Porphyry Knob Prospect, Innoko Mining District, Yukon-Koyukuk Census Area, Alaska, USAi

Regional Level Types
Porphyry Knob Prospect	Prospect
Innoko Mining District	Mining District
Yukon-Koyukuk Census Area	Census Area
Alaska	State
USA	Country

For more information, contact Ron Rosander in McGrath, AK.
Location: 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.
Geology: 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).
Workings: 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.
Age: Cretaceous or younger. Mineralized sills intrude Cretaceous sedimentary rocks of the Kuskokwim Group.
Alteration: 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).

Commodities (Major) - Ag, Au
Development Status: None
Deposit Model: Epithermal gold (quartz-adularia) (Cox and Singer, 1986; model 25b, 25c?).

Other Databases

Link to USGS - Alaska:	OP031

Other Regions, Features and Areas containing this locality

References