Geological Classification of Gold Deposits (Part 2)
In this series of posts I'm delving into the classification of gold deposits with a bit more depth and an emphasis on their scientific aspects. Again, I'll do my best to define the arcane academic language (25-cent words) used by the PhDs so bear with me here. I'd pay close attention because what I'm presenting is valuable information for all of you small-scale gold miners and prospectors out there.
Gold mineralization comes in a variety of forms and is always an important consideration. Geologists typically view gold ore body mineralization in terms of the following types:
1) Discordant: This is a tough one to explain in some respects. Discordant ore bodies are typically not uniform in terms of their fundamental types, but tend to "disagree" with one another within a specific gold locale. In other words, you might come across sulfide type ores in one area and not far away come across a refractory type ore like a telluride or something totally different than both of these. Discordants are "mix-and-match" forms of mineralization. As you already know, Ma Nature rarely makes things that follow strict rules or paths and discordants are a prime example of this.
2) Stratiform: The easiest way to explain stratiform (sometimes called "strata bound") mineralization is to define it as layered or being formed in layers, often in meta-morphed sedimentary rock formations. Remember, altered sedimentary gold deposits were once large bodies of waters such as very ancient seas or lakes.
(Stratiform deposit. This one contains chromite.)
3) Massive Sulfide: We're getting into familiar ground here. Massive sulfides are iron-rich ore bodies that often display the presence of iron staining or related chemical oxidation signatures. A visual key to identifying sulfides is the presence of rock coloration containing reds, reddish oranges, oranges, yellow oranges, blues, blue greens, and/or other colors in combination and, of course...pyrite (FeS2) or arsenopyrite. Now this doesn't mean every rock you see with sulfide coloration is going to contain gold, but you should always examine this type of rock carefully when you're out and about. Massive sulfide ore deposits usually come in the form of large veins, reefs, outcrops, or even breccia (a rock form containing broken pieces of other rocks and minerals all fused together into a solid or semi-solid.)
Mineralization forms are all related to certain geochemical profiles or "signatures." These geochemical profiles are usually identified in this way:
Hydrothermal: Caused by super-heated water in the earth's crust.
Hypothermal: Triggered by super-heated water and very high pressure.
Epithermal: Formed at shallow depths by warm water at low pressure.
Mesothermal: Deposited at medium depths beneath the earth's crust at medium temperature and pressure.
The composition of the fluids involved in these geochemical profiles is just as important as the profiles themselves to the formation of gold, silver, copper, platinum, iron, lead, and so on. I'm not a chemist or geologist by training or profession so I can't give you all the ins and outs on mineralized fluids. Suffice it to say that they carry the chemical constituents of gold. Then, when these gold-laden fluids are extruded, heated, or put under pressure anything can happen...including the formation of gold rich ore bodies.
You may be wondering why you need to know all this geology mumbo jumbo...a valid point to be sure. But when you understand that these geochemical profiles are what cause gold (and other precious metals) to be formed initially as highly mineralized fluids, then the puzzle pieces should start fitting together even in the dullest of minds (none of which apply to any of you, right?). Once again, for the umpteenth time: "Knowledge is power." I don't care if you're just a hobbyist variety gold panner or an old salt with decades under your poke, every tiny bit of information and knowledge you acquire and process about gold, gold formation, and gold deposition is going to help, not hinder you.
Or would you rather fly by the seat of your pants and wish, pray, or hope you'll find more gold? Good luck on that one...
Best to all.
(c) Jim Rocha (J.R.) 2015
Questions? E-mail me at firstname.lastname@example.org