We thank Guochang Liu and Mirko van der Baan for stimulating
discussions. We thank Partha Routh, one anonymous associate editor,
and one anonymous reviewer for helpful suggestions, which improved the
quality of the paper. This work is supported in part by National
Natural Science Foundation of China (Grant No. 41004041) and 973
Programme of China (Grant No. 2009CB219301). This publication was
authorized by the Director, Bureau of Economic Geology, The University
of Texas at Austin.