Skarn Copper Deposits
Source: Copper Investing News, Leia Michele-Toovey (3/24/11)
"The largest copper skarns can exceed 1B tons and often host tungsten."
Generally, skarns are smaller than many other deposit types like porphyries; however, skarns may occur in association with porphyry copper deposits (e.g., Arizona's Twin Buttes and Utah's Bingham Canyon host both porphyry and skarn deposits).
For the most part, skarn deposits form when carbonate-rich rock is invaded by magma (igneous intrusion) or hydrothermal fluids. The invasion of these hot fluids into the surrounding rock causes a variety of chemical changes that may result in the deposition of valuable ore. Skarn deposits are found in a variety of tectonic settings.
Skarns are defined by their mineralogy. This mineralogy includes a wide variety of calcite- and calcium-bearing silicate and associated minerals but usually is dominated by garnet and pyroxene. Skarns are broken down into two broad subcategories: Exoskarns and endoskarns. Exoskarns are skarns developed in the sedimentary rocks surrounding the thermal source (magma or hydrothermal fluids). Endoskarns are developed within an igneous intrusion. Both types may host minerals, however, the majority of the world's economic skarn deposits occur in calcic exoskarns. Skarn deposits often are described according to the dominant economic metal or mineral present, whether copper, iron, tungsten, zinc-lead, molybdenum, gold, etc.