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Porphyry Knob

Prospect in Alaska, United States with commodities Silver, Gold
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Sections on this page
  1. Identification information
  2. Geographic coordinates
  3. Site location context
  4. Geographic areas
  5. Commodities
  6. Materials information
  7. Alteration
  8. Mineral occurrence model information
  9. Nearby scientific data
  10. Economic information about the deposit and operations
  11. Mining district
  12. Links to other databases
  13. Bibliographic references
  14. General comments
  15. Reporter information

Geologic information

Identification information

Deposit ID 10308031
Record type Site
Current site name Porphyry Knob

Geographic coordinates

Geographic coordinates: -156.0257, 63.53916 (WGS84)
Relative position The Porphyry Knob prospect is on a prominent , 1800-foot hill, informally called Porphyry Knob, at the head of Cripple Creek and Colorado Creek. It is in the SW1/4NW1/4 sec. 32, T. 22 S., R. 15 E., Kateel River Meridian. The location is accurate within 50 feet.

Site location context

Political divisions (FIPS codes)

Yukon-Koyukuk(Census area)

Alaska(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

Ophir C-1(quadrangle 1:63,360 scale)

Ophir NE(quadrangle 1:100,000 scale)

Ophir(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

Alaska(hydrologic region)

Geographic areas

Country State
United States Alaska

Commodities

Commodity Importance
Silver Primary
Gold Primary

Materials information

Materials Type of material
Arsenopyrite Ore
Chalcopyrite Ore
Gold Ore
Pyrite Ore
Stibnite Ore
Quartz Gangue

Alteration

  • (Local) The igneous rocks display intense levels of phyllic, silicic, and carbonate alteration. The sedimentary rocks exhibit local sericitic, silicic, carbonate, hornfels, and skarn alteration (Avalon Development, 1998; Duncan, 1999).

Mineral occurrence model information

Model code 106
USGS model code 25b
Deposit model name Epithermal vein, Creede
Mark3 model number 58
Model code 150
USGS model code 25c
Deposit model name Epithermal vein, Comstock
Mark3 model number 16

Nearby scientific data

(1) -156.0257, 63.53916
ListMap

Economic information

Comments on the geologic information

  • Geologic Description = The rocks in the vicinity of the Porphyry Knob prospect consist of highly altered, intermediate to mafic, volcanic, volcaniclastic, and flow-layered rocks interbedded with black shale, graywacke, and minor conglomerate of the Cretaceous Kuskokwim group (McGinnis, 1997). In the Colorado Creek area, high-angle, northeast-trending faults are cut by later, northwest-trending faults. Gold at the Porphyry Knob prospect occurs in a gently-dipping sequence of intermediate to mafic volcanic, volcaniclastic, and marine sedimentary rocks. The principal country rock at the prospect is altered andesite. Dacite porphyry is also an important hostrock, but petrographic work by Placer Dome Exploration demonstrates that the light-colored, porphyritic rocks characteristic of this prospect are in fact altered andesite and basalt, despite their leucocratic appearance in hand specimen (McGinnis, 1997). The altered volcanic rocks contain abundant disseminated pryite and arsenopyrite and are cut by quartz and limonite veins up to 2 centimeters thick. The dacite porphyry contains veins and veinlets of quartz + adularia + pyrite +/- apatite +/- ankerite +/- calcite. The andesite contains veins and veinlets of quartz +/- albite + ankerite +/- calcite + pyrite + arsenopyrite (McGinnis, 1997). Fine colorws of native gold can be panned from soil on the southeast margin of the prospect area (Avalon Development, 1998). Ten core holes were drilled into the prospect by Placer Dome Exploration in 1997. The most significant intercept averaged 1.9 grams of gold per ton over 19.2 meters, and includes an intercept 2 meters long of 7.5 grams of gold per ton (McGinnis, 1997). The most abundant sulfides in the core included arsenopyrite, chalcopyrite, pyrite, and stibnite (Duncan, 2000). The gold:silver ratio at Porphyry Knob is generally low and ranges from 1:1 to 1:10 (Duncan, 1999). The prospect is marked by soil samples containing up to 3610 ppb gold, anomalous amounts of arsenic, antimony, and mercury, and minor bismuth (Avalon Development, 1998). The igneous rocks display intense levels of phyllic, silicic, and carbonate alteration. The sedimentary rocks exhibit local sericitic, silicic, carbonate, hornfels, and skarn alteration (Avalon Development, 1998; Duncan, 1999). The Porphyry Knob prospect may be part of a gold-bearing, quartz-adularia- type epithermal system (Avalon Development, 1998; Duncan, 1999; Dashevsky, 2000). Also see the Moose Jaw prospect (OP032), and Eldorado Creek (OP033).
  • Age = Cretaceous or younger. Mineralized sills intrude Cretaceous sedimentary rocks of the Kuskokwim Group.

Economic information about the deposit and operations

Development status Prospect

Comments on exploration

  • Status = Active

Mining district

District name Innoko

Comments on the workings information

  • Workings / Exploration = Gold mineralization was identified on Porphyry Knob by S. Nerod during the 1970's at the Nerod trench in the southern part of the prospect area. In 1994, 1995, and 1996, Ron Rosander, the claim owner, contracted James Barker to evaluate lode sources of placer gold in the Colorado Creek area. Barker (1966) collected 254 soil samples, 93 rock samples, and 12 samples of placer gold for scanning-electron microscope and trace element analysis. Barker also conducted geologic mapping and a ground-based magnetometer survey. 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 (Avalon Development, 1998). Placer Dome Exploration drilled 10 core holes totaling 1372 meters in the Porphyry Knob prospect during 1997, soil sampled along a large grid, and conducted geologic mapping, trenching, and road construction (McGinnis, 1997). Nova Gold explored the Porphyry Knob prospect in 1998 and conducted further geologic mapping, rock sampling, and soil sampling (Avalon Development, 1998). During 1999, Northern Associates, Inc., was contracted by Poisedon Minerals, Ltd. to conduct further soil sampling in the Colorado Creek area, including Porphyry Knob.

Reference information

Bibliographic references

  • Deposit

    McGinnis, M.F., McCoy, M.E., Nerup, M.R., and Barnett, D., 1997, Colorado Creek project Alaska, 1997 summary report: unpublished Placer Dome Exploration Inc. report, 37 p.

  • Deposit

    Avalon Development Corp., 1998, Colorado Creek project, Ophir and Medfra quadrangles, southwest Alaska: report prepared for NovaGold Resources Inc., 9 p.

  • Deposit

    Duncan, R.G., 1999, Colorado Creek project, Alaska, interim report: unpublished report for Poseidon Minerals Ltd., 22 p.

  • Deposit

    Dashevsky, S.S., 2000, Colorado Creek project (Au) Innoko district, Alaska: unpublished report for Rosander Mining Company, 7 p.

Comments on the references

  • Primary Reference = McGinnis, 1997

General comments

Subject category Comment text
Deposit Model Name = Epithermal gold (quartz-adularia) (Cox and Singer, 1986; model 25b, 25c?).
Deposit Model Number = 25b, 25c?
Deposit Other Comments = For more information, contact Ron Rosander in McGrath, AK.

Reporter information

Type Date Name Affiliation Comment
Reporter 07-AUG-01 Cameron, C.E. Northern Associates Inc.

Beyond USGS

Supplemental information added by qvyshift.com. Not part of the original USGS MRDS record.

Maps centered on this site

Authoritative Alaska resources

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