Skarn Copper Deposits

In the broad sense, a skarn is a silicate-rich metamorphic rock. Skarns may or may not host economic reserves of metals; if they do, they are called Skarn Deposits. Skarns are considered 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 world s most abundant skarn type. They are particularly common along subduction zones and found in both oceanic and continental margins. The largest copper skarns are associated with mineralized porphyry copper plutons. These deposits can exceed 1 billion tonnes of combined porphyry and skarn ore, with more than 5 million tonnes of copper recoverable from skarn.

Generally, skarns are smaller than many other deposit types such as porphyries, however, 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. 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 are often described according to the dominant economic metal or mineral present, whether it is copper, iron, tungsten, zinc-lead, molybdenum, gold, etc.

Examples of world class copper skarn deposits

The Antamina

Antamina is the world s largest copper-zinc skarn deposit. It is located in the Andes, North Central Peru and co-owned by Compa a Minera Antamina S.A., in which BHP Billiton (NYSE:BHP) holds a 33.75 percent interest, Xstrata (LON:XTA) 33.75 percent, Teck (NYSE:TCK) 22.5 percent, and Mitsubishi 10 percent.

Antamina has a long history of exploration with small -scale mining first recorded in 1860. It wasn t until a century later, in the 1950 s that serious exploration began. The Antamina deposit is a large copper skarn with zinc, silver, lead, molybdenum and bismuth mineralization. The ore is mined by open pit methods, and batch treated by ore type through a conventional grinding and flotation mill.

Total reserves at Antamina are 745 million tonnes, based on a the most recent reserves study (2008).

The Ok Tedi Mine

The Ok Tedi mine is located in the remote Star Mountains of Papua New Guinea s Western Province. Ok Tedi Mining Limited (OTML) operates the mine. The Ok Tedi mine started operations in 1984, and has become the single largest business contributor to the economies of both the Western Province and Papua New Guinea.

In January 2002, BHP Billiton Ltd divested its 52 percent shareholding in OTML by transfer of its shares to the Papua New Guinea Sustainable Development Program Limited. OTML s shareholders are PNG Sustainable Development Program Limited (52 percent), Inmet Mining Ltd (TSE:IMN)(18 percent) and the PNG Government (30 percent).

The mine s main ore body includes both porphyry and skarn ores. The skarn ores are very-high grade, and supply approximately 50 percent of the contained ore reserves. As of 31st December 31st 2007, the Ok Tedi mine had produced 11,580,784 tonnes of copper concentrate containing 3,598,992 tonnes of copper metal and 8,771,026 ounces of gold metal.

Ok Tedi s estimated proven and probable reserves as at 31 December 2006 were: Proven reserves: 157 million tonnes at 0.88% copper and 1.14 g/t gold. Probable reserves: 21 million tonnes at 0.76% copper and 1.29 g/t gold. OK Tedi hosts an estimated reserve of 4.5 billion pounds of copper and 7 million ounces of gold.

Junior miners/explorers with skarn deposits

Alturas Minerals (CVE:ALT)

Alturas Minerals is an exploration company dedicated to the discovery of gold and copper-gold deposits in Peru. Alturas controls a portfolio of 11 gold and copper-gold properties, including four that are drill-ready.

Southern Silver Exploration Corp. (CVE:SSV)

Southern Silver Exploration Corp. is a Canadian-based precious/base metal exploration company whose interests include the copper/gold Minas de Ameca project as well as the Pinabete project, a silver-zinc-lead Carbonate Replacement Deposit deposit located in Chihuahua, Mexico. In addition, the company holds prospective projects in both Arizona and New Mexico.

Zincore Metals (TSE:ZNC)

Zincore Metals is an exploration company focused on the identification, acquisition, exploration, evaluation and development of primarily zinc and related base metals projects in the Americas. Properties with skarn deposits include the Dolores Cu-Au porphyryl, located in the prolific Yauri-Andahuaylas copper district; and the Minasccasa, located in the Andahuaylas-Yauri metallogenic belt in southern Peru.

Copper One Inc.(CVE:CUO)

Copper One is focused on developing high-value copper deposits in leading mining jurisdictions. Copper One has six properties in the Arizona and New Mexico copper mining districts, including the Lone Mountain copper oxide project which is currently in the permitting process for a major drill program anticipated in 2011.