JARBIDGE - Neglected old documents covered in rat droppings proved irresistible to one historian visiting this remote gold rush town in the mountains just south of the Idaho-Nevada line.Now, they're part of a project Mary Greenfield and five other historians who dubbed themselves "The Jarbidge Archive Brigade" are undertaking to preserve the documents and chronicle life in the Old West.Greenfield first came to Jarbidge in 1996 while pursuing a master's degree in Western U.S. history at University of Montana and then returned the last two summers. She spotted the records of Elkoro Mines Co., dating back to 1916 in the …