|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 Shamrock prospect covers a considerable area on the west side of 'Buck Mountain', the hill under VABM 3026 'Buck'. But the coordinates are at the center of the considerable trenching on it about 1 mile west of the top of the hill. The prospect is about 7.2 miles northwest of Richardson on the Richardson Highway and about 0.1 mile northeast of the center of section 19, 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). The country rocks around the Shamrock prospect are mainly unaltered quartz-biotite-feldspar-hornblende gneiss (Freeman, 2011).
The Shamrock prospect was first identified in 1991 during regional reconnaissance exploration and geochemical surveys (Vartanyan and others, 1991; Freeman, 2011). Panned concentrate samples containing visible gold, as well as notable pyrite, tourmaline, biotite, pyroxene, and magnetite were found widely in the area west of VABM Buck. In 1992, the area was mapped, three long road cuts and trenches were dug west of VABM Buck, and 582 grab and channel samples were collected in them (Vartanyan and others, 1992; Freeman, 2011). The samples contained up to 6,351 parts per billion (ppb) gold, up to 364 parts per million (ppm) silver, up to more than 2,000 ppm arsenic, up to 379 ppm bismuth, up to 2,866 ppm lead, and up to more than 2,000 ppm antimony. Channel samples up to 15 meters long contained more than 500 ppb gold; the highest grade was 1.5 meters that averaged 6,351 ppb gold; the longest was 12 meters that averaged 1,997 ppb gold. Aushburn (1998 [Redstar]) check sampled some of the trenches but could not duplicate the high values of the 1997 work.
Freeman (2011) identified three types of mineralization: 1) quartz-limonite breccia, 2) pyrite-arsenopyrite-tourmaline-quartz veins, and pyrite-arsenopyrite-quartz veinlets. The quartz-limonite breccia is widespread. The pyrite-arsenopyrite-tourmaline-quartz veins form linear zones up to 40 centimeters thick. They commonly also contain sphalerite, chalcopyrite, tetrahedrite, and rare bismuthinite. The pyrite-arsenopyrite-quartz veinlets are closely-spaced and feature cockscomb textures; fluid inclusion work indicates that they formed at 150-280 degrees C. These veins contain no silver, lead, bismuth, or antimony minerals but they average 1,201 ppb gold and 1,350 ppm arsenic. Pervasive quartz-sericite and quartz-sericite-muscovite flooding in the gneiss host rock form alteration envelopes around the gold-bearing zones. The alteration and mineralization trends north, varies from 20 to 50 meters wide, and extends for 200 to 4,000 feet along strike.
In 1997, the Tri-Valley Corporation collected more rock samples, drilled 4 core holes that totaled 641.5 meters, and dug or re-dug 12,950 feet of trenches (Aushburn, 1998 [Executive summary]). The best intercept in the drilling was 20 feet that averaged 1.77 grams of gold per tonne. Noyes and others (2006) reexamined the 1997 core with results that differed only in detail.
In 1999, Placer Dome (USA) sampled the soils on bedrock on a 200-meter grid and found that the gold is associated with arsenic and antimony (Rogers and others, 1999). They also collected 59 rock samples; they contained up to 2,200 ppb gold, 14.4 ppm silver, 10,000 ppm arsenic, 110 ppm bismuth, 46,600 ppm lead, and 10,000 ppm antimony.After synthesizing the considerable analytical and geologic data on the Shamrock deposit, Freeman (2011) concluded that it is distal Intrusion-related gold deposit.
|Geologic map unit||(-146.434661891329, 64.3811897617706)|
|Mineral deposit model||Distal, intrusion-related gold deposit.|
|Age of mineralization||Probably about 90 Ma as are most of the gold deposits in the Richardson District and many in central Alaska.|
|Alteration of deposit||Pervasive quartz-sericite and quartz-sericite-muscovite flooding in the gneiss host rock form alteration envelopes around the gold-bearing zones.|
|Workings or exploration||The Shamrock prospect was first identified in 1991 during regional reconnaissance exploration and geochemical surveys (Vartanyan and others, 1991; Freeman, 2011). In 1992, the area was mapped, three long road cuts and trenches were dug west of VABM Buck, and 582 grab and channel samples were collected (Vartanyan and others, 1992; Freeman, 2011). In 1997, the Tri-Valley Corporation collected more rock samples, drilled 4 core holes that totaled 641.5 meters, and dug or re-dug 12,950 feet of trenches (Aushburn, 1998 [Executive summary]). Noyes and others (2006) reexamined the 1997 core. In 1999, Placer Dome (USA) sampled the soils on bedrock on a 200-meter grid over the prospect.|
|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., 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.
|Reporters||Cameron S. Rombach (ADGGS); D.J. Grybeck (Contractor, USGS)|
|Last report date||4/1/2012|