In my book series, there is this extremely rich Earthlike planet called Aurea whose claim to fame is supplying the vast majority of the galaxy’s precious metals (gold and silver for the purposes of this question). This trade in precious metals forms the backbone of the Aurean economy. However, these metals on Aurea are found almost entirely in two different deposits: a gold deposit around the size of the US state of Maine in a tropical mountain range called the Monsaltu Mountains, and a silver deposit around the size of the US state of Connecticut located in a subtropical mountain range called the Sparteian Alps. Each of these deposits annually yields many times more of their metal than has ever been mined in all of our Earth’s history. How could precious metal deposits this absurdly huge and bountiful exist? Could they even exist?
The amount of Gold and Silver is not at all extreme, after all Earth has enough gold to plate the entire surface including oceans with a layer 10 feet thick. It's just impossibly difficult to get to it, as most of it is down near the core.
The problem is that gold is both very dense, and chemically inert.
Thus in molten material, such as more than 99% of Earth currently is, and 99.999%+ of Earth was at some point, the gold will sink down towards the core. This is what makes gold deposits on the surface so rare.
All that is needed to endow one planet with silly-rich gold deposits is for a previous planet to have collected its gold in such an internal collection, solidified, then this previous planet was broken up. One of the "chunks" of broken planet was then an asteroid of suitable size, composed largely of just gold.
Then, you have this asteroid do a comparatively "soft" impact into the newly forming planet. Soft enough that the impact crater does not punch all the way through the crust of the recipient planet. The asteroid can fragment, melt or even vaporize, that's fine. The gold, being gold, will just settle down wherever it ends up, and form a very,very,very rich deposit.
The exact same thing has happened on Earth, and is the source of some of our deposits of gold, silver, tungsten, platinum. Just usually on a much smaller scale of impactor.
Of course, this does not help your story much. Because if the source of precious metals is from planetary debris in space, and you have an advanced space-faring culture, then they would go to the source, and not bother with those pesky buried deposits down in gravity wells of planets.
You have no doubt heard the legends, that long before recorded history a great intergalactic empire used our Aurea as their largest military stronghold. They ruled this arm of the galaxy and from this stronghold world, the empire taxed fifty thousand worlds. Some paid in silver, others in gold. The wealth of the entire empire flowed inward to our beautiful planet and the flow continued for tens of thousands of years.
When rebellion finally arose, the rebels found this stronghold too well defended to conquer, so they bombarded it with missiles full of nanite disassemblers. They turned our beautiful world into grey goo. The rebels ultimately won war then in their own time, fell to the rigors of deep time.
That was eight billion years ago. In the intervening years, Aurea has healed what the disassemblers did. It has reformed a solid crust and reignited its magnetic field. It has redeveloped life and once again become a paradise.
Meanwhile deep within its bowels, an unfathomable fortune in gold and silver is hidden. Tectonic forces have pushed a small part of that fortune to the surface in a few remote mountainous places.
What can be seen and scanned is very easily the largest concentration of precious metals in the known universe, but if anyone ever discovers the whole of what is hidden here, war will once again blaze across the galaxy... a war with Aurea as the final prize.
I'm thinking the easiest way to explain a deposit of metals in incredibly high concentration is having an Asteroid like Psyche Smash into the planet at some long-ago point in pre history.
Sadly that doesn't quite match what you've described. Firstly - that kind of an event would be more likely to leave a crater than a Mountain range, and secondly, all your metals would be in the one location. And there'd be LOT's more than just Gold & Silver...
You could explain the mountain range as a result of volcanic activity thrusting up the results of the massive asteroid strike, maybe even as a result of the strike causing a weakness in the crust that the underlying mantle pushed up through. You'd still be left with a single site of mixed metals - not your scenario again.
When we consider how metals are formed there's a couple of theories, in stars that go supernova - or in the collision of a couple of Neutron stars. Either way - gold & other metals ejected are pretty widely dispersed. You can find a bit of an explanation here: How Gold is made and how it got to our planet
It kind of seems difficult to find a natural process explanation for the kind of deposits you are describing. If your premise includes the possibility of ancient precursor civilisations - they could have mined some planet for them and "stashed" them in some fashion that created these deposits. My suspicion would be they'd have to have been in orbit around your planet, somewhere in the solar system of your planet or actually stashed on the planet itself. That last is not too likely because it would have been obviously put there if so. Sooo, I'm kind of thinking the most likely would have been stored in orbit in a fairly raw ore form - and then something causes that orbit to decay (maybe an asteroid?) - and the stashed metals "fell out of the sky"
The method of mining the planet could be as simple as smashing it into asteroids. I've always had a fairly strong suspicion our own asteroid belt was caused by the Breakup of a planet. & if that was the case - I also have the strong suspicion Psyche was the core of that planet - or at least a big chunk of it...
If natural process is what you really want - you can go for it on the premise of anything is plausible in a big enough galaxy, after all there is now pretty credible evidence of a planet largely made of Diamond. You could use some of the ideas I mentioned above, especially the idea of a nearby pair of Neutron Stars colliding and a couple of streams of molten gold & silver are sent out together smashing into your poor planet and aeons later being discovered in these mountain ranges. With the amount of radiation that would be pouring out it would even be a fairly credible kick start to life on this planet
Hope this gives a few useful ideas!
The Sudsbury Basin in Canada is the 2nd largest known impact basin on Earth. 1.8 billion years ago, a comet impacted there.
The impact was so great that that it created hundreds of cubic kilometers (if not more) of magma. This magma chamber was large enough to stay hot for sufficient time to allow minerals to crystallize and separate (essentially concentrating the different minerals together).
The impact also had the effect of fracturing the surrounding rocks. These fractures allowed the fluid magma to penetrate into the rocks, leaving veins of highly concentrated minerals. While some of these minerals did arrive with the comet, most of these minerals were already present in the crust. The concentrating effect of the magma chamber just rearranged the minerals into very rich deposits.
For Aurea, the impacting bodies could have been larger (the original Sudbury crater was "only" ~200 km long) and the bodies themselves may have been very rich in gold and silver (something similar to 16 Psyche, maybe?).
And then, perhaps, if the impact sites were closer the edges of tectonic plates, these impact sites could be uplifted into mountains ranges.
One caveat for this mining endeavor, is that having such a huge source of gold and silver will absolutely tank those markets. Gold and Silver would likely drop in price to that of Iron, or even lower.
The deposits come from the same place. A super massive black hole's accretion disk. As matter goes into the black hole some matter doesn't fall in but instead forms a very thin band that orbits it called an Accretion disk. As it orbits the powerful gravity and high speed orbit causes the orbit to act like a centrifuge, separating out the materials by type, and by isotope. Note that the centrifuge isn't a centrifuge because it is spinning, but rather because more massive objects are moved less by the same force and tend to have lower orbits due to lower velocity. Because the black hole is so massive the gravity differential at larger distances is still noticible, cussing more fine sorting. Over time the matter in the same layer of the accretion disk clumps together to make small planetoids of specific elements and isotopes. later a large object passed through the accretion disk launching the planetoids out of the black hole's orbit. A dozen of these ended up in the local system, and a gold and silver planetoid ended up impacting the planet several hundreds of millions of years apart. The rest of the planetoids got thrown out of the system eventually. I'm not 100% sure that this gravity centrifuge thing works on all black holes, or just in general, but a process like this could do what you need.
Wide region of black smokers
Probably the best scenarios would be a large region of black smokers that formed after a continental rift.
Some time in the distant past a continental rift formed via large scale mantel up welling and was flooded by ocean. The spreading plates caused the regions crust to thin. But large volume of continental crust that happened to contain more acidic minerals nearby allowed water to be more acidic.
The magma up welling would provide heat, the nearby rock would be more fractured to allow water to cycle down get warm and acidic and then dissolve minerals, heat further emerge from the black smokers and deposit minerals. This acidic pressurized warm-hot water is able to dissolve many metals including gold. At some point it is warm enough to flow up and out the black smokers, where pH, temperature, pressure changes all cause all the mineralization to precipitate out, hence the 'black' portion of the black smokers.
If the black smokers remain active for tens of millions of years, more is better. This will concentrate the metals from the entire nearby basin, into the region around the black smokers.
Then sometime later the magma up welling shifts, plates move and the region with the black smokers gets squished between two plates pushing it up into a mountain range.
Smaller versions of this are begin mined and have been mined. This is a known way to have formed dense concentrations that are above 20% by mass metal(limited pockets, not bulk).