A novel chemical model for burial diagenesis and Zn–Pb sulphide precipitation within the Carboniferous Waulsortian Limestone, Ireland Jack Stacey a, ⁎, Malcolm Wallace a , Christopher Reed b , Conor Moynihan c , William Leonard a , Ashleigh Hood a a School of Geography, Earth and Atmospheric Sciences, University of Melbourne, Victoria 3010, Australia b Teck Australia Pty Ltd., Level 2, 35 Ventnor Avenue, West Perth, Western Australia 6005, Australia c Teck Ireland Ltd., Arklow Business Enterprise Centre, Kilbride Industrial Estate, Arklow, County Wicklow Y14 T440, Ireland abstract article info Article history: Received 30 August 2022 Received in revised form 7 November 2022 Accepted 11 November 2022 Available online 17 November 2022 Editor: Dr. Catherine Chagué Basin-scale processes such as regional fluid flow and organic maturation are commonly associated with the pre- cipitation of Pb–Zn sulphide minerals. Despite extensive study, the origin of carbonate-hosted Pb–Zn mineralisa- tion (Mississippi Valley-type and Irish-type) remains controversial, with deposits viewed as exceptional features derived from unusual basin conditions (e.g. regional hydrothermal fluid flux). In order to explore the links be- tween mineralisation and broader-scale basinal processes, we have examined the relationship of base metal sul- phides to regional diagenetic phases in the Waulsortian Limestone (Feltrim Formation) of the world-class Irish Midlands Zn–Pb province. Using combined sedimentology, petrography, and trace/rare earth element chemistry of regional calcite cements, we have reconstructed the basinal fluid history before, during and after sulphide min- eralisation. The non-bright‐dull cathodoluminescence and trace metal composition of calcite cements indicates that the Waulsortian Limestone was subjected to three major chemical environments during progressive burial diagenesis: 1. Near-surface/shallow-burial oxic conditions; 2. subsurface euxinic conditions; and 3. subsurface ferruginous (ferrosulphidic) conditions. Zn–Pb mineralisation occurred under stage 2 euxinic conditions. We suggest that euxinic conditions were initiated when hydrocarbons entered the Waulsortian succession re- gionally, associated with the introduction of sour gas (H 2 S) accumulations. Mixing between more oxic base metal-bearing and H 2 S-bearing fluids produced ideal conditions for voluminous Zn–Pb sulphide precipitation. In- creasing fluid anoxia then led to iron oxide dissolution with consequently increased iron sulphide precipitation, causing eventual depletion of H 2 S, and marked the onset of ferruginous conditions that terminated metal sulphide precipitation. Base metals were potentially derived from a range of sources that include Carboniferous seawater (and the dissolution of associated Mn/Fe oxyhydroxides), hydrocarbons, and basement-interacted ba- sinal brines. The chemical model proposed by this study is compatible with existing data on the Irish Zn–Pb Orefield and indicates that sulphide mineralisation is linked to normal basin-scale processes such as subsidence, aquifer redox evolution and hydrocarbon generation. These results may explain many other global occurrences of carbonate-hosted Zn–Pb mineralisation and provide new insights into the chemical and redox processes that occur during burial diagenesis. © 2022 Elsevier B.V. All rights reserved. Keywords: Carbonate geochemistry Diagenesis Zn–Pb mineralisation Calcite Hydrocarbons 1. Introduction The global occurrence of Pb–Zn and Fe sulphides in many carbonate successions (Leach et al., 2005, 2010; Paradis et al., 2007) poses funda- mental questions about the chemical and diagenetic processes that occur within the subsurface environment of sedimentary basins. De- spite extensive research, there is little consensus on the processes and conditions under which economically important carbonate-hosted Pb– Zn deposits form. In particular, the genetic relationship of Mississippi Valley-type (MVT) Pb–Zn to ‘Irish-type’ Zn–Pb mineralisation has been the subject of significant debate (Hitzman and Beaty, 1996; Leach et al., 2005, 2010; Paradis et al., 2007; Wilkinson and Hitzman, 2015). Most research on Irish-type sediment-hosted Zn–Pb mineralisation has, for obvious economic reasons, focussed on details of the ore deposits themselves, including deposit structure and stratigraphy (e.g. Johnston et al., 1996; de Morton et al., 2015; Kyne et al., 2019), sulphide petrology and paragenesis (e.g. Peace and Wallace, 2000; Wilkinson, 2003; Doran et al., 2022), fluid inclusion studies (e.g. Banks et al., 2002; Reed and Wallace, 2004; Johnson et al., 2009; Wilkinson, 2010), and lead/sulphur isotope studies (e.g. Caulfield et al., 1986; Sedimentary Geology 442 (2022) 106297 ⁎ Corresponding author. E-mail address: jack.stacey@unimelb.edu.au (J. Stacey). https://doi.org/10.1016/j.sedgeo.2022.106297 0037-0738/© 2022 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo