Looks like we won't know until some dedicated member reaches that post count. I did a search on "forum rank" and...
The system that was on a old forum to the server I used to play on was the same as here the ranks was achieved like this:
Carpenter: Normal 0 post's
Farmer: 50 post's
Stone miner: 100 post's
Iron miner: 300 post's
Gold miner: 800 post's
Diamond miner: 1500 post's
And they also had End destroyer which was 3500 post's don't know if that is here
And off course Moderator, Admin and VIP
Hope it helped 
Those post based ones are not quite right, but as I have explained before I am not going to tell you the correct list because it ruins the fun
.
Also sorted
List of Members by Position, and based on the range of post counts, it seems like:
Carpenter 0-50
Farmer 50-100
Stone Miner 100-250
Coal Miner 250-500
Iron Miner 500-750
Gold Miner 750-1000
Redstone Miner 1000-?
(no Member beyond this rank at the moment)
But either way, it seems that only plain Members can have this rank. If we were Supporter, VIP, Moderator or Veteran, those will be our forum ranks regardless of our number of posts.