former premier made unannounced visit to capital ulaanbataar last month .
met new mongolian prime minister chimed saikhanbileg on trip , sparking fears from ecologists he was encouraging western investment in mining .
noyon uul region has major gold reserves but many sacred burial mounds .
exploitation may ` destroy monuments of the mongols and their ancestors '
mr blair admits to ` regular ' mongolia visits but denies discussing mining .