Now that Rio Tinto (NYSE:RTP) has raised around $21 billion through a dilutive rights offering and a proposed iron ore joint venture with BHP Billiton (NYSE:BHP), an Anglo American/Xstrata pairing would likely create the most heavily indebted public mining company in the world. The sheer scale of their combined debt burden is enough to give investors an ulcer in this highly uncertain credit environment, even though creditors might be more likely to waive debt covenants to avoid defaults of such magnitude.