("Do my rocks contain gold?")
Over the years (including recently) I've received numerous e-mails from readers with attached photos of various collections of rocks they've gathered while out and about. The main questions asked by these folks are "Is this gold ore?" or alternately, "Do my rocks contain gold?"
No Easy Task
You old timers out there know that these sorts of questions are extremely difficult to answer based on photographic evidence, especially if the photos in question are not highly detailed close-up images. I suspect that most (but not all) of the readers who hit me up about possible gold in their rocks have limited knowledge or experience when it comes to gold ores or the presence of free-milling gold in host rock. This makes absolute sense...otherwise, why would they contact me in the first place? So no judgment is applied here on my part regarding those of you who send me such queries and I welcome them just the same with the simple caveat that if you're going to send me photos, please make them as close up and detailed as possible. That helps me greatly.
While I'm mining this vein of thought, you should always keep your eyes open for evidence of gold mineralization in the rocks around you and underneath you. In fact, it should become second nature to you after a while especially if your focus is prospecting. That said, as a long-term placer miner I'm always studying my surroundings for clues as to where the gold may be and what type of source it came from. Any would-be gold prospector or placer miner who isn't aware of his or her surroundings is NOT going to be very successful in the long, or the short run. Take that from someone who knows.
Being able to discern the mineral or metal constituents of the rocks around you is no easy task, even for highly experienced prospectors and miners. The one edge these veterans have, however, is long-term, hands-on field experience. They know the difference between "bull" quartz and highly mineralized quartzitic rock and are programmed to hone in on the visual clues nature provides for identifying the latter. Moreover, "elite" prospectors and miners have done their homework, researching the geology of the areas they plan to prospect or work as well as studying historical records, geological bulletins, personal accounts of the sourdoughs first on scene, and even production records generated by large-scale mining operations. I know I keep beating this same old drum, but the more you know the more successful you'll be as a gold prospector and miner. It's a very simple equation really. One of those paths to the knowledge you need can be found right here in the words I write and the posts I publish in Bedrock Dreams. I'm not stroking my own ego here...just stating facts.
Getting back to those rocks that my readers are so concerned about let me first say that there's no mistaking what visible, free-milling gold looks like. This seems like a no brainer to you veteran prospectors and miners out there, but you'd be surprised how many folks who contact me about their rock collections who obviously don't know what natural gold (placer or lode) really looks like. Again, I mean no slight to anyone here as this is a common failing among those with little prospecting or mining knowledge or experience. We all started as newbies or greenhorns, myself included. The way to get a leg up on this gold identification issue is simply to educate yourself and start garnering some actual field experience.
(There's no mistaking natural gold in host rock or placer form.)
One of the biggest identification factors made by newbies and greenhorns is the "fool's gold" factor. This where the uninitiated or the uniformed pick up a rock containing mica or iron pyrite and are off to the races thinking they've hit the "big one." The experts don't call these two mineral examples "fool's gold" for nothing...they've plagued newcomers to prospecting and mining for centuries. For those of you out there who are still prone to making this sort of mistake, let me clarify things for you with some info I've thrown out there before:
- Gold, whether in natural or artificial form is extremely dense and very, very heavy. In fact, gold is nearly 19 times heavier than water which is why it tends to sink or work its way down in stream or wash gravels. Gold has an even, lustrous glow to it that doesn't change under direct light such as the sun. Natural gold is also soft and highly malleable and can be easily grooved or etched with a pen knife or other sharp object. If struck with a hammer or heavy object gold will flatten and/or stretch.
- Mica, on the other hand, is only 2.88 times heavier than water which is why you often see the upper levels of stream sands sparkling with layers of it in the form of very thin, flat, rectangular flakes. Unlike real gold, mica tends to glitter or sparkle under direct light. If dug at with a knife or sharp object mica flakes will essentially fall away from host rock and if struck will disintegrate into a fine powder. You can actually place most small mica flakes between two fingers, apply pressure, rub your fingers, and watch mica turn into a powdery finger coating.
- Pyrite has a specific gravity of 5.02 which means it is five times heavier than water and nearly twice as heavy as its "fool's gold" partner, mica. Iron pyrites (FeS2) are a sulfide mineral that form in cubic crystals that appear more bronze or, in some instances, silvery in color. Pyrite tends to refract direct light and "flash" when moved under a light source. It's also extremely brittle and if struck with a heavy object will shatter into a dark powder, something that would never happen with real gold.
(Iron pyrites...note their cubic structure.)
Here's the bun kicker for you newcomers though. Rocks containing iron pyrites or iron sulfides in any form should be examined closely, especially if the host rock containing pyrites shows evidence of heavy iron staining or sulfide oxidation. Why? Simply because many gold ores are sulfide-based. Now this doesn't mean every rock with pyrite crystals is some form of gold ore. In fact, in most cases this is NOT the case.
There are only a couple of methods of actually determining if those rocks in your collection contain free-milling gold that you can't already see. You can crush them up into a fine powder using a heavy duty mortar and pestle or rock crusher or ship samples off for a chemical or fire assay. In the first case you'll pan that powder out and see if any color shows up in the pan after the lighter materials have been washed out. The second case is more reliable. A good assay will show you the exact mineral/metal constituents in your rocks by percentage factors. However, assays cost good money and shouldn't be used frivolously by newcomers (or veteran miners and prospectors for that matter).
Once again, I'm always willing to help you when you send me rock photos. But understand that the best way of determining whether your rocks contain real gold is to digest what I've written here. Make sense?
Best of luck out there!
(c) Jim Rocha (J.R.) 2015
Questions? E-mail me at email@example.com