Tuesday, September 1, 2015

"Gold Wears an Iron Hat"

(Gossan outcrop at the Giant King Mine in the California Motherlode Region.)

There's an interesting saying that German miners used back in the old days. "Gold wears an iron hat." This particular adage found its way here to the United States along with German immigrants (especially those with gold mining experience). There is much truth in this old statement as you'll soon find out.

Iron Hats and Red Caps

The "iron hat" or eisenhut that German miners spoke of in relation to gold is simply heavy iron mineralization. For the most part, this iron (Fe) mineralization is not reflected in run-of-the mill or ordinary ore types but a specific heavily iron-stained ore called "gossan." Yes, I've discussed gossan peripherally here in Bedrock Dreams in previous posts, but now I'll discuss it at length with the hope that you'll grasp its overall importance to gold prospecting and, ultimately, gold recovery. Gossan is a physical manifestation of the presence of extensive iron sulfide (FeS) mineralization, either in local country rocks themselves, various vein materials, or large swatches of mineralized ground. Most, if not all, old-time American prospectors kept a sharp eye out for "iron hat" or "red cap," as it became more commonly known here in the United States. Iron staining and/or the presence of gossan were two key visual clues that made the old timers take a closer look at the rocks in their hands or the terrain they were traversing. They may not have had PhDs in geology, but the old timers knew that gossan often contained gold or at the very least heralded its presence nearby. Although precious metals such as silver and gold where what they were primarily after, those old single-blanket, jackass prospectors also knew that other economically feasible metals could be contained in "red cap." Numerous rich gold lodes and their incidental elluvial or alluvial placers are historically linked to highly enriched gossan locations throughout the American West and Southwest, not to mention the Outback of Australia and elsewhere.

 (Note the visible gold in this gossan ore.)

What is Gossan?

So what is gossan? It's a heavily or intensely oxidized ore, ore body, vein, or outcrop that is typically rusty red, reddish-orange, or orange in color. Some gossan ores are also bright yellow or, on rare occasions, solid black. Unlike other sulfide or iron-stained ores, gossan is usually very weathered looking and exhibits attributes of decomposition (what the old timers called "rotten" or "friable"). I've actually been able to crumble certain types of gossan in my bare hands without the benefit of additional tools, which underlies that old-timer term "rotten." The reason for this extreme decomposition of solid rock is quite basic. The presence of FeS constitutes a chemical means of oxidation or "rusting out," and this process (along with Ma Nature's own air, water, heat, cold, and moisture) ensures that heavily oxidized and mineralized rocks of various types will eventually turn into "red cap." So-called classic gossan may exhibit some or all of the following characteristics:
  • Iron oxides or sulfides (again, FeS in high quantities).
  • Presence of quartz or small, quartz-lined cavities.
  •  A "spongy" or rotten look indicative of heavy weathering and oxidation. 
  •  In darker-colored gossan, the presence of manganese oxides or manganese itself.
  •  A hematite and goethite mixture known as limonite.
  • Free or visible gold, silver, or copper as well as other metals.
  • Colorization (already discussed).
Personally speaking, gossan has a look to it that catches your eye immediately, if you're familiar at all with various other types of gold ores and even gold mineralization in general. When you spot a piece of gossan float I guarantee you'll want to pick it up and eyeball it closer...that's how interesting and eye-catching it can be out in the field. That is, if you know what to look for because gossan often blends in to its surroundings to the point of expert camouflage at times. In the characteristics list above I mentioned classic gossan with a "spongy " look to it. Let me elucidate a bit. The rock material itself has the visual or textural appearance of a large sponge, with small holes and pockets and a textured surface like you'd see with a real sponge. That's the best way I can describe it. Anyway, I've come across various types of gossan numerous times in my prospecting and mining career and it's the sort of thing that just whispers "gold" in your ear. I've also recovered placer gold in various spots where larger or coarser pieces of that placer still had gossan matrix attached to it and the gold itself exhibited iron staining along tiny fissures or depressions (this can be cleaned, by the way).

 (Gold-bearing gossan. Note the "spongy" surface texture.)

Where You Step In?

If you're really lucky you'll come across a "red cap" area where the gossan oxidation is simply out of control, for lack of a better way of putting it. The entire ground is stained or "capped" with the presence of sulfides, even in cases where no float (as such) or outcrops exist. Take heed here. That sort of colorization of the ground in a given locale signifies that sulfides are leaching out from somewhere...most likely nearby or perhaps under your own two feet. A little digging is probably in order. This said, don't mistake large swathes of territory containing reddish laterite soil as evidence of some gigantic gossan deposit. You're barking up the wrong tree if you do. Laterite soil is typically found in hotter and humid climates like the tropics (there was a lot of laterite earth in Vietnam, by the way) but many areas of the U.S. contain reddish soils that are reminiscent or akin to laterite, some of them right here in New Mexico. Those are not signs of gossan, but of iron-enriched earth so you need to calm thyself if you're experiencing severe gold fever! The two things are as different as apples and oranges, don't ya know?

 (A laterite mesa. You could dig a ton of this stuff and not turn up a single microdot of gold.)

OK, that point made, let's move forward. I recently discussed the potential of elluvial gold and the fact that gossan has undoubtedly played a big hand in the generation of many elluvial (and alluvial) deposits. Sure, there are very many gold ore types out there, but gossan-type ores get my vote for their visual clue importance and their gold-bearing potential. Undoubtedly, some of you out there (not you old timers!) have walked right by or over gossan without any idea of the potential riches under your feet or a few yards away. Remember, I said potential riches. Most of the large gossan-oriented strikes in the U.S. of great importance and economic value were dug out long ago. This doesn't mean that smaller, less rich "red caps" don't exist...they undoubtedly do and remain scattered throughout the United States and other parts of the world. But it'll take a prospector or miner with the right knowledge, some field experience, and a sharp eye to spot them. Like their rich placer counterparts, the easy-to-find or get gossans have already been found or gotten. Maybe this is where you step in to prove that gold indeed "wears an iron hat."

Best of luck to one and all.

(c) Jim Rocha (J.R.) 2015

Questions? E-mail me at jr872vt90@yahoo.com

2 comments:

  1. Great post. Around here (Breckenridge, CO) in the 1800's the saying was "iron rides a good horse." I'm working a Gossan that was discovered in 1869 and never developed. The vein also contains a bedding plane slip which caused the secondary enrichment to form a horizontal deposit (blanket vein).

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  2. I heard it as "gold rides an iron horse" .......same thing, i guess. Something to think about to be sure.

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