(Section of caliche-bound gravel in a small bench.)
In my previous post I digressed for a moment to talk about clay layers. In this post on gold-bearing bench gravels I'll be mentioning another aspect of clay as a visual clue to working benches, so let's get started.
OK, after all that discussion about clay layers and benches I'm going to throw a curve ball at you. There's another way that clay can impact gold-bearing bench gravels in a positive manner and you may (or may not) already be aware of it.
Tightly Bound Gold
Previously I spoke about clay layers in benches being a visual clue to the possibility of better gold. I was also somewhat emphatic about the notion that, in my experience, I've found little placer gold mixed in with clay layers themselves. This is true, but earlier I failed to mention the fact that some benches are tightly consolidated throughout their lengths and depths by clay or caliche ("desert cement").
In dry or desert placers where little, if any, water exists this is an easy enough concept to grasp. Calcium or lime-rich soils or clays tend to fuse into very tightly bound material under the harsh glare and heat of the desert sun. This is the classic caliche I've already mentioned and it can literally be as hard and resistant as...well...cement!
(Another desert bench.)
If desert placer gold was bound up or mixed in with sedimentary rock and clay before the elements transform it into caliche, then it stands to reason that any gold will still be locked inside that natural cement. It may be a bitch to get out, but in certain instances the gold will still be locked inside that caliche. Now let's imagine a desert or dry placer bench where the gold-bearing gravels are tightly consolidated either throughout their entire length or breadth (or in sections) by caliche that's not in nice, neat layers but instead mixed in throughout those gravels. Getting the picture here?
Don't Let the Facts Fool You
Now let's fast forward out of those hot and dry desert placer environments to wet placers with constant flowing water, trees, and most importantly...shade for those old timers like me. If wet placer benches containing clay that's spread or mixed throughout their gravels are either far enough away from water (again, left "high and dry") or exposed directly to the heat of the summer sun even for only part of the day, then voila! We end up with a wet placer bench situation that's similar to its desert or dry placer counterparts. The main difference is that wet placer benches like this do not contain overwhelming amounts of lime or calcium so true caliche will be absent from them for the most part.
(A good bench can contain gold like this...and better.)
Don't let this fact fool you however. Working wet placer benches that are composed of gravel, rock, and fused or hardened clay throughout are tough nuts to crack...almost as bad as their desert cousins. I'm talking pick and shovel work ladies and gents. Ass-busting slave labor that will have your miner's pick bouncing off that hard pan so violently and so frequently that your hands will be vibrating and aching and you'll feel you've been struck down by the palsy after an hour or so. No exaggeration here, by the way. But hey...mining's a tough job and if you don't know that by now you'll find out soon enough.
It all sounds a bit unappealing doesn't it? Well, take heart because both desert and wet placer benches like this can be excellent gold producers for those of you willing to give them a go. Trust me on that point.
We'll talk more about this in my next post. Until then, stay safe and keep smiling.
(c) Jim Rocha (J.R.) 2014
Questions? E-mail me at firstname.lastname@example.org