Mining and mineralization of the Clear Lake region
The Geysers-Clear Lake area has been one of the most productive in the United States for mercury, and gold was mined in the late 1800s. Many of the deposits are directly associated with outcrops of early Clear Lake volcanic rocks. For over a century, a correlation has been known between the mercury ore deposits, thermal springs, and volcanism at Clear Lake. No mercury deposits are known in the Coast Ranges north of Clear Lake. Gold and mercury mineralization (formation of minerals) occurs in rocks of the Great Valley sequence and the Coast Range at the eastern edge of the Geysers-Clear Lake area.
The McLaughlin gold mine is located farther south along the eastern edge of the Geysers-Clear Lake; it is a disseminated gold deposit that was discovered at the site of the former Manhattan mercury mine in the Knoxville district. Mineralization at this deposit is located in and adjacent to basaltic lava now dated at 2.2 Ma. Siliceous sinter deposits at the heart of the gold deposit indicate that mineralization is related to former hot springs; only relatively cool mineralized springs now emerge in the district.