SPECTACULAR SILVER BOULDER TREASURES
An Exhilarating New Arizona Find
An exhilarating new find made in 2018 that represents the largest surviving group of native silver nuggets in the world continues to generate global interest and excitement. Three huge silver boulders and an assortment of smaller specimens were discovered near Globe, Gila County, Arizona, during a careful search that lasted about two years. These enormous natural specimens of precious silver weigh 610 pounds collectively, or 8,896 troy ounces. The largest silver boulder weighed 417 pounds when it was unearthed, and now, after cleaning, tops 411 pounds.
These boulders were found near each other in an arroyo (or stream bed), where they had traveled only a short distance after becoming dislodged by weathering from their vein outcrops. They have not been waterworn or rounded, as they have not traveled down a river.
The trio of spectacular silver boulders was on exhibit at the 2020 Tucson Gem & Mineral Show® (TGMS). I am so glad I had the opportunity one morning before opening our booth to take a quick walk around the exhibits. I must confess, while there were fabulous mineral specimens of gold, tourmaline crystal clusters, and more, these silver boulders truly stood out for me. I was taken by their spectacular size and the mesmerizing textures on the surface of the boulders. Despite being dark and not flashy, they were truly imposing specimens. Their dark appearance is due to a natural coating of cerargyrite (AgCl - silver chloride). But what I found even more intriguing was the astonishing story of the rarity of the discovery.
I am not sure that the term boulder truly reflects the rarity of these specimens. We normally call boulders large rocks without huge monetary value. Boulders are often smooth rounded masses of rock with a diameter greater than 25cm that have been shaped by erosion and transported by ice or water from their original position. Boulders are bigger than cobbles.
The term nugget may be more appropriate, defined as a solid, native lump of precious metal. We are more familiar with gold nuggets, which range from a few grams to huge native gold specimens. In comparison, the world’s largest surviving gold nugget is the Pepita Canaã (Canaan Nugget), mined in Brazil in 1983, which has a gross weight of 60.82 kilos (134.10 pounds) and contains 52.33 kilos of gold, or 1,682 troy ounces of gold. So, compared to the largest surviving gold nugget, the silver nuggets recently discovered are two to three times larger, and fall into the boulder-sized category. Still, maybe, they should rather be called “mega-nuggets.”
Adding to the extreme rarity of discovering such silver boulders is the fact that silver disintegrates and falls apart naturally, so even if huge boulders were once formed, over the millennia, they have been broken down by nature’s forces.
At the TGMS show, I talked to Dr. Chris Osterman, the geologist, directly connected with the discovery of the silver boulders, as well as the future exploration of the underlying silver veins. As the program chair of the Old Pueblo Lapidary Club (OPLC) in Tucson, I invited Chris to give a presentation to our club’s monthly meeting, and he was very happy to come. He has shared with us in the past, his discoveries of mining Mongolian turquoise and Mexican leopard opal, and is a very knowledgeable and compelling speaker. He returned in March 2020 to share with OPLC members this amazing discovery, an unbelievable Arizona treasure.
Dr. Osterman has been involved with gold, silver, and copper mining for the last 35 years. After earning a Ph.D. in geology at the Colorado School of Mines, Chris has lived and worked in southern Africa, North and South America, and Central Asia. He served as the chief executive officer of First Mining Gold Corporation, a company dedicated to the exploration of gold deposits throughout the Americas, and is currently president of the Phoenix Silver Corporation, which holds the silver boulders and property.
THE 417 PROJECT AND DISCOVERY OF THE NUGGETS
On invitation, Dr. Osterman traveled to Globe to look at and evaluate the first couple of silver boulders, soon after local prospectors discovered them. After viewing the boulders, he and his partners formed the Phoenix Silver Corporation and named the venture “The 417 Project” after the weight of the largest boulder. Two years had passed between the discovery of the first nugget and the rest. Claims were staked, and more advanced metal-detecting equipment brought in.
The 417 Project consists of 100 lode claims, located about seven miles north of the copper-mining town of Globe. Within six months of discovery, more than 25 large nuggets (weighing a little more than 1 pound or 0.5 kg each) of native alluvial silver were found with metal-detectors. The total weight of the nuggets was nearly 730 pounds (330kgs). The claims now cover both the placer surface as well as the lode claims below.
According to the Geological Report filed by the Phoenix Silver Corporation and shared with me by Dr. Osterman, “the Globe-Miami Mining District is a prolific copper producer and still has three mines in full production. All copper comes from the Laramide age porphyry deposits, located 8km to the west of the 417 Project claims. The “417” area is underlain by Proterozoic diabase with the quartz-carbonate vein and presumable source of the native silver occurring near
the contact of the diabase and the Proterozoic Apache Group sedimentary rocks which the diabase intrudes.”
That means that the 417 Project silver claims formed in the Proterozoic age, are much older than the nearby younger porphyry copper ores. The Proterozoic Eon extended from 2500 mya to 541 mya (million years ago), which is roughly the time between the appearance of oxygen in the Earth’s atmosphere to just before the proliferation of complex life, such as trilobites and corals. The age of the silver has been pushed back from the 1919 USGS report by geologist F.L. Ransome, who thought it might be early Mesozoic (The copper deposits of Ray and Miami, Arizona, USGS Professional
Paper 115). Also, according to the same Phoenix Silver Corporation geological report, the composition of the silver nuggets has skeletal quartz up to 40% by volume, although occasionally, the nuggets are pure silver. One hand-held XRF analysis showed the silver contained 6% tin and 2% mercury, and that the outer rind of the nuggets is often covered with a 1mm layer of dark cerargyrite (AgCl — silver chloride). The native silver contains, as identified through a petrographic study of a sample, 2 percent mercury as an amalgam. It also contains acanthite, chlorargyrite, jalpaite, and mckinstryite.
The largest nugget, named aptly “Big Boy,” was found standing up in the ground. Weighing 417.8 pounds, it took a huge effort to drag the boulder out, a half-mile haul, without causing damage. Albeit being so heavy, it could easily be damaged and quickly transform from “the largest surviving silver nugget in the world,” as Osterman said, to the “two or three large silver nuggets.” The team wrapped the silver boulder like a mummy and very carefully transported it on a “sled” to their pickup truck. Once at their headquarters, they had to build a special winch to weigh the specimen and move it around. The boulder was then sent to Bryan Lees of Collector’s Edge in Golden, Colorado, to undergo special cleaning by soaking it in a chemical bath. The result is an imposing mega-nugget at 4 feet tall, with beautiful, threedimensional frilly surfaces and rustic-look textures that I find irresistible, similar to a weathered tree-bark. All of the large nuggets underwent the same chemical cleaning.
HISTORY OF MINING IN GILA COUNTY
For nearly 200 years, prospectors in the Southwest rumored about the existence of silver boulders lying right on or below the surface in Arizona, and that bonanza-grade silver lodes existed underground. It was
believed that Spanish conquistadors might have explored the Richmond Basin in Gila County, Arizona, for silver in the 16th and 17th centuries. The rumors were proved true with this amazing discovery in 2018. The silver mining area is within the famous Globe-Miami Mining District. This major mining area at the northern foothills of Pinal Mountains and the Globe Hills includes the famous Inspiration Mine and the activelyoperational Ray Mine and smelter in Miami. Both mines have produced huge amounts of copper, as well as beautiful chrysocolla in quartz mineral specimens.
The Globe-Miami Mining District sits about 90 miles east of Phoenix and about 104 miles north of Tucson. In 1873, the Globe Ledge Silver claims were discovered, and in 1875 Globe came to be as a mining camp, where shortly after formation, prospectors found silver in the Richmond Basin. Globe owes its name to a discovery - as the legend goes - of an unusual nine-inch, globe-shaped, silver nugget that a prospector found in Pinal Creek around 1872. There are five more main known native silver occurrences in the area, as well as several smaller mines, such as the McMorris and Stonewall Jackson. However, all these mines have not seen any significant modern exploration.
The Richmond Basin is on the southwest slope of the Apache Mountains, where native nuggets of silver were first found in the superficial Ramboz Wash, just 8.5 miles from Globe. Miners sunk shafts, and the one at the McMorris mine reached a depth of 800 feet, producing high-grade silver ore until 1882 when work ceased. Most of this ore was treated in a mill at Wheatfields on Pinal Creek, about 12 miles northwest of Globe. Reports of the total yield range from $300,000 to more than $600,000. Rich silver ore was also found at the McMillanville mine in 1878-79, and a 5-stamp mill was erected there in 1879 (Geology of the Globe Copper District, Arizona, Ransome, USGS 1903 Professional Paper Report).
Just before 1878, the principal settlement of the district transferred from Ramboz to Globe, perhaps because the more plentiful water supply was available, also possibly because of the constant raids of the Apaches under Geronimo and Victorio to isolated miners. But in only four years, the silver begun to give out, and adding to the setback, was the repeal of the 1878 Bland-Allison Act, which had re-monetized silver in the US economy and allowed the Arizona mines to profit. However, in 1893, the US Government demonetized silver, and the silver prices dropped dramatically. Arizona saw a reduction in output of its silver mines, and miners moved on to pursue gold strikes or to work in the growing number of copper mines in response to the advent of electricity. By the early 1900s, all mining shifted to copper.
The last published discovery was of a 60-pound nugget found in Nugget Wash. The mine had reportedly produced in the early 1900s about $100,000 worth of silver, and with the price of silver being at the time $1 an ounce, 100,000 ounces of silver would have been mined (Geology and Ore Deposits of the Globe-Miami District, Arizona, N.P. Pederson, 1962, USGS Professional Paper 342).
ADDITIONAL HISTORIC NUGGET DISCOVERIES
Today, the newly discovered silver boulders in Arizona are probably the largest surviving silver nuggets in the world. However, other discoveries have preceded this one, such as the 1894 discovery of a 2340 lb native silver nugget at the Smuggler Mine, on the slopes of Smuggler Mountain near Aspen, Colorado. The nugget was 93% pure silver, weighing over a ton (2240 pounds), and had to, sadly, be broken into three pieces to be hauled out of the mine.
COBALT SILVER DISTRICT IN ONTARIO
The only other silver deposit similar to the silver nuggets discovered near Globe is the Cobalt Silver
District in Ontario, Canada. Cobalt is a town in the district of Timiskaming, in the province of Ontario. The area was mined heavily in the early 1900s for silver. The silver ore also contained cobalt, hence the name. Over 445 million ounces have been produced from thin veins in this
Proterozoic diabase. Mining declined significantly by the 1930s, and by 2017 it was referred to as a ghost town. However, due to the high demand for cobalt in batteries for mobile devices, and the high price for silver, there is now a renewed interest in mining in Ontario.
DENDRITIC NATIVE SILVER AND LAPIDARY
The characteristic look of native dendritic silver that many lapidaries love to cut has been traditionally mined at the Silver Beaver Mine and the Castle Mine in Cobalt, Ontario, among others. The native silver in cobaltite is a very rare and collectible material. The ornate dendrites appear as frilly ferns or snowflakes. Cutting silver nuggets on the saws is extremely slow and hard on the blades. It is also dirty cutting, as with most metal ores, but the results are beautiful, eye-catching cabochons. Dendritic native silver is also found at the Alhambra Mine in Grant County, New Mexico. When slabbed, the dendritic native silver often shows spotty dendritic patterns with grey to pink secondary mineralization. Now, slowly, some material is being cut from
the new silver nuggets location in Globe. Well-known lapidary and jewelry artists in Arizona, such as Kevin Lane Smith, Andrew Laws, and Randall Pipkorn, are using the material to create unique artworks. I, too, look forward to working with some of these silver nuggets.
The discovery of the silver mega-nuggets from Globe is a worldwide news-worthy find. They may not be colorful like other minerals and gem materials, but their striking beauty lies in their enormity, rarity, and intricacy of the textures. Hopefully, their new home will be a public museum, preferably here in the US, where many of us can visit and admire them.
I am truly extremely thankful to Dr. Chris Osterman for his friendship, for sharing his knowledge and supplying all the information and photos about the discovery of the silver nuggets.