In gathering material for my posting on Zippy the Pinhead’s road trip to Kansas, I came across this sentence in the Wikipedia entry on Strataca, aka the Kansas Underground Salt Museum.:
There are 14 other salt mines in the United States, but none of which are accessible to tourists.
The intended meaning is clear, but the syntax is definitely off. The sentence looks like a blend of two different, though very similar, formulations of the idea:
(a) There are 14 other salt mines in the United States, none of which are accessible to tourists. [nonrestrictive relative clause]
(b) There are 14 other salt mines in the United States, but none of them are accessible to tourists. [conjoined independent clause with but]
Both are syntactically unproblematic (disregarding the disputed usage choice between none … are and none … is, which is identical for (a) and (b)). But it appears that the writer(s) began option (b), with the conjunction but, and then continued with the relative-clause syntax of option (a). A classic syntactic blend, it seems to me.