophysical surveys which led to the discovery of the Twin Peaks prospect (Jones and Olson, 1999; Eden, 2010). In 1999, Kennecott drilled 16 holes on similar mineralization to the west (EA115 and EA168) but dug only one trench at Twin Peaks. Encouraged by strong geochemical anomalies, Teck Resources drilled one hole in 2001 at Twin Peaks. A large block of claims was staked over the property in 2007 by Millrock Resources Inc., the current owners of the property (Millrock, 2011) and they did considerable rock and soil sampling in the area.
Indication of production None
Reserve estimates None.
Production notes None.

References

References

Eden, Karsten, 2010, Napoleon claim block, Forty Mile mining district, east-central Alaska, April 8, 2010: Unpublished NI43-101 report, 96 p. (posted on www.sedar.com, April 14, 2010).
Jones, S. and Olson, D., 1999, Napoleon Project Report: Kennecott Exploration Company unpublished report, 13 p.
Layer, P.W., Drake, J., and Szumigala, D.J., 2002, 40Ar/39Ar dates for mineralization and igneous and metamorphic rocks in a portion of the Fortymile mining district, Eagle quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Preliminary Interpretive Report.
Lefebure, D.V., and Hart, C., 2005, Plutonic-related Au quartz veins and veinlets, model L02, in Fonseca, A.,and Bradshaw, G., eds., Yukon mineral deposit profiles: Yukon Geological Survey Open File Report 2005-5, p.121-128.
Millrock Resources Inc., 2011, Fortymile: http://www.millrockresources.com/projects/fortymile/ (as of February 6, 2011).
Reporters M.B. Werdon (ADGGS, 2000); D.J. Grybeck (Contractor, USGS)
Last report date 2/28/2011