Since ore grades continue to get lower and lower it would seem that we will never run out of metals in the foreseeable future. Is this because mining methods are more efficient and economical?
Ore grades are getting lower and lower, but at a certain point, the decrease will no longer be compensated by more efficient methods. Economies of scale can only go so far. At some point it just becomes impractical and impossibly expensive to make equipment bigger. The reason we can mine less economical ore is because metal prices are increasing (or alternately, metal prices are increasing because we use less economical ore). Price is the driving force behind a lot of materials choices. The auto industry uses galvanized steel in place of aluminum because it's cheaper. But we have about 20 years of currently economical zinc ore reserves and over 100 years of bauxite (aluminum ore) reserves, so at some point (probably around 20 years from now), it will likely be cheaper to use aluminum (or some sort of composite) in place of galvanized steel for car bodies.
As demand for metals goes up, price goes up making lower grade material economic to mine. Mining methods are more efficient to a point but is the dollar value per ton that is the driving force. Gold is the best current example of this: large scale mines 10 years ago would have been mining at an average grade of 1.5-2 grams per tonne. Today mining is economical in the same mine down to 0.5 grams per tonne.
In a gross historical sense, minimum grade for economic mining operations has decreased because of improved efficiency in the collection and treatment of the ore materials. Over the short term (say the past decade about), the rise in metal prices has made many marginal (often lower grade) deposits worthwhile to mine, meaning that even though the grade is low, there is still enough metal that can be recovered for less than they will earn from selling it. I do not think there is much gain in efficiency and effectiveness of collection and recovery that will be obtained into the future. Most of the technological developments that produce large improvements in efficiency and effectiveness have already been instituted (the industry is mature). The main driving force for making low grade ores recoverable comes from increase in product value these days, rather than in reduction of costs of production.
