Subject to the provisos of section 541a of this title, any mining location made under the mining laws of the United States, including chapter 12 of this title, on lands of the character described in section 541 of this title, except locations made for lands within the exterior boundaries of a prior coal prospecting permit or lease, if based upon a discovery of valuable source material in deposits other than deposits of Leasing Act minerals, shall include the right to mine, remove, and dispose of lignite containing valuable source material and lignite necessary to be stripped or mined in the recovery of source material contained in lignite, subject to the reporting and payment requirements of section 541 of this title, and subject to the provisions of the Atomic Energy Act of 1954 [42 U.S.C. 2011 et. seq.], and upon filing in the land office designated in section 541 of this title, an adequate description of his claim or claims containing such lignite: Provided, That nothing in this section shall be construed to limit or restrict the rights acquired by virtue of a mining claim heretofore or hereafter located, under the 1872 Mining Act, as amended, or to impose any additional obligation with respect to the mining and removal of source material which does not occur within any seam, bed, or deposit of lignite.