The easiest way to make $100M in mining, start with $200M. Mining is a tough business. For any one project it takes 10-15 years of spending on any one project before you start to see money rolling in. You need deep pockets to finance. That does not cover all the exploration costs spent looking for the one site that looks promising. To find that one site you will have explored hundreds all incurring costs. I was involved in a project in Colombia with Anglo Gold Ashanti. After exploring hundreds of sites and spending money at the tune of $7M a year for 10 years they finally found a site that yielded the largest known gold reserve outside of their mine in South Africa. The mother lode. Unfortunately they will be unable to extract it because it sits in a forest preserve. Of course the government was happy to take their money to issue the exploration license for the area and assured them if they found something it could be mined.
There are a lot of dollars spent on exploration thats for sure. I was in China and the government had a reserved area that was known for its gold. and since they wanted progress they cancelled the reserve. To bad about the government in Africa, their are many crooks in this game.