|Quadrangle map, 1:250,000-scale||MH|
|Quadrangle map, 1:63,360-scale||A-6|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||The Zackly prospect is about one-half mile north-northwest of peak 5375, a northwest-aligned ridge about 1.8 miles east of the West Fork Maclaren River. The deposit is exposed from about 4,000 to 4,650 feet in elevation near the north end of the ridge. The location for this record is at the approximate center of the deposit, about 2,400 feet south of the center of section 36, T. 19 S., R. 5 E., Fairbanks Meridian. The property is accessible by a bulldozer trail from the Kathleen-Margaret mine trail (see MH087), not shown on the A-6 quadrangle. The location corresponds to locality [figure] A26 of Kurtak and others (1992) and locality 26 in table 2 of Nokleberg and others (1991).|
The Zackly skarn prospect is hosted in Triassic tuff and sedimentary rocks (Nokleberg and others, 1991). These rocks have been intruded by quartz monzodiorite and monzonite of Cretaceous age. Numerous east-trending high-angle faults cut out or repeat skarn, limestone, volcanic rocks, and intrusive rocks. In general, the contact between sedimentary and intrusive rocks that host the main ore body strikes nearly east-west and is very steep (Kurtak and others, 1992, figure A26). It can be traced for about a mile. The main so-called skarn ore body consists of pods and irregular stratabound lenses of skarn at the contact of Upper Triassic marble with albitized quartz monzodiorite. The body is crudely zoned; the mineral sequence outward from the quartz monzodiorite consists of: (1) brown garnet with chalcopyrite, (2) green garnet with bornite and chalcopyrite, (3) clinopyroxene and wollastonite, and (4) marble with magnetite and bornite. Fine-grained silica (chalcedony) and clay occur irregularly throughout and appear to be late, retrograde minerals.
Stellar project work in 2016 (Millrock Resources Inc., in a joint venture with Vista Minerals Pty Ltd.) consisted of collecting 7 lines of induced-potential measurements over the Zackly copper-gold skarn. They identified chargeability anomalies within copper-in-soil anomalies, which appear to extend the strike length of the skarn to both the east and west (Athey and Werdon, 2017).
Gold occurs only with skarn; the higher gold values are in a supergene (?) assemblage of malachite, limonite, chalcedony, and native copper. Unoxidized ore contains chalcopyrite and bornite. A high-grade sample collected by the U.S. Geological Survey assayed 6.6 percent copper, 4.4 parts per million (ppm) gold, 35 ppm silver, and 30 ppm molybdenum (Nokleberg and others, 1991). Sample 1681 collected by the Bureau of Mines assayed 0.18 ounce of gold per ton, 2.45 ounce of silver per ton, 7.1 percent copper, and 0.11 percent tungsten (Kurtak and others, 1992); a representative 7-foot sample (1675) cut across the skarn assayed 0.05 ounce of gold per ton, 0.31 ounce of silver per ton, 1.35 percent copper, and 430 ppm tungsten. Tungsten is probably contained in scheelite; molybdenum is either in molybdenite or in solid solution in scheelite. Lead, zinc, and arsenic are generally not strongly enriched in the ore.
A resource exists mainly in a gold-skarn body about 2,600 feet long and about 9 feet thick that has been followed down-dip about 1,000 feet. The deposit is fairly high grade; it contains about 1.24 million tons averaging 2.69 percent copper, 0.18 ounce of gold per ton, and 0.96 ounce of silver per ton (UNC Teton Exploration and Drilling, Inc., 1982).Metallurgical testing by the U.S. Bureau of Mines suggests that most of the gold in the ore is particulate but fine grained. Oxidized copper minerals contained in the ore seem to be serious cyanogens. Ordinary leaching with 20 pounds of NaCN per short ton of concentrate recovered only about 45 percent of the gold. Satisfactory recovery of gold was made on a test sample pre-leached with sulfuric acid to dissolve oxidized copper minerals (R.W. MacDonald, written communication, 1989); MacDonald pointed out the potential hazard involved with the acid pre-leach and later production of HCN if acid was not neutralized. An alkaline ammonia pre-leach of oxidized copper minerals apparently was not tried.
|Geologic map unit||(-146.700664733441, 63.217663577559)|
|Mineral deposit model||Cu-gold skarn, similar to the Cu skarn of Cox and Singer (1986; model 18b).|
|Mineral deposit model number||18b?|
|Age of mineralization||Cretaceous.|
|Alteration of deposit||Alteration zoning is recognized within the endoskarn and exoskarn (Kurtak and others, 1992). Endoskarn development is related to the degree of original rock calcium metasomatism and spatial distribution of the original limestone and volcanic rocks (UNC Teton Exploration and Drilling, Inc. 1982). Four stages of exoskarn development are recognized: (1) skarnoid, (2) main, (3) hydrosilicate, and (4) late hydrothermal. The skarns consist mainly of garnet and clinopyroxene and have undergone retrograde metamorphism and silica-clay alteration (Kurtak and others, 1992).|
|Workings or exploration||
Resource Associates of Alaska discovered the Zackly skarn deposit and staked claims in 1979-1980; they drilled 9,723 feet of core in 1981. A partnership with UNC Teton Exploration and Drilling, Inc. was formed, and 19,210 feet of diamond drilling was conducted in 1982 along with trenching and geophysical surveying. Copper, silver, and mercury soil geochemistry was successfully utilized to locate skarn mineralization in overburden areas (Ford, 1988; Resource Associates of Alaska, 1981). In 1986 the property reverted to Resource Associates of Alaska, which was subsequently bought out by Nerco. In 1987 a joint venture of Nerco and Boulder Gold conducted 12,000 feet of reverse circulation and 3,000 feet of diamond drilling. By 1990, the Zackly property was controlled by Pacific Northwest Resources and was optioned to Phelps Dodge Corporation, which completed a limited drilling program. The property was optioned to Hemlo Gold in 1993-34, which completed rock and soil sampling, IP geophysical surveys, and 1,500 feet of reverse circulation drilling. Since 1994 only limited exploration and assessment work has been done on the property.
The U.S. Bureau of Mines collected a 300-pound bulk sample for beneficiation studies at the U.S. Bureau of Mines Salt Lake Research Center (Kurtak and others, 1992). Only 18 percent of the gold in a sample that assayed 0.072 ounces of gold per ton were recovered in a bulk flotation test. Forty-five percent of the gold was recovered in a cyanide amenability test of a 1,000-gram sample ground to -325 mesh and leached for 72 hours (R.W. McDonald, written communication, 1989). Using a 3,965-gram sample ground to various sizes from +20 to -325 mesh improved recovery only to 48 percent. A factor that inhibits gold recovery is the high content of oxidized copper minerals in the ore, which interferes with the NaCN solution. Gold recovery increased to 98 percent after a sulfuric acid preleach of oxidized copper minerals (R.W. McDonald, written communication, 1989).As part of their 2016 Stellar project work, Millrock Resources Inc., in a joint venture with Vista Minerals Pty Ltd., collected 7 lines of induced-potential measurements over the Zackly copper-gold skarn. They identified chargeability anomalies within copper-in-soil anomalies, which appear to extend the strike length of the skarn to both the east and west (Athey and Werdon, 2017).
|Indication of production||None|
UNC Teton Exploration and Drilling, Inc. (1982) defined a resource of 1.24 million tons of 2.69 percent copper, 0.176 ounce of gold per ton, and 0.96 ounce of silver per ton. This mineralization occurs in a steeply dipping east-west striking body with 2,600 feet of strike length and an average thickness of 8.5 feet extending 1,000 down-dip. U.S. Bureau of Mines beneficiation studies indicated enhanced gold recovery will be necessary for the deposit to become economic (Balen, 1990 [USBM OF 40-90]).Ross Glanville and Associates (1996) estimated that the resource defined by exploration through 1989 consisted of 1,407,000 tons that graded 2.19 percent copper, 0.83 ounce of silver per ton, and 0.132 ounce of gold per ton. The reserves in the so-called main ore body are not yet sufficiently delineated to be able to carry out a feasibility study; however, a preliminary evaluation was completed by consultants in 1996. On the basis of the number of limiting input parameters and utilizing a mineable reserve of 1,080,000 tonnes of ore, the net present value of the Zackly project was calculated to be $0.7 million. A 50 percent increase in reserves was considered likely to be achieved by additional exploration, which would increase the net present value to $7.4 million (Ross Glanville and Associates, 1996).
Additional commentsThe copper-rich Nikolai Greenstone assimilated by the quartz monzodiorite is speculated to be the source of the metals in the skarn deposit (UNC Teton Exploration and Drilling, Inc., 1982). Pacific Alaska Resources (PO Box 145, Battleground, WA 98604; ph 360 687-2763) controls the property and has additional information about the Zackly prospect.
Balen, M.D., 1990, The feasibility of mining in the Valdez Creek mining district, Alaska: U.S. Bureau of Mines Open-File Report 40-90, 58 p.
Ford, M.J., 1988, Geology and Mineralization of the Zackly copper- and gold-bearing skarn, central Alaska Range, Alaska: Fairbanks, Alaska, University of Alaska, unpublished thesis, 157 p., 1 plate.
Kurtak, J.M., Southworth, D.D., Balen, M.D., and Clautice, K.H., 1991, Mineral investigations in the Valdez Creek Mining District, south-central Alaska: U.S. Bureau of Mines Open-File Report 1-92, 658 p., 2 sheets, scale 1:250,000.
Nokleberg, W.J., Lange, I.M., Roback, R.C., Yeend, Warren, and Silva, S.R., 1991, Map showing locations of metalliferous lode and placer mineral occurrences, mineral deposits, prospects, and mines, Mount Hayes quadrangle, eastern Alaska Range, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-1996-C, 42 p., 1 sheet, scale 1:250,000.
Resource Associates of Alaska, 1981, Report on the Central Alaska Range Joint Exploration Program-- Zackly, Tucena Creek, and Powell prospects: Unpublished company reports available from Pacific Northwest Resources Company, Vancouver, Wash., 176 p.
UNC Teton Exploration and Drilling, Inc., 1982, Geology and mineralization of the Zackly gold-copper skarn prospect, Alaska Range, Alaska Final Report 1982: Unpublished company report, available from Pacific Northwest Resources Company, Vancouver, Wash., 139 p.
|Reporters||W.T. Ellis (Alaska Earth Science), C.C. Hawley (Hawley Resource Group), and W.J. Nokleberg (USGS); M.B. Werdon (DGGS)|
|Last report date||8/26/2017|