Near the surface, oxygen-rich groundwater attacks and breaks down primary copper sulfide minerals (such as chalcopyrite). As the copper dissolves and migrates, it can be chemically reduced and redeposited as native copper in weathered rock fractures. Hot, copper-bearing hydrothermal fluids move through ancient basaltic lava flows and porous volcanic rocks, depositing pure copper in gas bubbles (amygdules), cracks, and open spaces. The classic example is the Keweenaw Peninsula in Upper Michigan.
