1. Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, U.K.
2. Mineral Resources, Commonwealth Scientific and Industrial Research Organisation, 1 Technology Court, Pullenvale, QLD 4069, Australia
3. Facultad de Ingeniería, Universidad Del Valle, Ciudad Universitaria Meléndez, Calle 13 # 100-00. A. A., Cali 439, Colombia
4. Servicio Geológico Colombiano, Diagonal 53 N0. 34 − 53, Bogotá D.C. 11121, Colombia