ALBUQUERQUE, N.M. (AP) — A U.S. district judge cited tribal sovereignty in dismissing a lawsuit aimed at shutting down a coal-fired power plant and adjacent mine near the Arizona-New Mexico border. The lawsuit by environmental groups was targeting the 2015 approval by the U.S. government of a lease extension for the Navajo Mine and the Four Corners Power Plant, which has for decades provided electricity to customers throughout the Southwest. The groups argued that the Interior Department and other agencies did not consider clean-energy alternatives or possible effects on endangered species in the region when they approved the 25-year extension. In the order issued Monday, Judge Steven Logan of Phoenix tossed the case since the mine is owned by a corporation created by the Navajo Nation, which makes it immune from such legal challenges. The judge said the case could not move forward without the mine as a defendant. Navajo Transitional Energy Co. was allowed to intervene in the case last fall, citing its interest in the operation of the mine. The company argued that if the environmental groups were successful in their challenge, the tribe's solvency and economic development strategies could be jeopardized. The tribe created the company in 2013 for the purpose of purchasing the mine from BHP Billiton for $85 million through a three-year loan. The company obtained a new loan to pay off the original note and to maintain working capital. If mine operations were hampered, tribal officials were concerned the company could default on the loan and lose ownership of the mine, which would cost the Navajo Nation millions of dollars. The judge ruled that the tribal entity's interests in the outcome of the case far exceeded the federal government's interest in defending the validity of its environmental review and decision-making process. The Four Corners Power Plant is one of three coal-fired generating stations in the region that are scaling back operations as utilities shift toward natural gas and renewable sources such as solar due to regulations and economic forces. The Navajo Generating Station in northwestern Arizona is scheduled to close in 2019 but the Navajo Nation is pushing to keep it open longer. The San Juan Generating Station near Farmington, New Mexico, will be closing two of its stacks by the end of the year, and Four Corners has closed three of its units. Environmentalists argued in their complaint that the Four Corners and San Juan plants together emit more pollution than any other source in North America and that pollution from the plant and the mine degrade air and water resources throughout the San Juan Basin, which includes parts of New Mexico, Arizona, Colorado and Utah. They said extending the lease would allow more strip mining on tribal land in northwestern New Mexico along with the renewal of right of ways for transmission lines that carry the electricity across Navajo and Hopi lands to population centers in Arizona. The environmental groups did not immediately respond Tuesday to a request for comment on the judge's decision. As it braces for revenue and job losses expected with the ramping down of coal-fired generation, the Navajo Nation recently opened its first utility-scale solar farm near the sandstone buttes of Monument Valley. In addition, Navajo President Russell Begaye signed an executive order last week aimed at building up the tribe's clean energy economy.