By Leia Michele Toovey
Porphyry deposits are one of the most important sources of copper and molybdenum, and are also major sources of silver, gold, and tin. In addition, porphyry deposits may host valuable by-products including platinum, palladium, and tungsten.
Approximately 50-60 percent of the world’s copper, and 95 percent of 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 prompted exploration companies to look more critically at their drilling and assaying results. 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 they generally average less than one percent. In porphyry copper deposits, for example, copper grades range from 0.2 percent to more than one percent copper.
The metal content of porphyry deposits is diverse. For deposits with sub-economic grades and tonnages, subtypes are based on probable co-product and by-product metals, assuming that the deposits were economic.
Where are porphyry deposits 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 coast 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.
Types of porphyry copper deposits
In porphyry copper deposits, copper grades can range from 0.2 percent to more than one percent, while molybdenum grades can range from approximately 0.005 percent to 0.03 percent. Silver grades range from 0.004 to 0.35 grams per tonne, and gold grades range from 0.2 to 5 grams per tonne.
There are a limited amount of copper-porphyry deposits in the world. However, one prime example is Freeport McMoRan’s (NYSE:FCX) Grasberg mine in Indonesia, the largest gold mine and one of the largest copper mines in the world.
Exploration companies are always in search of the next great copper porphyry deposit. Not only are these deposits an excellent source for copper, but as mentioned above, they often feature significant co- and by-products, which can help lower production costs. What’s more, porphyry deposits are amenable to open-pit mining, often a lower-cost method of resource extraction.
Examples of porphyry copper deposits include El Salvador, the Bingham mine in Utah, and the giant Chuquicamata deposit in Chile, which originally comprised over 9000 million tonnes of mined ore and reserves.
A sample of junior explorers and developers with copper-porphyry potential
Panoro Minerals (TSXV:PML)
Taseko Mines (TSX:TKO)
Carube Copper (TSXV:CUC)
Pacific Booker Minerals (TSXV:BKM,AMEX:PBM)
NGex Resources (TSX:NCQ)
Reservoir Minerals (TSXV:RMC)
Quaterra Resources (TSXV:QTA,OTCMKTS:QTRRF,FWB:QR2)
Nevada Copper (TSX:NCU)
Kaizen Discovery (TSXV:KZD)
AMARC Resources (TSXV:AHR)
Editorial Disclosure: Western Copper and Gold, American CuMo Mining, Entree Gold, Los Andes Copper and Copper Fox Metals are clients of the Investing News Network. This article is not paid for content.
This article was originally published on Copper Investing News on February 17, 2011