Seriously how the fuck do they know where the good stuff is going to be, especially back before ground penetrating radar?
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I saw a video that showed scientists/geologists hanging out of helicopters to clip the tall pine trees. Apparently, they can analyze the clippings and determine if gold is present in the ground below.
Sooo cool!
Telltale signs can sometimes exist at ground surface. Highly mineralized soil can leach out metals that prevent vegetation growth, so a miner could prospect those patches for what vein might exist at depth.
Often times those deep mines would be a case of “follow the vein”. A “vein” is like a streak or sheet of one type of rock that cuts through another type of rock. For example, gold is often found in quartz veins. These are visible from the surface, and the mine shaft would be dug to follow its path. Once the vein ended in a particular direction the digging would stop.
So let’s look at an old gold-rush prospector example:
“You find some gold flecks in a river” let’s find out where the gold came from!
One way to do that is to basically divide the area around the river into a grid and take samples of rock and soil from different areas of that grid. Wash/filter/sift your sample and look for gold flecks – did you find LOTS of gold? Look at how much gold you find in each bit of the grid, and use that to basically play “Warmer/Colder” until you find the source of hard gold deposits.
Now imagine instead of just taking “I’m a prospector who found gold flecks in a river” you scale that up to a mining company that is familiar with general characteristics of the minerals known to certain regions. Instead of “Gold Flecks in the River” your start might become “Exposed Cliff Face with mineral composition known to be found in the vicinity of the stuff you’re looking for” (Different types of ore form under different geological conditions — if you’re looking for “Rare Old Hot Rocks” you want to start looking where you know there are “Old Hot Rocks”). Instead of just “Dig and sift” they can drill and take core samples and see what kind of rock they get but a lot of the general premise is the same.
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Looking at the overall geology of a site, they can get some idea of the type of rock that makes it up. Taking core samples is another way — it gives an idea of the layers of material that are underneath, that sort of thing. Prospecting for oil can be done with sound, too — using explosives to create a sound wave, the vibrations can be analyzed to see what’s underground.
But also — hitting really rich deposits of material was difficult, and in some cases still is today.
They don’t *know*, but there are ways to guess and check.
Even a century ago, we had a pretty good idea of what types of rock were found in what areas of the world. And certain types of rock are, for various reasons, associated with particular mineral deposits. For example, if you’re looking for diamonds – which are made very deep in the Earth – you’ll need [a very deep vent](https://en.wikipedia.org/wiki/Volcanic_pipe#Kimberlite_pipes) that can pull up material from near the mantle. If you’re looking for [ores that hang out with iron](https://en.wikipedia.org/wiki/Goldschmidt_classification#Siderophile_elements), you probably shouldn’t look in rocks that don’t *have* any iron in them. And so on.
My mom’s friend does this. It’s kind of like how if you yell at a wall you can tell if you’re in a bathroom or against a steel building or a bunch of pillows based on the sound you hear back.
They have big machines that do that and use computers to listen and figure out what the sounds mean.
Definition of a gold mine: a hole in the ground with a liar at the bottom of it. Mark Twain. Unless it’s painfully obvious like coal, generally speaking it’s luck and luck. If you ever travel through Nevada stop in at any small town bar and I guarantee you will get your fill of gold and silver mining stories.
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