("Gold is where you find it" and more often than not it can be found in bench gravels.)
Although gold-bearing bench gravels can be hit or miss at times, they can provide good gold recovery opportunities when the gold eludes you in streambeds and dry washes. To learn more, read on.
What I Think
As I mentioned in the previous post on this topic, at times I've done really well working bench gravels. Then again, there have been times when benches yanked my chain and left me with little gold. I'm no geologist or mining engineer so I don't have any doctoral thesis to offer you, just my own observations and theories on why bench gravels can be so contrary in this regard. All this said, I don't think there's been a single time I've worked benches that I didn't get some gold out of my efforts. So file that away for future reference.
So why is it some benches are better than others? I'll take that question a step farther and ask why some sections or parts of benches produce more gold than adjoining or nearby sections?
Here's what I think. If we make the assumption that most bench gravels are evidence of old (or even existing) streambeds that were left "high and dry" due to a range of natural or man-made forces then it stands to reason that benches, just like existing streams or dry wash courses, were once susceptible to the same gold deposition principles that govern placer deposition everywhere. In other words, if a section or length of bench gravels was in a low-pressure area of an ancient stream it makes sense that "better" gold deposition occurred there. Taking the opposite view, if bench gravels were formerly part of a high-pressure or rapid flow region of a stream or wash, then little gold could be expected to have been deposited in those bench sections. The same holds true for other types of stream or wash gold trapping characteristics, including drop offs and obstructions. Anyway, this makes sense to me.
Sample and Sample Again
One problem you'll have in this regard out in the field is that it's nigh on impossible to tell what was going on in bench gravels from a ground or eye-level point of view. Another issue is the presence of new growth (trees, grasses, weeds, bushes, etc.) or rock or mud slides that may have covered benches, sometimes under boo-coo amounts of sterile overburden. In desert or dry placers things may be easier to get a visual handle on with regard to bench gravel distribution, but even in those instances without new growth or mud slides, you'd probably need some sort of aerial reconnaissance to sort the true gold deposition issues out as they relate to stream hydraulics. So now we're back to first base on trying to determine what benches or bench sections we should be working.
(These miners are serious about their bench gravels!)
There are a number of ways around this issue. The first is plain old common sense. It means taking numerous samples both high and low from different sections of that bench and either panning, sluicing, dry washing, or highbanking them to determine what sort of gold values exist in that bench and where the best values are. One thing to remember here is the basic scientific or mathematical model when it comes gathering data (after all, that's what you're doing by sampling). The greater the number of samples (i.e., the more data you have), the more accurate your data will be. So half-assing things isn't going to help you much when sampling benches. Be thorough and be accurate, and do the work necessary to determine where the good gold is in a bench. Got it?
In my next post I'll talk about some visual clues to determining what parts or sections of a bench you should focus on in your gold recovery efforts. You'll still have to sample using these clues, but the time and effort you'll expend will probably be a lot less if you know what to look for.
Hang tough out there.
(c) Jim Rocha (J.R.) 2014
Questions? E-mail me at email@example.com