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.