Iron Oxides as a Visual Prospecting Clue
(Gold-bearing vein material showing extensive iron oxidation.)
In the past I've spoken about iron sulfides (especially iron pyrites) and their relationship to gold mineralization. One thing I haven't addressed properly, however, is the importance of iron oxides as a visual prospecting clue.
What are iron oxides (Fe2O3)? Here's the simplest answer I can give you...rusted iron. Iron oxides form when iron-rich rock or material is exposed to the elements and then begins to degrade or breakdown in an often long, drawn-out process known as (yep, you guessed it) oxidation.
For example, if you forget to return that cardboard box of iron nails to its proper location in your garage or shed and instead leave it to the tender mercies of the elements, especially moisture, you know what will happen, right? The box will melt away, the iron in those nails will start oxidizing, and the first evidence you'll have of that oxidation will be rust.
Gold Concentrates Gold Pans Gold Concentrators
That rust or rusty coloration is not only an indicator of the presence of iron and oxidation, but it also signifies the beginning of the chemical alteration of the very structure of those nails. Left outside long enough, those nails will oxidize to the point that all that's left is a mound of rusty looking powder.
Now to avoid confusion, let me tell you right now that both types of gold mineralization (sulfides and oxides) will created "rusty" looking stains in any rock or gold-bearing material they are present in. Generally speaking though, any material or rock containing large amounts of iron will display much more of that rusty appearance and will also degrade or decompose much faster. Sulfides, on the other hand, tend to retain their host rock's solidity.
Some very heavily iron-oxidized gold ores will crumble away into powder fairly easily when struck by a pick or rock hammer. I've even had occasions in my small-scale prospecting and mining career where this sort of heavily oxidized, iron-rich rock could be crumbled in my hand. Let me tell you now that if you ever run across ore like this you'll probably end up one happy camper.
Please remember that I'm speaking about gold prospecting in terms of hard rock, lode, or vein mining here, not placer mining. Yes, there are visual clues resulting from iron oxidation that can lead you to gold in placer situations as well and I'll talk about that later. Right now let's stay focused on the task at hand.
(There's a small seam of gold running laterally near the top of this chunk of oxidized ore.)
OK, does all this mean that any iron-rich (or rusty looking) rock you come across will carry boo-coo gold in it? Of course not. If that rusty rock you're seeing or recovering is coming from an area or location where gold or gold mineralization is not present, then chances are all you have is rusty looking rock...nothing more.
Here's a General Rule
Additionally, not each and every piece of rusty looking rock in California's Motherlode Region is carrying gold either. However, any heavily oxidized rock you come across in that area (or any other heavily mineralized gold zone in the world) deserves a closer look.
So do any outcrops or vein material you spot that visually show deep rusty reds, browns, oranges, and even an orangish-blackish cast. Here's a general rule for you: the heavier the rusty appearance, the closer the look you should take. (Be careful while doing so...although Au doesn't typically tarnish, in heavily iron oxidized hosts the gold can take on a very rusty looking cast too until cleaned or refined.)
There's more to come from this "old timer." Good luck till then...
If you liked this post, you may want to read: "Prehistoric Rivers of Gold (Part 5)"
(c) Jim Rocha (J.R.) 2012
Questions? E-mail me at firstname.lastname@example.org