From a refrigeration and air conditioning textbook ... The refrigerant R-12, dichlorodifluoromethane ... "is chemically inert at ordinary temperatures and thermally stable to above 800 F."
Inert gases are those that don't react with any other substance, so diffusion at the molecular level would almost solely depend on the stability of the molecule; and CFC refrigerants are known to function well (without molecular disassociation occurring) inside refrigeration systems for 20 - 30 years.
When CFCs are released into the atmosphere, it is the CFC molecule that will mix with air, with little if any disassociation occurring. And the molecular weight difference between air and those highly stable and inert R-12 molecules will still outweigh air at the 5.7 to 1 ratio.
If the R-12 goes anywhere once it's in a mine shaft, it will go into cracks and crevices until it reaches water or bedrock.
There's a question ... Will water absorb an inert gas?
Diffusion is not reaction. Diffusion is where a concentrated substance becomes less concentrated or diffuse. Here r12 would be concentrated at 100% at the bottom and diffuses so its concentration might be 99% at 1 ft. 98% at 2 ft. etc. The concentration at 1000 ft above the bottom might be .0000001%. That is if you had a detector sensitive enough to measure anything, you might measure 1 part per trillion at 1000 ft. above the pool of r12. Over a long enough time frame, the r12 will all leak away into the atmosphere. This is why the moon has no atmosphere. If you put an atmosphere on the moon, it would eventually slowly diffuse into outerspace. It might take billions of years, but it would happen because the gravity is too weak. Diffusion is similar to evaporation in water. Put a pan of water out and it slowly evaporates or diffuses into the atmosphere. This is what happens with r12 and the stuff does enter the upper atmosphere. You can take a can of motor oil and open the top and after about 10 years, it will evaporate or diffuse into the atmosphere too.