Purdy Mine, Fortymile District, Southeast Fairbanks Borough, Alaska, USA

Latitude & Longitude (WGS84):	64° 6' 57'' North , 141° 56' 49'' West
Latitude & Longitude (decimal):	64.1158333333, -141.946944444

Location: The Purdy lode-gold mine is located about 3 miles north-northwest of Chicken at an elevation of about 2,500 feet, on the ridge between Stonehouse Creek and Myers Fork. The mine is in section 18, T. 27 N., R. 18 E., of the Copper River Meridian. The location is accurate within 100 feet. This site is locality 50 of Eberlein and others (1977) and locality 13 of Cobb (1972 [MF-393]).
Geology: The Purdy lode-gold mine is in phyllite within an east-west-trending, fault-bounded block of upper Paleozoic greenschist-facies metasedimentary and metavolcanic rocks (Werdon and others, 2001). South of the Purdy mine, the phyllite is in high-angle fault contact with the down-dropped Chicken basin that preserves a wedge of Tertiary gabbro and sedimentary rocks. North of the Purdy mine, the phyllite is in high-angle fault contact with the Chicken pluton of Jurassic age; hornblende from this pluton gives a 40Ar/39Ar plateau age of 187.8 +/- 0.9 Ma (Layer and others, 2002). The pluton intrudes Paleozoic amphibolite-facies metamorphic rocks. About 1,500 feet northwest of the Purdy mine, the Chicken pluton has been intruded by a porphyritic intrusion of unknown age that is spatially associated with aplite dikes that contain anomalous gold (see EA119) (Szumigala and others, 2000). Prindle (1905) reported specimen-quality gold in calcite veins that occurs in shale overlying a dark, fine-grained porphyritic rock. He also reported that near the head of Chicken Creek gold occurs in thin calcite veins associated with pyrite-bearing quartz veins in black phyllite (Prindle, 1908). These reports likely refer to the Purdy lode gold mine. Foster (1969 [B 1271-G) described the Purdy deposit as a rich gold-quartz-calcite vein phyllite; it extended to a depth of 6 feet. Fred and Arthur Purdy discovered and mined out the vein(s). Extensive prospecting around the Purdy mine has exposed only a few additional thin, discontinuous calcite veinlets, which contain specks of gold, and abundant bull quartz veins (Foster, 1969 [B 1271-G]). A very high grade sample containing visible coarse-grained gold in quartz-calcite veins within phyllite from the Purdy mine is currently (2002) on display at the University of Alaska Fairbanks museum in Fairbanks, Alaska as part of their permanent collection of historic gold. The Purdy mine consists of two 20-foot-deep trenches that cover an area that extends for about 160 feet east-west by 70 feet north-south. In 1999, the Purdy trenches were mapped by Bryan A. Finseth as part of the Alaska Division of Geological and Geophysical Surveys' Fortymile mining-district geologic mapping project; the following observations are from his unpublished mapping. Two small east-west-trending high-angle faults cut the phyllite and are cut(?) by a 20- to 25-foot-wide north-northwest-trending breccia zone. No igneous rocks are exposed in the trenches. Disseminated pyrite occurs within an iron- and manganese-oxide-stained chert (hornfelsed phyllite?) layer within the phyllite. Along the north wall of the northern trench, both the phyllite and the breccia zone contain pyrite, and the breccia zone contains secondary copper minerals, including malachite and tenorite. Thin calcite coatings are present on some fracture surfaces. A sample of fault gouge with supergene copper minerals contains 1,055 ppb gold, 72.6 ppm silver, 308 ppm arsenic, 73,600 ppm copper, 253 ppm mercury, 2,850 ppm antimony, and 236 ppm zinc (Szumigala and others, 2000). A calcified fault breccia contains 225 ppb gold, 78.6 ppm silver, 134 ppm arsenic, 8 ppm bismuth, 3,100 ppm copper, 148 ppm mercury, 1,610 ppm antimony, and 626 ppm zinc. A whole-rock sample of altered hornfelsed phyllite adjacent to a gold-bearing calcite vein from the Purdy mine has a 40Ar/39Ar plateau age of 182.3 +/-0.9 Ma, probably the age of the gold deposition (Layer and others, 2002).
Workings: Fred and Arthur Purdy discovered and mined the vein. Extensive prospecting adjacent to the Purdy mine has exposed only a few additional thin, discontinuous calcite veinlets, which contain specks of gold, and abundant bull quartz veins (Foster, 1969). The deposit was mined out prior to the early 1970's.
Age: A whole rock sample of altered hornfelsed phyllite adjacent to a gold-bearing calcite vein from the Purdy mine has a 40Ar/39Ar plateau age of 182.3 +/-0.9 Ma, probably the age of the gold deposition (Layer and others, 2002).
Alteration: Hornfelsing of phyllite.

Commodities (Major) - Ag, Au, Cu; (Minor) - As, Bi, Hg, Sb, Zn
Development Status: Yes; small
Deposit Model: Possibly epithermal veins (Cox and Singer, 1986; model 25c?).

Mineral List

5 valid minerals.

Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org

Paleozoic - Cambrian-Ordovician 252.17 - 541 Ma

Keevy Peak Formation and similar rocks Age: Paleozoic (252.17 - 541 Ma) Description: Greenschist facies metasedimentary and lesser metavolcanic rocks. Metasedimentary rocks of this unit are fine- to medium-grained marble, slate, quartz mica phyllite, and minor quartzite. Paleozoic age assignment based on reported crinoid columns in marble near Napoleon Creek (Prindle, 1909). Metavolcanic rocks are mostly andesitic to rhyolitic composition with mm-scale mineral and color banding suggesting a tuffaceous origin. Minor massive greenstone interbeds included in this unit. Unit thickness is unknown, but most likely in excess of 200 m. Magnetic susceptibility of this unit varies tremendously: greenstone is typically 0.55 x 10x-3 SI; intermediate to felsic composition metavolcanic rocks are 0.50.1 x 10 SI, and metasedimentary rocks are <0.1 x 10x-3 SI. This unit contains goldcalcite veins at the Purdy mine, but no evidence for base-metal syngenetic mineralization. Absence of hornfels in this unit or skarn in carbonate-bearing portions near or adjacent to contacts with Jurassic and Triassic plutons suggest that these contacts are faults Lithology: Metamorphic Reference: Wilson, F.H., Hults, C.P., Mull, C.G, and Karl, S.M. (compilers). Geologic map of Alaska. doi: 10.3133/sim3340. U.S. Geological Survey Scientific Investigations Map 3340, pamphlet 196. [21]

Carboniferous - Devonian 298.9 - 419.2 Ma

unclassified; Metamorphic: undivided Age: Paleozoic (298.9 - 419.2 Ma) Description: Eastern Alaska, Yukon, Mackenzie region, Yukon-Tanana upland Comments: Orogen, magmatic arc/suite; Wilson & Hults, unpublished compilation, 2007-08 Lithology: Schist, gneiss, migmatite, diatexite, tectonite, mylonite, granulite Reference: J.C. Harrison, M.R. St-Onge, O.V. Petrov, S.I. Strelnikov, B.G. Lopatin, F.H. Wilson, S. Tella, D. Paul, T. Lynds, S.P. Shokalsky, C.K. Hults, S. Bergman, H.F. Jepsen, and A. Solli. Geological map of the Arctic. doi:10.4095/287868. Geological Survey of Canada Map 2159A. [2]

Data and map coding provided by Macrostrat.org, used under Creative Commons Attribution 4.0 License

References