Issue link: http://resourceworld.uberflip.com/i/119004
MINI NG What makes a deposit economic? by Vanessa MacLean "Now a miner, before he begins to mine the veins, must consider seven things, namely: – the situation, the conditions, the wastes, the roads, the climate, the rights of ownership, and the neighbours." Written in 1556, by Georgius Agricola, De Re Metallica outlines chief factors that are still, almost 500 years later, the pivotal aspects influencing whether a deposit will be economic. Today however, our advanced knowledge regarding the characteristics of particular mineral deposit types and associated grades may very well be the foundation used to search out these economic source rocks. Every rock in the Earth contains at least some quantity of virtually all the naturally occurring elements that exist. However, as I am sure you know, with less than 1% of the Earth's crust containing deposits that can be mined at profit, the concentration of these elements are normally far too low to define anything economic. Processes must occur to naturally congregate particular elements to a level where extraction is profitable. The processes that amass these valuable elements are numerous, and are often fluid based (hydrothermal, surface water, or groundwater transport), magmatic, and sedimentary. These processes transport elements from an initial source within the crust, elsewhere, where they are then subsequently deposited or crystallized due to changes in temperature, pressure, or through chemical reactions over a relatively small area. If the concentration of the elements deposited is high enough, this may form an economic mineral deposit. Depending on the transport processes, the elements being carried, and the host rock within which they were deposited, different types of mineral deposits can form. You may have heard geologists naming or classifying these deposits, and figured you'd leave the analysis up to them. But, having knowledge of different mineral deposit types is a very important tool when evaluating exploration companies and their press releases when seeking possible investments. Although every deposit is unique, with tonnage, grade, size and shape differing immensely, there are typical tonnages, grades and elemental characteristics of particular mineral deposit types that should serve as a general guide when analyzing the potential of a deposit type. What will follow below is an overview of typical mineral grades and tonnage for the most commonly described mineral deposits. Grades and tonnages provided have been accumulated from typical world-class deposits. PORPHYRY COPPER DEPOSITS VOLCANIC MASSIVE SULPHIDE DEPOSITS (VMS) Large tonnage, low to medium grade deposits Significance: World's major source of copper and molybdenum and an important source of gold Examples: Mt Polly, BC; Highland Valley Copper, BC Hosted: Range of rock types within and around a related intrusion Mineralization: Copper and molybdenum sulphide minerals Cu±Au±Mo Structures and shape: Fractures, stockworks, veins, disseminations; large, oval or spherical Mining method: Normally open-pit Drill intersection: Mineralization should be near surface (top 100 metres) and define a deposit at least hundreds of metres in size in all dimensions Tonnes (Mt):10-1000, usually at least 100 Mt. Grades: Cu%: 0.25-1.0 Au (g/t): 0.1-1.0 Mo%:0.01-0.15 Ag (g/t): 1.0-3.0 48 www.resourceworld.com Low to moderate tonnage, moderate to high-grade deposits Significance: A major source of the world's copper, zinc, lead, gold, and silver Examples: Flin Flon, Manitoba; Mount Lyell, Australia Hosted: In volcanic terranes (often in clusters) Mineralization: Massive sulphide mineralization Structures and shape: Lens-like or tabular underlain by stockwork system Mining method: Normally mined using underground methods Drill intersection: Several metres to tens of metres in length with combined grades of copper, zinc and lead in the 4-10% range or higher Tonnes (Mt): 2.0-15.0. Most that are mined exceed 5 Mt. Grades: Cu% 1.0-2.5 Zn%: 2.0-6.0 Pb%: 2.0-6.0 Au (g/t): 0.5-2.0 Ag (g/t): 15.0-50.0+ APRIL 2013