Monday, March 31, 2014

Metamorphics and Gold (Part 2)


 (Metamorphic schist with stringers or "waves" of quartzitic material.)

The relationship between certain metamorphic geologies and gold mineralization is an incontrovertible fact in many parts of the world, including the American West and Southwest. In the first post of this series I established some basics about metamorphics and in this post I'll be elaborating on things a bit more.

Foliated Metamorphics

First off, there are two main types of metamorphic rock, foliated ("layered" in common English) and non-foliated (not layered). As I mentioned in my earlier post on this topic, metamorphics can either alter other, non-metamorphic rocks or "morph" into different types of metamorphics themselves. Here's an example:

1) Shale (non-metamorphic) is changed into the foliated metamorphic known as slate, a much harder rock.

2) Slate is then transformed into a foliated metamorphic called phyllite (or metamorphosed slate). Phyllite is layered but the layers are uneven, not flat, and these layers often possess a slight undulation to them like small rolling hills on the Great Plains. Phyllite has a silky look to it and shows a slight presence of small pieces of mica. (J.R.: Although not universally associated with gold mineralization, mica is nearly always present in locales containing gem minerals. At least that's been my experience over the years.)
 
3) Schist is nothing more than (yep, you guessed it) metamorphosed phyllite. Schists contain a lot more mica than phyllites and some schists show signs of mineralization, including the presence of garnets or other crystals.

(Micaceous schist...mica is associated more with gem minerals and crystals than it is with gold...but keep your eyes open anyway.)

4) Gneiss is formed when schist metamorphs. Gneiss is very close in its look and properties to granite, but its not granite despite its appearance and often fools the uninitiated out in the field. The foliated "layers" in gneiss are often crumpled up. sometimes quite dramatically.

Anyway, this sequence gives you a pretty good idea of the foliated metamorphic process.

True Qualifiers

Now here are a few non-foliated metamorphics: marble (from limestone), quartzite (from sandstone), serpentine (from pyroxene and olivine), and hornfels (from siltstones, dark shales, or basalts). Of both the foliated and non-foliated metamorphics listed, you may recognize some that are related to gold-bearing areas. Slate and serpentine form the bedrock or "country rock" for some areas where Au is present, while certain schist, gneiss, and quartzitic environments can actually host gold or quartz stringers where gold is a constituent.

This doesn't mean that each of these metamorphic types will contain gold...only that there are instances where this is true. The true gold qualifiers for metamorphic belts or zones are heat (temperature), pressure, deformation, mineral inclusion, and friction generated by earth movements or collapses.

(Metamorphic rock deformation.)

You don't need to understand all the ins and outs of the geological aspects, but what you do need to take away from all this is that any geological zone or rock type that has undergone a great deal of transformation or alteration over millenia is a likely prospect for precious metals mineralization and the presence of gold itself. Add to that the presence of sulfides or tellurides and you may want to consider your surroundings more closely.

A Starting Point

What about quartz then? Quartz veins, reefs, lodes, and stringers within metaphorphic rocks, zones, or belts have been a significant host material for gold, silver, and copper here in the western and southwestern United States. That said, however, the presence of quartz of and by itself does not mean gold will be nearby or even at hand. You can find some sort of quartz rock in nearly every state of the Union but not every state has gold in economic quantities or even in small amounts for that matter. If the required metamorphic alterations have not taken place, well pard...you're probably going to be S.O.L. unless some other type of geologic gold ore or host genesis has taken place in the area you're prospecting or mining.

Quite a few small-scale prospectors and gold miners pooh-pooh this sort of geological knowledge and function on the old principle that states "Gold will be where it's been found before." Very true indeed and a good rule of thumb to work from as a starting point. But if you're looking to increase your gold mining and prospecting knowledge and ability to find and get the gold, then you'd do well to take heed to what I said before.

"Metamorphics have produced some of the most significant gold deposits here in the United States and the world at large."

I'll leave it at that...

(c) Jim Rocha (J.R.) 2014

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

4 comments:

  1. JR, I find this very interesting,but very confusing as well. Most of the rock I have found in the areas I have found gold in, is granite like in appearance. It is more of an orangeish-tannish color, but has quite a bit of mica in it as well. I thought the gold was coming from that rock, but maybe it is not. I'll have to look around and see what other types of rock is there next time I go. I'm not sure I'm smart enough to figgure out all this, but I'm learning slowly! Thank you for that! Gary

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  2. That granite-like rock you speak of is a metamorphic, is it not Gary? I'd bet money on it. Understand the confusion though. Best, J.R.

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  3. ju
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    Just like a quartz, you have to crush it. Found alot of gold in crushed rock that i did not know was there. Ken and I have been practicing this and been rewarded. Even tho the old ones have exposed these veins, they tended to move on due to better strikes, less work. This gold value is very brite and makes it worth seeing. J.R. maybe a good write on crushing ore. We use a post hole digger upside down with a rebar shaft to crush....redneck I know but always working on those raining days with a 24 pack. If you can't prospect make due with what you have and enjoy!

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  4. Very good advice my friend. You won't make it as a miner if you can't improvise. J.R.

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