The stripping ratio refers to the amount of waste rock vs. ore (as opposed to mineralization) or, more precisely, how much worthless rock you have to move to get at the ore you plan to mine. If over the life of an open-pit mine you have to move three tons of barren dirt or rock to extract one ton of ore you run through your plant, your stripping ratio is 3:1.
Obviously, the higher the stripping ratio, the more expensive the mining, and the less profit there is in doing it. At some point, it costs so much to get at the pay dirt, it doesn’t actually pay at all, and you leave it right where you found it in the ground. Conversely, the more valuable your ore, the more dirt you can pay to shove out of the way. Generally, the ratio will be higher in the first bench of an open pit mine (because of the overburden) but should improve with depth. The trick is to keep the cost of removing the waste below the value of the ore…
One thing to note is that the stripping ratio affects mining costs (haulage and blasting, if necessary), not processing costs. Haulage costs can vary a great deal, even over the same distance, if, for example, one deposit is uphill from the processing facility and the other is downhill.
The depth at which a deposit is found is obviously a major factor in a project’s stripping ratio, but so is the geometry of the deposit. A deposit that’s a hill sticking out of a plain is the ideal shape: no stripping, and you can just move the deposit sideways onto a leach pad or through a plant. But take a pyramid-shaped deposit and move it down into the earth so that only the tip is near the surface, and the deeper you chase it, the larger the amount of dirt and rock you have to move out of the way to get at it.
Lastly, the impact of the stripping ratio can vary hugely for different projects. A ratio of 3:1 might kill one project but be manageable in another, even if the mineralization is of the same type and grade. So, one size does not fit all, but generally, if you have a stripping ratio of 1:1 or less, it’s terrific, less than 3:1 is good, and 7:1 or less can work if the grade, geometry, haulage, and other factors are good enough.