Metal ores typically exist as oxides and sulfides bound together with waste rock. Mined crude ore is "beneficiated" to separate ore minerals from waste rock. At this stage, signifi cant amounts of solid waste (tailings) are inevitably generated; the amount of valuable elements is relatively small compared to the total volume or mass of ores actually mined. Over time, as ore grade has decreased in most metals, the waste volume has tended to increase. For example, copper ore mined in the beginning of twentieth century contained about 3% copper (Graedel et al. 2002), but the current typical copper ore grade is now only about 0.3%. Thus, 1000 kg of copper is currently accompanied by more than 300,000 kg of waste. In the case of gold, the typical ore grade in Australian mines at the end of the nineteenth century was around 20 grams per 1000 kg, but current grade is ten times smaller. Thus, the amount of ore milled and wasted is about one-half to one million times the net gold content. In addition, waste rock associated with open cut mining is also several times larger than the ore mined (Mudd 2007b). Based on company reporting, Mudd (2007b) presented site-specific ore grades for twenty gold mines, ranging from 0.45-14.5 grams per 1000 kg.
The factor representing the ratio of pure metal to total mass of mined ore has been used to calculate the so-called "ecological rucksack" or hidden flows.
An indicator termed "total material requirement" (TMR) accounts for the total tonnage of ores mined plus removed overburden from Earth's crust. TMR has been used as a proxy indicator of the environmental impact of massive resource use (discussed further below).
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