Masters Theses

Date of Award

3-1986

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kula C. Mirsa

Committee Members

Kenneth R. Walker, Otto C Kopp

Abstract

The Gordonsville zinc mine is located in the Central Basin section of the Interior Lowlands Physiographic Province of Middle Tennessee. The Gordonsville mine, along with the nearby Elmwood mine, are the major producers of zinc ore in the Central Tennessee zinc district. Ore-grade mineralization is believed to be confined to dissolution cavities and collapse breccias in the Mascot Formation, the upper unit of the Upper Cambrian - Lower Ordovician Knox Group. Sphalerite is the only zinc ore mineral in the district. Associated gangue minerals include fluorite, barite, calcite, galena, minor dolomite and quartz. Approximately two hundred samples were collected from several stopes in the Gordonsville mine in order to: (1) establish mineral paragenesis within the mine and, (2) through fluid inclusion and chemical analyses, determine the nature of ore fluids.

From petrographic and field study the generalized sequence of ore and gangue mineralization is as follows: early, white calcite - sphalerite - galena - fluorite - main-stage, white to lavender calcite - barite - late-stage, clear to amber calcite. Textural evidence suggests that these minerals were, for the most part, in equilibrium with circulating fluids throughout the mineralization period. Based on fluid inclusion data, trace element data, and petrographic evidence, the three textural varieties of sphalerite (disseminated, massive, and vug-filling) appear to belong to the same depositional episode. Electron microprobe analyses of minor element concentrations in all sphalerite types suggest that microscopic color regions in sphalerite (white to deep orange) may be due to variations in cadmium content. The whiter areas contain the highest concentrations of cadmium (0.25 - 0.75 wt.%). Main-stage, white to lavender calcite and late-stage, clear to amber calcite could not be differentiated by the trace elements analyzed although they differ in color, crystal habit, formational temperatures and salinities.

Fluid inclusion analyses on ore and gangue minerals suggest that the mineralizing fluid changed in temperature and salinity through time. Homogenization temperatures for all minerals fall within the general range of 90° to 130°C. A decrease in fluid temperature of approximately 6°C following sphalerite deposition may have been sufficient to cause precipitation of fluorite. A further decrease in fluid temperature is suggested by inclusions in amber calcite (and barite?). Freezing temperatures, which provide an estimate of fluid salinities, remained fairly constant through main-stage calcite deposition (-17° - -22°C, corresponding to 20 - 23 equiv. wt.% NaCl). Precipitation of barite and late-stage, amber calcite, however, was marked by a large decrease in fluid salinity (7 - 12 equiv. wt.% NaCl).

The consistency and narrowly defined range of paired homogenization and freezing temperatures for fluid inclusions in sphalerite suggest that the ore mineral precipitated from one fluid rather than by the mixing of two or more fluids. Metals may have traveled with partly oxidized sulfur, or as chloride or organometallic complexes with reduced sulfur. Precipitation of fluorite and main-stage calcite was brought about by temperature and, possibly, compositional changes in the ore fluid. Limited carbon and oxygen isotope analyses of the three calcite stages show only minor variation and all data fall in or near the region of marine carbonates. This suggests that the slight decrease in fluid temperature and the marked decrease in fluid salinity during late-stage mineral precipitation was not necessarily due to an influx of meteoric water, although a meteoric component cannot be ruled out.

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