I read a series of books in the late 1970s or early 1980s at my local public library. There could have also been a second or related series. They were hard cover, young adult books. At my small town library, they could have been published quite a long time before that.
The series involved a number of children that formed a club and solved mysteries. Both boys and girls were in the group. The series was set in the west or in the mountains. The words “Treasure” and “Indian” come to mind when I think about these books, one of them could have been part of the name of the club.