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).

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).

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]).