Complex sentence: In 1990', mining operations were stopped as they were no longer profitable but as of 2015 there is a possibility that the mines will be opened again because the price of gold (in USD) has risen five times since then.
Split into simpler sentences: In 1990', mining operations were stopped as they were no longer profitable. However, as of 2015, there is a possibility that the mines will be opened again because the price of gold (in USD) has risen five times since then.