Eldorado Basin

Prospect, Active

Alternative names

Moose Jaw Mountain

Commodities and mineralogy

Main commodities Au
Other commodities As; Cu; Sb
Ore minerals arsenopyrite; chalcopyrite; pyrite; pyrrhotite; scorodite; stibnite
Gangue minerals actinolite; epidote; quartz

Geographic location

Quadrangle map, 1:250,000-scale OP
Quadrangle map, 1:63,360-scale C-1
Latitude 63.5366
Longitude -156.006
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Eldorado Basin prospect is in the headwaters of Eldorado Creek, which flows into Colorado Creek. The center of the activity is at an elevation of about 1,800 feet along the creek but the several exploration efforts cover an area of about a half square mile in the central part of section 32, T. 22 S., R. 15 E. The location is accurate.

Geologic setting

Geologic description

The rocks in the vicinity of Eldorado Creek consist of intermediate to mafic volcanic and volcaniclastic rocks, and Cretaceous, Kuskokwim Group black shale, limestone, graywacke, and conglomerate; the strata are intruded by Cripple Mountain monzonite (McGinnis and others, 1997; Avalon Development Corp., 1998; Duncan, 1999). The headwaters of Eldorado Creek contain limonite-stained, gossanous breccia (Avalon Development Corp., 1998).
The Eldorado Basin prospect is on a mineralized zone that trends northeast across the headwaters of Eldorado Creek for about 2,200 meters. This zone extends northeast into the Medfra quadrangle and end at Colorado Creek, northeast of the summit of Moose Jaw Mountain. At the Eldorado Basin prospect, the mineralized zone contains scattered, thin veins of quartz and stibnite, arsenopyrite, and quartz, and locally of actinolite, chalcopyrite, or epidote. Soil samples collected in the vicinity contain up to 674 parts per million (ppm) copper, 2,250 parts per billion (ppb) gold, 2,818 ppm arsenic, 945 ppm antimony, and 12 ppm bismuth (Avalon Development Corp., 1998).
The northeast extension of this zone is defined by soil samples on Moose Jaw Mountain. Two mineralized areas have been identified: a NW-SE-trending, 400-meter by 50-meter area; and an adjacent area 1400 meters long by 200 to 400 meters wide. The 1400-meter-long area is open to the south (Duncan, 1999). Soil samples from these areas contained up to 515 ppb gold, 2,770 ppm arsenic, 2,942 ppm antimony, and 9 ppm bismuth (Avalon Development, 1998). Hypabyssal, porphyritic rocks at the top of Moose Jaw Mountain are cut by veins less than 5 centimeters thick of crystalline quartz, stibnite, arsenopyrite, and scorodite (Avalon Development, 1998).
Exploration in the Eldorado Basin includes geologic mapping and sampling by Barker during 1994, 1995, and 1996; soil sampling, rock sampling, and geologic mapping by NovaGold during 1998; and mobile-metallic-ion (MMI) sampling by Poseidon Minerals Ltd. in 1999 (Barker, 1996; Avalon Development Corp., 1998; Duncan, 1999). Also see OP002 and OP031-033 for similar mineralization nearby.
In 2009, TintinaGold Resources Inc. (TintinaGold, 2009; Chutas and others, 2009) drilled 9 holes in the upper basin of Colorado Creek in the general vicinity of the zones of mineralization and anomalies that had previously been identified. The holes were mostly drilled through colluvium along the creek but some were in quartz monzonite, hornfels, rhyodacite quartz porphyry, andesite, and siltstone. Significant pyrrhotite-rich replacement mineralization was cut in the hornfels as were quartz-carbonate veins in the hornfels, diorite, and andesite dikes. The quartz-carbonate veins included pyrrhotite, chalcopyrite, pyrite, and arsenopyrite. Broad propylitic alteration as well as chlorite, sericite, and carbonate alteration were noted.
Geologic map unit (-156.008375654666, 63.5359889151237)
Mineral deposit model Epithermal Au? (Cox and Singer, 1986; model 25b, 25c?).
Mineral deposit model number 25b, 25c?
Age of mineralization Cretaceous or younger based on the age of the Kuskokwim Group rocks in which much of the mineralization occurs.
Alteration of deposit Broad propylitic alteration as well as chlorite, sericite, and carbonate alteration.

Production and reserves

Workings or exploration During 1994, 1995, and 1996, Ron Rosander contracted geologist James Barker to evaluate the lode sources of placer gold in the Colorado Creek area. Barker collected 254 soil samples, 93 rock samples, and 12 samples of placer gold for scanning-electron microscope and trace element analysis. Geologic mapping and a ground-based magnetometer survey were also carried out (Barker, 1996; McGinnis and others, 1997). In 1996, the Alaska Division of Geological and Geophysical Surveys conducted geologic mapping in the Cripple Creek Mountains and Placer Dome Exploration flew an east-west-oriented aeromagnetic survey over the Cripple Creek Mountains. Nova Gold conducted geologic mapping, rock sampling, and soil sampling during the summer of 1998 (Avalon Development, 1998). During 1999, Poseidon Minerals, Ltd. carried out soil sampling in the Colorado Creek area, and used mobile-metallic-ion (MMI) analyses (Duncan, 1999). In 2009, TintinaGold Resources Inc. drilled 8 holes along Colorado Creek.
Indication of production None
Reserve estimates None.
Production notes None.


MRDS Number 10308034


Avalon Development Corp., 1998, Colorado Creek project, Ophir and Medfra quadrangles, southwest Alaska: Avalon Development Corp. report prepared for NovaGold Resources Inc., 9 p.
Barker, J.C., 1996, Geologic evaluation of the Colorado Creek prospect area, Alaska: Summary report prepared for Placer Dome Exploration, San Francisco, Calif., 73 p.
Dashevsky, S.S., 2000, Colorado Creek project (Au) Innoko district, Alaska: Unpublished report for Rosander Mining Company, 7 p.
Duncan, R.G., 1999, Colorado Creek project, Alaska, interim report: Unpublished report for Poseidon Minerals Ltd., 22 p.
McGinnis, M.F., McCoy, M.E., Nerup, M.R., and Barnett, D., 1997, Colorado Creek project Alaska, 1997 summary report: Placer Dome Exploration Inc. unpublished report, 37 p.
TintinaGold Resources Inc., 2010, Colorado Creek: http://mantramining.com/Colorado_Creek_Project.aspx?m_id=6364 (as of February 1, 2010)
Reporters C.E. Cameron (Northern Associates, Inc.; D.J. Grybeck (Contractor, U.S. Geological Survey)
Last report date 4/2/2010