Silver prices have doubled over the past year and interest in discovering deposits rich in the white metal is pacing its soaring value; and Alaska, the United States top silver producing-state, is an obvious place to search.
Though 14 million ounces were recovered in Alaska last year, accounting for about one-third of the U.S. production, the Far North state has only two mines extracting poor man’s gold, as the metal is sometimes irreverently referred.
Hecla Mining Company’s Greens Creek mine near Juneau is Alaska’s lone primary silver mine and at 7.2 million ounces takes the title of Alaska’s largest silver producer in 2010. At the zinc-rich Red Dog mine in Northwest Alaska the white metal is a by-product. Teck representatives informed Mining News that some 7 million ounces of silver was captured in the mine’s lead and zinc concentrates last year, though the amounts retained in the refining process is unclear.
Between these two silver producers at opposite corners of Alaska lies a vast area of silver potential.
Promising VMS deposits
Volcanogenic massive sulfide deposits similar to Greens Creek hold great promise for explorers seeking silver in Alaska.
The United States’ top silver producing mine lies in the midst of a 450-mile-, or 725-kilometer-, long belt of late-Triassic VMS deposits that spans the Southeast Alaska panhandle and carries northward, encompassing the Windy Craggy deposit in British Columbia.
At some 8 million metric tons of reserves averaging 426 grams per metric ton silver, 3.4 g/t gold, 10.5 percent zinc and 3.8 percent lead; Greens Creek is by far the most silver-enriched of the known deposits along this trend.
The ore at Constantine Metal Resources Ltd.’s Palmer project at the north end of Alaska’s portion of the VMS belt contains about 31 g/t silver — along with 2 percent copper, 4.8 percent zinc, 0.30 grams per metric ton gold. At the southern end of the VMS trend, Heatherdale Resources Ltd. has defined 4.14 million metric tons of indicated resources averaging 38.7 g/t silver, 1.13 percent copper, 2.32 grams per metric ton gold, 2.27 percent zinc plus additional inferred resources in the Lookout zone of its Niblack project.
Woewodski Island is one Southeast Alaska prospect that demonstrates promise to be a Greens Creek contemporary. A hole drilled by Bravo Ventures Group Inc. in 2003 — which cut 1.8 meters grading 222 g/t silver, 6.34 percent lead and 16.15 percent zinc — underscores the potential of the prospect, but the limited drilling conducted here has yet to find thick zones of high-grade massive sulfides.
The Brooks Range, a chain of mountains that spans the more than 600-mile-width of Alaska north of the Arctic Circle also hosts VMS deposits enriched with silver.
Andover Ventures Inc.’s Sun deposit in the Ambler Mining District is one such example. A resource calculated for the project in 1977 estimates Sun has a resource of 20.3 million tons averaging 74 g/t silver, 1.9 percent copper, 4.5 percent zinc and 1.2 percent lead.
Similar VMS deposits and prospects, including NovaGold Resources Inc.’s Arctic project, span this northern mountain chain.
Heatherdale Resources Inc.’s silver-enriched Delta property – located in the eastern Alaska Range about 36 miles, or 58 kilometers, southwest of Tok – represents an emerging VMS district in Interior Alaska.
An inferred resource of 15.4 million metric tons grading 62 g/t silver, 0.6 percent copper, 1.7 percent lead, 3.8 percent zinc and 1.7 g/t gold has been calculated for Delta. Large massive sulfide boulders averaging 113 g/t silver 7.3 percent lead, 5.6 percent zinc, 113 g/t silver and 0.7 g/t gold have also been found at the 39,840-acre property
“Delta is an early-stage project, but it also represents an emerging massive sulfide district with the potential to be in the top tier of global districts of its type,” Heatherdale President and CEO Patrick Smith lauded.
Northwest SEDEX
The silver enriched ore at Red Dog is found in a high-grade sedimentary exhalative, or SEDEX, body.
Exhalative deposits are formed when mineral-rich brine is “exhaled” into the ocean creating layers of sulfide ore. This process continues today in the form of black smokers on the ocean floor.
Northwest Alaska is home to several other SEDEX deposits and is prospective for others.
“Red Dog is one of the greater lead-zinc deposits in the world and there is more potential for those in Northwest Alaska, as well as other places there are Paleozoic rocks that are similar to Red Dog,” explains Millrock Resources Inc. Vice President of Exploration Phil St. George.
According to a 2004 report written for the Society of Economic Geologists, the Anarraaq deposit consists of a barite body, estimated to be as much as 1 billion metric tons, and a zinc-lead-silver zone with an estimated resource of about 18 million tons at 18 percent zinc, 5.4 percent lead, and 85 g/t silver.
Lik is estimated to contain about 4.6 billion pounds of zinc, 1.5 billion pounds of lead and 41 million troy ounces of silver. Lik runs about 8 percent zinc, 2.6 percent lead and 1.5 ounces per ton silver; or about half the concentrates found at Anarraaq and Red Dog.
Statewide potential
In addition to the SEDEX- and VMS-style deposits that host Alaska’s two producing silver mines, several styles of mineralization spanning the state are known to host rich stores of the shimmery metal.
When mineral-rich brine is not exhaled, but instead is trapped in the ocean floor sediments, it forms replacement-style deposits. Full Metal Minerals Ltd. is exploring one such deposit at its Fortymile project in eastern Alaska.
Hole LWM10-64 drilled at Fortymile last year cut 5.89 meters averaging 198 g/t silver, 7.4 percent zinc, 13.8 percent lead. LWM10-68 intersected 4.4 meters averaging 314 g/t silver, 23.7 percent zinc and 23.6 percent lead
Epithermal gold prospects found along the Alaska Peninsula and the Aleutian Islands are typically contain high concentrations of silver, but are relatively unexplored.
Shumagin and Apollo, two such epithermal gold deposits being explored by Full Metal on Unga Island, contain a combined historical resource of 280,000 metric tons averaging 27.7 g/t gold and 92.6 g/t silver.
Silver Chalice – found on the banks of the Yukon River about 330 miles, or 530 miles, west of Fairbanks – reflects Interior Alaska’s potential for silver-enriched epithermal deposits. . Rock chip samples returned assay up to 462 g/t silver. Average silver-gold ratio of sample taken from Silver Chalice is about 40:1, making silver predominant metal at current prices of the two precious metals.
Alaska Department of Natural Resources Division of Geological & Geophysical Surveys Senior Minerals Geologist Dave Szumigala told Mining News that most of Alaska’s tin prospects such as Kougarok, Sleitat, Win, Won and Coal Creek contain silver and the white metal is typically a significant byproduct in porphyry copper deposits such as Pebble.
As silver prices soar, poor man’s gold will increasingly enhance the luster of mining projects across Alaska.
