Saddle

Prospect, Active

Commodities and mineralogy

Main commodities As; Au
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 64.5766
Longitude -163.7203
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Saddle prospect is an area about 800 meters long by 300 meters wide. It was originally defined by anomalous arsenic and gold in soil samples; later 14 scattered holes were drilled in it. The center of the area is about 1.0 mile northeast of the abandoned settlement of Bluff. It is about 0.2 mile southwest of the center of section 32, T. 10 S., R. 25 W. The location is accurate.

Geologic setting

Geologic description

The Saddle prospect was first identified by BHP-Utah in 1995 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. (2010) began exploring in the area under an agreement with Bering Straits Native Corporation. Millrock's work on the Saddle prospect included geologic mapping and sampling, reanalysis of some of the BHP core, and geochemical soil surveys.
The Saddle prospect is near the center of an arcuate band of Paleozoic schist about three miles long and up to a half mile wide that extends from the mouth of Koryana Creek to the mouth of Daniels Creek (Herreid, 1965; Mulligan, 1971; Till and others, 1986; Stevens, 2010)) . This schist unit hosts most of the lode deposits in the Bluff area, including the Koryana (SO176), Daniels Creek (SO177), 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 Saddle area and most of the information about it comes from the 10 BHP-Utah drill holes. As described by Ford (1993) and Ford and Snee (1996), the mineralization at the Saddle 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; Ford, 1993, Ford and Snee, 1996; Goldfarb and others, 1997).
On the basis of their drilling, BHP-Utah concluded that the resources of the Saddle prospect was 1,500,000 tons of material with a grade of 7.77 grams of gold per tonne, or a total resource of 165,000 ounces of gold (Heine and Kell, 1991; Stevens, 2010). (However, Stevens also emphasized that the BHP-Utah 'resource' for Saddle was not 43-101 compliant.)
Geologic map unit (-163.722882708259, 64.5758632235706)
Mineral deposit model Discontinuous, irregular to joint-con tolled arsenopyrite-quartz veins and breccia with disseminated arsenopyrite and pyrite in host rocks; low sulfide-Au quartz vein (Cox and Singer, 1986; model 36a).
Mineral deposit model number 36a
Age of mineralization Mid-Cretaceous; a sample of white mica from a quartz vein gave an Ar/Ar plateau date of 109.1 +/- 0.2 Ma (Ford and Snee, 1996).
Alteration of deposit Quartz veining; the veins are commonly bordered by altered zones with plagioclase, chlorite, carbonate, white mica, biotite, titanite, and tourmaline.

Production and reserves

Workings or exploration The Saddle prospect was first identified by BHP-Utah in 1995 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. As of early 2010, Millrock's work on the Saddle prospect included geologic mapping and sampling, reanalysis of some of the BHP-Utah core, and geochemical soil surveys.
Indication of production None
Reserve estimates On the basis of their drilling, BHP-Utah concluded that the resource of the Saddle prospect was 1,500,000 tons of material with a grade of 7.77 grams of gold per tonne or a total resource of 165,000 ounces of gold (Heine and Kell, 1991; Stevens, 2010). However, Stevens also emphasized that the BHP-Utah 'resource' for Saddle was not 43-101 compliant.
Production notes None.

References

MRDS Number 10308323

References

Apodoca, L.E., 1994, Genesis of lode gold deposits of the Rock Creek area, Nome mining district, Seward Peninsula, Alaska: Boulder, Colorado, University of Colorado, Ph.D. dissertation, 208 p.
Ford, R.C., 1993, Geology, geochemistry, and age of gold lodes at Bluff and Mt. Distin, Seward Peninsula, Alaska: Golden, Colorado School of Mines, Ph.D. dissertation, 302 p.
Ford, R.C., and Snee, L.W., 1996, 40Ar/39Ar thermochronology of white mica from the Nome district, Alaska--The first ages of lode sources to placer gold deposits in the Seward Peninsula: Economic Geology, v. 91, p. 213-220.
Goldfarb, R.J., Miller, L.D., Leach, D.L., and Snee, L.W, 1997, Gold deposits in metamorphic rocks in Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 151-190.
Heine, R., and Kell, R., 1991, 1990 Bluff Gold Exploration Summary Report-Parts 1, 2, 3, and 4:
Millrock Resources Inc., 2010, Bering Straits Native Corporation Lands: http://www.millrockresources.com/index.php/projects/bering_straits/ (as of Feb 10, 2010).
report prepared for BHP-Utah Minerals Int.
Stevens, D.L., 2010, Bluff Gold prospect, Seward Peninsula, western Alaska: NI43-101 Technical Report for Millrock Resources Inc., 153 p. (posted on www.sedar.com on February 4, 2010).)
Reporters Travis L. Hudson (Applied Geology); D.J. Grybeck (Contractor, U.S. Geological Survey)
Last report date 4/2/2010