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Orthomagmatic Fluids and Saline Melts in Iron Oxide-Apatite and Porphyry-Copper Systems
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- Author / Creator
- Bain, Wyatt
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The physical and chemical properties of aqueous fluids and melts, and how these evolve through time and space, are key controlling factors that govern the formation of mineral deposits. For some metal-rich system types such as iron oxide-apatite deposits (IOA), the sources and chemical compositions of mineralizing fluids are still unknown and genetic models for these systems are therefore largely speculative. For other well-studied system types, such as porphyry deposits, the sources and compositions of mineralizing fluid are unambiguous, but questions remain as to how these fluids evolve between their sources and the site of ore formation.
In this thesis, I describe the results and implications of detailed studies aimed at determining the sources, properties and evolution of mineralizing fluids in three IOA systems and one small, but exemplary porphyry system. The overarching objective of these works are to document the physical and chemical properties of the ore-forming fluids, and to clarify the geologic factors that govern these properties. The main tool employed throughout this thesis is analysis of fluid and melt inclusions by a combination of detailed petrography and chemical microanalysis. A number of major themes emerge: For each of the studied systems (IOA and porphyry), I present evidence for a central role of immiscible fluids and melts, and show that particular aspects of the geologic setting of each deposit gave rise to critical features of the ore-forming fluids and melts.
Regarding the IOA deposits (Buena Vista, Nevada; Iron Springs, Utah; and El Laco, Chile), I report that the inclusion record reveals ubiquitous polycrystalline melt inclusions with compositions ranging from carbonate rich (Buena Vista) through carbonate-sulfate (Iron Springs) to sulfate rich (El Laco). These polycrystalline inclusions represent aliquots of previously unknown, Fe-rich, carbonate-sulfate melts, which appear to be a common, fundamental feature of IOA ore formation. I interpret that these melts were generated by anatexis and assimilation of chemical sediments (limestones and/or evaporites). In each case, the inclusion record shows that these melts exsolved an aqueous fluid (brine and/or vapor), which explains the widespread hydrothermal alteration associated with each deposit. I argue that these previously unrecognized carbonate-sulfate melts represent a key factor in IOA ore formation and explain the overlapping magmatic and hydrothermal features of these deposits.
Regarding the porphyry deposit (Saginaw Hill, Arizona), I report evidence of cyclic fluid exsolution and degassing of a metal-rich brine from a highly-evolved melt at the magmatic-hydrothermal transition. The main intrusive phase at Saginaw Hill displays well-developed unidirectional solidification textures (USTs) that host abundant assemblages of coeval silicate melt and aqueous brine inclusions, indicative of direct exsolution of highly saline brines from the silicate melt. High-resolution, time-resolved data from petrographically well-constrained assemblages of fluid and melt inclusions reveal that the Saginaw Hill system underwent hundreds of repeated pulses of gradual fluid buildup, punctuated by rapid bursts of fluid release. These latter bursts are recorded by quartz with elevated and highly variable trace element contents indicative of rapid (disequilibrium) growth, which contains assemblages of coexisting brine and vapor inclusions indicative of decompression and boiling. This cyclic process of brine exsolution, accumulation, and pressurization followed by rapid decompression, boiling and venting occurred repeatedly, and each pulse led to the formation of an individual UST band. This process took place only after the melt had fractionated extensively- to a muscovite-saturated, near-eutectic composition in an intermediate to felsic silicate system. The highly-fractionated character of the melt provides the best explanation for exsolution of highly-saline, Cu-rich fluids.
While the circulation of aqueous brines is a factor in both porphyry-Cu and Iron oxide-apatite systems, this thesis demonstrates that other factors are likely prerequisites for the formation of mineralization in each. In the case of IOA systems carbonate-sulfate melts are required for iron transport and precipitation while aqueous brines are likely more central in the formation of Na-Ca alteration. In porphyry systems aqueous fluids (i.e. brines) are responsible for metal transport but the interplay of aqueous fluid accumulation and release and melt fractionation are main controls on the volume, salinity, and metal enrichment of mineralizing fluids. Thus, this thesis illuminates key processes in the deep levels of both systems which act as direct controls on the mineralization in both. -
- Graduation date
- Fall 2020
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.