Auger-Drill Sampling for Gold
(Small, portable auger-drill rig.)
Proper sampling is fundamental to all placer and lode gold recovery operations, no matter their size or scale. I've tried to pound the importance of this premise into your heads before, but some of you still seem to be suffering from the same malaise that certain members of a TV reality gold show suffer from. It's called mental laziness (a.k.a, stupidity).
What I Want to Do
Now that I've really pissed some of you off, maybe I've gotten your attention as well! That was the method behind my little bit of madness here, so forgive my transgressions and listen up yet again.
No, I'm not here to snipe at or belittle a certain television gold mining crew, nor am I looking to turn some of you from potential friends into potential enemies. What I DO WANT TO DO is to emphasize to you once again the importance of proper sampling before you begin running any sort of mining equipment, big or small, cheap or horribly expensive, in any type of mining operation, be that small-scale or commercial or even larger. What's the mantra? Correct you are! "Sample, sample, sample and then sample some more!"
One of the best means for sampling the potential gold values in placer ground is the auger-drill method. Even individual small-scale miners can use this approach if they care too...after all, it's not just for the mining "big boys." I myself have used a portable hand auger to take placer gold samples, although I must say it took quite a bit of effort to get that auger down into the gravels I wanted to sample at the time. Invariably I find myself using your basic "cave-man" approach, never the high-faluting, sophisticated approach of someone with a motorized auger-drill, be that hand held or vehicle mounted. You see, I'm hard headed and tend to learn my lessons the same way.
(Hand-powered ice auger. Something like this could be used or modified for placer sampling.)
Anyway, what I'd like to do in this post (and perhaps subsequent posts, if necessary) is to provide you with the basics of how to go about auger-drill sampling using mechanical or mechanized means, as opposed to my time-consuming, blistered hands approach. I know many of you will never be in the position to have to go to these lengths to sample the typical gold ground you work, but it's good information to know just the same. Who knows? You just might inherit a rich placer claim in Alaska or the Klondike and end up showing certain mining dunderheads how it's done the RIGHT way. Or, you may be asked to supply your auger-drill sampling expertise to someone else for cash...you know, a consulting fee. Whatever the case, store this info away in those mental mining files you maintain. It can't hurt, but only help you.
1) The ground to be sampled can either be gridded out beforehand into major and then minor grids, with the square yardage higher (obviously) for major grids and smaller for the minor grids within those major sections. The size, number, and overall organization of the grids are pretty much dictated by the gold ground itself. Fairly level ground is more auger-drill friendly and is more easily gridded out, while broken or highly disrupted ground is less so in both respects. The same holds true for wet versus dry placer ground.
2) Sampling can also be done following certain terrain or streambed features that form potential gold deposition points or likely deposition areas. These features can include gravel types, distinct layers, benches, terraces, true bedrocks or "false" bedrocks (clay layers, etc.), and so on. Although gridding is not usually employed in this approach, it's a good idea to pull your auger-drill samples in as systematic a manner as you possibly can under the circumstances.
3) The overall intention in both of these approaches is to gain a realistic approximation of the gold values present in the ground sampled. Each auger-drill test hole will tell its own unique tale and this information can be assessed separately, in clusters, or as a total average of the gold values to be expected. It can also be used as a means of discerning the location of placer gold pockets and paystreaks. Remember, each and every placer location is different so no single auger-drill sampling plan is a "One-size fits all" solution to determining existing gold values at a given location.
(Part of a placer gold sampling chart.)
4) Typically, in commercial mining operations a detachable auger-drill rig is mounted on the back end of a front loader or similar piece of heavy equipment. But you small-scale folks could probably modify this approach using smaller motorized vehicles like a Bobcat, a truck, a tractor, or anything else you can think of that would get you to the sampling location (or you it) and that could power the auger-drill. Or alternately, you could use my trusted and true "cave-man" auger-drill approach. This latter is probably not your best bet, but that said...it works!
5) In some commercial sampling operations, a drill string consisting of 5-foot long, 1-5/8 inch hex-pinned auger stems with 5-1/2 inch outside flight diameters is used for drilling to sample depths of 110 feet or more (usually 120-130 feet max). If overburden gravels are fairly shallow or bedrock is close to the surface, then auger-drill depths will follow accordingly. Some custom-made and many manufactured drill bits employed in drilling operations can cut their way through boulders, caliche, and even bedrock itself.
6) Samples are taken from the holes at preset depth intervals to gain an idea of gold values through depth. By the way, most test holes are drilled at a 90-degree declination (straight down!). Where feasible, auger-drilling samples are taken all the way to and sometimes, into bedrock itself. In conditions where wet material is sampled, you need to be aware of the fact that some gold will be lost in your samples due to gold's tendency to "fall away" from the auger-drill sampling core end in wet conditions. In some instances this gold loss can be as high as 20% (yep, you heard right). So, wet placer gravel samples should be evaluated accordingly.
7) In most placer gold sampling operations samples are run through a portable trommel or sluice system after being classified through a 3/8-inch screen. Alternately, samples can be run using a spiral wheel concentrator or shaker table. In the case of small-scale miners like you and I, samples can simply be panned out using a standard gold pan. Essentially, samples can be processed in the manner deemed most efficient (and feasible) for the existing conditions and scale of the gold operations involved.
8) Gold particles are then counted in each sample and described as either "Fine to coarse-grained colors or flakes." For typical auger-drill sampling operations, a "color" is defined as a visible particle of gold less than 0.25 millimeters (mm) in diameter which will pass through a 60-mesh screen. Flakes are described 0.25 mm to 2 mm wide in two different directions. Finally, the sampled gold ground is evaluated at how many colors and/or flakes are estimated to be contained in the site's gravels per cubic yard. "What about nuggets?", you ask? Well pards, nuggets is nuggets. They rarely show up in auger-drill samples but if they do then it's time to celebrate because you're on some pretty damn good gold ground!
(This simple little power mini-sluice is being used to run samples at the Polymet Potala Mine outside the U.S.)
So what's the essential value of taking auger-drill samples or any samples at all? Come on now, I know you're smarter than that. In fact, I bet you're a lot smarter than most of those reality TV gold mining crews who run around like chickens with their heads cut off burning up thousands of dollars a day in fuel and maintenance costs wishing and hoping good gold values will magically turn up in their wash plant's sluice boxes. Granted, some of these folks are learning (or have learned) the importance of thorough sampling, but like the saying goes, "You can lead a horse to water but you can't make it drink."
So be a winner out there. Sample first, then mine.
(c) Jim Rocha (J.R.) 2015
Questions? E-mail me at firstname.lastname@example.org