Skarn Copper Deposits

Mining & Minerals - Evaluations / Other Metals

Date posted: 04/08/16

A skarn is a silicate-rich metamorphic rock. Skarns may or may not host economic reserves of metals, but if they do, these are called skarn deposits. Skarns are considered to be the premier source of tungsten, but may also host valuable amounts of other metals, including copper, gold, lead, iron, zinc and molybdenum. Arguably, copper skarns are the most abundant skarn type. They are particularly common along subduction zones, and are found in both oceanic and continental margins. The largest copper skarns are associated with mineralized porphyry copper plutons, or bodies of intrusive igneous rock. These deposits can exceed 1 billion tonnes of combined porphyry and skarn ore, with more than 5 million tonnes of copper recoverable from the skarn portion of the deposit. Generally, skarns are smaller than many other deposit types such as porphyries. However, it is worth noting that skarns may occur in association with porphyry copper deposits. For example, the Twin Buttes in Arizona and Bingham Canyon in Utah 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 (country) 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, which includes a wide variety of calcite and calcium-bearing silicate and associated minerals. However, the mineralogy of skarns is usually dominated by garnets 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. Read on for more details...