(Gold has an interesting affinity for iron.)
I addressed the basic concept of heavies pretty well in my previous post so I won't beat that dead horse any longer. I will answer a reader question or two and either in this post and in the next I'll talk about how heavies actually drop out in a stream flow, including variances on that theme. So stick around and learn something useful instead of listening to that tinny banjo music playing "Oh Susannah!" on Ma and Pa Kettle's "Git Yerself Sum Gold" website. (I know, I know...I'm bad.)
OK, back to reality. My good friend and Idaho mining pard "Muskrat" (Gary Thomas) asked a very good question in his comments on Part 1 of this series of posts. Here's "Muskrat's" query:
J.R., I'm a little confused.....I have heard several times now what you say here: "Oxidized iron or fused clumps of iron can be an indicator of gold." Is it because they tend tend to collect in the same places being heavy, or is it likely to form together, or something more? I know heavy mineralization of iron is a good sign and I have heard the saying, "Gold rides an iron horse."
Well Gary, you're right on all three counts to some degree or another.
Yes, they are both heavies but gold's specific gravity (i.e., density in water) of 19.32 far outweighs iron's 7.85 by a factor of more than two times. Remember, however, that I consider any mineral, metal, or material with a specific gravity of 4.00 or higher to be a heavy since garnets are at this density and are often recovered with gold and heavy black sands (at least in certain locales).
Iron and placer gold are often found in close proximity because iron is heavy enough to often be deposited where the placer gold is. Sometimes those rusty (oxidized) iron scraps and old nails can tell you a lot about gold deposition and often they'll start showing up in your test pans or your dredge's or highbanker's sluice box heralding the presence of gold. I've seen this happen countless times in my small-scale mining career, even as recently as last summer here in New Mexico's Tusas Mineralized Zone.
I don't know what causes the clumping together of iron and gold but it does happen.
As far as clumps of oxidized iron fusing together with both silver and gold I've come across this interesting little bit of physics more times than I can count, both in beach treasure hunting (detecting) and in placer gold mining. I don't fully understand the chemical reaction/attraction thing here...perhaps a chemist or geologist could clue me in. All I know is that this fusing together of odd bits of oxidized ferrous metal is not uncommon out in the field. For example in my beach hunting days I frequently found gold jewelry and silver coins fused in with conglomerations of old oxidized iron nails, screws, scrap metal, etc. Although not as frequently, I've found placer gold (including small nuggets) fused together in similar clumps of ferrous metal. That's the long and short of it. Why it happens I can't say...I just know from experience it DOES happen.
(I've used this pic before, but it really expresses the idea about fused iron clumps.)
Yep..."Gold rides an iron horse."
As Gary rightly points out, "Gold rides an iron horse." Gold and iron (and iron pyrites) tend to have a close connection, often from a geological standpoint as well as a chemical bonding standpoint. Iron-stained ores often contain gold as do pyritic ores. I suspect the main culprit here is some sort of chemical reaction or attraction between the two metals, but that's only supposition on my part since I'm an engineer and not a scientist. It's an intriguing mystery though. Ma Nature does all sorts of strange things when it comes to metals and minerals.
(Sulfide gold ores like this are iron based.)
The upshot of Gary's perceptive questions is simply this. Don't worry about how gold and iron get together, just understand that they do in frequent (but not all) instances. That's the message to take away here. When you're out and about and rusty bits of iron start showing up in your sluice box, take heed and pay close attention. I also recommend that you take a VERY CLOSE LOOK at any clumps of fused ferrous metal you come across or recover in your small-scale mining and prospecting efforts. Iron won't always herald the presence of gold but it does in more instances than you might currently imagine. So be aware and look past the obvious.
There's more to come on heavies. Stay tuned and keep the faith.
(c) Jim Rocha (J.R.) 2016
Questions? E-mail me at email@example.com