Rio Tinto 's aluminium smelter in New Zealand, which has suffered losses in two of the last three years, faces a July 1 deadline to decide whether to extend its power supply contract with utility Meridian Energy . The decision could determine the future of the 350,000-tonne-per-year smelter, which is the country's single biggest energy consumer, beyond 2017 following a battle by the plant's owners to slash…