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Sediment-hosted Zn-Pb-Ag mineralization in the Red Dog district, Alaska, USA: pre-ore environments and mineralizing processes
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- Author / Creator
- Reynolds, Merilie A.
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Sediment-hosted massive sulfide (SHMS) deposits are an important source of global zinc resources, and the Red Dog Pb-Zn-Ag district in Alaska (USA) contains giant deposits of this type. The existing model for ore formation at Red Dog involves early diagenetic replacement of sediment deposited in a restricted basin with stratified suboxic bottom waters. We present new observations of trace fossils Schaubcylindrichnus ichnospecies (isp.) and Chondrites isp. in several Red Dog deposits. The presence of the trace fossils, the size of the largest burrows, and the pervasiveness of the ichnofabric indicate that at least some intervals of the host sediment were deposited in an oxygenated middle to outer shelf environment. To reconcile these data with those from previous regional sedimentological and lithogeochemical studies, we propose a new model whereby the ore-hosting sediment was deposited in a shelfal setting in which redox conditions were affected by a fluctuating oxygen minimum zone. The strong spatial correlation between bioturbation and Red Dog SHMS deposits suggests that the presence of trace fossils may have played an important role in controlling the flow of ore-forming fluids by increasing host sediment permeability.The Anarraaq area of the Red Dog district contains a barren barite body with as much as 1 Gt of barite and a sulfide deposit with an inferred resource of 19.4 Mt at 14.4% Zn, 4.2% Pb, and 73.4 ppm Ag. Rhenium-osmium isochron ages of organic-rich mudstone overlying the sulfide deposit (339.1 ± 8.3 Ma), pre-ore diagenetic pyrite in the sulfide deposit (333.0 ± 7.4 Ma), and hydrothermal pyrite (334.4 ± 5.3 Ma) are all within uncertainty of each other and of the Re-Os isochron age of the Main deposit in the Red Dog Mine Area (338.3 ± 5.8 Ma; Morelli et al., 2004). This indicates that hydrothermal mineralization was broadly synchronous on a regional scale and that mineralization was approximately coincident with sedimentation.We present an updated stratigraphic, structural, and paragenetic framework for the Anarraaq area based on detailed logging, thin-section microscopy, and the construction of two cross sections. Importantly, this work reveals that Anarraaq contains three discrete intervals of abundant pre-ore barite, carbonate, and pyrite in the organic-rich mudstone of the Mississippian Kuna Formation (Ikalukrok unit). One of these intervals – the Anarraaq barite body – is barren; the other two were later overprinted by Zn-Pb-Ag mineralization and comprise the Anarraaq sulfide deposit. Textural observations indicate that the carbonate and pre-ore pyrite are diagenetic (not sedimentary and hydrothermal, respectively, as previously interpreted) and their spatial relationship to barite (previously interpreted to form at a cold methane seep) suggest that all three minerals precipitated in association with methane-related diagenetic processes.In situ isotopic data (secondary ion mass spectrometry; SIMS) from barite (δ18O and δ34S), pyrite and marcasite (δ34S) in the Anarraaq area indicate that much of the pre-ore barite formed in the very shallow subsurface where rates of bacterial sulfate reduction (BSR) were high and produced relatively small fractionations and minor reservoir effects. At the same time, BSR also took place farther below the seafloor where the dissolution of barite in methanic pore waters provided a secondary source of sulfate. The fractionation produced during this process was also small. In both settings, BSR was likely coupled to the anaerobic oxidation of methane (i.e., sulfate-driven anaerobic oxidation of methane); carbonate and pyrite are common byproducts of this process. The introduction of hydrothermal fluids generated zones of high porosity and permeability as they dissolved diagenetic carbonate. Hydrothermal pyrite and marcasite in the Anarraaq sulfide deposit display a wide range of δ34S values with local small-scale spatial-temporal heterogeneity. This may reflect 1) mixing between two reservoirs of diagenetic (biogenic) bisulfide with distinct S isotopic compositions or 2) bacterial, and possibly other types of sulfate reduction of a single reservoir of hydrothermal sulfate under conditions of highly variable sulfate and reductant availability. Overall, microbial activity and methane played important and complex roles in the formation of high-grade Zn-Pb-Ag ore in the Anarraaq sulfide deposit. We suspect that methane-related alteration of host strata may have been a crucial pre-condition for the emplacement of all SHMS mineralization in the Red Dog district.
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- Subjects / Keywords
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- Graduation date
- Spring 2019
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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.