back out provision
If I understand it correctly, it sounds like the back out provision is that if Xstrata doesn't do the required exploration spending, then Sabina can buy the property back for however much Xtrata spent on exploration.
So if I understand that correctly, the result is that Xtrata could back out of the deal down the road, but the penalty they will pay is that they will forfeit the $50 million they will be paying up front. That would seem to be a pretty good deal for Xtrata, they get to develop a mine and if market conditions or whatever lead to changing their minds, it just costs them $50 million.
But you have to figure that barring a crash in the commodities market, due to some bizarre circumstance such as the United States balancing its budget (please don't laugh too hard), you have got to figure that Xtrata wants to develop Hackett. Mining is what these guys do for a living. And Zinc and Silver are things they like to mine.