Slitherine

In 2 regions I did use regional decision prospect of resources few times and in both regions I got iron vein and silver vein. Both regions have mining exploitations and that is only requirement in order to build iron mine and silver mine.
There is no silver mine in list of available structures and both regions have same problem but both regions can build iron mine. ---solution---
crraftsmen district was needed and silver mine is now in list of available buildings.

Silver Mine is a Tier 2.5 structure, so you need the Craftsmen District, do you have it? If you have it, post your save, if not, please build it.

You are right and it is ok now, my other region have craftsmen district and silver mine is on list of available buildings I did not check because 6 turns was needed for population growth to get new building slot.

2 things did confuse me:

--- craftsmen district is missing as requirement when buidling browser is opened but I did see second tier on image of silver mine. --- I did use search feature and 2 regions did not have craftsmen district but maybe both of them started game with silver mine.

It's a good remark actually, the way the engine is set, I could not add as direct pre-requisite both the mining exploitation and the Craftsmen district, but the 2nd is still needed through another method not seen by the building browser. I'll need to think of something here.

Do you even use the 3 boxes on the left where the pick is?

Yes I believe so, but let me check.