1. The University of Texas at Austin, Bureau of Economic Geology, 10611 Exploration Way, Austin, Texas 78758, USA.(corresponding author); .
2. The University of Texas at Austin, Bureau of Economic Geology, 10611 Exploration Way, Austin, Texas 78758, USA and University of Oslo, Department of Geosciences, P.O. Box 1047, Blindern, 0316 Oslo, Norway..
3. Southern Methodist University, Department of Earth Sciences, Dallas, Texas, USA..