Or “Is there really gold in them thar hills?”
Thanks to board games and fantasy novels, my brain has filed away the “fact” that you have to go to the mountains of you want to mine for metals.
But if there any actual geological basis to this? Are metals more likely to be found in mountainous regions or is that just a trope?
Few places in the world are flatter than Western Australia, and yet the place is stiff with valuable minerals (including gold).
Mountain are where things are pushing up and erosion is pulling down. A great place to find stuff, but not necessary.
Yes and no. Gold deposits form from geothermal activity - hot water containing gold and silica flows from deep in the earth. As the water cools, gold and silica precipitate. A quirk of the chemistry makes gold precipitate as free metal.
So the base deposit is associated with geologically active regions, not flat plains.
But then glaciation and erosion happens - the hills, and the associated quartz veins containing free gold are ground down and exposed. This frees the gold into streams and rivers. Being heavy, the gold deposits into pools and the base of the hills as alluvial gold deposits.
So the gold was in the hills, but it washes into the plains as well.
Heavy and refractory metals in easily identifiable and recoverable seams are generally found in porphyric formations resulting from lava flows. These seams are typically exposed due to uplift and erosion, hence why early mines were often found in natural caverns and canyonlands. These often yield very limited amounts of recoverable material (by modern standards) and so eventual yields are often too low to sustain long term mining prospects. Most modern mines are open pit operations that are centered on porphyric zones with high overall concentrations of metals recovered by a chemical or electrolytic smelting or recovery through high temperature ‘roasting’ from silicate or carbonate minerals containing it.
Stranger
Well not really. WA’s number 1 mining export by value is from iron ore which comes by and large from the Hammersley Range. Then there is bauxite which comes in large part from the Darling Scarp. Nickel, gold and lead come from an area around the Gawler Ranges. As you can probably tell from the names, these are not flat areas. They aren’t high mountains but they are rugged.
The other thought that comes to mind is that when you have hills, you have valleys, and when you have valleys you have alluvial deposits. Panning for gold, rather than mining for it, can be a much more profitable pursuit for a lone adventurer. The whole point of the alluvial deposits is that erosion has concentrated any gold in them thar hills into the stream beds.
As noted above, there is nothing special about the existence of simple recent hills relative to the geological history that might lead to gold deposits. You need to look deep into the crust to look for the reasons for the deposits. Something is responsible for driving the hot water around.
Well they are not high any more. A billion odd years tends to wear anything down.
Probably more accurate to talk of the Gawler Craton. There is lots of good stuff across the craton, as well as significant amounts of gold.
The highest point in WA is Mount Meharry which, granted, is in the Hammersley, but it’s only 1,250 metres above sea level, which is more of a pimple than a mountain, if we’re honest. (And it’s nowhere near the sea; it only rises about 400 m above the surrounding plain.) This in a state that measures 2.645 milion sq. km. The Darling Escarpment rights to a high point of 582 m over sea level; the Gawler Range high point is 465 m (and they’re not even in WA; they’re in South Australia).
So, for a place of its enormous size, WA has remarkably few eminences, and hardly any that could be called “mountains”. Gold in particular comes from The Goldfields (there’s a clue in the name) and, believe me, it’s pretty flat.
Of course, the geology of WA is very ancient and pretty eroded, so when the gold an other deposits were formed it’s possible that this was the outcome of geological processes which also produced a much more articulated landscape than we see now.
One thing - in mountainous areas, *exposure *of deposits is a lot more, so it’s easier to find deposits than if they’re buried under sediment and fields. So for the time period and tech level of RPGs and fantasy books, yes, minerals are to be found in the mountains not the plains. Not because the plains don’t have them, but because nobody can find them. You only have to look at where older deposits were from. Now, **UDS **may consider some those mountains pimples, but they are, nevertheless, mountains not plains.
In addition to what everyone here is saying, mountains have more relief and surface area, and erosion exposes various rock formations, that an experienced miner can use as clues to valuable deposits near the surface.
In my area there are riverbeds that became lava rivers from distant, ancient volcanoes. As the surrounding lands eroded away, you have these somewhat serpentine ridges of hardened rock over the riverbed - one can dig horizontally into these ridges to locate the former riverbed and potential valuable deposits. Much easier to find these deposits when it is elevated above the surrounding plain.
Exposed veins are more easily found in the mountains since erosion removes material and exposes things. That material gets deposited in lowlands covering things up. But rivers constantly erode in lowlands as well.
It’s not hard to find gold just about anywhere. Dig a few cubic yards of material out of any stream or riverbed and you’ll probably find some gold. Some tiny bits of gold that wouldn’t add up to enough to buy a new shovel after you wear one out looking for it. Finding enough gold concentrated in one place is the hard part.
Flat areas generally accumulate sediment. Sometimes hundreds or thousands of feet thick. Here’s an article about Great Plains geology where repeated episodes of immese deposits from seas, river deposits, volcanic ash, and even wind repeatedly built up thick deposits. Only to get (partially) washed away. We are currently in a erosion oriented cycle, esp. thanks to glaciers in the northern parts. So Kansas is getting slightly less flat. But below Kansas in most places you have do dig very deep to get to anything that could contain useful minerals.
A lot of the more famous flat areas are like this. They got flat by having thousands of feet of useless cruft deposited on them.
But there are exceptions. The Canada shield has bedrock close to or at the surface and as a result has some nice mining going on. Thank glaciers.
Not completely flat, it once had really tall mountains that have been worn down plus younger formations. But there’s interesting stuff to be found in some of the flatter parts.
So, I’d say: there’s gold in them thar eroded areas.