Chalcocite is a significant copper ore mineral having the formula Cu2S - copper sulfide. It's got a metallic luster, a dark gray to black color & streak, no obvious cleavage, is moderately heavy for its size, and is moderately soft (H = 2.5 to 3). Chalcocite is found with other copper minerals such as chalcopyrite. It usually forms in the near-surface, secondary enrichment zone at & below the water table, in areas having copper mineralization.
Chalcocite (3.7 cm across) from the Butte Mining District, southwestern Montana, USA. Butte was the top copper producer on Earth for many decades. In that area, hydrothermal veins intruded and altered the Butte Quartz Monzonite (Butte Pluton, mid-Campanian Stage, late Late Cretaceous, 76 million years), a large intrusive igneous mass forming part of the Boulder Batholith. Cu-rich hydrothermal vein intrusion occurred at about 62 to 66 million years, around the time of the Cretaceous-Tertiary transition.