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I guess it would depend on the amount of salt in the brine, might not want to freeze but as others say, why not just make it fresh, the time it takes to make a brine and cool it would still be quicker than thawing out a batch.
When you wrote "unused" did you mean that the meat had not been IN it yet?
If you made the brine fresh and there has been no meat in it, I suppose there is no problem with that, and should be fine.
If the brine has had ANY sort of muscle meat in it, to include a spoon or mixer that has been in contact with the meat, then no. Discard that and make fresh.