I have a region on a 'Eurasia' style continent that is surrounded by large mountains. The reason that I came up with was a very dense and volcanically active microplate that was being compressed on three sides. As such, three large mountain ranges, and a abutment into the nearby inland sea.
I want this region to be home to an absolutely insane amount of silver. Not argentite, acanthite, or copper-ore byproducts, but massive quantities of pure silver running like rivers through stone. Instead of one lonely mountain, an area of mountains that collectively stretch across a 8,000-ish km arc.
If it is pertinent, prograde orbit, earth-esque biome layout, the region in question is an east-facing cup of mountains bordering a large inland sea, with the lowest latitude being 45, and the uppermost reaches into the low 60's. The civilization era is High Medieval, but with massively improved healthcare and sanitation.
Here is a political / geographic region map I created using the Inkarnate software.
The 'Lakelands' are a massive volcanic caldera surrounding a mega-hotspot which erupted some couple of thousand thousand thousand thousand years ago. If my geology is correct (which it likely isn't) Silver is usually upwelled from divergent ocean crust, and upthrust at convergent boundaries, or hydrothermally refined from cooling magma chamber precipitates. I wanted a mix of the two to cover my bases.
The weird step-like tiers of the Northolt, Stonerill, Midlands, and lowlands were a vain attempt to use "folding" (like in the great basin of the US), and copious amounts of erosion via river to make some nice landforms. The real mountains, as in formed by tectonic processes, are the Andwynns in the south, the Spires in the North, and the Lakeland rim.
The big question for actually qualified people to surmise.
Where, and to what degree, would the native silver deposits be, and how would one reasonably maximize this as a worldbuilder?
Alter the composition of the planet itself. Insomuch as gravity remains similar, magnetosphere remains earthlike, and that life would still be feasible.
Introduce traumatic events to the planets surface, i.e. asteroids, planetary collisions, etc... With the consideration of the amount of time required between such events and a humanoid civilization interested in mining strategic metals. Without creating unreasonable effects on biodiversity and/or making life infeasible post-event.
Alter the geographic situation of the region. Without altering the landscape to an extreme degree. I.e., mountains can get smaller, hills can be added, rivers diverted, and volcanoes shifted. Mountain ranges may not be subtracted or added, only shortened/heightened.
Use magical means. Gods are active forces walking the surface. I don't want to use the 'a god did it!' reasoning, especially given the selfish nature of my pantheon (they created the world as a private retreat and got stuck there.) Mortal mages exist, but they are limited by their perspective, knowledge, and understanding. I.e., a Mage could manipulate gravitational vectors if they had a complete understanding of the subject.
Use valuable metal alloys, notably electrum, if all else is unfeasible. I wanted to avoid baser metals, because the smelting process is convoluted for pure forms of silver. Apparently the Egyptians used molten salt and water to refine ore rock, which I like the idea of.
I wanted to avoid involving the practicality of a state or civilization hoarding a metal, thus making it common, to keep the question straightforward. I will link another question about the viability of a artificially constrained market. Once I can get a general feel of exactly how much silver I'm dealing with.