Thursday, December 22, 2016

Lode Gold's Sulfide Mineral Indicators

( A mineral indicator...but it ain't gold.)

Sulfide minerals can provide important clues for small-scale gold miners and prospectors in locating both placer and lode deposits. This is especially true when it comes to lode or vein gold (my main focus here). In this post I'll be talking about some of the main sulfide mineral indicators you should keep a sharp eye out for when prospecting.

Deduce...Don't Assume

Before I get to the heart of the matter I want to tell you that mineral indicators are just that. Indications that gold mineralization MAY be present. They don't guarantee the presence of gold but they strongly suggest it. Meaning no disrespect to anyone reading this post, I do know some of you tend to go a bit overboard when you come across some of these minerals, particularly in those rocks you pick up and bring home. I guess that's better than ignoring the presence of sulfide minerals, but I suggest you temper your enthusiasm in this regard and approach things from a knowledge-driven logical standpoint rather than an emotional or wishful thinking framework. Learn to recognize what's around you and whatever signposts may lead you to gold before assuming it positively exists because you've come across certain mineral indicators. Be like Sherlock Holmes and deduce rather than assume, OK? You'll be much more successful in your gold locating and recovery efforts by doing so.

Sphalerite: The main sulfide mineral associated with tin and the biggest matrix for tin ore. It is typically found in igneous and metamorphic rock formations (remember, metamorphics have been huge gold producers in the American West and eslewhere). The old timers knew the significance of sphalerite as a potential gold indicator and called it "blackjack" or "rosin jack." Sphalerite comes in a range of colors, including grey or greyish-black and usually has a metallic luster (although not in every instance).

(Sphalerite.)

Chalcopyrite: The main sulfide ore of copper throughout the world. And where's there's copper there's usually some silver or gold found along with it. Chalcopyrite is typically a brassy yellow in color and when exposed to the elements loses its metallic luster. Its surface then becomes rough or granular. Sometimes chalcopyrite can have an iridescent look with multiple colors playing across its surface (like "peacock" ore).

(Chalcopyrite.)

Galena: The main lead ore, galena is essentially lead sulfide. Galena is easy to spot since it usually displays a silvery gray color and if broken will show perfect cleavage in three directions. Galena will oxidize or "tarnish" to a dull grey when exposed to the elements. It has a specific gravity of around 7.5, which makes it fairly heavy and dense (like those lead shot and fishing weights that show up in your gold pan or sluice box). In crystalline form galena exhibits cubic crystals. It often contains small amounts of silver.

(Galena.)

Bornite: Another important ore of copper, bornite is metallic in its appearance and displays a copperish-red color until exposed to the elements. Then it will turn an iridescent violet color. Other, less common bornite colorations include pinkish-orange, pinkish-grey, and creamy pink.

(Bornite.)

Chalcocite: A "secondary" form of copper sulfide that has a metallic luster and is usually colored grey, dark grey, or a bluish-black.

(Chalcocite.)

Pyrite: Commonly known as "fool's gold" pyrite (iron sulfide) forms in cubic crystals that can range in color from brassy yellow to silvery. It invariably displays a metallic luster which is why it's often mistaken for natural gold when found in brassy yellow color. A big giveaway difference between natural gold and pyrite is that real gold will maintain an even, lustrous glow when exposed to direct sunlight while pyrite will "flash" or sparkle. It also has a high specific gravity which is another reason its often mistaken for natural gold. However, the presence of pyrites is often a good sign you may be in an area of gold mineralization.

(Pyrite.)

These are the main mineral indicators of the possible presence of lode (and sometimes placer) gold in a given area. Once again, they aren't guarantees so please remember that point. Mineral identification in the field is rapidly becoming a lost art these days so it behooves you to get "schooled" up on the subject. Well-rounded miners and prospectors know this.

Have a great Christmas out there!

(c) Jim Rocha 2016

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

5 comments:

  1. JR, many years ago (20+) there was a big pile (15 feet tall or so) of grey/black rock along side a road South of here. It must have been worth something for someone to pile it up that way. It was very heavy, a baseball size piece weighed several pounds. I thought it must be lead ore, since it wasn't that far from the town of Leadore, Idaho. I took a piece of it home and heated it to see if I could melt the lead out of it. (I cast my own lead bullets). Nothing....
    The pile is gone now, and I still don't have any idea what it could have been. Identifying rock is something I have always wanted to be better at and I have studied it, yet still,I don't have much of a grasp on it. Anyway, as always, interesting stuff here. Thanks again for taking the time to do this.

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  2. You're welcome Gary. Merry Christmas my friend!

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  3. To the Gentleman that makes this happen and all that read these great works,i wish you all a Merry Christmas and a happy,healthy and prosperous New Year.You may take the liberty of substituting any holiday you choose and what ever prosperous means to you...chuckle ;-)

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  4. Thanks for the kind thoughts Bill! Right back at you.

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  5. MERRY CHRISTMAS JIM!!

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