The geochemistry and mineralogy of the Gies gold-silver telluride deposit, central Montana

Zhang, Xiaomao
Major Professor
Paul G. Spry
Committee Member
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Geological and Atmospheric Sciences

This dissertation has been written in three parts: the mineralogy and geochemistry of the Gies deposit, Montana; aqueous tellurium speciation and the stabilities of gold and silver telluride at elevated temperatures; and mineral stabilities and sulfur isotope distribution in log fO[subscript]2-pH space at elevated temperatures;Gold-silver telluride mineralization at the Gies mine, central Montana, occurs in steeply dipping veins at the contact between alkaline intrusive rocks and sedimentary rocks. The mineralogy of the vein system is complex, and consists of 36 minerals, including 6 tellurides, in three stages of mineralization. Fluid inclusion studies show that the temperatures of the three stages of mineralization are approximately 300°, 260°, and 225°C, respectively. The salinity of ore-forming fluids for all three stages range from 6 to 8 equivalent weight percent NaCl. Gold and silver telluride mineralization is mainly associated with stage III mineralization. [delta][superscript]18O and [delta]D values of the ore-forming fluids decrease from stage I to stage III, suggesting that the ore-forming fluids were dominated by magmatic water initially and were subsequently diluted by evolved meteoric water. [delta][superscript]34S values of sulfides are concentrated in a narrow range close to 0 per mil, suggesting a magmatic source of sulfur. Thermodynamic modeling shows that stage I mineralization precipitated under higher pH and lower fO[subscript]2 conditions than stage II and III mineralization. While fS[subscript]2 decreased systematically from stage I to stage III, fTe[subscript]2 increased slightly from stage II to stage III, and induced precipitation of tellurides;The principle of balance of identical like charges is employed to determine equilibrium constants of reactions involving aqueous tellurium species at elevated temperatures. Thermodynamic calculations in the system Te-O-H and Au-Ag-Te-Cl-S-O-H, utilizing these equilibrium constants, show that Te[subscript]2[superscript]2- is an important aqueous Te species and that calaverite and hessite can only coexist in fO[subscript]2-pH space below 250°C. Gold and silver tellurides generally precipitate as a result of a decrease in temperature and an increase in fO[subscript]2.;Two computer programs, CONSTANT and FO2PH, were developed to calculate equilibrium constants of reactions involve mineral, gaseous and aqueous species at elevated temperatures (CONSTANT), and to model mineral stabilities and sulfur isotope distribution in fO[subscript]2-pH space (FO2PH). The program FO2PH utilizes aqueous sulfur and carbon speciations to determine equilibrium of mineral reactions in the system Cu-Fe-S-O-H, solubilities of barite and calcite and sulfur isotope fractionation.