Massive sulfide Zn deposits in the Proterozoic did not require euxinia

doi: 10.7185/geochemlet.2008 Our most important Zn resources occur within clastic-dominated (CD-type) deposits, which are located in a small number of Proterozoic and Phanerozoic sedimentary basins. The most common model for CD-type mineralisation involves sedimentary exhalative (SEDEX) processes, i.e. the venting of metal bearing fluids into a restricted, anoxic H2S-bearing (euxinic) water column. In the Carpentaria Zn Province (Australia), multiple world class deposits are hosted in Proterozoic (1.6 Ga) stratigraphy, where models of the ancient sulfur cycle have also been developed. Focusing on the most recent discovery – the Teena deposit – we report bulk rock and isotopic data (δSpyrite values) that provide information on the sulfur cycle during the diagenetic and hydrothermal evolution of the Teena sub-basin. In contrast to the SEDEX model, intervals containing abundant pyrite with highly positive δ34S values (>25 ‰) correspond with euxinic conditions that developed due to high organic loading (i.e. productivity) and not basin restriction. This basin wide feature, which can also be mistaken as a hydrothermal pyrite halo, is genetically unrelated to the subsequent hydrothermal mineralisation that formed beneath the palaeo-seafloor. The formation of CD-type deposits in the Proterozoic does not, therefore, require euxinic conditions. Received 31 July 2019 | Accepted 27 January 2020 | Published 6 March 2020 1. GFZ German Research Centre for Geosciences, 14473 Potsdam, Germany 2. Institute of Geological Sciences, Freie Universität, 74-100 Malteserstrasse, 12249 Berlin, Germany 3. Teck Australia Pty Ltd. Australia * Corresponding author (email: joseph.magnall@gmail.com) Introduction The Proterozoic Carpentaria Zn Province contains the largest accumulation of sediment-hosted base metals in the earth’s crust (nearly 120 Mt Zn and Pb; Huston et al., 2006). Deposits of the northern Carpentaria Province are of particularly broad interest, as they are hosted within one of the few low metamorphic grade marine sedimentary basins from the Proterozoic. Since the early 20th century, world class (upper 10th centile contained metal) clastic-dominated (CD-type) deposit discoveries in the Carpentaria Province have averaged one per 20 years (McGoldrick et al., 2010). Yet with increased urbanisation and decarbonisation the global demand for Zn will continue, and some models predict a peak in Zn production within 20 years (Mohr et al., 2018). As the largest Zn reserves and resources occur in CD-type deposits, it is necessary to critically evaluate and improve existing genetic models to enhance the predictive capacity of exploration programmes. A restricted number of sedimentary basins in 2 broad time intervals (midProterozoic and Palaeozoic) are known to host the largest CD-type deposits (Leach et al., 2010). The most widely applied genetic model involves sedimentary exhalative (SEDEX) processes, where a sulfidic (euxinic) water column provides the in situ trap for hydrothermally vented metal-bearing fluids; the stratiform textures that are characteristic of CD-type deposits are then thought to result from the direct precipitation of sulfide minerals (pyrite, sphalerite, galena) from euxinic seawater (e.g., Large et al., 1998). Although models of Proterozoic ocean chemistry have been refined over recent decades – from the Canfield Ocean of widespread euxinia (Canfield, 1998) to more recent models in which euxinia is more spatially and temporally restricted (Poulton and Canfeld, 2011) – the SEDEX paradigm persists and mineralisation is frequently taken as a priori evidence that euxinic conditions were prevalent (e.g., Johnston et al., 2008). In many cases, observations on the distribution and isotopic composition of pyrite provide the foundations for hydrothermal and palaeoenvironmental models (e.g. Lyons et al., 2000; Ireland et al., 2004). In this study, we focus on the most recent discovery in the Carpentaria Province – the Teena Zn-Pb deposit (Fig. S-1). Petrographic and geochemical data, collected through more than 600 m of stratigraphy, are presented for both mineralised and correlative un-mineralised samples. We demonstrate a high degree of variability in the overall abundance and isotopic composition (δ34S value) of non-hydrothermal pyrite and show that euxinic conditions developed in response to high productivity during periods of Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2020) 13, 19-24 | doi: 10.7185/geochemlet.2008 20 high relative sea level. Contrary to the SEDEX model, hydrothermal pyrite has a much more restricted distribution and formed via sub-seafloor replacement processes. For exploration programmes, this means that the detectable footprint of CD-type deposits is more limited than predicted by the conventional SEDEX model.