Interpreting Visual Prospecting Clues: a Dying Art ? (Part 4)
(Mother Nature offers up many visual clues if you know what to look for.)
Unlike their counterparts of the late 1800s and early 1900s, many aspiring gold prospectors and miners today have little knowledge or experience in interpreting Mother Nature's visual clues to the presence of precious metals. In a very slight way I'm trying my best to correct that perceived deficiency by passing along a few tips that will help you "see" what's going on out there.
Micro versus Macro?
If you've been reading this series of posts carefully you'll know that I've reiterated time and time again that regardless of the nature of the visual clues that may lead you to gold (or other precious metals or minerals), there is invariably an abrupt transition taking place geologically. This geological "abruptness" is just as relevant to smaller or localized areas or zones as it is to those of a large-scale nature. Rarely do highly mineralized zones occur in geological environments containing a single rock type or that transition very slowly from one distinct geological zone to another.
My son (a geologist fresh out of college) would probably take issue with that last statement. Undoubtedly there are exceptions to this rule of thumb (the California Motherlode?). I'm not here to debate all the PhDs and budding field geologists out there, including my own son. However, they're theoreticians first and foremost...not gold miners. You, I, and every other gold prospector or miner out there looks at geology from a very intimate or micro-scale standpoint while the scientists tend to look at things from a macro or large-scale view. That may be an over-simplification of sorts, but I think you get my drift.
Shifting gears again, I'd like to get back to this idea of the significance of abrupt geological changes. Let me give you a perfect example:
The Ortiz Mountains lie a scant 14 miles from my home outside Santa Fe, New Mexico. First discovered and "rushed" in 1829 I believe, the area possesses a singularly unique geology that's rooted in localized batholithic upthrust, metamorphism, and the presence of pyritic ore bodies (iron sulfides). This potent geological combination produced gold veins that eroded out along the north-and south-trending flanks of the Ortiz over millenia leaving behind some relatively rich dry placers (Old and New Placers Districts).
Yet, just a few miles south, southeast, and southwest of the Old Placers District there is virtually no precious metals mineralization at all...drive a few miles down Highway 14 (the famous "Turqouise Trail") and geological conditions suddenly shift to become coal-bearing and the underlying country rock changes to some degree as well. In fact, the old coal mining town of Madrid is in this locale. Drive a few miles farther south and you'll enter the southern boundaries of another placer and lode gold boom area, the New Placers District. So another abrupt change takes place in terms of gold mineralization
Jumping back north again, travel just a tiny bit north and west from the Old Placers and you'll find the old Spanish village of Los Cerrillos. Immediately north and west of Cerrillos (or adjacent to) are the Cerillos Hills and some of the oldest turquoise mines in the United States! Talk about abrupt mineralized zones...I've always marveled at these sorts of abrupt geological transitions. They always get my gold "radar" up and running.
OK, enough of that. Here's your next tip:
5) Evidence of other metals or minerals
At first glance this may seem like a no brainer. Still, there may be some of you out there who might not make the connection between copper and gold, silver and gold, garnets and gold, or any number of other combinations that lead to the presence of the gleaming yellow metal. Let me clarify things a bit here: most gold ores (and placer gold for that matter) contain small amounts of silver, copper, iron, or other such "contaminants." Conversely, most silver and copper ores contain small amounts of gold. This may also be true of iron ores but again, I'm no geologist and I'm not going to stick my neck out on that one! The percentage of gold contained in these other sources or "hosts" is a crap shoot at best...sometimes it's more and sometimes it's less (or very, very little).
(The Ortiz Mountains of New Mexico...gold was found here in both the Old Placers and New Placers Districts.)
That's really not the issue here though. Nor is the fact that in some instances semi-precious stones or gem minerals like garnet are found with or near gold, despite the fact that both are formed from what I believe are different types of mineralization. Most gem minerals are heavy on the mica side when it comes to mineralization and as a gold prospector and miner I've never been fond of micaceous geological environments. That said, I can name at least half a dozen of these sorts of environments where I've found or mined gold, so out-of-the norm minerals need to be "seen" as a potential visual clue.
Getting back to that issue at hand I spoke of earlier...keep your eyes peeled for any and all potential connections that might link you to gold, whether that link is tin, iron, copper, silver, garnets, moon rocks, kryptonite, or whatever. Mother Nature likes to play funny little tricks on those who try to tease her treasures from her. So keep an open mind and your eyes sharp, and if in doubt...check things out.
(c) Jim Rocha (J.R.) 2013
Questions? E-mail me at firstname.lastname@example.org