Terrane wreck lures explorers to Alaska
Nearly 400 million years of mineral-rich formations crashing into each other created among world's most prospective geology
Last updated 2/28/2010 at Noon
Geologically, Alaska is a terrane wreck, with multiple tectonic plates dumping their mineral payloads over the landscape. Geologists are still sifting through the wreckage in many places across the state to determine which mineral deposits were dumped by which terranes and when - a task not always easily accomplished as pileups have resulted, in many cases, from multiple mineralization events happening in the same geographical regions over time. A terrane is a series of related rock formations.
"We have got all these mineral deposits and mining districts because the geology of Alaska is composed of rocks of multiple ages, formed by a wide variety of geological processes, arranged in these various terranes that slammed into each other in what is present-day Alaska. Each of the terranes had components like back-arc basins, volcanic arcs, and basins; and each of those components have associated mineral deposits," said Alaska Division of Geological & Geophysical Surveys geologist David Szumigala.
From the zinc-rich volcanic massive sulfides deposited in the north about 385 million years ago to the deposits still being formed today in the geologically active Southwest region of the state, the more than 400 million years of mineral-rich terranes crashing into the land mass that today is Alaska has resulted in 365 million acres of some of the most mineral-rich real estate on earth.
Explorers have discovered several world-class deposits in Alaska over the past 20 years. Among these recent finds are the Pebble deposit, which is estimated to contain 80.6 billion pounds of copper, 107.4 million ounces of gold and 5.6 billion pounds of molybdenum, and the 40 million-ounce Donlin gold project.
It is commonly believed that large mineral deposits in Alaska are only found in remote and logistically challenged regions of the state, but geologists are continuing to make discoveries adjacent to Alaska's relatively few highways and byways. A stone's throw away from the highway running north from Fairbanks to the oil fields of Alaska's North Slope, the 12.5 million-ounce Livengood gold deposit is one such find.
Geologists familiar with Alaska expect that more world-class deposits lay hidden just below the surface of the vast and underexplored state.
"The thing about Alaska is there are a lot of low-lying areas out there that are covered with vegetation, glacial gravels and things that haven't been explored thoroughly, or at all," Millrock Vice President of Exploration Phil St. George said. "I, and others, think there is a lot of potential to find other Pebbles, other Donlins, other huge gold systems."
"Alaska hasn't experienced near the amount of exploration that has taken place in British Columbia and the Yukon (Territory)," he added.
Tintina Gold Belt
The Tintina Gold Belt is a particularly rich and vast aurum province that stretches across the breadth of the state.
Sometimes referred to as the Golden Arc, the Tintina is bounded on the north by the Kaltag-Tintina fault and on the south by the Farewell-Denali fault, and arches from northern British Columbia, through southern and central Yukon, and across Alaska into the southwest region of the state.
In Alaska, the Tintina cuts a swath about 850 miles, or 1,350 kilometers, long across the middle of the state, averaging more than 200 miles, or 320 kilometers, wide. The province - which hosts such large gold deposits as Fort Knox, Pogo, Donlin Creek and Livengood - covers a multitude of placer gold districts. A lode source for much of the alluvial gold has yet to be found.
The gold mineralization of the Tintina Gold Belt in Alaska can be divided into two distinct groups: the eastern portion of the belt, running from the Alaska-Yukon border about 300 miles, or 484 kilometers, into the state; and the 74,000-square-mile, or 190,000-square-kilometer, Kuskokwim Gold Belt of Southwest Alaska.
Alaska's two currently operating gold mines (Fort Knox and Pogo) as well as the estimated 12.5 million-ounce Livengood gold deposit are all found in the eastern extent of Alaska's portion of the Golden Arch. The intrusion-related gold mineralization in all three deposits is estimated to be about 90 million to 100 million years old.
Over the past 125 years more than 12 million ounces of placer gold have been recovered from this region, but the lode source of the alluvial deposits of historic mining districts such as Fortymile and Circle have yet to be found.
At the western end of the Golden Arc lies the Kuskokwim Mineral Belt, a 400-mile- or 645-kilometer-long belt of gold-rich terrain that runs from the Goodnews Bay region of Southwest Alaska to the northeast. The gold mineralization found in the Kuskokwim belt was deposited by a widespread pulse of magmatism and mineralization about 70 million years ago, making it about 20 million years younger than the Fort Knox-type mineralization to the northeast.
The nearly 40 million-ounce Donlin Creek gold deposit - being developed by Barrick Gold Corp. and Novagold Resources Inc. - is the crown jewel of the Kuskokwim. Over the past several years, majors and junior explorers alike have scoured the region in search of other large intrusive-related gold deposits, uncovering several promising prospects.
Besides Donlin Creek-style mineralization, the Kuskokwim belt also hosts Nixon Fork, a carbonate skarn deposit rich in copper, silver and high-grade gold; several mercury-antimony deposits; and a large past-producing placer platinum deposit.
Gold mineralization in Alaska extends well beyond the Tintina Gold Belt. With the exception of the oil-rich plains at the foot of the Brooks Mountain Range's North Slope, gold has been found in every geographical region of the state.
Some 10 million ounces of gold have been recovered from the Seward Peninsula in Northwest Alaska since the discovery of gold in 1898 by the "Three Lucky Swedes" a few miles from present-day Nome. Placer gold accounts for most of the yellow metal recovered from the 20,600-square-mile, or 53,350-square-kilometer, peninsula.
Novagold Resources Inc.'s Rock Creek Mine, not currently in operation, is the only modern hardrock mine on the Seward Peninsula. NovaGold's three properties on the peninsula - Rock Creek, Nome Gold and Big Hurrah - contain 500,000 ounces of probable gold reserves, 1.9 million ounces of measured and indicated resources and 0.3 million ounces of inferred resources.
"The (Nome) district has produced some 5 million ounces of alluvial gold. We certainly see the potential for additional targets there," NovaGold President Rick Van Nieuwenhuyse said.
Geologists are looking for both orogenic and carbonate replacement sources of the river and beach placers that lured tens of thousands to this far western region of Alaska.
Placer deposits in the Chandalar and Koyukuk-Nolan districts along the southern Brooks Range also drew fortune seekers at the turn of the 20th Century. High-grade orogenic veins are the suspected source of the rich alluvial deposits and large gold nuggets found in these placer districts located about 200 miles, or 320 kilometers, north of Fairbanks.
An estimated 84,000 ounces of gold has been recovered from the Chandalar district. About 76,000 ounces of the yellow metal, or about 90 percent of the total, was alluvial gold found in the stream beds and frozen bench gravels. Most of the remaining 10 percent of the total was recovered from the Mikado Lode.
Goldrich Mining Co., a junior mining company that has claims over most of the Chandalar district, said Mikado is one of about 30 auriferous quartz-sulfide veins that are now documented on the property. Very little modern exploration work has been completed at Chandalar.
The Koyukuk-Nolan district, which lies directly west of Chandalar, has produced about 350,000 ounces of placer gold. The region is well-known for producing large nuggets. The third (146 troy ounces), fourth (137 troy ounces), 14th (61 troy ounces) and 17th (55 troy ounces) largest gold nuggets in Alaska were found on the Hammond River, a tributary to the Koyukuk River.
A lode source for these big nugget producing placers has yet to be discovered. Silverado Gold Mines Ltd., though, is drilling what it believes to be the source of the placer gold found in Nolan Creek, another large nugget producing drainage in the district.
Explorers are also finding high-grade orogenic gold in Southcentral and Southeast Alaska, two regions notable for historical high-grade gold mines. Coeur d'Alene Mines Corp.'s Kensington gold mine in Southeast and Full Metal Minerals Ltd.'s Lucky Shot project are examples of these modern discoveries.
Three porphyry belts
At the largest known gold deposit in Alaska, the yellow metal is actually a byproduct in a porphyry copper-gold-molybdenum system. The latest resource estimate released by joint venture partners Anglo American plc and Northern Dynasty Minerals Ltd. estimates the Pebble deposit contains more than 107 million ounces of gold, 80 billion pounds of copper and 5.6 billion pounds of molybdenum.
The copper-gold-molybdenum was being deposited at Pebble during the same time period as much of the gold of Alaska's Eastern Interior.
"At about the time Pebble was being formed (around 90 million years ago) there was an oceanic plate being subducted under Alaska called the Kula Plate, and that seems to have been a fairly productive time period where Pebble, Fort Knox, Livengood and Pogo were created from the oceanic plate and the current Pacific Plate that is subducting Alaska," St. George explained. "The Kula Plate was still being subducted during the Donlin age."
A belt of porphyry deposits that stretches from Southwest Alaska along the Alaska Range and into the Yukon Territory dates to about the time Donlin was being formed, 65 million to 70 million years ago.
St. George explained, "There is a series of 65 million-year-old gold occurrences through the Alaska Range, including the Estelle deposit we (Millrock Resources) are working on."
A belt of much younger porphyries can be found on the Alaska Peninsula southwest of the Pebble deposit. The mineralization in this geologically active section of the "Ring of Fire" ranges from 10 million years to current.
The Alaska Peninsula and the Aleutian Islands represent an island arc formed by the Pacific Ocean plate diving under the North American plate. This region has received limited modern exploration and is prospective for both epithermal gold, and porphyry copper-gold mineralizing systems.
Several prospects have been identified along the nearly 1,000-mile- or 1,600-kilometer-long peninsula and island chain. Two holes were drilled for Full Metal Minerals at the 3 million-year-old Bee Creek porphyry copper-gold deposit in 2006. One hole, targeting a copper-gold-molybdenum geochemical anomaly that centers on a larger magnetic high measuring about 2,000 meters in diameter, cut 118 meters with 0.31 percent copper.
VMS in NW, SE Alaska
Two regions in opposite corners of Alaska host metal-rich volcanogenic massive sulfide deposits: the western half of the Brooks Range in Northwest Alaska and the entire Southeast Alaska panhandle.
The VMS mineralization found along the Brooks Range, a chain of mountains that spans the more than 600-mile-width of Alaska north of the Arctic Circle, was deposited during the Devonian age. The Arctic project, currently being explored by Novagold Resources Inc., is the best known of these 400 million-year-old deposits.
An estimate completed for the company in 2008 outlined an indicated resource at Arctic of 16.8 million metric tons containing 4.1 percent copper, 6.0 percent zinc, 0.83 grams per metric ton gold, 59.6 grams per metric ton silver and 0.94 percent lead. The estimate includes an additional inferred resource of 11.9 million metric tons with an average of 3.6 percent copper, 5.0 percent zinc, 0.67 g/t gold, 48.4 g/t silver, and 0.80 percent lead.
Arctic is located in the Ambler District, a belt of VMS deposits that stretches at least 70 miles, or 115 kilometers, east-west. Similar Devonian-age VMS mineralization has been discovered as far as 140 miles, or 230 kilometers, east of the deposit. Hints of this Devonian age mineralization have also been found on the Seward Peninsula, about 200 miles, or 320 kilometers, to the southwest.
Though much of the VMS mineralization discovered in northern Alaska is very high grade, explorers have only scratched the surface due to its remoteness.
"It hasn't been drilled off very well because it is so remote. People have just worked on the very near-surface mineralization, so there is a lot of potential at depth. There is potential for other Arctics. If there was some infrastructure or development starting to occur at Arctic, the district would probably get another generation of deeper, more thorough drilling, St. George observed.
In the opposite corner of the state, stretching from the Niblack Project in the south to the Palmer Project in the north, a belt of late-Triassic VMS deposits can be found along the entire 450-mile, or 725-kilometer, length of the Southeast Alaska panhandle.
The precious-metal-rich Greens Creek Mine is one such deposit. More than 10 million tons of ore have been mined at Greens Creek, and the mine has about another 8 million tons of reserves that are estimated to average 426 g/t silver, 3.4 g/t gold, 10.5 percent zinc and 3.8 percent lead.
The western reaches of the Brooks Range host several high-grade zinc sedimentary exhalative deposits, the most notable of which are being mined by partners Teck Resources Ltd. and NANA Regional Corp. at Red Dog, one of the largest producing zinc mines in the world. The high zinc grades found at this SEDEX deposit is what gives it distinction. The Aqqaluk deposit - the next deposit to be mined by Red Dog - contains 51.6 million tons of ore with an average zinc content of 16.7 percent along with 4.4 percent lead.
Anarraaq and Lik, located 7 miles, or 9 kilometers, and 14 miles, or 17.5 kilometers, respectively, from Red Dog, are two other known high-grade SEDEX zinc deposits.
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.
"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," St. George said.
"There are some SEDEX deposits that are similar genetically (to Red Dog), but not near as big, found in the central Alaska Range," he added.
Though an economic platinum lode-source has yet to be discovered in Alaska, about 650,000 ounces of the extremely rare metal was recovered from Salmon River placer deposits in the Goodnews Bay region of Southwest Alaska. An ultramafic intrusion is believed to be the lode source of these historical placers. A 3,000-meter drill program was conducted on the intrusion in 2008, but the explorer failed to find what it sought and did not follow up on initial drilling.
Small amounts of placer platinum as a byproduct of gold mining also have been recovered from Dime Creek on the Seward Peninsula. Linux Gold Corp. reports that soils samples collected near Dime Creek contained up to 160 parts per billion gold, 174 ppb platinum, and 144 ppb palladium.
The Wrangellia Terrane, a distinct belt that runs from Southcentral Alaska through southwestern Yukon and along the Coast of British Columbia, is prospective for platinum and other related metals.
An ultramafic intrusion at the Man project located on the southern slopes of the Alaska Range is the source of platinum group elements, nickel and copper found on the property.
One hole drilled by Pure Nickel Inc. in 2007 intersected 77.4 meters of disseminated sulfides with an average grade of 0.26 percent nickel, 0.12 percent copper, 0.139 g/t palladium and .062 g/t platinum.
According to a report by U.S. Geological Survey geologist Robert Kelley, late Triassic flood basalts characteristic of the Wrangellia Terrane in southern Alaska have been found at least 50 miles, or 80 kilometers, southwest from the southern Alaska Range through the Talkeetna Mountains.
No ultramafic rocks have been mapped to date in this possible extension of the Wrangellia, but field geophysical data suggest the possibility of buried ultramafic bodies, and nickel-copper-platinum group element stream sediment geochemical anomalies occur in close proximity to the basalts.
Across the border in southwestern Yukon, explorers are also finding platinum in the Wrangellia Terrane.
Rare earth elements, tungsten, uranium, niobium and diamonds also have been found in Alaska.
Rare earth elements, which are becoming increasingly important due to their use in many modern high-tech devices, have reportedly occurred in several places in Alaska. The most notable known deposit is at the Bokan Mountain Granite Complex on Prince of Wales Island in Southeast Alaska.
A 1989 study by the U.S. Geological Survey ranked Bokan as housing the single largest combined heavy and light rare earth deposit in North America. Results from drilling by Ucore Uranium Inc., show REE mineralization, including yttrium, zirconium, beryllium and niobium.
REEs have been discovered near Manley Hot Springs in Interior Alaska. A reserve of about 100,000 pounds of niobium - used in high-grade structural steel, super alloys and superconducting magnets - is estimated to be present in placer tailings at Idaho Gulch. Known as Tofty Ridge, the niobium prospect is near a 12-mile, or 19-kilometer-long group of cassiterite- and gold-bearing placer deposits known as the tin belt. The area is underlain by Cretaceous and Tertiary granitic plutons.
REEs also have been reported to occur on the Man property in the Wrangellia Terrane and in dredge tailings on the Seward Peninsula.
Though kimberlites are not believed to exist in Alaska, diamonds have been discovered in the state. The first three documented macro-diamonds in Alaska were found by three different operations about 1-2 miles, or 2-3 kilometers, from each other at Crooked Creek in the Circle Mining District in the early 1980s.
USGS also documented microdiamonds found with placer platinum deposits at Goodnews Bay in the 1970s.
As geologists sort through Alaska's terrane wreckage, they are convinced they will discover more surprises in this vast and mineral-rich state.