As a general rule, any reduction in the supply of gold on the market leads to an increase in its value. Since 2000, the mining of gold has been on the decline. This combined with the fact that it takes nearly a decade to bring a new mine online further limits the supply of gold on the market.
It is also worth noting that the sale of gold bullion by global central banks, the biggest suppliers since the 1900s, has been slowing down since 2008; thus, further reducing supply.