Many old-timers and even more modern miners lost a remarkable amount of gold in their mining operations. Some tailing piles contain fortunes in lost gold values if identified and properly reworked with modern mining methods.
Lode or hard rock tailings and mine dumps can be very productive sources of gold specimens, as many times good material was often accidentally discarded with the mine waste. If the material is rich enough, it can easily be located with metal detectors or more simply with a crack hammer.
Placer tailing piles can be much more readily reworked profitably, having been already excavated and even stacked up, ready to process. Quite often there is a large hole in the ground nearby that represents the original source of the material. Most small-scale prospectors and miners look at old mine workings and tailings in a much more limited way, looking for small portions of missed virgin ground. Larger placer operations often bypassed or overlooked a few yards here and there, having been inconsequential in the scale of the original operation but can provide many days or weeks of returns for those operating with very small-scale equipment.
Attempting to capture small gold while employing enough water force to move large rocks through a sluice box or high-banker is problematic. While many ingenious riffle designs have been tried singly or in combination to deal with the problem, there is no perfect solution and processing all material through a sluice box with no pre-screening often results in substantial gold losses. The only pre-screening that took place was often done by hand in older operations as larger rocks that would not wash to the sluice box were stacked aside. Other rocks made it to the sluice box but hung up in the first section of the sluice and were tossed out to both sides. It is easy to find locations where a low gully with a pile of rocks on both sides marked the location of the initial sluice run.
Bulldozers and other heavy equipment made things even worse, pushing everything into a box. In many sluice designs the main concern seems to have been how to build the box to handle large rocks rather than on how to capture gold. The use of railroad track as riffles became a common practice due to the ability it had to stand constant long-term pounding from large rocks. Recovering more gold from tailing piles of this type is simply a matter of running them once again through a modern wash-plant that properly screens the material before processing. The real pay here is in the fine gold, but these types of operations lost surprising amounts of large gold as well, especially if the gold contained or is mostly composed of quartz. Large gold-quartz specimens just rolled through these systems along with the other rock.
These simple sluice systems were often also subject to overfeeding of material, surging, and riffle packing from infrequent cleanups. While overfeeding posed a huge problem, was another major contention. A box could sit for some time with no material running though it, resulting in the material scouring out between the riffles. Then a huge volume got pushed in, completely burying the riffles. Rocks could sit in the riffles, and water gushing around and over the rock again could blow gold out of the riffles. Let the sluice run too long between cleanups, and magnetite and other heavy materials could pack into riffles so hard that gold had little or no chance of being captured.
LET’S LOOK AT SCREENING SYSTEMS
The amount of gold remaining in these types of tailing piles can be very substantial. A small mine that produced 10,000 ounces of gold with a loss rate of 20%, which is not extreme at all, could mean 2,000 ounces or more sitting in the tailing piles waiting for the savvy miner with a proper recovery system.
The screening systems were tailored to remove the rock efficiently while passing nearly all the gold to the recovery system. In general, a screen of 2” more or less will allow for high volume screening and the design of a reasonably efficient recovery system, although actual screen sizes employed may vary from as little as 1” to as much as 3” in size. Smaller sizes result in more efficient fine gold recovery, but impede volume, while larger sizes may sacrifice some small gold recovery in return for higher volumes. The nature of the material to be processed and the gold itself determine the best overall solution for any given operation.
The most obvious problem with screening systems are nuggets or gold specimens that are actually larger than the screen size. In most gold deposits, these over-sized nuggets are so nearly non-existent that they are of no concern. In other situations the nuggets exist, but are so rare that the actual percentage of gold lost as a result falls well into acceptable limits. However, in very rare cases, large nuggets were common enough that various attempts were made to employ “nugget traps” and even spotters to watch for large nuggets on the way to being discarded. Despite these precautions, many large nuggets were lost and discarded into tailing piles.
Clumping of material from lumps of clay, cemented gravels or frozen material could also result in gold smaller than the screens passing through along with the large rocks, so it’s not always just large gold found in the cobble piles left from screened mining operations. A poorly maintained or overfed screen can clog and allow smaller gold to escape.
One way gold was commonly lost was in the stripping of overburden. In many cases gold-bearing gravels are overlain by worthless material called overburden. It would cost money and wear out equipment to process this material, so an important part of mining lies in efficiently stripping and pushing this material aside. In modern operations this material is stockpiled and used later to cover and reclaim mined areas. In the past, however, it was simply pushed aside and stacked up. The dividing line between gold-bearing material and overburden is not always obvious, and can also vary in depth. The result often was that some gold was lost simply by being pushed aside and left in piles of stripped overburden. In wetter climates, these piles of overburden often contained topsoil and plentiful organic material. They are often recognizable by being much more vegetated than normal tailing materials, which tend to have little or no vegetation.
Whenever you are dealing with tailing piles that are created by digging and then dumping material on top of a pile, the last material processed is often the richest. Gold tends to lie on or near bedrock, and as an excavation is dug to reach this gold, the leaner material goes in the pile first. The richest material ends up last and, therefore, very often right on top of the tailing piles. Another important item to remember is that when piling up mixed materials the largest material, rocks or gold nuggets, tends to roll off the top and down the side to rest near the bottom. Just look at any pile or slope of gravel and rocks to see the larger rocks resting near the bottom of the pile or slope. The wetter and looser the material is, the more this happens.
Old mine workings and tailing piles can represent fantastic opportunities for the modern prospector, from the chance of finding a nugget on up to the reworking of the old tailings in a modern washplant or high-banker.
Good luck and Happy Prospecting!