Field Testing Gold Ore Part 4
What’s a Good Sample?
By: Kerby Jackson
In case you missed it, here is Part 1 Part 2 and Part 3. Unlike placer mining, where a good pan is determined by the amount of visible gold you put in your vial to flash at your mining buddies, lode mine sampling differs considerably.
Though it is possible to kind-of-sort-of eyeball values of free gold in a pan sample, as indicated earlier, often the most values are tied up in the sulfides. In reviewing our samples for worth, always remember this one simple fact. Many a valuable mine has been found where it was next to impossible to pan free gold out of the ore samples. The presence of sulfides in our samples, especially in large quantities and/or accompanying good quantities of free gold should always be something to keep our attention and encourage us to keep sampling and keep up our note taking. Half filling a vial or more with the sulfides and maybe a little free gold from a single pan sample and doing it consistently should be something to keep our efforts up, even if it’s not really filling up your poke with gold dust to show off to your friends.
That being said, since most of us are hopefully looking to locate some ore that produces enough free gold to justify investing in a little mill and putting some free gold in our pocket, it really is free gold that we are after.
With that in mind, what sort of results are we really looking for? In the first place, it’s an unscientific, yet basic rule of thumb of many lode miners, that if you reduce a one pound sample down and can raise 90 to 100 tiny little colors in your pan, you’re probably looking at ore that is running about an ounce of free milling gold per ton of ore. This is high grade free milling ore by any lode miner’s standard. By contrast, if you raise 25 or 30 of those little colors in a one pound sample, you’re running about a quarter ounce a ton and that is a pretty fair result. Though it won’t make you rich, it is certainly milling grade ore if you can move enough of it each day. If you could get that result pretty consistently, you could do okay at the current price of gold if you could extract and mill it efficiently and sooner or later, you’re going to run into some higher grades to do very, very well on occasions if you keep at it. On the low end of the scale, if we only see 5 or 10 colors in a sample, that’s pretty low grade stuff, but it’s proof that it’s at least there and that we should keep looking, keep sampling and keep up our note taking efforts. By contrast, if the free gold was a little bigger than those tiny colors, it’s possible for a sample with only 5 or 10 colors to run a lot closer to a quarter ounce a ton.
Though this “lode miners rule of thumb” is completely unscientific, it is however, based on the long experience of many lode miners dating back over a century who carefully eyeballed their colors in a pan and then sent part of the same sample off to a reliable assayer and over time noticed enough of a pattern in the results to call it a “rule of thumb” and use it as a sort of loose gauge to determine values.
Speaking of assays, at $25 to $50 a pop (sometimes more), they are a relatively expensive method of mine evaluation. While the big mining companies don’t think much of sending 100 or 200 samples off to their assayer at one time and doing it again and again twice a week because it’s the way they do things, it would be a pretty fast and sure fire way for a small miner to go broke and to get there in a hurry.
With that in mind, while the fire assay is still the best tool most commonly used to evaluate values in an ore deposit, unlike the big boys, the small scale prospector is better off to utilize assay services as a means to prove his or her gut assumptions that are based on their actual field tests using the “crush and pan” method. Or in other words, as the late great Galice geologist, Geoff Garcia, often instructed: “Don’t waste money to send in blank samples. You’ll go broke. Crush and pan, – you always crush and pan first – and then use the assay lab to fine tune the findings of your successful field tests”.