1. Department of Geology, Mining and Environmental Science, University of Bamenda, P.O Box 39 Bambili, NW Region, Cameroon
2. Graduate School of Humanities and Social Sciences, Hiroshima University, Hiroshima 739-8511, Japan
3. Robert M. Buchan Department of Mining, Queen's University, Kingston, Ontario K7L 3N6, Canada
4. College of Economics, Nanjing University of Post and Telecommunications, Nanjing 210023, China
5. State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China