BHP Billiton Ltd., the owner of the world’s biggest copper mine known as Escondida, said it will take legal action after a group of more than 300 people entered the mine site during a strike and forced some contractors to abandon the compound. People wearing masks entered the mine site at 6 p.m. Santiago time on Saturday, threatening the staff of contract companies and setting off fire alarms, causing damage, the Melbourne-based company said in an e-mailed statement Sunday.