By Leia Michele Toovey-Exclusive to Copper Investing Newswidth=312Porphyry deposits are the world's most important source of copper and molybdenum, and major sources of silver, gold, and tin. Porphyry deposits may also host valuable by-products, including platinum, palladium, and tungsten.Approximately 50-60 percent of the world's copper, and 95 percent of the world's molybdenum comes from porphyry deposits. In the past, porphyry deposits were often overlooked with conventional drilling and assaying exploration methods due to their low grade. The discovery of many large porphyry deposits has led exploration companies to look more critically at their drilling and assaying results. This is why, nowadays, you will see junior miners report their drilling results showing that drill cores “started and ended in mineralization” and that drill cores provided an “extension of mineralization.” These are important facts to consider when trying to determine the overall extent of mineralization in an area.Although the grade may be low, porphyry copper deposits typically contain hundreds of millions of tonnes of ore. Grades for the different metals vary considerably, but generally average less than 1%. In porphyry Cu deposits, for example, copper grades range from 0.2% to more than 1% copper. The metal content of porphyry deposits is diverse. For deposits with currently sub-economic grades and tonnages, subtypes are based on probable co-product and by-product metals, assuming that the deposits were economic. In porphyry deposits, ore minerals are dominantly structurally controlled, which means that the bulk of mineralization in these deposits is hosted in stockworks, veins, vein sets, fractures and breccias.Where are they found?The worldwide distribution of porphyry deposits is controlled by orogenic belts. Two types of orogenic belts are known to host porphyry deposits, those created along a subduction zone (where a continental and ocean plate meet) and those found along island arcs where two oceanic plates collide. The porphyry deposits along the cost of South America, and North America are an example of porphyry deposits formed along a subduction zone. Those in the Philippines and Papua New Guinea are examples of island arc porphyry deposits.How are they formed?When two plates collide, a great deal of pressure, stress and heat is generated. The extreme heat melts a portion of the earth's crust, turning it into a liquid. The liquid form is less dense than the solid, therefore, due to the laws of physics the liquid material, now called magma, rises through the crust towards the earth's surface. As it rises, a variety of geochemical changes take place. The hot, rising magma melts portions of the crust as it moves upward, incorporating new minerals into the magma. Furthermore, as the magma rises it encounters decreasing pressure, which causes the water dissolved in the magma to separate. This water may also contain, and deposit valuable minerals. The magma will crystallize as it rises and cools, generating large crystals of material called “porphyritic.” Once the magma solidifies, the high-temp fluids are released into the solidified porphyry and its surrounding host rocks. The water, which is often mineral rich, will take the path of least resistance and travel through cracks and fractures. This is why porphyry deposits often host mineralization in cracks and fractures.TypesIn porphyry copper deposits, copper grades range from 0.2% to more than 1% , molybdenum grades range from approximately 0.005 to about 0.03%: and silver  contents range from 0.004 to 0.35 g/t, gold content ranges from 0.2 to 5 g/t. Copper grades in porphyry copper-gold deposits are comparable to those of the copper-porphyry, however, the gold content tends to be considerably higher. There are a limited amount of copper-porphyry deposits in the world, however, one prime example is  Freeport McMoRan's (NYSE:FCX) Grasberg in Indonesia, with a resource greater than 2.5 billion tonnes grading 1.1% copper and 1.04 g/t gold.The future Exploration companies are always in search of the next great copper porphyry deposit. Not only are these deposits an excellent source for copper, extra  money can be made from the significant co- and by-products.  Examples of porphyry copper deposits include El Salvador, the Bingham, Utah, and the giant Chuquicamata deposit in Chile, which originally comprised over 9000 million tonnes of mined ore and reserves. Porphyry deposits are amenable to open-pit mining, offering a low cost way to extract the resource.Some miners with copper-porphyry potentialTombstone Exploration (OTC:TMBXF)Mindoro Resources (CVE:MIO)Bellhaven (CVE:BHE)Savant Exploration (CVE:SVT)Virgin Metals (TSE:VGM)Nevada Copper (TSE:NCU)

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