Posts Tagged ‘ore’


Howardsville, Colorado on the Animas River.

So far on my tour through Colorado’s mining history, I have reported on how the ore was mined. Today, I got the chance to see how the ore was transported and processed at a mill. After completing my tour of the Old Hundred Mine near Silverton, I drove back down Stony Creek to where it joins the Animas River at a place called Howardsville, where some mining operations were still evident.

Arrastra Gulch

Google Earth view of Arrastra Gulch and Silver Lake. The Mayflower Mill is located at the bottom of the gulch in the upper left corner.

I stopped along the way toward Silverton at the base of Arrastra Gulch. This is the location of the main mining area around Silverton and one of the richest deposits in all of the San Juan Mountains. Before a proper mill could be built to process the ores, a Spanish-style arrastra was built here, which is a circular area with a flat stone floor and a central post with arms coming out. Each arm had a heavy stone or iron weight that hung from it and which would drag over the ore and crush it. Mules, donkeys, or even humans would be used to push the arms around in a circle. Once mills were built, the ore was transported to them from Arrastra Gulch and the high glacial circque above it (around Silver Lake) by tramlines or flumes. At one point as many as four separate overlapping trams were operating.

Arrastra Gulch marker panel a

Arrastra Gulch marker Part 1

The largest mill in the area was the Mayflower Mill (also known as the Shenandoah-Dives Mill) about two miles northeast of town. It was built in 1929 to process gold, silver, zinc, lead, and copper ores. Another large mill nearby was the Silver Lake Mill on the Animas River.

arrastra trams

Map of aerial trams in Arrastra Gulch near Silverton, Colorado.

Built of pre-framed Oregon fir and completed in six months for $373,000, the Mayflower Mill began processing ore in Feb., 1930 and continued in operation for 49 of the next 61 years, finally closing down in 1991. It is in fact still capable of operation, and all the original equipment is intact. The historical society allows self-guided tours that start in the machine shop, then move to the tram station, ore storage bins, ball mills, flotation cells, recovery system, assay office, etc.


A restored arrastra in Groveland, California. Heavy rocks were dragged around in a circle to crush ore.

It was an extensive operation, the biggest in the San Juan Mountains, and employed the latest technologies available in 1929, including the new techniques of ball mill crushers, froth flotation of sulfide ores, and recovery of base metals as well as gold and silver. These techniques are still used today in such places as the concentration plant at Utah’s Rio Tinto/Kennecott Copper operation, although the scale there is enormous.

Shenandoah-Dives mine

A sketch showing what the Shenandoah-Dives mine looked like during the 1930s. The aerial tramline connected with the Mayflower Mill.

For its 61 years of operation, it processed over 9,700,500 tons of ore to produce 1,940,100 ounces of gold, 30,000,000 ounces of silver, and over 1,000,000 tons of base metals.

Tramway in Arrastra Gulch

The aerial tramline connecting the Shenandoah-Dives Mine above Arrastra Gulch with the Mayflower Mill. The gulch is the canyon in the foreground, and the high circque is the basin around Silver Lake.

I used my camcorder to create a complete walkthrough of the mill, going in order from start to finish. At each stop I would stop the tape and take photos as well, and took my time to document everything. There were interpretive signs at each stop explaining what each piece of equipment did. Here is a rundown:

Mayflower Mill

The Mayflower Mill near Silverton, Colorado. A self-guided tour is available during the summer.

Processing Ore

The ore coming from the mines was about 5% metals and 95% waste rock (tailings). The metals have to be separated out, and this is done in stages so that all the metals (gold, silver, copper, lead, and zinc – the big five) could be individually removed and purified. This is done in three main steps: crushing, separation or reduction, and purification. The final step was done by a smelter off-site, but the first two steps were done at the mill.

tram station

Tram station at the Mayflower Mill. Full buckets descended from the mine by gravity, which also pulled the empty buckets back up.

The ore arrived in large open buckets by tramline. Gravity brought the ore down and allowed the empty buckets to move back up the loop. The ore was brought into the mill at the tram station and dumped, then transported by conveyor belt to the cone crushers. It was screened for size, and if too big would be returned to the crushers.

cone crusher

Cone crusher at the Mayflower Mill. It would crush the ore between rotating cones until it was pebble sized.

Once it was pebble sized, it would be transported to the Fine Ore Bin, which would hold 1200 tons of ore, enough for one full day of operation. The ore was then transported out of the bottom of the bin and mixed with water to form a slurry, then passed through a rod mill (which used long iron rods rolling around) where the ore was further crushed to a fine powder and sorted by a spiral classifier, an auger-like device that pushed the ore upward. If the ore was fine enough, it was pushed all the way to the top – if not, it would fall back down and be returned to the rod mill for further crushing.

rod mill

Rod mill at Mayflower Mill. Iron rods were fed into the mill, then allowed to roll around inside to crush the ore to the size of sand grains.

The powder, now the consistency of sand, was passed through a ball mill, with 2-3 inch diameter iron balls rolling around to crush the ore even finer. These balls were added frequently during the day through pipes from a ball bin. Now the ore was now the consistency of talc and fine enough to start to separate.

Spiral classifier

Spiral classifier at the Mayflower Mill. Ore slurry from the rod mill would be pushed up the spiral. If it was fine enough, it would be pushed over the top. If not, it would return to the rod mill.

The first metal to be separated was gold, using a system of settling jigs that pumped the ore through, allowing the heavier gold particles to settle out through vibration and suction. The lighter remaining material was passed on to flotation cells, where reagents and flocculents were added that would float the desired metals to the top of the tank solution while depressing or sinking the other metals. Lead was removed first, then copper, and finally silver and zinc removed in large tanks. The soapy bubbles would simply be skimmed off the top of the cells.

Ball mill

Ball mill at the Mayflower Mill. Ore crushed to the size of sand grains would enter the rotating drum and be crushed to powder by 2-3 inch iron balls.

The flotation cell solutions were then passed through filters with pumps that pushed the water through, drying out the solution to a damp cake-like material that was then shipped to a smelter for final refining, where it would be heated to drive off the sulfides. Each day, samples were removed and filtered through a squeeze press, then sent away to an assayer to determine the percentage of metals in each day’s run.

gold jigs

Gold jigs at the Mayflower Mill. Using air pressure, the lighter ore powder was suctioned away from the heavier gold particles.

Meanwhile, the gold filtered out by the jigs was sent through a concentration process. It would be passed over a shaking Deister table where the gold would be caught by riffles and formed a streak to be collected. It was mixed or amalgamated with mercury to remove the gold from the remaining waste ore. The amalgam was then formed into rounded boats or cakes and heated in a retort at 1200 ° F for 12 hours to evaporate the mercury, which was bubbled through water to condense it for reuse. The remaining gold was now called “sponge” and was about 80% pure. It would be sent off to a foundery for final purification. Four to five sponges would be produced each week. Each sponge weighed about 22 pounds. During the last year of the  mill’s operation (1991), a new process was developed that eliminated the need for mercury (which was highly toxic).

Lead cleaner cells

Lead flotation tanks at the Mayflower Mill. Reagents were added that would float the various metals, such as copper or lead, to the top of the liquid on soap bubbles which were skimmed off into the trough in front. The remaining metals were depressed to the bottom. Impellers would keep the solution agitated while blowing air through it.

Once processed, the waste material is called tailings and was made up of water and sandy ground rock. It was pumped down to settling ponds, where the solid tailings would settle out. This was an innovation of the Mayflower Mill, as previously the tailings would simply be allowed to flow into the Animas River. The high sulfur and iron content in the tailings would travel down the river and created the reddish stains on the rocks that I noted on my train trip up the gorge several days ago. At the Mayflower Mill, the ponds were shifted so that the solid tailings would build up a series of mounds downhill from the mill. These have now been collected into a large tailings pile near the mill.

Deister table

Deister table at the Mayflower Mill. It would shake, causing the gold particles to separate out against the riffles.

I found this self-guided tour to be fascinating from a chemistry perspective. The mill used a system of physical separations to crush, concentrate, and amalgamate the ore. The final smelting used a system of chemical separations. It is a perfect example of a chemical engineering process, and was continually upgraded and improved during its 61 years in operation. The mill could be run, during the night shift, with only three people. During the day there were additional people to do repairs and take samples, to run the gold process, and to run the machine shop. Shift supervisors oversaw the operation from the dog house, one man ran the crusher facility, and one man ran the flotation cells. This was the biggest operation of its kind in southwest Colorado and processed more ore than any other mill in the area.

gold sponge

A model of what gold sponge looked like after being removed from the retort furnace. The holes in it are caused by mercury vapor bubbling out.

Retort furnace

Retort furnace and gold button mold at the Mayflower Mill. The gold particles would be amalgamated with mercury, then heated in this retort furnace to drive the mercury off.


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mining terms

Mining terminology, at the Creede Underground Mining Museum

As mentioned in my last post, I am embarking on a two-week tour of Colorado mining towns. Before I go, there are some basic mining terms that any greenhorn or tenderfoot like me should know before venturing into a mine. Many of these terms come from the Cornish miners who came to America to work when the tin mines in Cornwall played out in the 1800’s.

First, the basic parts of a mine: you always refer to a mine as if you are facing into it. The part of the mine you are working to drill, load, and blast is called the “face.” The left-side wall is the “left rib” and the right-side wall is the “right rib.” The ceiling is the “back” and the floor is the “foot.” The back is also called the “hanging wall” and the floor the “foot wall” depending on the orientation of the ore vein.

Ore body diagram

Diagram of the original ore body.

A “tunnel” is horizontal and must see daylight at both ends. If it only opens to the outside on one end, it is called an “adit.” If it doesn’t connect to the outside at all, it is a “level.” Levels are like the various floors of a building, only underground in a mine, and they provide access to the ore body. A vertical hole that connects with the surface is a “shaft.” If it is a hole that is dug down from a level or an adit, it is a “winze,” and if it is dug upward it is a “raise.” A hole dug to follow a vein horizontally away from a level or an adit is called a “drift” and to dig out a large ore body going up or down is called a “stope.”

The valuable mineral that you are trying to dig out is the “ore,” along with useless rock called “tailings.” Usually the ore is injected as a hydrothermal body along a fault or other natural zone of weakness in the rock, and the entire mineralized zone is called the “ore body” or “lode.” If it is found as a large vertical mass with branches, it is an “ore chimney” and if it is a thin line following any direction it is a “vein.” Sometimes ore is found as crystals deposited along the walls of a natural chamber. This is called a “vug.” When a vein reaches the surface, it is an “outcrop,” and when parts of the outcrop erode away and are carried down into river valleys by water, avalanches, and gravity it will pile up in still areas of the stream, such as the inner parts of meanders along with gravel. These are called “placer” deposits (pronounced “plah-cer” and not “play-cer”).

exploratory mining

Prospectors mine the placers and conduct exploratory mining

The first miners in a new mining district are prospectors, because they are looking to find, develop, and sell a good “prospect.” Typically the first discoveries are placer deposits, because they are easy to find and work using pans, rockers, and sluices. Once the placers are played out, the prospectors head upslope to find the source outcroppings, or the “Mother Lode.” Once they find evidence of ore (such as associated minerals like iron pyrite or chalcopyrite, quartz, etc.) they will “stake a claim” by pounding stakes in the corners of the land and starting to dig exploratory shafts or adits using hand tools such as picks and shovels. They will use a windlass to haul the “muck” or loose rock out of a developing shaft with a bucket. Claims have to be an allowed size (a long, thin swath of land) and registered in the county mine office to be legal. It’s good to set up with a partner so that when one of you leaves to register a claim, the other can guard it from “claim jumpers.”

Samples of the ore are taken to an “assay” office where they are analyzed chemically to see how much valuable metals are actually in the ore. If the ore is rich, or “high grade” or if the vein widens and appears to continue, the prospector will usually sell out to a mining company with the resources and capital needed to further develop the mine.

Once the mining company buys out the prospectors, it starts to build the infrastructure needed to enlarge the mine. The irregular prospector shafts and adits are enlarged and shored up with timbers. The top of a shaft is boxed in with a “collar” and an adit’s entrance is shored up and extended outward to prevent loose rock from falling into it. This becomes a “portal.” At the top of a shaft, a “headframe” or “gallows frame” is erected out of large timbers or steel with pulleys called “sheave wheels” at the top. A braided rope or cable is brought over the sheave wheel and attached to a metal cage called a “skip” which can carry men or ore buckets in and out of the shaft. The other end of the cable is brought to a “hoist,” which is an electric or diesel winch. As the skip is raised and lowered in the mine, a series of electric bell chimes are used to signal the “hoistman” how far to raise and lower the skip. A mark on the cable tells the hoistman when the skip is “on the level.”

mine expands

After a mining company buys the prospect, it expands the mine and adds infrastructure

As the mine deepens, it will usually encounter underground aquifers or water tables which become a major problem as they start to flood the lower mine shafts. The main shaft must be dug lower than the lowest level and a pump installed to remove the water. This low-lying shaft is called a “sump” and the pumps used ran on steam, diesel, electricity, or compressed air. The biggest of these were the famous Cornish pumps found in some mines.

Eventually the shafts are too deep to economically raise all ore cars, sump water, and men to the top of the shaft. A drainage and ore removal adit is sometimes dug at the bottom of the mine that will drain out the waste water and allow easy passage of ore cars out of the side of the mountain. These adits usually have a slight downward slope to the outside so the loaded ore cars can be more easily moved. Waste rock was simply dumped out of the shaft or portal and created a “tailings pile” downslope from the mine or mill.

integrated mine

Integrated mine and mill. As the mine develops, drainage adits, interior shafts, reduction mills, smelters, and other structures are built.

As the mine gets bigger, with additional levels every 100 feet and a complex set of drifts, adits, winzes, raises, interior shafts, stopes, etc. it becomes advantageous for the owners to build their own mill instead of sending their ore elsewhere for processing. A mill is built on the side of the mountain below the lowest portal. It first sorts, then pulverizes the ore into powder, then concentrates the ore mechanically or chemically. The concentrate is then shipped by rail to a smelter for final processing and purification. Sometimes the concentrated ore is heated in a retort or furnace but not separated into its final constituent metals. This combination of metals is poured into bar-shaped or cone-shaped molds and cooled, creating “dore bars” or “buttons” which contain gold, silver, and other metals.

Once the mine is exhausted of ore, or the shaft extends down below where it can be economically drained of groundwater, or the price of the final metal drops so the mine can no longer turn a profit, it is closed down (sometimes temporarily). Today, mines have to post bonds that force them to reclaim the mine and make it safe once mining has concluded. But in the old west, the mines simply shut down and left everything where it was. Tailings piles are the most obvious evidence of mining, and the rocks are often stained a yellow, orange, or reddish brown color from iron sulfides and sulfates. Rotting timbers poke from the ground, and rusted metal scraps adorn the slopes. Drainage water still seeps from adits, often contaminated with metals or other effluents. And the shafts and portals remain, too often a temptation for the unwise to explore. A few people die each year from cave ins while exploring old mines, or get killed by handling old dynamite left in mines. In some states, such as Utah, a concerted effort is underway to close all of these abandoned mines in the name of public safety but at the expense of history. Other states, such as Colorado, seem to strike a better balance between history and safety.

Mining terms B

More mining vocabulary terms. From the Creede Underground Mining Museum.

Now there are many more terms, such as how a typical miner spends his shift to drill, load, shoot, and muck the face. We’ll talk about these later as they come up on my journey. I’m amazed at how many mining terms have made it into general vocabulary, such as “big shot” [blasting out a large section of the face], “hang-up” [when ore is blasted to fall into a lower chute but gets stuck], “getting the shaft” [to buy a worthless mine], etc. For better or worse, hard-rock mining has had a big impact on our history and our culture.

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