|Quadrangle map, 1:250,000-scale||BD|
|Quadrangle map, 1:63,360-scale||B-5|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||The Bald Knob prospect is centered on hill 2860, a high bare hill at the head of Banner Creek. The prospect is about 5.7 mile north-northeast of Richardson on the Richardson Highway and about 0.4 mile northwest of the center of section 31, T. 6 S., R. 7 E., of the Fairbanks Meridian. The location is accurate.|
The Richardson District is characterized by gentle slopes and broad, alluvium-filled valleys (Prindle and Katz, 1913). The area is unglaciated and largely overlain by windblown silt, sand, and loess, locally up to 50 meters thick (Foster and others, 1979). The rocks consist of three structure-bounded units (Weber and others, 1978; Swainbank and others, 1984; Noyes and others, 2006; Freeman, 2011). There is a core that consists mainly of biotite-hornblende-feldspar-quartz gneiss and lesser amphibolite, quartzite, marble and calc-silicate hornfels. The core is bounded on the north and south by amphibolite-grade pelitic schist. The contacts between the units are probably low-angle faults; north-trending vertical faults occur widely. Weber and others (1978) assign the pre-metamorphic age of the rocks to the Precambrian or Paleozoic. They were metamorphosed in the Middle to Late Devonian and recrystallized in the Early Cretaceous (Dusel-Bacon and others, 2004; Freeman, 2011). The metamorphic rocks are intruded by 90 Ma, felsic to intermediate dikes and plugs, notable the quartz-feldspar porphyry dike at the nearby Democrat Mine (BD014). As seen in the core from three holes drilled in 2000 at or near the Bald Knob prospect, the rocks in the area consist of gneiss, migmatite, and felsic granitic dikes that vary from pegmatite to aplite (Noyes and others, 2006).The Bald Knob prospect was discovered by the Tri-Valley Corporation during reconnaissance exploration in 1998 after 10 float samples of quartz were found that contained 0.35 to 1.54 ounces of gold per ton at the nearby Buckeye prospect (BD048); the samples also contained up to 846 parts per million (ppm) bismuth and up to 127 ppm tellurium (Vantanyan and others, 1998). In late 1998, the Tri-Valley Corporation and Placer Dome (USA) formed a joint venture that initially concentrated their work in the headwaters of Banner Creek over the Buckeye prospect but soon was extended to this prospect about a mile and a half to the north. In 1999, Placer Dome collected 1,037 soil samples on bedrock, collected 136 rock samples, and drilled three hole that totaled 516.5 meters (Rogers and others 1999). The soil samples defined a strong gold-bismuth-tellurium anomaly over the Buckeye and Bald Knob prospects. One drill hole was at the Buckeye prospect, the other two were at the Bald Knob prospect. Placer Dome did more soil and rock sampling in 2000 and drilled three more holes (Graham 2002; Noyes and others, 2006). Low-grade intercepts of more than 100 parts per billion gold were seen in the six holes drilled in 1999 and 2000. The high gold values in the core can be correlated with high bismuth and tellurium but there seems to be no correlation of gold with such pathfinder elements as silver, arsenic, antimony, copper, lead, or zinc, elements that are commonly associated with other gold deposits in the district. The higher gold values are associated with narrow quartz veins that cut otherwise unaltered gneiss. Additional soil sampling was done in 2005 but added little to the extent of the previous anomalies. The source of the rich gold-bismuth-tellurium-quartz samples found at the surface near the Buckeye prospect in 1998 remains unknown. Freeman (2011) in his synthesis of the previous work on the gold mineralization of the Richardson district suggests that the mineralization at the Bald Knob prospect is a distal intrusion-related gold deposit.
|Geologic map unit||(-146.247462879557, 64.3546939878853)|
|Mineral deposit model||Distal intrusion-related gold deposit.|
|Age of mineralization||Probably 90 Ma as are most of the gold deposits in the district.|
|Alteration of deposit||Notable lack of alteration in the gneiss host rock.|
|Workings or exploration||Three generations of soil sampling on bedrock from 1998 to 2005. Numerous rock samples collected and analyzed. Nine holes were drilled in 1999 and 2000 at this and the nearby Buckeye prospect (BD048).|
|Indication of production||None|
Dusel-Bacon, C., Wooden, J.L., and Layer, P.W., 2004, A Cretaceous Ion-Microprobe U-Pb zircon Age for the West Point Orthogneiss: Evidence for another Gneiss Dome in the Yukon-Tanana Upland in Galloway, J.P., ed., 2004, Studies by the U.S. Geological Survey in Alaska, 2001: U.S. Geological Survey Professional Paper 1678, p. 41 - 60.
McCoy, D.T.; Newberry, R.J.; Layer, P., DiMarchi, J.; Bakke, A.; Masterman, J.S. and Minehane, D.L., 1997, Plutonic-related gold deposits of interior Alaska: in Economic Geology Monograph 9, p. 191-241.
Rogers, J. A., McCoy, D. T., Graham, G. and Nerup, M., 1999, Placer Dome-Tri-Valley Option Agreement, 1999 Annual Report, Richardson Project: Confidential Private Report, 46 p.
Vartanyan, S.S., Volchcov, A.G., Zelikson, B.S., Bohme, D.M., 1998, Preliminary summary Report, 1998 Richardson District Exploration Program: Internal report for Tri-ValleyCorp., September 15, 1998, 35 p.
|Reporters||D.J. Grybeck (Contractor, USGS)|
|Last report date||4/1/2012|