Concentrate
What makes a mine profitable? It’s not having gold or copper or even diamonds- it’s having them in quantities that can be sold at a profit. There’s gold in sea water but it costs more to get the gold out than you’d make selling it. It’s the same for all commodities. Valuable minerals are found in many places but you have to concentrate them to make a saleable product.
Gold has high unit value; at this writing it’s near $2,000 per ounce. If you have a deposit where it costs more than $2,000 per ounce to recover the gold, it’s not profitable.
So how would you concentrate a saleable mineral? If you have the patience you could wash every rock to see if it’s valuable. This is not productive. Using a method that relies on the properties of the mineral would be a good start. Gold is heavier than most of the gravel it’s in so if you have a method of separating based on gravity, that would give you a way to wash a lot of gravel cheaply. Horn spoons, gold pans, sluice boxes are all methods that are based on gravity. Minerals recovered by gravity include gold, platinum, tin, diamonds and garnets.
You could use electrical properties to concentrate. Strong magnets are used to recover magnetite, a magnetic iron ore. Metal detectors are used to find gold nuggets by weekend prospectors. In Gold Basin, Arizona, during a winter weekend, they’re like ants combing the hills with their detectors. If the minerals are soluble like lithium (used in those batteries that can burst into flames) you pump water into the ground to dissolve the lithium and then pump out the pregnant solution and precipitate the lithium.
If the valuable mineral is still in the rock, the rock has to be ground up to release the mineral so it can be recovered. Early miners used an arrastre (comes from Spanish word for dragging- arrastrar). They had to use what was available so they came up with dragging a big rock around in a circle over flat stones to crush the ore. Arrastres can still be found in the washes near the old mining camps of Mohave County and it’s a thrill to find one and dream about the hardy souls that worked them.
The next equipment breakthrough was the stamp mill. This consisted of long vertical rods with weights on the bottom called stamps. The rods were attached to a cam shaft so that the rods were lifted and then dropped thereby crushing the ore. Stamp mills are not commonly found in the mountains because when the ore ran out at one mine, the stamps were usually taken to the next mine in operation.
Stamp mills were used for free-milling gold, gold that was liberated by crushing. If the gold or other valuable mineral did not break free so it could be concentrated by gravity, it had to be ground up to a very fine power. The mineral was recovered using chemicals. Enter the ball mill, a drum-like cylinder with a grinding media, something to pound the rocks. In the early days of the Golconda Mine in Todd Basin, Cerbat Mountains, Arizona, they used diabase rocks to crush the ore- diabase being a very hard rock. The miner’s term for these is Dutch mill balls. I have found them on the dumps of the Golconda mine. They are round, fist-sized, heavily stained diabase rocks. Nowadays the mills use iron balls. The drum is loaded with ore, water, chemicals and mill balls. The drum lies on its side and rotates, the balls will go up the side of the drum until gravity makes them fall on the ore. The drum is rotated until the ore is a fine power. The slurry is taken out and the minerals recovered. The white power material found around old mills is called tailings, the discarded ground up ore after the minerals are recovered. Oatman has large piles of tailings along Old Route 66. The large white dirt mound near the county land fill are the tails from the Mineral Park mine.
Why is the white power called tailings? Miners have terms for everything. What goes into the mill are called heads, what comes out are tails and during the time it’s being processed, it’s called midlings. The rock that has been dumped alongside mine workings, erroneously called tailings, are just mine dumps.
In 1976, when I started working at Mineral Park as mine geologist, I was thrown into a dire situation. The main shaft of the primary crusher had broken and the mill had to shut down for lack of crushed rock, throwing everyone out of work. The mine manager said that the only reason I was hired was to find soft ore to feed the mill and if I didn’t find soft ore, I was fired. Everyone looked to me for geologic information that would bring them all back to work. I didn’t sleep for an entire week, going through geologic maps and reports and mapping the pit to find soft ore. At the end of the week, the manager had me ride around with the foreman so I could tell him which ore to haul to the mill. Finding the soft ore is where geology paid off. I was mine geologist for five years before being sent to South America as Vice President of Exploration. Since that first week at Mineral Park, none of my jobs have had the stress as when I had to find soft ore or get fired.
