Koyana

Prospect, Active?

Commodities and mineralogy

Main commodities Au
Other commodities As
Ore minerals arsenopyrite; gold; marcasite; pyrite; pyrrhotite
Gangue minerals carbonates; quartz

Geographic location

Quadrangle map, 1:250,000-scale SO
Quadrangle map, 1:63,360-scale C-4
Latitude 64.5717
Longitude -163.6859
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Koyana prospect is an area about 1,100 meters long by 300 meters wide. It was originally defined by anomalous arsenic and gold in soil samples and 10 scattered holes were drilled in it. The center of the area is about 0.5 mile west-northwest of the mouth of Koyana Creek and 0.5 mile south of the center of section 33, T.10 S., R. 25 W., of the Kateel River Meridian. The location is accurate to within 500 feet.

Geologic setting

Geologic description

The Koyana prospect was first identified by BHP-Utah in 1986 as an area anomalous in arsenic and gold in soils. Subsequently they drilled 10 holes in the area before they dropped the property in 1990 (Stevens, 2010). The Koyana prospect is at the east end of an arcuate band of Paleozoic schist about three miles long and up to a half mile wide that extends from the mouth of Koyana Creek to the mouth of Daniel's Creek (Herreid, 1965; Mulligan, 1971; Till and others, 1986). This schist unit hosts most of the lode deposits in the Bluff area, including the Saddle (SO175), Daniel's Creek (SO006), and Idaho (SO135) prospects. The schist is bordered to the north and south by thick, extensive marble; the schist-marble contacts are probably thrust faults.
There is little outcrop in the Koyana area and most of the information about it comes from 10 BHP-Utah drill holes. As described by Ford (1993) and Ford and Snee (1996), the mineralization at the Koyana prospect is localized in extensional joints that strike easterly and dip moderately to the south in quartz-muscovite schist. The veins are discontinuous and commonly less than 3 inches thick. Gold grades are irregularly distributed; vein intersections up to 3.3 feet across contain up to 1.8 ounces of gold per ton. The quartz veins frequently contain a few percent of ore minerals, mainly arsenopyrite, but marcasite, pyrite, and pyrrhotite occur in small amounts. The veins are commonly bordered by altered zones with plagioclase, chlorite, carbonate, white mica, biotite, titanite, and tourmaline. A sample of white mica from a quartz vein gave an Ar/Ar plateau date of 109.1 +/- 0.2 Ma and metamorphic white mica in the host schist gave Ar/Ar total gas dates of 122.6 +/- 0.4 Ma and 122.4 +/- 0.2 Ma (Ford and Snee, 1996). This indicates that the mineralization at Saddle is Late Cretaceous and probably related to regional metamorphism as are many other deposits on the Seward Peninsula (Apodoca, 1994; Hudson, 1994; Ford, 1993, Ford and Snee, 1996; Goldfarb and others, 1997).
Based on their 10 drill holes, BHP-Utah concluded that the resource of the Koyana prospect was 1.81 million tonnes of material with a grade of 0.69 gram of gold per tonne, or a total resource of 1.24 million grams of gold (Heine and Kell, 1991; Stevens, 2010). However, Stevens also emphasized that the BHP-Utah 'resource' for Koyana was not 43-101 compliant.
In 2008, Millrock Resources Inc. began exploring in the area under an agreement with Bering Straits Native Corporation. As of early 2010, Millrock's work on the Koyana prospect included geologic mapping and sampling, reanalysis of some of the BHP core, and geochemical soil surveys (Stevens, 2010).
A field exploration program undertaken in 2010 by Millrock Resources Inc. extended the known areas of anomalous values into areas not previously sampled. The anomalies included gold values of up to 483 parts per billion (ppb) and a positive correlation with arsenic values. This work identified a conformable, depositional contact of graphitic schist with the carbonaceous marble on a cliff face, despite poor outcrop due to recessive weathering (Negri, 2011).
Geologic map unit (-163.688480910985, 64.5709631670063)
Mineral deposit model Low-sulfide Au-quartz vein (Cox and Singer, 1986; model 36a).
Mineral deposit model number 36a
Age of mineralization Mid-Cretaceous; white mica from similar quartz veins nearby in the Koyana Creek area gave Ar/Ar plateau dates of 109.1 +/- 0.7 Ma and 109.6 +/- 0.2 Ma (Ford and Snee, 1996).
Alteration of deposit Quartz veins are commonly bordered by altered zones with plagioclase, chlorite, carbonate, white mica, biotite, titanite, and tourmaline (Stevens, 2010).

Production and reserves

Workings or exploration The Koyana prospect was first identified by BHP-Utah in 1986 as an area anomalous in arsenic and gold in soils. Subsequently they drilled 10 holes in the area before they dropped the property in 1990. In 2008, Millrock Resources Inc. began exploring in the area under an agreement with Bering Straits Native Corporation. In early 2010, Millrock's work on the Koyana prospect included geologic mapping and sampling, reanalysis of some of the BHP core, and geochemical soil surveys (Stevens, 2010).
Indication of production None
Reserve estimates Based on their drilling, BHP-Utah concluded that the resource of the Koyana prospect was 1.81 million tonnes of material with a grade of 0.69 gram of gold per tonne, or a total resource of 1.24 million grams of gold (Heine and Kell, 1991; Stevens, 2010). However, Stevens also emphasized that the BHP-Utah 'resource' for Koyana was not 43-101 compliant.

References

References

Reporters D.J. Grybeck (Contractor, U.S. Geological Survey); V.C. Zinno (Alaska Earth Sciences, Inc.)
Last report date 3/15/2016