Porphyry Knob

Prospect, Active

Commodities and mineralogy

Main commodities Ag; Au
Ore minerals arsenopyrite; chalcopyrite; gold; pyrite; stibnite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale OP
Quadrangle map, 1:63,360-scale C-1
Latitude 63.5394
Longitude -156.0254
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Porphyry Knob prospect is on a prominent 1,800-foot hill, informally called Porphyry Knob, near the divide between Cripple Creek and Colorado Creek. It is about 0.4 mile west of the center of section 32, T. 22 S., R. 15 E., Kateel River Meridian. The location is accurate.

Geologic setting

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 and others, 1997). In the Colorado Creek area, high-angle, northeast-trending faults are cut by later, northwest-trending faults.
The rocks at the Porphyry Knob prospect consist of a gently-dipping sequence of intermediate to mafic volcanic, volcaniclastic, and marine sedimentary rocks. What is usually called dacite porphyry is an important hostrock, but petrographic work by Placer Dome Exploration demonstrates that the light-colored, porphyritic rocks characteristic of this prospect are in fact mostly altered andesite and basalt despite their leucocratic appearance in hand specimen (McGinnis and others, 1997). The altered volcanic rocks contain abundant disseminated pyrite 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 and others, 1997). Fine native gold can be panned from soil on the southeast margin of the prospect area (Avalon Development, 1998).
Placer Dome Exploration drilled ten core holes in 1997. The most significant intercept averaged 1.9 grams of gold per ton over 19.2 meters; this included an intercept 2 meters long with 7.5 grams of gold per tonne (McGinnis and others, 1997). The most abundant sulfides in the core are arsenopyrite, chalcopyrite, pyrite, and stibnite (Duncan, 1999). 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 that contained up to 3,610 parts per billion 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 (OP032), and Eldorado Creek (OP033) prospects.
In 2009, TintinaGold Resources Inc. drilled 4 holes on Porphyry Knob (TintinaGold, 2010; Chutas and others, 2009). At the surface, the rocks on the knob are mainly rhyodacite quartz porphyry surrounded by siltstone and andesite. The sedimentary and volcanic rocks were also intersected at depth in the drill holes and gold veins were intersected in both the porphyry and its host rock. A 111-meter interval in one hole contained 0.64 gram of gold per tonne; this included a 2.3-meter interval that contained 8.28 grams of gold per tonne and a 2-meter interval that contained 6.38 grams of gold per tonne. Another hole nearby intersected 98.4 meters that contained 0.54 grams of gold per tonne; this included 22 meters with 1.01 grams of gold per tonne.
There are six types of veins, at least two of which are gold bearing. The gold-quartz veins are associated with various suites of arsenic, antimony, and mercury minerals. In addition to the veins, the drilling cut significant intercepts of disseminated sulfide mineralization with bulk-tonnage potential. Several styles of alteration were noted in the porphyry, including pervasive sericitization, clay alteration, carbonate alteration, and a later clay-oxide alteration. Pervasive oxidation persists to at least 225 feet below the surface.
Geologic map unit (-156.027776601701, 63.5387886342143)
Mineral deposit model Epithermal gold (quartz-adularia) (Cox and Singer, 1986; model 25b, 25c?).
Mineral deposit model number 25b, 25c?
Age of mineralization Cretaceous or younger. Mineralized sills intrude Cretaceous sedimentary rocks of the Kuskokwim Group.
Alteration of deposit 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).

Production and reserves

Workings or exploration
Gold mineralization was identified on Porphyry Knob by S. Nerod during the 1970s 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 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 1,372 meters in the Porphyry Knob prospect during 1997, soil sampled over 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. In 2009, TintinaGold drilled 4 holes on Porphyry Knob and another 8 nearby in the upper basin of Colorado Creek.
Indication of production None
Reserve estimates None.
Production notes None.

References

MRDS Number 10308031

References

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