Investigations of Mississippi valley-type mineralizing fluids via analytical geochemistry, numerical modeling, and experimental geochemistry

Abstract

Mississippi Valley-type (MVT) deposits represent enrichments of base metals and other elements up to 1000's of times greater than their average concentrations in the Earth's crust. These enrichments most commonly consist of Zn and Pb as the minerals sphalerite and galena, but Ba and F, as the minerals barite and fluorite, can be abundant and even predominate over Zn and Pb in some deposits. Key to understanding how MVT deposits become enriched in the above elements is knowledge of the concentrations of these elements in the mineralizing fluids and how the fluids interact with host rocks. This dissertation consists of three studies that address this knowledge gap. The first was a case study of the Hansonburg, New Mexico MVT district, in which barite and fluorite are the principal ore minerals. The aim of the study was to test the hypothesis that anomalously F-rich fluids formed fluorite-rich MVT deposits by analyzing the composition of fluid inclusions in ore stage minerals. The study showed that the Hansonburg mineralizing fluids were very F-rich, with F concentrations of 100's to 1000's of ppm. The fluids would have been very acidic, which would have suppressed metal sulfide mineral precipitation. The second was a numerical reactive transport modeling study of the Illinois-Kentucky district, a fluorite dominant atypical MVT district. The aim of the study was to assess the importance of F-rich ore fluids in producing the features of the deposits observed in the field. These models showed that silica-armoring of conduits along which the F-rich fluids ascended was critical for allowing the fluids to retain their F-rich, acidic profile until the fluids entered the limestone host rocks. Further, the models showed that high ore fluid F concentrations of 100's to 1000's of ppm were necessary to form the spatial distributions of fluorite mineralization and limestone host rock dissolution observed in the field, and to form the fluorite mineralization within a geologically reasonable time period. The final experimental geochemistry study was undertaken with the goal of producing a tool for determining aqueous Zn concentrations from solid solution Zn concentrations in dolomite using element partitioning theory. Dolomite easily incorporates Zn into its crystal lattice and is a common ore-stage mineral in MVT deposits, thus it has a high potential for this purpose. However, experimental distribution coefficients (D) for the partitioning of Zn between dolomite and aqueous solution are unknown. A series of dolomite precipitation experiments was performed at temperatures between 125 and 200 degrees C, 10 MPa pressure, and aqueous Zn concentrations from about 10 to 1000 ppm for periods of time ranging from 10 to 80 days. The D values calculated from these experiments have applications in hydrothermal ore formation, sedimentary diagenesis, and low-grade metamorphism.