(Where's the gold going in a ravine like this?)
The terrain configuration at any placer location (wet or dry) has much to do with how gold gets deposited at that location. We all know the standard deposition rules (if not, you should know them) but certain terrains can throw a curve ball our way. That's what this post is all about.
Ravines don't get a lot of air play in prospecting and mining books, videos, or websites. I'm here to change all that...or at least, bring your attention to this type of terrain and how it affects gold deposition. When I use the term ravine I'm talking about placer locations with steep slopes on both sides of the current watercourse or dry wash. Some ravines can be fairly short in overall height while others can go up hundreds and hundreds of feet. The key factor here is their steepness and we'll find out why shortly. Right now I'd like to touch on a few salient characteristics of ravine type placers and discuss them one-by-one:
1. Streams, rivers, creeks, washes, and gulches that lie within the protective ramparts of ravines don't typically have lots of switchbacks, curves, or inner and outer bends. Granted, this is a general rule and certain variations to it do occur in ravines. What this means is that low-pressure areas in the stream course or wash are limited in ravines, especially the steeper their flanking sides. Now we're all accustomed to hearing that low-pressure areas are where the "goods" are in most "normal" water courses, but what happens to the gold when low-pressure areas are limited or virtually non-existent?
(Ravine-type washes or stream courses are typically "chutes" or straight shots.)
2. Some rich gold locations have been discovered in ravine-type settings. This includes locations in California, Idaho, and British Columbia (B.C.) to name but three. I've prospected and mined (dredged) ravines in the Northern California Motherlode Region and I can vouch for that fact the good gold recoveries can be had in them...IF you know how gold deposition works in them.
3. Obstructions, including large rocks and even tightly bound cobble take on new significance in ravine placers. This is due, in part, to the faster water flow that typically dominates ravines. You'll find few slow (low-pressure) water points in a ravine but long stretches of fast-moving water. Since gold (and other heavies) cannot drop out and sink easily in fast-flowing water, any obstructions or tightly bound cobble become the main obstacles to placer gold's movement downstream. Ditto for any book-shelf bedrock that may be found in a ravine setting. Know this, however. Except along the steeps sides of ravines, true bedrock may be buried under large amounts of overburden. If smooth or worn bedrock is visible in ravines it will do you little, if any, good since the fast water flow in the ravine will wash the gold right on over and past it. Again, obstructions become central stopping points for gold in ravines. Remember this point.
(It's in the cobbles.)
4. In ravines, placer gold will tend to accumulate along a straight line in the CENTER of the stream course. This is the salient point I referred to in the second paragraph of this post. Since ravines are configured typically as straight shots or "chutes" and the water flow is typically very fast, placer gold tends to move in and along the center of ravine watercourses. This very same gold will tend to get caught and bound up in the heavy cobble and behind larger obstructions in the center of the stream or wash. The steeper the sides of a ravine, the greater the likelihood of gold deposition in the middle of that watercourse. Sure, you may find traces of very fine color along the sides of a ravine watercourse but if you want the good stuff you're gonna have to find a way to work the middle of the stream. Usually, the only way to do that is by suction dredging. That's the bugaboo. You Desert Rats and dry washing types out there are home free in this regard. Just remember that the gold will be tending toward the center of that ravine wash (for the most part anyway).
(Suction dredges are your best bet for working ravine wet placers. That is, if they're allowed in your area.)
5. Keep an eye out for oxidized layers, clay layers, or caliche. The first two items are for you wet placer miners or dredgers out there and the second is, of course, for you Desert Rats. If you hit a rusty looking layer under the cobble or obstructions it's a sign post that you should heed. Many ravine wet placers will have these types of layers underneath the cobble, sometimes only a few feet down. If those layers are very clayey and not given to being dredged up you'll usually find the gold from the top of the cobble down to that layer. If that rusty looking layer is loosely consolidated and can be broken up or dredged, then you want to get into it because it may hold the best gold. You Desert Rats are familiar with caliche (desert cement). If you're working a dry ravine in the desert, gold can be found in the top layer down to the caliche layer...again, in the center of that ravine wash.
Most of you may not ever work a ravine-type placer but if you do, keep these points in mind, OK? You never know when this info will prove valuable. Also bear in mind I've presented some general rules here...Ma Nature can do funny things, even in ravines.
Best of luck to all.
(c) Jim Rocha 2016
Questions? E-mail me at email@example.com