Dealing with mineral ores is rapidly becoming more complex as ore grade is decreasing, mines are getting deeper and the cost of energy and labour increases.

The minerals industry has seen an increase in production costs of 70% since the mid 1980s and ore quality has, on average, declined by 50%.

Ore "complexity" has also increased during the same period of time, with valuable components distributed sparsely and finely within the ore and often associated with a high content of impurities that are difficult to remove.

There has also been an increase in global competition, metal price volatility, and pressures to improve environmental performance.

Since 2004 the price for copper has increased by 50% and during the same time the price for iron ore has increased by four times.

http://phys.org/news/2014-01-costly-ores-rich.html

New report

Gold, for example

https://minerals.usgs.gov/minerals/pubs/mcs/2017/mcs2017.pdf