Complex sentence: Phoenix was the site of the first successful copper mine in the Keweenaw Peninsula in the 1840s, although the mining boom ended not long after it had begun, as more easily accessible copper was discovered near Copper Harbor.
Split into simpler sentences: Phoenix was the site of the first successful copper mine in the Keweenaw Peninsula in the 1840s. The mining boom ended, however, not long after it had begun, as more easily accessible copper was discovered near Copper Harbor.