1. Lead-Zinc ’90, ed. T.S. Mackey and R.D. Prengaman (Warrendale, PA: TMS, 1990).
2. EPD Congress ’90, ed. D.R. Gaskell (Warrendale, PA: TMS, 1990).
3. T.D. Kaufmann, “1991 Recession Impact on U.S. Mining and Metals,” Mining Eng., 42 (1990), pp. 1334–1335.
4. Mineral Commodity Summaries 1989 (Washington, D.C.: U.S. Department of the Interior, Bureau of Mines, 1989).
5. “21st Annual Survey and Outlook,” E&MJ, 191(3) (1990), pp. 17–64.