Iron Oxides as a Visual Prospecting Clue (Part 3)
(Alaskan gold placer gravels with an oxidized layer to the lower right.)
Although the importance of iron oxides as a visual prospecting clue is primarily associated with ore bodies, the tell-tale rusty red color of oxides can occasionally point the way to gold in placer environments as well. The trick is knowing what to look for.
This is not always an easy task, especially for "newbies" because in many gold regions (especially in parts of California's Motherlode) reddish or rusty looking soils abound. These may contain some iron but are not true laterites and shoveling a ton of this material probably wouldn't bring more than a trace of color, if that.
No, what I'm talking about here are not soils, but stream and wash gravels as well as isolated pockets where the presence of iron oxides is a tip off to the presence of placer gold. Another important aspect of oxides in placers is that they are not as extensive in scale nor always geologically based as their hard-rock counterparts are.
Gold Prospecting Books
Let's stop a moment here. Does this mean that only those placer gravels displaying the rusty reds and oranges of iron oxidation contain gold? No, absolutely not. Each placer mining environment is different and is influenced by a range of factors that may or may NOT include oxidation. Once again, the presence of iron oxides in the field doesn't guarantee anything...it's simply a visual clue that should be recognized and subsequently examined.
Oxidation in the Placer Environment
I think the best way to tie in oxidation to the placer environment is by providing you with a few examples, a couple of which are direct and the other indirect:
Example 1: Back in the early 1980s myself and a "pard" were highbanking in Northern California along a large gravel bar that had been formed when the North Yuba River split into two channels. This wasn't your usual minor gravel bar, but was more on the order of a small island with lots of growth and even large trees covering it.
We sampled the gravels on the main river channel side of the bar but found little gold. It was obvious that the river's flow on the outside of the bar was too strong and not suited for good deposition. So, we shifted to other side of the gravel bar facing the smaller river channel where the water flow was slower and approaching that side of the bar as if it were an inside bend.
Heavily Iron Stained
The topmost gravels at this location were pretty much devoid of color and had the usual greyish, sterile look that many stream gravels have. But about a foot down we starting running into a heavily oxidized layer of gravel that was quite unique in that it was loosely consolidated and composed primarily of lens-like, flattened gravel of nearly uniform size.
(I had a pretty good day when I recovered this placer gold.)
All of the gravel was heavily iron stained as was the material around it and the whole mix was pretty much saturated with coarse black sands. When we sample panned this layer, 5-6 small and medium-to-large flakes of gold were showing up in nearly every pan, so we set to bucketing and running this layer like madmen.
When we shut down for a clean up a substantial amount of beautiful, bright yellow placer gold was easily visible in the upper reaches of the highbanker's sluice box. No nuggets or "chunkers," but again, lots and lots of small, medium, and large gold flakes. We had to work for it, but that oxidized gravel layer held good gold. Very good gold, as a matter of fact.
I'll have more in the way of examples in my next post. Until then, be safe and keep smiling.
If you liked this post, you may want to read: "Working Clay Layers"
(c) Jim Rocha (J.R.) 2012
Questions? E-mail me at firstname.lastname@example.org