Friday, February 13, 2015

Geological Classification of Gold Deposits (Part 1)

 (Quartzitic vein in host rock.)

You know, there's both an art and a science to gold prospecting and small-scale mining. In this series of posts I'll be talking a bit more about the science aspect as it relates to gold geology, so be prepared to deal with some arcane academic verbiage that I'll do my best to define and explain for you. I've touched on some of this science before, but not to the degree I'll address it here.

Setting: Sedimentary and Volcanic Rock

Rocks of sedimentary and volcanic origin (the PhDs call this supracrustal rock) are the two primary types of geological settings found around the vast majority of gold deposits, whether those deposits are placer or lode (i.e., reef or vein). In areas where sedimentary rock was or is prevalent, shale can be present as can arenite, quartz pebble conglomerates, carbonates, and certain sandstones. These types of rocks are typically associated with basins, or areas that were once covered by or part of ancient seas, oceans, lagoons, and lakes. Some of these sedimentary basins are what the academics call intracratonic basins (large-scale depressions that "sag" downward to depth) while others are known as miogeoclines. The latter are shallower and usually form near continental margins. In locales where volcanics were or are the primary geological setting, extrusive rocks like andesite, rhyolite, greenstone, basalt, and dacite often predominate as well as a form of volcanic sedimentary at times.

 (Geological diagram of an intracratonic basin.)

 (Part of an ancient andesite flow.)

The thing to remember in all this is the massive scale of geological transformation that can take place with both types of rock when metamorphism takes place over time as these basic geological settings are exposed to the elements, bent or buckled under pressure, or "injected" with mineralized chemical constituents via high-pressure steam or super-heated fluid flows. As I've said many times here in Bedrock Dreams, I'll wager a guess that metamorphic volcanics form the basis for the majority of the major gold (and silver and copper) concentrations here in the western and southwestern United States. Granted, there are also gold-rich batholiths and metamorphic sediments around, but they are far fewer in number unless I miss my earlier bet. (Now you PhD geologists out there can take a swing at me!)

 (Gold peeking through this chunk of metamorphic rock.)

Host Rocks

The topic of host rocks for precious metals is pretty straightforward in most instances. The experts contend that most gold (and Ag and Cu) host rocks are spawned in intrusive environments where dikes (large veins or reefs as the Aussies call them), sills, or batholiths are the primary feature. Remember, one of the richest gold strikes of all time, the California Gold Rush, was the result of a huge batholith (and its subsequent breakdown) that helped form part, if not all, of the Sierra Nevada Mountains.

OK, when we talk about intrusions like those listed above, we're dealing with things on a macro scale. What about intrusions on the micro scale? I don't know if the PhDs out there will appreciate me for saying this (nor do I give a shit if they do or don't), but gold host rocks that you can hold in your hot little hand will often show intrusions of quartz, quartzite, greenstone, feldspar, and on and on and on. This is what I call the micro scale and, push come to shove, it's the geological scale that most gold prospectors and small-scale miners are concerned with, not the macro scale. Is it good for us to know something about the large-scale geological forces that were instrumental in bringing gold to the area we're working? Absolutely. But are we going to while away hour after hour writing stale scientific treatises on the topic or posing new theories about intrusives at the next geological conference? Nope. We have better things to do, truth be told.

 A Very Small Rant

Let me digress here a moment for a very small rant. I know some of you earnest academicians and geology graduate school students out there are feeling your faces flush a bit because you sense a certain negative "bias" on my part for academia. Well, guess what? You're absolutely right! But I've earned my stripes and the right to profess a counterpoint view to what the public relations hacks and general media bias would have you believe about our colleges and universities and those who teach in them. You see, I was an academic myself for 11 years at a well-known private university. Yep, yours truly. One thing I saw during my academic tenure was the insipid, ongoing arrogant posturing on the part of most faculty members and the general absence of personal sacrifice (like military service), hard work (real jobs), and plain old common sense in their day to day lives or life history. This doesn't even include the worst facet of U.S. academia these days...its penchant for inculcating anti-American, left-wing drivel in the hearts and minds of impressionable students who don't know any better. Don't even think about arguing with me on this point if you're an academic 'cause I'll light your ass up. Been there...done that.

(Zippy the pinhead.)

OK, that's off my chest thankfully! Don't we all feel so much better?! Even the PhDs are trying to hold back a smile in certain instances.

Anyway, there's more to, not rants, but gold geology!

Be safe out there.

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

Questions? E-mail me at


  1. AMEN on your rant!!!
    I was an APMS at a high tech state university for 3 years and worked with a number of PhDs later on in my career.
    Discovered that having a PhD does not mean someone is really smart and to often they lack common sense.
    Nothing beats hard work, personal sacrifice ,real life experience and common sense.

  2. Here in Idaho, having a PHD means you have a Post Hole Digger.......Hell, I've got two!
    Common sense is more important here.