Complex sentence: By the early 20th century, gold was mined in alluvial deposits spread over the island, and the ores of antimony, copper, iron and tin were thought to be abundant, in addition to asphalt, coal, and petroleum.
Split into simpler sentences: By the early 20th century, gold was mined in alluvial deposits spread over the island. At the same time, antimony, copper, iron and tin were thought to be abundant, in addition to asphalt, coal, and petroleum.