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.
Maybe the Spotlight Club mysteries (and/or the Brillstone books) by Florence Parry Heide and/or Roxanne Heide? All the ones I remember were available in hardcover and featured a mixed-gender set of kids – and the era sounds about right.
Could this possibly be the Trixie Belden series? Trixie and her best friend Honey form a club with their brothers and some other friends to do service projects and solve mysteries, calling themselves “the Bob-whites of the Glen.” The series is set mostly in upstate NY, but the group often travels, with stories set in Arizona, the Ozarks, and other locales. The first dozen or so books were written in the 1960s, and were hardcovers. The series has been reprinted a lot, in paperback and hardcover, with new stories added in the 1980s.