here we go, page-27

hera, discount rates for projects/companies are calculated using the risk free rate for bonds. However other factors affect the discount rate used,
check out a few other gold project studies/analyst reports, industry norm is to use 8% discount rates for gold projects and 10% for base metal projects.
Few examples:

http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=0CGAQFjAI&url=http%3A%2F%2Fwww.evolutionmining.com.au%2Fframework%2Fdocuments%2Fdisplaydocument.asp%3Fdoc%3D395&ei=QUfyT9vJG9OhiQfB8cXHAw&usg=AFQjCNGh_sNypq8IgZCJQC3n0AyqSVYu0g

http://www.reedresources.com/reports/RDR_Initiation_110203.pdf

Some light reading on reasons why:

http://web.cim.org/mes/pdf/VALDAYLarrySmith.pdf

In any case the whole idea behind discount rate is the opportunity cost of investing in another asset with comparable risk & is a bit arbitrary to say the least.