Fluid-inclusion evidence on the environment of formation of mineral deposits of the southern Appalachian valley

Approximately 1,330 fluid inclusions were studied in samples of ore and gangue minerals from both massive ore and late-stage vugs from a series of Appalachian deposits and five active mines in the East Tennessee zinc districts. Most primary inclusions in sphalerite, fluorite, dolomite, and quartz from East Tennessee homogenized at 82 degrees -149 degrees C. Most primary inclusions in fluorite, barite, and sphalerite from the Central Kentucky, Central Tennessee, and Sweetwater barite districts, ranged from 72 degrees -132 degrees C. Pressure corrections to be added are probably less than 10 degrees C.Freezing data were obtained as a crude measure of the salinity of the inclusion fluids. With few exceptions, all primary inclusions contained very strongly saline brines (mostly >20 weight percent salts), with appreciable amounts of at least some salts other than NaCl, and some contained immiscible globules of oil. The exceptions include several quartz, fluorite, calcite, and barite samples that contained only moderately saline brines (12-16 percent), and some inclusions containing essentially fresh water, in vug calcite. Secondary inclusions had lower salinities than adjacent primaries. Eight primary inclusions in the differently colored outer millimeter of one Central Kentucky fluorite had low salinity (4-6 percent).The data indicate that all these deposits formed from hot, saline brines, with sphalerite forming from slightly hotter and more saline fluids than later gangue minerals. This places severe restrictions on possible modes of origin and makes deeply circulating connate brines the most probable ore fluids. The data give no information on the cause or direction of fluid circulation, which would be controlled by topography, salinity, and temperature during ore deposition, and they permit wide latitude in the construction of possible models. Other observations, on "colloform" textures, seem to indicate the admixture of at least small amounts of surface waters with the brines.