Complex sentence: The mining district encompassed a significant portion of southwest Wisconsin, including the counties of Grant, Iowa, Lafayette, and Jo Daviess County (Illinois), which played a major role in the mining of lead and zinc ore.
Split into simpler sentences: The mining district encompassed a significant portion of southwest Wisconsin. More specifically, the counties of Grant, Iowa, Lafayette, and Jo Daviess County (Illinois) played a major role in the mining of lead and zinc ore.