Complex sentence: The discovery of gold in 1851 was a change in direction for the Australian economy and marked a period of economic expansion, attributable to gold, that eclipsed even wool production in terms of percentage of economic expansion.
Split into simpler sentences: The discovery of gold in 1851 was a change in direction for the Australian economy. It marked a period of economic expansion, attributable to the gold, which eclipsed even wool production in terms of percentage of economic expansion.