In the previous topic we learned that the economics of mining is particularly related to the amount of ore that can be produced and the amount of waste rock that must be moved. That makes underground mining particularly suited to structurally compact ore bodies with high grade ores which don’t require an extensive network of access tunnels through worthless rock and which don’t involve large qualities of gain being brought to the surface with the valuable ore and deeper deposits where excessive depth of overburden makes the costs of accessing the ore prohibitive. In this topic, we will take a look at those situations in some more detail. Mineral and oil deposits come in many different forms and contain many different types of minerals. A full consideration of the geological origin of oil deposits is well beyond the scope of the current course. In this topic, we’ll only consider those mineral deposits which are relevant to our discussion of underground mining.
Classifications of mineral deposits can be formed on many different basis. These include the genetic geological origin of the deposit that is how it is formed, the minerals it contains, the structure that is size and shape of the deposit. In the context of mining engineering important considerations are how to stall the mineralization and the form of the deposit. We can separate mineral deposits according to their form recognizing styles such as massive deposits, where ore minerals occur in concentrated form in discrete bodies, often with well defined extent disseminated deposits, where the minerals occur in a well dispersed arrangement throughout a large volume of ordinary rock and stockwork vein deposits, where the minerals are concentrated in a large number of veins, formed in a network of fractures in an ordinary rock mass. Let’s now look at how some of these deposits might be suited to underground mining. Ore deposits can occur in concentrated form as massive bodies, where these occur between layers of rock they are often described as stratabound or strata form deposits. In some cases, strata form deposits are deposited as horizontal layers and they remain in that orientation until they are mined. In other cases, movements in the earth crusts cause stratabound deposits to become tilted and in some cases even folded. They may even end up almost vertical. Alternatively, massive deposits may be discordant to the other geological layers cutting across them at some angle. Examples of this are veins and loads hosted by faults. Massive deposits can occur any depth below the earth’s surface, while some are exposed directly at the surface others may be buried by great depths perhaps by hundreds of meters. Massive deposits at great depth make good targets from underground mining because the grade of the ore is high, so every tonne of ore mined underground brought to the surface has great value, they are often so deep, it is not economical to uncover them in an open pit and once they are accessed by a tunnel, they can be progressively extracted in a productive manner without having to mine through large amounts of worthless rock. Examples of massive deposits that are mined underground include layers of coal which are stratabound deposits, lenses of massive sulphides of copper, lead, zinc and silver is a rough strata form deposits as well, lenses of nickel-copper sulphides, lenses of chromite ores and platinoids and kimberlite pipes containing diamonds. As noted in module three, disseminated deposits contain low grade ore polluted by a lot of gain minerals It is generally not economical to mine disseminated deposits with underground mining techniques. Stockwork vein deposits occurs veins in heavily fractured rocks by mineral rich aqueous fluids. While the mineralization may be rich the veins are often thin and widely distributed, re-vein is thicker and more persistent it may be economical to mine a particular vein using underground mining techniques. This is particularly so in the value of the commodity produced is high such as for gold silver, molybdenum, bismuth or antimony. Such mining operations however would generally be small.
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In the topics that follow we will look at how some of the minerals that carry these very in these various shaped deposits may be mined using underground mining techniques.