I could be wrong but in the XML file (terrain folder) for bonus infos I think you would have to add the peak terrain under the section for silver for it to show there. Peak is listed as a terrain in the terrain info file.
The Peak_terrain ends with "<bGraphicalOnly>1</bGraphicalOnly>" and has no yield infos, so it's not actually a real terrain, it's really just a section that defines the movement noises and things like that.
The BonusInfos say Silver should only be found in tundra and ice on hills, and sure enough you get the bonus to silver production on Mountains which are on top of ice or tundra. Since there's no ice on the Classic Americas map, the simple solution is to place Ice over all the Mountains and further restrict silver to icy hills.