Yesterday was spent doing set construction for our upcoming production of You Can’t Take it With You. The spouse of one of the actors is a gold miner and a pretty fair carpenter.  He returned early from a placer mining expedition this summer west of Dawson, YT, Canada, across the border on the US side.

My acquaintance- we’ll call him “Ted” because of confidentiality- was a bit reluctant to discuss the recent expedition only because he felt his part in the thing was minor. In fact, his comments were very telling of the kinds of hardships facing anyone with a fancy for placer mining in Alaska or the Yukon Terrritory.

Ted has plenty of experience in placer mining in Colorado, especially in the Fairplay region west of Denver.  Placer mining has a special appeal to those who want to mine for gold, but don’t want the grief and expense of underground mining of chemically complex ores.

Ted signed up to work for a New Zealand miner who had a sizeable claim an hour outside of Boundary, AK, along the Top of the World Highway. This site is reasonably remote, judging by the fact the nearest city where machine parts could be obtained was Dawson, across the border in Canada. Mining season is 100 days in duration, or 2400 hours. This is because of the climate and the hardships associated with work out in the bush. 

The gold bearing formation is a band of sand and gravel 3-6 ft in thickness and 6 ft below the topsoil. The gold bearing gravels sat atop the bedrock.  The miners use a floating separations plant consisting of a trommel and a sluice. The machine had a long elevator with a belt for transferring gravel and rock from the sluicing operation.

The gold is recovered in a densly knapped carpet positioned along the bottom surface of the sluice. The gold particles are trapped in the fibrous mat and are periodically flushed out into centrifuge bowls for further separation.

The operation requires a good deal of readily available water. Ted recounted that the sluice process water was returned to a pond for reuse.  The position of this pond had to be managed constantly.

Here is how the operation works. The overburden on the claim must be removed well before the sluicing is to begin. This is done with a D-9 Caterpillar. Owning a large piece of machinery in the wilds of Alaska is an expensive proposition. A small mining operation cannot afford a new Cat, so a used machine must be purchased and delivered. This machine consumes X gallons of diesel per hour and suffers from mechanical breakdowns on occasion.

A small mining operator must be able to do maintenance and repairs because having a mechanic on site may not be possible.  The operator must have plenty of working cash on hand to pay for very expensive fuel and parts. Flying parts in will consume much of the short mining season.

The operator must carefully scrape the overburden away to reveal the ore body.  Leaving too much overburden will consume extra sluicing plant time.  Once the ground is scraped a pond must be constructed in order to support a floating sluice plant.

The sluicing plant is fed by an end-loader or back hoe. The gravel and sands are loaded into a trommel to sort the material and remove the large rock. The finer mesh gravels are then washed onto a sluice where a flow of water will wash the material across riffles to cause deposition of the denser components like gold.  Carpet positioned below the riffles will trap the fines and prevent them from being washed away by overexuberant water flows.

Ted said that gold dust recoveries can be as much as 6 ounces per hour when everything was operating smoothly.  While this sounds like a lot of money at the current price of gold, bursts of profitable sluicing can easily be overcome with expenses and downtime due to logistical snags, equipment mishaps, and unanticipated difficulties with the deposit itself.

One of the problems that this kind of mining operation can encounter with subsurface deposits in Alaska is the presence of permafrost. Ted explained that his early departure form the site was due to extensive permafrost in the claim. If you cannot dig up the gravel, you cannot recover the gold from it. Like any other single continuous processing train, downtime leads to a cessation of operating capital.

In Ted’s experience this summer, all of the puzzle pieces were in position except for the condition of the gravel deposit. It happened to be frozen in place.  It remains to be seen if these operators will return next year.

I have been nursing a theory about the American gold/silver rush phenomenon of the mid to late 19th century. That theory held that the critical enabler of the gold/silver rush was the development of extraction technology, referred to as extractive metallurgy in the mining business.  Wouldn’t you know that not only has someone else developed this idea, but also written a book on it. A very good book, I might add.

The book I refer to is Ores to Metals, by James E. Fell Jr., 2009 (paperback), University Press of Colorado, ISBN 978-0-87081-946-9.  The books is actually a version of his dissertation. I wish I could publish my dissertation like that, but we won’t go there…

In 1858 groups of prospectors lead by the Georgians William G. Russell and John H. Gregory found placer gold in streams near present day Denver. These prospectors worked their way up Clear Creek and Ralston Creek looking for more placer gold and for the lodes that would be the origin of the placer deposits.  The modest success of the prospectors in locating placer gold quickly spread eastward and lead to the 1859 Pikes Peak Gold Rush.

Prospectors combing the mountains along the creeks soon found lode gold. Gregory is credited with being the first to find a lode on May 6, 1859.  Placer mining soon lead to subterranean workings and within a few years the mining districts of Blackhawk, Central City, Nevadaville, and Idaho Springs were abuzz with activity.

The surface exposures of gold veins were amenable to familiar methods of processing. Soon, stamp mills were built in the vicinity of the mines and ore was hauled to the mills for crushing and further processing.  Since the gold isolated early in the development of the mines contained gold in a form processable to sluicing or amalgamation, great optimism about the future of the districts lead to further relocation of people hoping to cash in on the rush.

However, by the mid-1860’s, the ore pulled from the mines was of a form that was quite resistant to extractive methods then in place. The ore that had been removed first was from a body of rock that had been long exposed to the weathering effects of water and oxygen. This type of altered ore is referred to as gossan.

To miners accustomed to placer mining, the extraction of gold from gossan was feasible in that the gold was found in native form and within a matrix that didn’t interfere with known isolation methods. What local miners had in their toolbox up to that point was comminution, sluicing, and amalgamation.  Within a few years of operation, miners had encountered a form of the ore that would be called a sulfphuret. Sulphuretted ore as it was then called was actually rock consisting of metal sulfide minerals. These metal sulfides were deposited into fractures and faults millions of years ago by the hydrothermal flows from deeper and hotter source rock.

Assay of gold by cupellation would reveal the gold content even in the sulphuret. However, the gold recovery experienced by the mines plummeted when they got to ca 100  ft below the surface and into the sulphuretted zone of the ore body.  By 1867, many mines and mills were shuttered due to the low extraction yields from the new type of ore encountered. The Pikes Peak gold bubble was collapsing and the sulphurets were to blame.

So, along comes a chemist in 1863. Professor Nathaniel P. Hill from Brown University in Rhode Island. Professor Hill had been engaged to go to Colorado by associates of William Gilpin, territorial governor of Colorado, who was investing in mining property. The early 1860’s had seen a wave of hucksters selling snake-oil methods of gold extraction to mine operators frustrated with the sulphurets. These hucksters were referred to as “process men”. Gilpin sought funding and expertise from out east for his own interests. Hill visited the property Gilpin was interested in and reported that the property held little prospect of gold. Hill returned to Providence with a notion of the possibilities in Colorado.

Hill was entranced with the prospects of wealth from the gold district of Central City and left Providence to operate his own mining company. While in Central City, Hill was engaged by an mine operator named James E. Lyon to provide consultation. During that time, ca 1864-5, Lyon had engaged two European smelters to develop and install a smelting process for sulphuretted ore. News of Lyon’s smelters gave the impression of success, but in a few years Lyon’s business failed in part do to poor management.

Meanwhile, Hill returned to Providence to perform experiments on smelting methods. He settled on the Swansea process of smelting to produce a copper matte containing gold and silver, and eventually went to Europe to investigate the technology in greater detail. Hill visited Swansea and learned much about the smelting process. Returning to Providence, Hill pulled together investors and produced a plan for building and operating a smelter in Blackhawk, Colorado, as the Boston and Colorado Mining Company. 

To make a long story short, Hill and coworkers produced a process for the calcining of sulphuretted ore by open pile roasting, followed by higher temperature roasting in a reverberatory furnace. The reverberatory furnace produced a slag layer and a lower layer of melt that was enriched in copper, gold, and silver that could be discharged from below the slag layer. 

This process produced a product called a matte that was then crushed and shipped to Swansea, Wales, for production of bullion. By 1870, Hill had developed and was operating a successful smelting operation that was buying ore from the local mines on a sliding price schedule. The Pikes Peak gold rush was resuscitated and gold and silver production was back.