How to calculate stripping ratios?

At the most basic level, the strip ratio formula involves dividing overburden thickness by ore thickness. For example, an overburden thickness of 100 meters and an ore thickness of 50 meters would yield a strip ratio of 2:1. That means mining 1 cubic meter of ore would require mining 3 cubic meters of overburden.

The stripping ratio of a deposit may be used, in part, to gauge how profitable it may be. The lower the strip ratio the better, since a low strip ratio translates into lower mining costs and good prospects for profitability.

Conversely, a project with a very high strip ratio likely will not be profitable. In this case, the unwanted material is much greater than the amount of ore that can potentially be extracted, making it too expensive to mine.

Ore quality is another consideration in stripping ratios in mining. That’s because if a deposit contains low-quality ore, more of it must be mined in order to achieve a return on investment.

It should be noted that every deposit is different, and a project that benefits from another factor — for example, having high grades — can potentially support a higher strip ratio. Generally speaking, there is an inverse relationship between reserve grade and strip ratio.

With all of this in mind, mining companies calculate strip ratios for open-pit projects well before they enter development and production, and tend to seek out projects with relatively low strip ratios. Even so, given all the factors involved in calculating a strip ratio, it’s difficult to determine an overall ideal figure. In the case of a "typical" large, low-grade copper porphyry deposit, a strip ratio below 3:1 is generally considered good.

What are some examples of stripping ratios?

Mines with good strip ratios over the life of mine include Lundin Mining’s (TSX:LUN,OTC Pink:LUNMF) Candelaria copper-gold-silver mine in Chile at 2.1:1, and Hudbay Minerals (TSX:HBM,NYSE:HBM) subsidiary Copper Mountain Mining’s copper mine in Canada at 2.77:1.

Goldsource Mines’ (TSXV:GXS,OTCQX:GXSFF,FWB:G5MA) preliminary economic assessment of its Eagle Mountain gold project in Guyana shows an average life of mine strip ratio of 2.1:1. World Copper's (TSXV:WCU) Zonia copper oxide project in Arizona reportedly has a very low strip ratio of 1.1:1 with a 0.07 percent cut off.

Western Copper and Gold (TSX:WRN,NYSEAMERICAN:WRN) has emphasized that its Casino copper-gold project in Canada's Yukon has a “truly impressive” life-of-mine strip ratio of 0.43:1.

It is also not unusual to see a high-grade volcanic massive sulfide deposit support a strip ratio greater than 5:1. For example, the high-grade Bisha copper mine in Eritrea posted a strip ratio of 5.4:1 in 2014. Similarly, the high-grade New Liberty gold mine in Liberia had a strip ratio of 15.5:1.