|Quadrangle map, 1:250,000-scale||BD|
|Quadrangle map, 1:63,360-scale||B-6|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||The Hilltop prospect is on hill 1920 about 13.9 miles northwest of Richardson on the Richardson Highway and about 6.0 miles east of the center of Harding Lake. It is about 0.4 mile east of the center of section 1, T. 6 S., R. 5 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). The poorly exposed rocks at the Hilltop prospect are predominantly quartz-feldspar-biotite+/-muscovite gneiss with some amphibolite.The Hilltop prospect was first suspected when Vartanyan and others (1991) identified anomalous gold in stream sediment and panned concentrated samples from streams draining Hill 1920. No outcrop is exposed on Hill 1920 but in 2000, Vartanyan and others (1991) collected 63 rock-chip samples, 24 samples from trenches to bedrock, and 195 soil samples from 203 pits to bedrock. Rock samples contained 0.003 to 1.726 ounces of gold per ton, up to 289 parts per million (ppm) bismuth, up to 30 parts per million (ppm) copper, up to one percent or more arsenic, and up to 276 ppm antimony. Many of the soil samples were highly anomalous in gold and arsenic. Based on limited field work, the Hilltop deposit appears to trend north-northwest, and is estimated to be 200-900 feet wide and extends along strike for over 3,000 feet.
|Geologic map unit||(-146.658666357109, 64.4232853914911)|
|Mineral deposit model||Intrusion-related gold deposit.|
|Age of mineralization||Uncertain but probably 90 Ma as are many of the intrusion-related gold deposits in the district.|
|Alteration of deposit||Not specifically noted.|
|Workings or exploration||Covered by regional geochemical surveys with stream sediment and panned concentrate samples in 1991. No outcrop is exposed on Hill 1920 but in 2000, Vartanyan and others (1991) collected 63 rock-chip samples, 24 samples from channels to bedrock, and 195 soil samples from 203 pits to bedrock.|
|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, Paul, DiMarchi, J.J., Bakke, Arne, Masterman, J.S., and Minehane, D.L., 1997, Plutonic-related gold deposits of interior Alaska: in Economic Geology Monograph 9, p. 191-241.
Vartanyan, S.S., Konstantinov, V.Y., Oreshin, V.Y., Rodionov, V.A., Pyatnitshy, V.A., Brovkin, V.A., Zagoskin, B.S. and Zelikson, B.S., 1991, Estimation of gold potential of the Richardson Mining District, Alaska: Internal TsNIGRI report to Tri-Valley Corporation, December, 1991, 40 p.
Vartanyan, S. S., Volchkov, A. G., and Brazhnik, A. V., 2001, Hilltop Zone Redmond Prospect area): Annual Report to Tri-Valley Corporation, 13 p.
|Reporters||D.J. Grybeck (Contractor, USGS)|
|Last report date||4/1/2012|