Freeport Indonesia said in February that it expected output from the Grasberg mine to rise 30% this year, after falling around 40% last year due to strikes and its mining an area with lower mineral content.
The content of the letter itself is not rare as it discusses income from tin mining stocks, at a time when the industry was still a driving force in Cornwall.
To produce a barrel of synthetic crude through mining takes roughly eight hundred and ten megajoules, which is the energy content of about an eighth of a barrel of oil.
Katharina von Knop, a data-mining expert at the University of German Federal Armed Forces in Munich, says many systems remotely analyse the content of web pages people visit.