Inorganic and organic geochemistry of the zinc ores of the Idol Mine, East Tennessee

The Idol Mine, operated by Clinch Valley Mining Company, is located at the southern end of the Copper Ridge District in Northeastern Tennessee. It is a relatively typical carbonatehosted zinc (Mississippi valley Type) deposit. The ores occur as interfragmental fillings within carbonate breccia bodies of the Lower Ordovician Kingsport and Mascot formations. The paragenesis study reveals that sphalerite, the only zinc mineral, precipitated at the earliest stage of ore formation accompanied by minor amounts of dolomite and marcasite. A major stage of coarse white dolomite precipitation took place after sphalerite. Minor amounts of other gangue minerals including pyrite, calcite, quartz, gypsum and fluorite all postdate ore formation. Field observations complemented by macro- and micro-scale examination of polished sections and doubly-polished thin sections reveal that five alternating light and dark growth bands are distinguishable within sphalerite and are correlatable throughout the mine. Electron microprobe analyses across the growth zones of the sphalerite reveal that the dark A, C, and E bands are characterized by relatively high Cd, Cu and Pb content and the light B and D bands by relatively high Fe content. It is also noted that there are two types of host breccias, one with bleached rims and the other without. Early bleach rimmed breccias are most likely the relics of karst-related breccias and later non-bleached breccias appears to be of tectonic origin. Representative sphalerite, gangue dolomite and host dolostone breccias have been sampled and tested by mass spectrometry to determine the presence and nature of hydrocarbons. The GC/MS analyses reveal that all the samples examined contain aliphatic hydrocarbons with minor amounts of aromatic compounds. The aliphatic hydrocarbons range from C₂₁H₄₄ to C₃₃H₆₈ and are most likely of terrestrial plant origin. On the basis of new observations combined with previous data, a model involving the mixing of two distintive fluids is proposed to account for the formation of the Idol zinc mine.