Author: Fischer, William
Tourmaline breccia pipes (TBPs) are unique geologic structures hosting mineralization in contrasting intrusion-related ore systems, specifically, porphyry Cu deposits (PCDs) and Sn-W intrusion-related systems. Regardless of deposit affinity, similar formational processes result in TBPs exhibiting similar physical and chemical features making it difficult to determine deposit affinity. To aide in distinguishing ore deposit affinity, new criteria for distinguishing between porphyry- and non-porphyry-related TBPs is established based on U-Pb zircon dating, trace-element zircon chemistry, tourmaline chemistry, and B-isotopes. These criteria are then tested on the A.M. breccia. The A.M. breccia is part of the Giant Copper porphyry system. It shares similarities with other TBPs, most notably those of Chile and Peru. These similarities include, concentric zonation of breccia textures, ore distribution, vertically-zoned alteration and the presence of vapor-rich and hypersaline fluid inclusions. The geologic characteristics and age of the A.M. breccia demonstrate similarities with other Cascadia age porphyry-type mineralization and TBPs globally. An LA-ICPMS U-Pb zircon date of 24.9 ± 0.2 Ma (2σ) for the Invermay intrusive suite, the intrusion responsible for breccia pipe formation, is consistent with Ancestral Cascadia Phase 2 magmatism (30-18Ma) and Cascaida PCDs. Trace element concentrations of Invermay zircon indicate redox state, water content and fractionation metrics that correlate well with known PCDs. Tourmaline at the A.M. breccia have major-, minor-, and trace-element compositions aligned with tourmaline from PCDs globally. Tourmaline in PCDs is alkali-rich with elevated Fe, Mg, Ti, and Ca, reflecting a metaluminous magma composition. Sn-W intrusion-related tourmaline is alkali- to X-site vacant-rich and contain high concentrations of Al, F, Si, Mn and X-site vacancies and reflect a peraluminous composition. Boron isotopes for A.M. tourmaline range from -8.1‰ to 5.6‰, indicating a MORB boron source, I-type affinity, strong correlation with boron isotope data from PCDs globally, and agreement with the Ancestral Cascadia model. Tourmaline sub-species vary based on their spatial location withinthe A.M. breccia, enhancing the efficiency of tourmaline as a mineral vector. Spectral reflectance also differentiates tourmaline associated with ore and ore-bearing breccia textures from tourmaline occurring distal to the pipe contact or within barren TBPs.
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Thesis advisor: Marshall, Daniel
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