The Chicago Manual of Style does not prescribe a standard format for unpublished archival materials. Instead, supply as much information about a document as possible in order to facilitate its future discovery. OWL Purdue has two overviews that should work for most of the documents you'll discover: citing legal, public, and unpublished materials, and citing miscellaneous materials. Once you have a template be sure to use it consistently.
If the documents have been processed by an archivist, you'll want to include information such as box and folder numbers, and the name of the holding institution. Here's an example of processed correspondence.
Booth Tarkington to George Ade, 8 May 1924, Box 10, Folder 5, George Ade Papers 1878-2007, Purdue University Archives and Special Collections, Purdue University Libraries.