Abstract
Abstract Sedimentary exhalative (SEDEX) deposits are a subset of sediment-hosted massive sulfide deposits and provide our dominant resource of Zn. In the SEDEX model, base metals (Zn, Pb, Fe) are hydrothermally vented into sulfidic (euxinic) seawater and deposited coevally with the organic-rich mudstone host rock, resulting in laterally extensive layered mineralization. In the Selwyn Basin (Canada) at Macmillan Pass, two deposits (Tom, Jason) are well preserved in a succession of Upper Devonian mudstones and are considered type-characteristic examples of the SEDEX deposit model. As with a number of SEDEX deposits, at Macmillan Pass barite is abundant in the succession hosting hydrothermal mineralization. Early work presented a hydrothermal model for barite formation, in which barite coprecipitated with base metal sulfides in a redox-stratified water column. Recently, however, studies have both proposed an alternative diagenetic model for barite formation and provided more precise constraints on the chemistry of the hydrothermal fluid that entered the vent complexes. Here, we present a new model for Macmillan Pass in which sulfide mineralization occurred entirely within the subsurface. The introduction of hot (300°C) hydrothermal fluids into the shallow subsurface (<1-km depth) resulted in the thermal degradation of organic matter and generated CO2; this promoted barite dissolution, which both provided a source of sulfate for thermochemical sulfate reduction and increased the porosity and permeability within the system. Importantly, there was clear potential for the development of positive feedbacks and self-organization between diagenetic and hydrothermal processes, resulting in highly efficient ore-forming systems. In contrast to the SEDEX model, alteration footprints will be controlled by the mass transfer involved in (barite) replacement reactions rather than hydrothermal venting, and exploration criteria at a district scale should strongly favor highly productive continental margins.
Highlights
Sediment-hosted massive sulfide (SHMS) deposits represent the anomalous enrichment of base metals and sulfur within marine sedimentary rocks (Leach et al, 2005)
As with a number of Sedimentary exhalative (SEDEX) deposits, at Macmillan Pass barite is abundant in the succession hosting hydrothermal mineralization
Work presented a hydrothermal model for barite formation, in which barite coprecipitated with base metal sulfides in a redox-stratified water column
Summary
Sediment-hosted massive sulfide (SHMS) deposits represent the anomalous enrichment of base metals and sulfur within marine sedimentary rocks (Leach et al, 2005). A series of studies investigated different components of the CD-type systems at Macmillan Pass to improve our understanding of the following: (1) the depositional environment and seawater paleoredox (Magnall et al, 2015, 2018); (2) pathways of sulfate reduction during diagenesis (Magnall et al, 2016a); (3) vent fluid chemistry and base metal solubility (Magnall et al, 2016b); (4) and the paragenesis and mineralogical evolution of the layered mineralization (Magnall et al, 2020). Rather than coeval formation of barite and sulfides, we describe how diagenetic barite is overprinted by hydrothermal activity beneath the paleoseafloor (Fig. 3) The implications of this new model are discussed within the broad context of CD-type mineralization in the geologic record and the potential for modern analogues
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