where the Secretary of Agriculture determines, with respect to lands which do not have significant forest cover within those national forests west of the 100th meridian, that surface mining is in compliance with the Multiple-Use Sustained-Yield Act of 1960 [
16 U.S.C. 528–531], the Federal Coal Leasing Amendments Act of 1975, the National Forest Management Act of 1976, and the provisions of this chapter:
And provided further, That no surface coal mining operations may be permitted within the boundaries of the Custer National Forest;