Dang...1 week? You must have had monsoons but I guess anything is possible. Any chance someone happened upon it and nabbed your block? I just can imagine the amount of rain required to dissolve a 50lb block.
I had a camera on it and the pictures ended on July 3rd when the rain started and ended on 7th, the next photo, no salt lick. I think they would have taken the camera and it was so far back in the woods with no access.
I put two out and and both are gone too. The rain ate em up. Thats ok, they like em better when they are dissolved in the ground anyway. It will be fine.