I have found over the years for my property the best salt is the course salt you buy to spread on ice in your driveway, it is just crushed with no additives and cheap at Home Hardware. And for best results dig a hole in some soil that tends to stay quite moist, like close to a swampy marshy area and fill the hole up with salt. The deer seem to like the salt mixed with soil especially clay best.
I tried the salt blocks for livestock when I first started putting out salt many years ago, but where I live the bears find them and play with them and they push them around and you can't find them. One time I carried in two salt blocks in a pack sack put them out in different spots came back a couple days later and both were gone, I looked around for about an hour and found one but I never did find the other one.
I started with a small hole filled with driveway salt by a swampy area with some clay in the soil about twelve years ago and I put out a 40 kilogram bag every spring and fall. The deer and moose have dug up the area so big now I can actually see the dug up spot from google earth