Origin of the opalite breccia at the McDermitt mercury mine, Nevada

The McDermitt mine in the Opalitc mining district is near the Oregon-Nevada border (Fig. 1). Mercury sulfides are disseminated in tuffaceous sedimentary and pyroclastic rocks associated with the 17.9to 15.8-m.y.-old McDermitt Caldera Complex (Rytuba and Glanzman, 1978). The Opalitc, Bretz, Cordero, and other mines in the district produced approximately 135,000 flasks of mercury between 1926 and 1972, of which the Cordero mine produced over 100,000 flasks between 1935 and 1970 (Speer, 1977). When Placer Amex Inc. began production at the McDermitt open pit in 1975, reserves were about 400,000 flasks (Mining Magazine, 1982). The average ore grade is about 0.5 percent Hg. McDermitt currently is the largest mercury mine in the United States. The purposes of this communication are to describe and determine the origin of the opalitc breccia at the McDermitt mine and to suggest its role in localizing ore. In the McDermitt caldera opalitc is a field term that has been loosely applied to a variety of silicified fragmental rocks including fault breccias and stratigraphic units (Hetherington, 1983). Roper (1976) interpreted the opalitc breccia at the McDermitt mine as lake beds that were silicified and brecciated uring an inferred period of surficial hot-spring activity that caused the mineralization. The present study identifies the mineralized opalitc breccia at the McDermitt mine as a lithic vitric ash-flow tuff (using the terminology of Ross and Smith, 1960, and Schmid, 1981). Hetherington (1983) presented a more detailed account of the results discussed here. Detailed geochemical studies of the deposit by Rytuba and by Placer Amex Inc. are still in progress.