As we told you a month ago, Alpha Natural Resources, Inc., one of the largest metallurgical coal producers in the world, has been offered $500 million for the company’s major assets–its crown jewels (see Bankrupt Alpha Natural Resources Offered $500M for Assets). Among those assets is 25,000 acres of Marcellus gas leases in Greene County, PA and a stake in Rice Energy. We now have more details. Yesterday Alpha filed a proposed Chapter 11 bankruptcy plan that lays out selling the company’s best assets, keeping a few assets so it can cover the cost of reclaiming certain coal mining operations. In essence the company is throwing in the towel and calling it quits…