Wednesday, February 10, 2010

Gold Geology and Prospecting, Alaska

A Christmas tree gold nugget attached to the side of a
rounded pebble at Snow Gulch. This nugget likely grew
in place in the gold placer.
Gold was discovered in Alaska in 1848 in gravel along the Kenai River in southern Alaska. Nearly 2 decades later (1865-1866) another discovery on the Seward Peninsula near Nome also failed to attract much interest. By1869, the first commercial gold deposits were found in gravel at Windham and Sumdum Bays near the future site of Juneau. And the first attempt to mine lode gold occurred at about the same time (1870) near the present site of Sitka (75 miles west of Juneau). Extensive copper deposits were found on Prince of Wales Island (150 miles southeast of Sitka), but because of remoteness, these were not developed for years. Then in 1898, rich gold placers were discovered in the Seward Peninsula at Nome. This discovery started a major rush involving thousands of prospectors, while the vast Yukon basin for the most part remained mostly unexplored due to inaccessibility other than by river. It wasn’t until 1902 that gold discovered near Fairbanks resulted in a rush to that region.
Location of the Donlin Creek discovery site, Alaska.

The harsh Alaskan environment delayed serious prospecting and mining for decades and still continues to stymie exploration activity even though there are likely dozens of hidden major and world-class gold and base metal deposits. Yet, Alaska has become an important source for gold, silver, other metals and energy resources.

From 1869 to 2008, Alaska produced a little more than 40 million troy ounces of gold from a few lode deposits and widespread placers. With so many placers, it is likely that many hidden lode deposits remain undiscovered upstream. Thus, exploration for the source should focus on a search for placers; first by examining the characteristics of the placer gold, and then by searching for concentrations of placers in a given region. For instance, rough gold with a texture of corn flakes with rough edges along with fragile nuggets and/or large nuggets suggest a nearby lode source.

In 2008, six large-scale lode mines operated in Alaska; four were mined for gold. Several small-scale, lode-gold operations were also reported along with 175 placer mines. In total, Alaska produced 800,000 ounces of gold in 2008, more than any other state except Nevada. Placers in Alaska are widespread, found in nearly every major river system and in beach sands at Nome, Kodiak Island, Yakataga, Lituya Bay and the Cook Inlet. Just in the past three decades, two major lode-gold discoveries were made at Donlin Creek and Pebble. These could result in world-class lode mines if mining permits can be obtained.

Lode mineralization is typically associated with (Mesozoic) felsic plutons (granite, granodiorite) and volcanic rocks intruded into Precambrian and Phanerozoic age rocks. Gold has also found throughout much of Alaska except in the vast uninhabitable swamps of the Yukon Flats and on the North Slope of the Arctic between the Brooks Range and the Beaufort Sea. In the vast central uplands and plains and in the Pacific Mountains to the south, numerous gold, silver and massive sulfide deposits are found. The central uplands and plains form a roughly east-west trending terrain between the Brooks Range to the north and the Pacific Mountains to the south. This terrain is a vast expanse of alluvial lowlands and eroded plateaus known for placer gold that includes the Seward, Koyukuk, Yukon and Kuskokwim uplands and peninsulas that overlook the broad flat plains of the Yukon River. This terrain includes Kuskokwim, Nome, Council and Fairhaven.

In the Copper River Region of the Pacific Range to the south, the bedrock is formed of Late Paleozoic to Quaternary graywacke, slate, and greenstone with lesser carbonates. Felsic (granite, granodiorite) and mafic (dunite, quartz diorite) intrusives invaded the basement terrain during late Mesozoic to early Tertiary. Mineralization includes copper, gold, silver, molybdenum, antimony, nickel, chromite, lead and zinc, but only copper, gold and by-product silver were commercially mined. Mines near McCarthy in the Nizina district and in the southwestern and northeastern parts of Prince William Sound accounted for most copper produced in Alaska.
Dredge at Flat Alaska

At the Greens Creek underground mine in the Admiralty district 18 miles south of Juneau, a complex silver-rich polymetallic massive sulfide was discovered in (Late Triassic) Hyd Group metamorphic rocks. The mine is one of the largest silver operations in the world. Discovered in 1975, production didn’t begin until 1989. Much of the ore is found as silver-zinc-lead-gold-barium massive sulfides hosted by hydrothermally altered, complexly folded and faulted metamorphosed phyllite, argillite and lesser mafic volcanics. Mineralization is distinctly laminated and exhibits replacement textures. Sulfides include pyrite, sphalerite, galena, tetrahedrite and tennantite. In 2007, 68,000 ounces of gold, 8.6 million ounces of silver, 63,000 tons of zinc and 21,000 tons of lead were recovered. Ore reserves were reported at 33 million ounces of silver, 257,000 ounces of gold and 237,000 tons of zinc at grades of 14 opt (opt=ounces per ton) Ag, 0.11 opt Au, 10% Zn and 4% Pb in 2006. Past operations in this region included the Alaska Empire (a gold-quartz deposit that averaged 0.25 opt Au) and the Funter Bay mines (a copper-nickel-cobalt deposit hosted by a gabbroic pipe).

Forty-five miles north-northwest of Juneau, the Kensington underground operation in the Berners Bay mining district is anticipated to begin production in 2010 at 120,000 oz/yr. This and Julian mine, are located adjacent to one another. These historical mines were dug on gold-quartz-veins reported at an average ore grade of 0.23 opt. A 2 million ounce resource has been drilled. These properties lie at the northern end of the Juneau gold belt, a narrow, 120-mile long structural zone consisting of deformed and metamorphosed sedimentary and volcanic rock intruded by diorite. The Kensington mine is described to have multiple mesothermal quartz-carbonate-pyrite veins with discrete quartz-pyrite veins in diorite (Cretaceous) over a vertical extent of >2,500 feet. Gold is predominantly gold-telluride (calaverite) but also found as inclusions in pyrite.

Stibnite with gold at Snow Gulch
To the north and west of the Copper River Region, the Kenai Peninsula-Cook Inlet region encloses the Susitna and Matanuska Rivers near Turnagain Arm. This region is bordered to the north by the Alaska Range and south and east by the Chugach and Talkeetna Mountains. It includes the Moose Pass, Hope, Homer, Seward, Girdwood, Turnagain Arm, Valdez Creek, Willow Creek and Yentna-Cache Creek districts. The Kenai Mountains consist of limestone, chert and tuff (Triassic) that rest on older metamorphic volcanic and clastic rocks. These Triassic rocks are overlain by volcanics (Jurassic) and deformed and metamorphosed slate and greywacke (Late Cretaceous). These are locally intruded by felsic intrusives. The Kenai Formation (Tertiary age) in this region is coal-bearing and is inter-layered with sandstone and claystone representing an important source for oil and gas. Gold was initially discovered in the Kenai River, but it wasn’t until the 1890s that minable placers were found in the Turnagain Arm area.

The first lode deposits in this region were discovered in 1896 within the Moose Pass-Hope district; however these lodes, even though rich, were small. Lodes at Moose Pass-Hope consist of fissure veins. At the old Hirshey mine near Hope, mineralization included gold with arsenopyrite (arsenic sulfide), galena (lead sulfide), sphalerite (zinc sulfide), pyrite (iron sulfide) and chalcopyrite (copper-iron sulfide) in quartz, calcite, ankerite gangue (gangue is waste material). Free gold was identified in quartz with galena and sphalerite. Placers were found in Crow, Resurrection, Palmer, Bear, Mills, Falls, Copper and Valdez creeks. At Valdez Creek, a paleo-channel (the Tammany Channel) was discovered and mined by hydraulic and underground methods in the early part of the 20th century. Placers were also discovered in the Yentna-Cache Creek district in 1905. One of the more productive districts was found north of Palmer and Wasilla, where quartz diorite intruded schist. These rocks have minable quartz veins in shear zones that are found along the margin of the quartz diorite. Gold is found with pyrite, arsenopyrite, sphalerite and chalcopyrite. More than a million ounces of gold have been recovered from this region.
This 20+ ounces was recovered in one
week at South Pass, Wyoming by two prospectors

A giant, 9 billion tonne ore deposit, known as the Pebble copper-gold porphyry was discovered in the mid-1980s west of Cook Inlet. This is one of the largest Cu-Au porphyry deposits in the world and hosts 72 billion pounds of Cu, 94 million ounces of Au, and 4.8 billion pounds of Mo. Permitting is scheduled for 2010; however the deposit lies in an environmentally sensitive area, which will make it very difficult to obtain all permits. The deposit is hosted by porphyritic granodiorite and tonalite (Upper Cretaceous) intruded into deformed sedimentary rock (Jurassic to Cretaceous).

The ore body (86 to 89.5 million years old) runs from surface to >5,500 feet deep. In the western part of the deposit, mineralization is found in complex granodiorite cupolas, diorite sills, older intrusions, breccias and sediments. This part of the deposit is exposed at the surface and expressed as gossan (rusty rock). The ore body extends eastward across a fault contact at depth. East of the fault, mineralization is found in abundant sills and in intruded sediments. Farther east, and deeper, the sills coalesce into a deeply-buried mineralized granodiorite pluton. The eastern extent of the deposit is overlain by (Tertiary) sedimentary and volcano-sedimentary rocks. Sulfide and oxide ore minerals include pyrite, chalcopyrite, molybdenite (molybdenum sulfide), bornite (copper iron sulfide), minor covellite (copper sulfide), chalcocite (copper sulfide), digenite (copper sulfide) and magnetite (iron oxide).

To the north in the Kuskokwim River basin, placer gold was found in the Tuluksak River, Bear Creek, California Creek, Salmon River, Marvel Creek, Canyon Creek, Kwethluk River, Crooke Creek, Donlin Creek, Flat Creek, Snow Gulch, Ruby Gulch, Quartz Gulch, Queen Gulch, Lewis Gulch, George River, Julian Creek, Murray Gulch, New York Creek, upper Holitna River, Taylor and Fortyseven Creeks. These deposits are located within the Aniak, Bethel, Georgetown, Goodnews Bay, McGrath, McKinley, Stony and Tuluksak districts

The Nixon Fork underground mine 30 miles to the northeast in the McGrath district had limited production in 1917 and in the 1950s. The deposit is a high-grade gold-copper skarn (altered limestone) enclosing 131,500 ounces of gold (Bluemink, 2009). Eighty miles north of the Nixon Mine, placers in the Ruby district Alaska, include several sizable nuggets including the largest found in Alaska – a 294-ounce nugget found on Swift Creek in 1998. Nuggets have also been found at nearby Long Creek, Poorman Creek and Moose Creek. The presence of several sizable nuggets indicates that a nearby hidden rich lode (or lodes) likely remains to be found.

An abandoned gold dredge at Flat Alaska, 1988.
At 75 to 100 miles southwest of McGrath, placers in the Georgetown district are mined at Snow Gulch and Julian Creek. Gold was also found in nearby Omega, Lewis, Quartz, Ruby, Snow and Queen Gulches. This region, known as Donlin Creek, includes some pristine gold flakes and fragile nuggets suggestive of a nearby lode source which attracted the attention of WestGold Exploration in the 1980s. As a result, geologists of WestGold discovered a large disseminated gold deposit with >5-mile strike length. Drilling over several years by various companies identified proven and probable resources of 29.5 million ounces with an indicated resource of 10 million ounces. Thus, Donlin Creek is similar in size to the legendary Homestake mine (which produced a total of 41 million ounces during its lifetime) and hosts as much gold as has been mined in all of Alaska from the 1869 to 2007!

Donlin Creek is one of the largest undeveloped gold deposits in the world. If developed based on 2009 plans, the property will become one of the largest gold mines in the world. Mine permit applications were submitted in 2009 and construction proposed by 2012. The deposit consists of a group of felsic sills and dikes that host gold in association with quartz veinlets and breccias. The higher gold values are associated with arsenopyrite and stibnite (antimony iron sulfide) and associated with sills that intrude a thick sequence (>5000 feet) of folded graywacke, sandstone, and shale. Mining is anticipated at 1.5 million oz/yr from ore with average grade of 0.07 to 0.08 opt Au. In this region, gold at Julian Creek sits in a similar setting as Donlin Creek. And mercury- gold anomalies at DeCourcy to the west and placer gold at Flat Creek to the north are also of interest.

Significant gold production at the Fort Knox open pit mine in the Tanana River region of the Fairbanks mining district northeast of Fairbanks, and at the Pogo underground mine in the Goodpaster district near Delta Junction, 70 miles east-southeast of Fairbanks resulted in 330,000 ounces from Fort Knox and 260,000 ounces from Pogo in 2007. The Fort Knox mine, located 25 northeast of Fairbanks is a low-grade (0.024 opt Au) ore body consisting of finely disseminated gold in (Cretaceous) porphyritic granodiorite and along margins of quartz veinlets and vein swarms controlled by NW-structural trends. Creeks draining the granodiorite include some of the more productive placers in the district which led to exploration and subsequent discovery of the lode. Gold production commenced in 1997: total reserves and resources are estimated at 4.3 million ounces. Nearby properties include the True North open pit mine (which suspended operations) and the Fairbanks, Cleary and Ester Creek placers. Ore at Pogo occurs in high-grade gneiss, schist and quartzite of the Yukon-Tanana terrain which is intruded by granite. The ore is found in two shallow-dipping quartz veins. A third vein was encountered at depth.

Dr. Paul Graff looks at abandoned monitor in Alaska, 1988
In the Seward Peninsula, within the Nome Mining District along the west coast of Alaska, is the site of a historical gold rush that involved some 20,000 prospectors who constructed a lawless, wild-west tent city on the beachfront tundra following placer gold discoveries in 1898. Only limited amounts of gold were initially found in the region leaving most prospectors penniless until someone panned beach sands resulting in a major discovery. Gold was found in the beach and traced over tens of miles up and down the beach from Nome. Gold was also found along Anvil Creek to the north, which was a source of several large nuggets including the second-largest found in Alaska (182 troy ounces) suggesting possibilities for hidden lode deposits.

Hardrock mines near Nome include Rock Creek and Big Hurrah. Rock Creek was originally a placer exploited in the 20th century. Early prospectors explored underlying rocks and found gold in a cluster of low-grade sheeted gold-sulfide-quartz veins in deformed metapelite (mica schist) that were uneconomic. However, at higher gold prices in the 21st century, the property was considered for mining. Construction began in 2006, a few miles north of Nome. The mine produced ore in 2008 until design flaws in the mill resulting in termination of operations two months later.

Gold nuggets from Julian Creek, Alaska
The Big Hurrah, the largest underground mine in the Seward Peninsula (40 miles east of Nome) initially operated in 1908 with only sporadically production. The mine is underlain by the Nome Group (metapelites containing mixed mafic schists, metapelite, calc schists and marble). The main ore body is known as the Abion vein, a sheared quartz vein breccia with fine-grained sulfides. Combined resources for the Big Hurrah and Rock Creek mines are 510,000 ounces of gold with another 32,000 ounces of indicated and inferred resources. Ore grades average 0.05 opt Au.

The Red Dog open pit mine in the remote Arctic 250 miles north of Nome accounted for 55% of Alaska’s mineral wealth in 2008. This is a world-class deposit that yielded 515,000 tonnes of zinc, 122,600 tonnes of lead, and 283 tonnes of silver valued at more than $1 billion. The Red Dog is listed as the largest zinc mine in the world with 61.4 million tonnes of ore reserves grading at 17.1% Zn and 4.5% Pb. Permits were applied to expand operations into the adjacent Aqqaluk deposit (containing an additional 56 million tonnes of ore). Other zinc ore bodies were found at Anarraak and Su-Lik, 6 to 13 miles north of Red Dog. These are stratiform massive sulfides in black shale and carbonate that represent sedimentary exhalative deposited on a sea floor during the Mississippian Period.

Read more at the ICMJ Prospecting and Mining Journal and at Donlin Creek discovery

Part of the Donlin Creek discovery team. Standing left is Robert Rutherford and standing to the right is Paul Graff. Sitting on the right is Dan Hausel (GemHunter) in 1989. Not shown are additional discoverers Mark Bronston, Richard Garnett, the late Bruce Hickok and the late Toni Hinderman. 

GOLD - Geology, Prospecting and Exploration

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Also, one of 7 members on the Donlin Creek, Alaska gold discovery - a deposit with drilled and inferred resources of 39.5 million ounces with >$42 billion in gold! Reported by the Northern Miner as one of the largest undeveloped gold deposits in the world and the largest undeveloped gold deposit in North America! A deposit as large at the Homestake, and a deposit with as much gold as been mined during the entire mining history of Alaska! In this book, the author will provide insight into what to look for and clues used to find gold deposits. It also describes locations of dozens of gold deposits.

Flying to Flat Alaska for the 4th of July celebration, 1989.

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