SYDNEY (Reuters) - Australian Gina Rinehart could soon become the world's wealthiest woman after amassing an $18 billion fortune from mining and media investments, Forbes magazine says in its latest list of the wealthiest people in Australia. The 57-year old widow saw her fortune almost double after a deal signed last month that will see South Korean steel giant POSCO take a 15 percent stake in her Roy Hill iron ore mine in Western Australia's Pilbara iron belt. The deal valued the project at $10 billion, boosting Rinehart's fortune dramatically. ...