I'm sorry, but this probably wouldn't work, and if it did, it would be even more laggy than the original minecraft.
1. Minecraft works by having an almost random world spawn when you decide to make a new world, Source has to load the map file from your HDD, so you could only play premade "worlds", and then you get into the problem with number 2.
2. Minecraft is a fully destructible world (except for bedrock), you can destroy every single block in the game (except for the aforementioned bedrock), in source on the other hand, the environment is not destructible, so you couldn't "mine" in it.
3. Even if someone managed to get around 1 and 2 (say... the level is just the bedrock, and all the blocks are separate props), it would be much more laggy than the original minecraft. Source is only built to handle 1024 props on screen, and assuming each block is a prop (i see no other way to do it), your looking at millions of props, causing source to lag to unprecedented amounts (the reason that it only supports 1024 props is that each and every prop has physics enabled, and physics is calculated per-object),