(A nice chunk of pyritic sulfide gold ore.)
I'm hoping to wrap this series up here, but ya never know! It all depends on whether I can cover what I need to get across in this single post or not. Either way, I'll do my best to keep solid info coming your way, so read on.
Gold Assay Methods
So here we are again, back to the "Do my rocks contain gold?" question. As I've said too many times to count, a good fire or chemical assay is your absolute best bet in determining whether those rocks you picked up in the field actually contain gold. Moreover, a good assay will also tell you how much gold your rocks have in them (if in fact they do) and what other mineral/metal constituents are tagging along for the ride as well as in what form and concentration. Actually, fire assays can be performed for a relative low cost depending on who you're using to do the assay. There are some very good "mom and pop" or small assay businesses that do excellent fire assay work for a relatively low cost (Please don't ask me for recommendations. Why? I'm the guy who ends up getting bitched at if there's some issue or someone isn't satisfied.) But fire assays are only one route of many to get the hot poop on what your pretty rocks are all about. So let's take a look at some gold assay methods:
1. Fire Assay
Fire assays are the most common (and relatively cheapest) method of assaying. They've been around for a long, long time and were employed by the old timers almost exclusively. A good fire assay will determine the presence of gold in ores or carriers from 0.10 grams up to 50 grams troy per ton.
(Pouring molten fire assay samples.)
2. Large Tonnage Fire Assay
This is the type of fire assay used when large amounts of potential gold-bearing material are being sampled. The principles employed are the same as those used for small-scale prospectors and miners but on a massive scale (four tons or more on occasion). Large tonnage fire assays can make an accurate determination of Au down to 0.001 grams per ton. This increased accuracy is due mostly to the very large material sample range. However, large tonnage fire assays are the province of big mining operations (unless you small-scale guys and girls have a crap load of extra money sitting around the house).
3. Chemical Assay
These assays are usually based on the same principal of cyanide leaching used by large mining companies for extracting gold, albeit on a much smaller scale. Chemical assays are very accurate at determining the gold content of ores or those pretty rocks you picked up but they're best employed with refractory ores as opposed to free-milling ores (i.e., visible gold in quartz).
(Image from the Western Mining Assay Museum.)
4. Acid Diagnostic Assay
This is another form of chemical leaching assay. Acid diagnostics are really good at determining gold content, distribution, and quantity in carbonaceous ores as well as in sulfides and silicates (quartz). This is the assay route you'd want to take when dealing with carbon-based ores.
5. Microscopy Assay
This sort of microscopic analysis (yep, using a high-powered microscope) can spot and determine gold concentrations down to 0.20-0.50 micrograms. Holy moly those are small particles! Microscopic assays can detect very small (microscopic) gold particles, identify other minerals in rocks/ores, and accurately characterize that gold or those minerals.
6. Digital Imaging Assay
This assay approach is a lot like those scans you get at the doctor's office. Scans will show the amount and size of Au in a host rock or ore in an extremely accurate "profile." Needless to say, this a costly approach to take.
(Digital assay image.)
7. Electron Microscopy Assay
Electron miscroscopy is similar to the basic microscopy approach but uses a electron scanner as part of the microscope configuration to paint a more detailed image. This type of assay's greatest strength is its ability to provide a quantitative analysis of gold in ores as well as the ore's geological morphology (how it changed over time).
8. Mass Spectrometry Assay
I've actually used the "mass spec" approach during my work career to make isotopic analyses of radiological elements in various types of samples. I know firsthand what an excellent job it does in mapping out various constituents and their quantities in even very small samples. A good spectrometric assay will give you the entire goods on what you're holding in your hot little hand right on down to the parts-per-million category. Again though, this type of assay approach is way beyond the financial means of small-scale guys and gals unless you know someone who has a mass spec and knows how to use it!
(Mass spec assay unit.)
9. X-Ray Emission Assay
Like the mass spec approach, X-Ray emission assays can detect gold into the parts-per-million range. That's the sub-microscopic gold I spoke of earlier in this series. X-ray emission is particularly effective when you're dealing with sulfide and silicate ores.
10. Laser Ablation Assays
Laser ablation assays can detect and quantify gold down into the parts-per-billion range. Yep, you heard right. Parts-per-billion. Like X-ray emission assays, the laser approach is best used with sulfides, oxides, and silicates.
11. Gravity Concentration Assay
OK, now we're back on familiar ground right? But before you get all excited and start happy dancing around the room please understand that your gold pan, your mini-sluice, shaker table, or spiral wheel may be excellent little devices for recovering gold (depending on how well you use them, that is) but they are woefully inaccurate when it comes to getting detailed assay info. Yes, you can crush up those pieces of ore you picked up and pan them out. Maybe you'll actually see a small crescent of gold appear in your pan. Good deal and more power to you my friend. But it's what YOU'RE NOT SEEING in your gold pan that's key here. You could pass up relatively rich ore by using simple gravity concentration methods to make an assay determination. Getting the drift are you?
(That gold in your pan DOESN'T tell the whole story.)
I realize that most assay methods are too expensive for most small-scale miners to use. But if you're dead serious about knowing what your rocks contain the age-old and proven fire assay is the route you want to take. It's cheap enough and accurate enough. And to those of you shaking your heads about me listing all those super-duper scientific assay methods you'll never use I have this to say.
Knowledge is power.
(Amen brothers and sisters.)
(c) Jim Rocha 2016
Questions? E-mail me at email@example.com