Gold's Mineral Indicators (Part 2)
(Mineral identification isn't just a power tool...it's a powerful tool!)
In this post I'll be continuing with information about certain gold-related minerals. Remember, these minerals are not guarantees that gold is going to be in the locations they're found in, but knowing what these minerals are and how to identify them is a powerful tool in your prospecting and gold mining arsenal.
Placer Deposit Minerals (continued)
Monazite: This is another main constituent of heavy black sands. Monazite is considered a rare-earth phosphate mineral and typically displays a slight reddish-orange color in its crystalline formation. It's usually found as small grains of sand and is usually associated with the presence of or weathering and erosion of granite, igneous and metamorphic rocks, schist, and gneiss. All of these rock types are known gold bearers, so monazite is a very good indicator that placer gold may be present nearby. Although monazite isn't usually found in deposits of high concentration, it is sometimes mined for its rare earth and thorium content.
(Emerald...one form of beryl.)
Sillimanite: Have you ever seen a cabochon cut stone in a ring that reflected a "cat's eye?" Well, more than likely that cat's eye ring stone is a form of sillimanite. Sillimanite is a metamorphic mineral that can be found in an array of deep colors. The mineral tends to have a smooth or "silky" texture to it even before it has been smoothed down in a rock hound's tumbler. It's main relation to placer gold is in its metamorphic origins, the same sort of geological origins that have produced some of the largest precious metal deposits in the world.
(Chunk of sillimanite.)
(Cabochoned sillimanite or "cat's eye.")
OK, let me flag a mineral here that some untutored or inexperienced folks think has an important association with placer gold:
Mica: I've been at this thing of ours for 37 years now, so I know my "shit" to put it bluntly. In all those years and in all the wet or dry placers I've worked in California, Mexico, Baja California, Colorado, Arizona, New Mexico, and Virginia I have never found this form of "fool's gold" to be a good indicator of the presence of placer gold. In fact, just the opposite is true. Yes, there's been a time or two (about 2% of the time) where a mica-rich stream held some placer gold for me, but 98% of the time it didn't. File that away. Any area containing an abundance of micaceous rock is NOT going to be a good gold producer and, in fact, you're probably going to draw a blank there gold-wise. Now if you're a rock hound or gem collector then lots of mica flakes and micaceous host rocks are an excellent sign that pegmatite dikes containing garnets, tourmaline, beryl, and other precious or semi-precious stones are probably in your area. Yes, garnets can be a signpost for placer gold but mica, in general, is a dry hole when it comes to gold indicators. Get that through your head and keep it locked inside.
(Mica flakes..."fool's gold.")
Now sure as heck someone is going to light me up on this statement about mica and will inform me not so politely that they hit a veritable motherlode in an area chock full of mica. OK, I'm ready for that. Know what my response will be? To tell them they're full of crap in my imminently diplomatic and soft-spoken way. Mica is responsible for a multitude of gold miss-identifications and you veteran miners and old salts out there know just how many greenhorns and newbies are ready to sell the house and start mining full time because of all those gold-colored flakes in their pan. Mica is horse puckey in my mining lexicon when it comes to placer gold. Sure it has its own geological importance and status...but a good gold indicator it ain't!
OK, that should give some pause for thought. There's more coming, so stop by again.
(c) Jim Rocha (2016)
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