Molybdenum occurs in nature only in chemical combination with other elements. A number of molybdenum-bearing minerals have been identified, but the only one of commercial significance is molybdenite (MoS2) - a natural molybdenum sulfide. In ore bodies, molybdenite is generally present in quantities from 0.01-0.25% and is often associated with the sulfide minerals of other metals, notably copper.
The Reserve Base shown in the figure above is 'That part of an identified resource that meets specified minimum physical and chemical criteria related to current mining and production practices, including those for grade, quality, thickness, and depth. ...The Reserve Base includes those resources that are currently economic (reserves), marginally economic (marginal reserves), and some of those that are currently subeconomic (subeconomic resources)1.
1Appendix C, U.S. Geological Survey, 2009, Mineral commodity summaries 2009: U.S. Geological Survey, 195 p.