Native copper (Cu) in calcite crystals on copper from the Precambrian of northern Michigan, USA. (Terry Huizing collection)

The bedrock in northern Michigan's Keweenaw Peninsula area with Earth's highest concentration of elemental copper (Cu) in its native state. The copper has been extensively mined, but only one active mine remains (all the easily exploited copper is now mined out). Copper usually occurs in various late Mesoproterozoic-aged basaltic rocks and conglomerates.

A beautiful and rare occurrence for native copper in northern Michigan is WITHIN other crystals. The above photo shows transparent to translucent calcite crystals having metallic orange-colored inclusions of native copper.

Stratigraphy & age: derived from the upper Portage Lake Volcanic Series, upper Mesoproterozoic (1.093 Ga); copper & calcite mineralization age is late Mesoproterozoic (1.05 to 1.06 Ga).