Rock Creek (lode)

Prospect, Active

Alternative names

Nugent

Commodities and mineralogy

Main commodities Au
Other commodities Ag; Pb; Sb; W; Zn
Ore minerals arsenopyrite; boulangerite; galena; gold; hematite; limonite; pyrite; scheelite; stibnite; sphalerite
Gangue minerals albite; ankerite; calcite; quartz

Geographic location

Quadrangle map, 1:250,000-scale NM
Quadrangle map, 1:63,360-scale C-1
Latitude 64.6148
Longitude -165.4181
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Rock Creek lode prospect extends for a strike distance of at least 1,000 feet in the Rock Creek valley above the confluence of Rock Creek and Sophie Gulch (NM208). The coordinates are at about the midpoint of the deposit, about 1.4 miles southwest of Mount Brynetson. The location is just inside the south-central border of section 14, T. 10 S., R. 34 W., of the Kateel River Meridian, and it is accurate to within 500 feet. It is included in locality 43 of Cobb (1972 [MF 463], 1978 [OFR 78-93]).

Geologic setting

Geologic description

Gold-bearing, northeast-striking quartz veins in schist were known on Rock Creek by 1903 (Collier and others, 1908). Sheeted veins were later described, massive veins were locally worked, and some residual placer gold and scheelite were produced from weathered sheeted vein complexes (Moffit, 1913, p. 75-76; Mertie, 1918 [B 662-I, p. 436]; Cathcart, 1922). Lodes in the Rock Creek area were principal examples of disseminated lode gold deposits identified in a regional mineral assessment during the 1970s (Hudson and others, 1977; Hudson and DeYoung, 1978). This deposit is the most extensively explored gold lode in the Nome mining district. Significant exploration to better define the gold grades, including extensive trenching and drilling, has taken place episodically through from the 1980s to the present (2007) since its relocation by geologist R. V. Bailey of Denver in the early 1980s.
As of early 2006, NovaGold Resources Inc. (2006, Projects) is developing the property and carrying out an aggressive infill drilling program which began in 2003. The 2003 drilling totaled 8,000 m and increased total drilling on the project to 18,960 meters in 217 drill holes. Intensive infill drilling continued in 2004 when 82 core and rotary drill holes totaling 20,000 feet (5,900 meters) were completed. The infill drilling results are incorporated in a feasibility study that was started in late 2003. A positive feasibility study could lead to production from a 500 meter by 1,500 meter by 100 meter open pit by 2007. The proposed mine which would produce 5,000 to 7,000 tons per day is expected to produce about 100,000 ounces of gold per year. NovaGold is also evaluating the feasibility of processing ore from the nearby Saddle deposit (NM223) and the more distant Big Hurrah deposit (SO023) if a mill is built at Rock Creek. As of March 28, 2007, NovaGold Resources Inc. (2007, Reserve) reported the resources at Rock Creek as 9.6 million tonnes of measured and indicated reserves with an average grade of 1.31 grams of gold per ton.
The most typical and highest grade part of the Rock Creek lode consists of a sheeted vein complex. The veins strike northeast and generally dip at a high angle to the northwest. They generally range from 1 inch to 6 inches thick, although some veins are more than 1 foot thick. Vein spacing is locally about one per foot. Cathcart (1922, p. 246) described a sheeted zone near the mouth of Sophie Gulch (NM208), where 23 quartz veins from 1 inch to 8 inches thick are in a zone 28 feet wide. In general, sheeted veins are well exposed in mechanical and hydraulic cuts in a 1,000-foot-long interval north of Sophie Gulch (NM208). Although good mineralization was found in some drill holes south of the Sophie Gulch fault, such as in Placer Dome RR-8-088, this fault appears to cut off or displace the best mineralization. The quartz and quartz-calcite veins of the sheeted set are composed mainly of white quartz with some internal crustification, but they are not banded. Albite tends to occur on the selvages and in adjacent wall rocks. Cathcart (1922) and others have reported muscovite in the veins. Sulfides tend to be relatively abundant close to the selvage, but are disseminated throughout the quartz. They consist mainly of pyrite, galena, stibnite, and sphalerite. Arsenopyrite is present but is more abundant in schist than in the veins. Lead sulfosalts such as boulangerite occur locally. Limonite tends to form on weathered veins, hematite on weathered arsenopyrite zones. The deposit is relatively long compared to its apparent thickness. Sheeted veins and most of the gold appear to lie above a marble-rich stratum which is at a depth of about 250 to 300 feet. Individual quartz stringers pinch and swell and may end abruptly at a slip plane parallel to schistosity.
The main Rock Creek deposit grades into several other deposits. Opposite the mouth of Sophie Gulch, sheeted veins 2 to 3 feet apart are in quartz-mica schist, but there are extensive arsenic- and albite-rich zones in the schist. Well-developed, fold-controlled quartz-albite zones were exposed in Kennecott trench RCT-94-8. The trench and adjacent hill slopes display arsenic-rich lodes of northwest strike. This area has locally been called Arsenic Hill. The Reinisch hydraulic pit (NM213) is in this area. A distinct vein called the Albion (NM211) was exposed by mine workings in upper Rock Creek; it probably is partly coincident with sheeted veins typical of the main Rock Creek deposit. The deposit at the Walsh Cut (NM214) resembles that at the Reinisch.
Most of the country rocks exposed at the prospect belong to the chlorite-rich metaturbidite schist and marble unit of Bundtzen and others (1994) or to the lower part of the 'mixed unit' of Till and others (1986). Graphitic mica schist and graphitic quartz schist are common; the graphitic quartz schist is locally a good marker unit. Schistosity generally strikes northeast and dips are low to moderate southeast. Quartz veins of the sheeted set are close to orthogonal to the schistosity. The schist appears to be a phyllonite. Although schistosity appears to be close to concordant with lithology, it is penetrative. Some coarse-grained units have incipient augen structure and are believed to have been sheared during a period of metamorphism that could be contemporaneous with early mineralization. Locally, schist is strongly mineralized with arsenopyrite and albite concordant to schistosity. Bedrock in the area probably is of early Paleozoic protolith age (Hummel, 1962 [MF 247]; Sainsbury, Hummel, and Hudson, 1972 [OFR 72-326]; Till and Dumoulin, 1994; Bundtzen and others, 1994).
Detailed mapping for Kennecott Exploration Company identified a strong northeast-striking fault that appears to cut off the Sophie Gulch fault. The fault, called the Arsenic Hill fault, is exposed in Placer Dome trench RRT-87-1 and in Kennecott Exploration Company trench RCT-94-8. The fault appears to localize complexly sheared graphitic quartz veins and may have both pre- and post-mineral history. It is subparallel and en echelon to the Albion (NM211) and proposed Calle (NM212) vein-fault structures.
Apodaca (1994) studied fluid inclusions and other detailed aspects of the vein geology at Rock Creek. Her work indicates that Rock Creek formed from low-salinity fluids relatively rich in carbon dioxide, methane, and nitrogen, with some hydrogen sulfide. Fluid inclusions indicate an estimated temperature of formation in the range of 225 to 275 degrees Centigrade. The Rock Creek deposit is probably similar in age (109 Ma) to the gold-quartz deposits at Bluff (Ford and Snee, 1996).
Geologic map unit (-165.420707825803, 64.6140345646801)
Mineral deposit model Low-sulfide Au-quartz veins (Cox and Singer, 1986; model 36a)
Mineral deposit model number 36a
Age of mineralization Mid-Cretaceous. The country rocks are part of the Nome Group derived from Proterozoic to lower Paleozoic protoliths (Till and Dumoulin, 1994). The Nome Group underwent regional blueschist facies metamorphism in the Late Jurassic or Early Cretaceous (Sainsbury and others, 1970 [P 750-C]; Forbes and others, 1984; Thurston, 1985; Armstrong and others, 1986; Hannula and McWilliams, 1995). The blueschist facies rocks were recrystallized to greenschist or higher metamorphic grades in conjunction with regional extension, crustal melting, and magmatism in the mid-Cretaceous (Hudson and Arth, 1983; Miller and Hudson, 1991; Miller and others, 1992; Dumitru and others, 1995; Hannula and others, 1995; Hudson, 1994; Amato and others, 1994; Amato and Wright, 1997, 1998). Lode gold mineralization on Seward Peninsula is mostly related to the higher temperature metamorphism in the mid-Cretaceous (Apodoca, 1994; Ford, 1993 [thesis]; Ford and Snee, 1996; Goldfarb and others, 1997).
Alteration of deposit Early alteration consists of locally extensive albitization and sulfidization (introduction of arsenopyrite; late alteration consists of minor sericitization and albitization along with introduction of sheeted veins; and local development of ankerite.

Production and reserves

Workings or exploration
Gold-bearing, northeast-striking quartz veins in schist were known on Rock Creek by 1903 (Collier and others, 1908). Sheeted veins were later described, massive veins were locally worked, and some residual placer gold and scheelite were produced from weathered sheeted vein complexes (Moffit, 1913, p. 75-76; Mertie, 1918 [B 662-I, p. 436]; Cathcart, 1922). Lode prospects in the Rock Creek area were principal examples of disseminated lode gold deposits identified in a regional mineral assessment during the 1970s (Hudson and others, 1977; Hudson and DeYoung, 1978). This deposit is the most extensively explored gold lode in the Nome mining district. Significant exploration has taken place episodically from the early 1980s to the present (2007) including extensive trenching and drilling has taken place episodically since the 1980s when the property was relocated by R.V. Bailey to the present (2007).
Bailey reopened trenches in the hydraulic cuts north of Sophie Gulch and exposed and sampled the sheeted veins. His work brought Placer Dome into the project in 1987 to 1989. Placer Dome drilled dozens of holes on regular, northwest-aligned fences approximately 200 feet apart for nearly 2,000 feet northeast from the mouth of Sophie Gulch. Both core and RC holes were drilled, and essentially all were steeply inclined to the southwest, perpendicular to the strike of the sheeted veins.
Some of this drilling was difficult. The water table is close to the surface and some holes had artesian flow. The program was sufficient to outline a geologic resource, but it was considered subeconomic by Placer Dome. An extensive soil geochemical survey was completed in the area by BHP in 1989, and the property was optioned by Newmont Mining Company in 1992. Some new drill holes, including holes to confirm Placer Dome tests, were drilled by Newmont. The property was further explored in 1994 and 1995 by Kennecott Exploration Company, who drilled a few holes along the northwest fences. The holes were inclined to the northwest so that they would be nearly at right angles to bedrock schistosity in lower Rock Creek. Both Placer Dome and Newmont carried out preliminary metallurgical work; it appears that about 70 percent of the gold is present as free gold; the balance is in auriferous sulfides, principally pyrite and arsenopyrite. Exploration continued in 2000 by NovaGold Resources. In 1999, they announced that better recovery and analytic techniques suggested higher average grades for the deposit, perhaps about 3 grams of gold per metric tonne.
As of early 2006, NovaGold Resources, Inc. (2006, Projects) is developing the property and is carrying out an aggressive infill drilling program which began in 2003. The 2003 drilling totaled 8,000 m and increased total drilling on the project to 18,960 meters in 217 drill holes. Intensive infill drilling continued in 2004 when 82 core and rotary drill holes totaling 20,000 feet (5,900 meters) were completed. The infill drilling results are incorporated in a feasibility study that was started in late 2003. A positive feasibility study could lead to production from a 500 meter by 1,500 meter by 100 meter open pit by 2007. Processing of 5,000 to 7,000 tons per day is expected to produce about 100,000 ounces of gold per year. NovaGold is also evaluating the feasibility of processing ore from the nearby Saddle deposit (NM223) and the more distant Big Hurrah deposit (SO023) if a mill is built at Rock Creek.
Indication of production Undetermined
Reserve estimates As of March 28, 2007, NovaGold Resources Inc. (2007, Reserve) reported the resources at Rock Creek as 9.6 million tonnes of measured and indicated reserves with an average grade of 1.31 grams of gold per ton.

References

MRDS Number A012834; D002590

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.
Armstrong, R.L., Harakal, J.E., Forbes, R.B., Evans, B.W., and Thurston, S.P., 1986, Rb-Sr and K-Ar study of metamorphic rocks of the Seward Peninsula and southern Brooks Range, Alaska, in Evans, B.W., and Brown, E.H., eds., Blueschists and eclogites: Geological Society of America Memoir 164, p. 184-203.
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.
Hannula, K.A., and McWilliams, M.O., 1995, Reconsideration of the age of blueschist facies metamorphism on the Seward Peninusla, Alaska, based on phengite 40Ar/39Ar results: Journal of Metamorphic Geology, v. 13, p. 125-139.
Hannula, K.A., Miller, E.L., Dumitru, T.A., Lee, Jeffrey, and Rubin, C.M., 1995, Structural and metamorphic relations in the southwest Seward Peninsula, Alaska; Crustal extension and the unroofing of blueschists: Geological Society of America Bulletin, v. 107, p. 536-553.
Hudson, T.L., 1994, Crustal melting events in Alaska, in Plafker, G., and Berg, H. C., eds., The Geology of Alaska: Geological Society of America, DNAG, The Geology of North America, Vol. G-1, p. 657-670.
Hudson, T.L., and Arth, J. G., 1983, Tin granites of Seward Peninsula, Alaska: Geological Society of America Bulletin, v. 94, p. 768-790.
Miller, E.L., Calvert, A.T., and Little, T.A., 1992, Strain-collapsed metamorphic isograds in a sillimanite gneiss dome, Seward Peninsula, Alaska: Geology, v. 20, p. 487-490.
NovaGold Resources Inc., 2006 (Projects): http://www.novagold.net/s/Projects.asp (as of April, 2007).
NovaGold Resources Inc., 2007 (Reserve): (http://www.novagold.net/i/pdf/NGReserve_ResourceTable.pdf (March, 2007)
Thurston, S.P., 1985, Structure, petrology, and metamorphic history of the Nome Group blueschist terrane, Salmon Lake area, Seward Peninsula, Alaska: Geological Society of America Bulletin, v. 96, p. 600-617.
Till, A.B., and Dumoulin, J.A, 1994, Geology of Seward Peninsula and St. Lawrence Island, in Plafker, G., and Berg, H.C., eds., The Geology of Alaska: Geological Society of America, DNAG, The Geology of North America, v. G-1, p. 141-152.
Reporters C.C. Hawley (Hawley Resources Group, Inc.) and Travis L. Hudson (Applied Geology, Inc.)
Last report date 10/10/2005