In Tomb of Annihilation, in chapter 2, there is a map of Chult on p. 39, the "DM's version" with all of the hexes filled in and all of the locations named and displayed.
However, although many of the locations are named, there are various "mines" that are shown throughout the map (and we can tell that they are mines because the legend tells us that that's what that symbol means), but unlike the named locations, they are not expanded on later in chapter 2.
Below is a section of the map showing a few of these mines (shown by the symbol of a spade and pickaxe), as well as a few examples of named locations (in spoiler quotes in case any players currently in a ToA game shouldn't be looking at it):
What is the purpose of these mines being on the map? If my players find one, what am I supposed to do with that? Is there any further information on these mines in the book? Hopefully I'm just being blind and it's right there under my nose, but I can't seem to find anything else about these mines...