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Jill Pasteris, Everett Shock, Ian Duncan, Robert Dymek, John Gleaves, John Bleeke
Date of Award
Restricted Access Dissertation
Doctor of Philosophy (PhD)
The hydrothermal transport of platinum-group elements (PGE) was evaluated by integrating a petrologic study of a portion of the Duluth Complex, Minnesota, and a thermodynamic analysis of both PGE solubility in supercritical aqueous solutions and alteration of mafic intrusive rocks. Investigation of two drillcores (FHL-1 and FHL-2) from the basal zone of the southern Duluth Complex indicates multiple overprintings of hydrothermal alteration within these rocks. Primary mafic igneous silicates are replaced by hydrous phases such as amphibole, biotite, and chlorite. Earlier massive magmatic Fe-rich sulfides (pyrrhotite) are replaced by Cu-rich sulfides (chalcopyrite, cubanite). Cu-rich sulfides also occur as disseminated grains and veinlets in more intensely altered zones. Values of Cu/Ni and Pd/Ir covary in the less-altered upper section of FHL-1, but in the lower, highly-altered zone these element ratios vary inversely. The highest values of PGE in FHL-2 samples are found in an intensely hydrothermally altered zone immediately below a massive oxide horizon. Within the latter, coexisting spinels and ilmenite delineate a temperature-oxygen fugacity trajectory for fluid-rock reaction that extends from $\sim$750$\sp\circ$C and 2 units below the value of the fayalite-magnetite-quartz buffer assemblage (i.e., FMQ-2) to $\sim$350$\sp\circ$C and FMQ-9. This trend is more reduced than that for a PGE-rich oxide zone from the northern region.
Development of a set of consistent thermodynamic data for Ru, Rh, Pd, and Pt minerals, aqueous ions, and aqueous chloride, hydroxide and sulfate complexes, combined with recent developments in theoretical geochemistry of supercritical aqueous solutions, has enabled quantitative evaluation of the solubility and transport of PGE in NaCl solutions to 1000$\sp\circ$C and 5 kbar. Solubilities are strong functions of pH, oxygen fugacity, temperature, and total chloride concentration. Theoretical investigation of supercritical alteration of troctolite, norite, and olivine gabbro indicates that at high temperatures and pressures (500$\sp\circ$ to 700$\sp\circ$C and 2 kbar) fluid pH becomes more alkaline and oxygen fugacity decreases during fluid/rock reaction. For these conditions, calculation of PGE solubility in a 3.0 m NaCl solution indicates that this process could produce a 1 ppm Pd deposit with steeply fractionated PGE patterns.
Sassani, David Carl, "Petrologic and Thermodynamic Investigation of the Aqueous Transport of Platinum-Group Elements During Alteration of Mafic Instrusive Rocks" (1992). Retrospective Theses and Dissertations. 57.