If feeding loose mineral, never mix salt with it as the horse may ingest unneeded minerals as it ingests the salt. Too many minerals can create problems as well. Never mix the minerals or salt with feed either as it's no longer free choice.
Actually, Saddlebag, it can be recommended to mix salt and minerals at times. In general I would agree with you, but for example, when I started my horses on the minerals they were eating them like crazy. I called Hoffmans to see if that would be a problem and they suggested cutting the minerals with salt for a couple of weeks to keep the mineral intake at a gradual increase. After two weeks, I stopped mixing them.
Years ago I did a reformulation for MoorMans grow strong minerals. They made it in 25 pound blocks and shipped a few tons out to California for testing. We put some out for horses that were turned out and got a call back the next day from a sales rep who said that we were going to make a lot of money selling these because his three horses ate almost a whole block in two days!! Mineral deprived horses will really over consume a quality mineral source when they are first exposed. In a few days, they generally back right off. The three horses sited above did just that. In most cases, there is very little risk. However I like to feed loose mineral that I can control to horses who have not been exposed to them previously. After a week or so, you can put out a block with very little risk of over consumption.