The chest you found at Mistamere didn't contain any minted coins, but raw gold and silver pieces; old trade bars, ingots, broken jewelry, nuggets, etc. There was simply no central authority during the Traladarn Dark Age, and no one to mint any coins. Precious metals were stored and exchanged by weight.
You can try to sell some of it in Threshold, but keep in mind that this is a small town with limited wealth. There simply isn't enough coin in circulation to buy the entire chest’s worth. Merchants might take a portion of it, especially if you’re willing to accept a lower value. Yor you might barter it; buy tracts of land, a few houses, or other long-term assets. If you want full value, you will do better selling it in a larger city like Kelvin or Specularum, where there are proper jewelers, wealthy patrons, etc.
That said, 100 gold isn't that much, so you can sell part of the pieces, if that's what you want.
There are also plenty of people you can hire in Threshold, but someone with a particular skillset might not be available. What did you have in mind?