SANTIAGO, July 30 (Reuters) - Workers at the world's No. 3 copper mine, Chile's Collahuasi, began a work stoppage over labor demands, their union said on early on Saturday. The strike at the mine, owned jointly by Xstrata and Anglo American , comes as a strike at the world's No.1 copper mine, Chile's Escondida, which has fueled supply worries and spurred global copper prices, entered a ninth day.