Complex sentence: It originally was founded in 1867 as part of the silver mining boom and then, in 1905, as part of the gold boom when "4,000 people flood (ed) into the region".
Split into simpler sentences: It originally was founded in 1867 as part of the silver mining boom. Then, in 1905, as part of the gold boom when "4,000 people flood (ed) into the region".