In order to preserve and protect, in the public interest, the historic Old Fort Union, situated in the county of Mora, State of New Mexico, and to provide adequate public access thereto, the Secretary of the Interior is authorized to acquire on behalf of the United States by donation, or he may procure with donated funds, the site and remaining structures of Old Fort Union, together with such additional land, interests in land, and improvements thereon as the Secretary in his discretion may deem necessary to carry out the purposes of this section and section 450kk-1 of this title. Donated lands may be accepted subject to such reservations, terms, and conditions as may be satisfactory to the Secretary, including right of reversion to donor, or its successors and assigns, upon abandonment as a national monument, and reservation of mineral rights subject to condition that surface of donated lands may not be used or disturbed in connection therewith, without the consent of the Secretary.