The Secretary of the Interior is authorized under such rules and regulations as he may prescribe, to lease at public auction upon not less than thirty days’ public notice for mining purposes land on any Indian reservation reserved for Indian agency or school purposes, in accordance with existing law applicable to other lands in such reservation, and the proceeds arising therefrom shall be deposited in the Treasury of the United States to the credit of the Indians for whose benefit the lands are reserved subject to appropriation by Congress for educational work among the Indians or in paying expenses of administration of agencies: , That a royalty of at least one-eighth shall be reserved in all leases.Provided