Look in your states Department of Natural Resources documents (they should all be open to the public). Many states have done studies of geothermal resources and may have some data on mineral contents (at least on the dates collected, which were usually in the 70's or 80's). I have gotten the documents for my state (Washington) and all of the hot sources that hot springs are located at are included in the study and it has data on mineral contents. The only problem is that these documents are not easy to read, you need to be used to dry science talk and pick out the interesting things. I was interested myself and did some research into Washington but I haven't looked into any other states.

Look in your states Department of Natural Resourses documents (they sould be open to the public). Look for a geothermal resource assessment study, many states did one in the 70's or 80's. I was able to find one for my state (Washington) and found mineral content information for all of the hot spirngs here. The only problem is that it is not easy reading, you need to be used to dry science reading and picking out the good stuff. Good luck!