Good morning all.
Congratulations to Karen. Well done!!
Old Gal, sorry if I misled you with a rather loose and possibly misleading statement earlier when discussing 24a. Follow the clue from Manxman.
However,when you have the answer - the word is also an American river flowing into the Arkansas river. The river got its name from its green and greyish colour suggesting the presence of copper ore. I recall this being mentioned as "footnote" comment on a Geography field trip in Cornwall. So my mind patterns were working on a totally personal level rather than following the clue! Sorry!!!