Butte Creek Prospect, Fortymile District, Southeast Fairbanks Borough, Alaska, USA

Latitude & Longitude (WGS84):	64° 38' 36'' North , 142° 10' 33'' West
Latitude & Longitude (decimal):	64.6433333333, -142.175833333

Location: The Butte Creek prospect is at an elevation of about 3,700 feet on the ridge north of the North Fork of the Fortymile River, about 3.5 miles north of the mouth of Butte Creek. The deposit occurs as three separate lenses that cover approximately 0.5 square kilometer each and are less than 5 kilometers apart. The coordinates are the approximate center of the lenses in the NE1/4 section 20, T. 3 S., R. 28 E., of the Fairbanks Meridian. The location is accurate within one-half mile.
Geology: More than 97 separate, widely scattered occurrences of ultramafic rock were identified during reconnaissance geologic mapping of the Eagle quadrangle (Foster and Keith, 1974; Foster, 1976). These ultramafic bodies include pyroxenite, hornblende pyroxenite, pyroxene hornblendite, and hornblendite. Biotite is a common accessory mineral, but locally it may be a major constituent. Butte Creek is an ultramafic-hosted, platinum-group-element prospect that consists of three separate lenses of mixed coarse-grained biotite clinopyroxenite and lesser hornblendite and minor biotitite (a rock consisting almost entirely of biotite). Hornblende from a biotite hornblendite from the prospect gives a 40Ar/39Ar plateau age of 181 +/- 0.7 Ma (Newberry and others, 1996). The ultramafic rocks, as well as biotite-schist country rock, appear to be pendants over a small hornblende granodiorite pluton of probable Mesozoic age (Keith and others, 1987). The ultramafic lenses each cover approximately 0.5 square kilometer, and together they extend for a distance of about 5 kilometers. The biotite clinopyroxenite consists of anhedral clinopyroxene and biotite crystals that vary from 0.25 to 2 centimeters in diameter; biotite locally comprises as much as 90 percent of the rock. The pyroxenite locally contains minor primary green amphibole, and small inclusions of magnetite within clinopyroxene crystals. Coarse-grained felsic dikes cut the biotite clinopyroxenite, some of which are brecciated and included within the dikes. Hydrothermal alteration and fracturing of the ultramafic rocks is spatially associated with the felsic dikes. Hydrothermal alteration has produced actinolite, chlorite, scarce muscovite, and trace pyrite in the ultramafic rocks. The felsic dikes are also extensively altered (Keith and others, 1987). At the Butte Creek prospect, samples with anomalous platinum-group elements were found in biotite clinopyroxenite and in felsic dikes with inclusions of, or near contacts with biotite clinopyroxenite. Samples contain as much as 3.0 ppm platinum and 1.5 ppm palladium, with [Pt/(Pt + Pd)] ratios that average 0.7 to 0.8 (Keith and others, 1987). Platinum-group enrichment is spatially associated with the contact between biotite clinopyroxenite and felsic dikes. Platinum-group values in the biotite clinopyroxenite decrease with distance from the dikes; although still slightly anomalous, the felsic dikes are essentially barren. The low copper and nickel content of platinum-group-bearing rocks suggests that at least some of the platinum-group elements may be present as alloys (Keith and others, 1987). Electron microscope imaging was used to locate platinum-group minerals less than 4 microns in diameter. They occur along cracks in clinopyroxene and magnetite and at grain contacts with magnetite of pyrite. These minerals contain platinum and arsenic (probably as sperrylite), palladium and antimony (stadiopalladinite?), and one complex platinum-group-bearing sulfide grain. Tiny grains of pyrite, galena, and sphalerite were also found in the clinopyroxenite. Tiny grains of barite occur in the felsic dikes (Keith and others, 1987). The platinum-group element, Pt/(Pt + Pd) ratios, and platinum-group element mineralogy are consistent with an Alaska-type ultramafic-deposit model (Keith and others (1987). Significant differences from the Alaska-type model include the compositions of the biotite and amphibole in the clinopyroxenites and the lack of major magnetite and chromite. Anomalous platinum and palladium values in samples from the Butte Creek prospect are reported by Keith and Foster (1973) and Foster (1975). Additional samples were collected in 1975 from biotite clinoproxenite and associated rocks at the Butte Creek prospect to study the mineralogy and possible origin of the anomalous platinum-group elements (Keith and others, 1987). Foley and others (1989) assayed a biotite-clinopyroxenite dike on Butte Creek; it contained elevated gold, platinum, and palladium. The assays indicates that platinum is present in greater concentrations than palladium and that gold concentrations are greater than those of osmium, iridium, ruthenium, and rhodium (Foley and others, 1989; U.S. Bureau of Mines, 1995).
Workings: More than 97 separate, widely scattered, occurrences of ultramafic rock were identified during reconnaissance geologic mapping of the Eagle quadrangle (Foster and Keith, 1974; Foster, 1976). Anomalous platinum and palladium values in samples from the Butte Creek prospect were reported by Keith and Foster (1973) and Foster (1975). Additional samples were collected in 1975 from biotite clinoproxenite and associated rocks at the Butte Creek prospect to study the mineralogy and possible origin of the anomalous platinum-group elements (Keith and others, 1987). Foley and others (1989) assayed a biotite-clinopyroxenite dike on Butte Creek; it contained elevated gold, platinum, and palladium. The assays indicates that platinum is present in greater concentrations than palladium and that gold concentrations are greater than those of osmium, iridium, ruthenium, and rhodium (Foley and others, 1989; U.S. Bureau of Mines, 1995).
Age: Hornblende from a biotite hornblendite collected at the Butte Creek prospect gives a 40Ar/39Ar plateau age of 181 +/- 0.7 Ma (Newberry and others, 1996).
Alteration: Hydrothermal alteration and fracturing of the ultramafic rocks is spatially associated with the felsic dikes. Hydrothermal alteration has produced actinolite, chlorite, scarce muscovite, and trace pyrite within the ultramafic rocks. The felsic dikes are also extensively altered (Keith and others, 1987).

Commodities (Major) - Au, Pd, Pt; (Minor) - Ir, Os, Pb, Rh, Ru, Zn
Development Status: None
Deposit Model: Possibly Alaskan PGE (Cox and Singer, 1986; model 9?).

Mineral List

7 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

Jurassic - Triassic 145 - 252.17 Ma

Igneous: intrusive; Intrusive: tonalitic-granodioritic Age: Mesozoic (145 - 252.17 Ma) Description: Eastern Alaska, Yukon, Mackenzie region, Yukon-Tanana upland Comments: Orogen, magmatic arc/suite; Wilson & Hults, unpublished compilation, 2007-08 Lithology: Monzonite, quartz monzonite, granodiorite, diorite, tonalite, granitoid rocks 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]

Triassic - Mesozoic 201.3 - 252.17 Ma

Peridotite of dismembered ophiolite of the Yukon-Tanana region Age: Triassic (201.3 - 252.17 Ma) Description: Hornblendite, pyroxene hornblendite, hornblende pyroxenite and pyroxenite. Lithology: Igneous 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]

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

References