I also feed trace mineral salt blocks at free choice. I used to give them the kind with molasses mixed into it, but they went through those too fast--in a day or two.
Kubie gets a joint supplement which has helped him significantly. It has some minerals mixed in with it, but not much. I try to buy good quality feed, so I know that they are getting the nutrition that they need.
All of my horses get about 1/4 cup of soy oil mixed with their grain. It's not really a mineral per say, but it helps keep them shiny. It also prevents Kubie's powder joint supplement from blowing away in the wind.