( 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.
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).
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).
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.
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.
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.
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 firstname.lastname@example.org