The molecular and isotopic effects of hydrothermal alteration of organic matter in the Paleoproterozoic McArthur River Pb/Zn/Ag ore deposit Kenneth H. Williford a, ⁎ ,1 , Kliti Grice a, ⁎, Graham A. Logan b , Junhong Chen b , David Huston b a Western Australia Organic and Isotope Geochemistry Centre, Department of Chemistry, Curtin University of Technology, Perth, WA 6845, Australia b Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia abstract article info Article history: Received 19 February 2010 Received in revised form 16 November 2010 Accepted 17 November 2010 Available online 10 December 2010 Editor: M.L. Delaney Keywords: HYC sediment-hosted ore deposit deuterium hydrogen carbon isotopes The molecular distribution and compound specific stable carbon and hydrogen isotope ratios were measured on solvent extractable hydrocarbons from the Late Paleoproterozoic McArthur River, or “Here's Your Chance” (HYC) Pb/Zn/Ag ore deposit in the Northern Territory of Australia. Five samples were collected from the McArthur River mine on a northeast–southwest transect in order to sample a gradient of hydrothermal alteration. One sample was taken from the unmineralized W-Fold Shale unit immediately below the HYC ore deposit. δD of n-alkanes, branched alkanes and bulk aromatic fractions were measured, and δ 13 C of n-alkanes, polynuclear aromatic hydrocarbons (PAHs) and bulk kerogen were measured to assess the isotopic effects of a varying degree of hydrothermal alteration on different components of HYC organic matter (OM). Relative to n-alkanes in Barney Creek Formation sediments that did not undergo mineralization, HYC n-alkanes are enriched in deuterium (D) by 50–60‰. This is likely to be a result of equilibrium hydrogen exchange during ore genesis with a highly D-enriched fluid that originated in an evaporitic basin. Trends with distance along the sample transect are ambiguous, but from the northernmost to southernmost point, n-alkanes are less D- enriched, and PAHs are less abundant and less 13 C-enriched. This could be due to decreasing hydrothermal alteration effects, decreasing delivery of highly altered OM by the mineralizing fluid, or both. The carbon isotopic composition of HYC PAHs is inconsistent with a Barney Creek Fm source, but consistent with an origin in the underlying Wollogorang Formation. PAHs are 13 C-depleted relative to n-alkanes, reflecting a kerogen source that was 13 C-depleted compared to n-alkanes, typical for Precambrian sediments. PAHs are more 13 C- depleted with increasing molecular weight and aromaticity, strengthening the case for a negative isotopic effect associated with aromatization in ancient sediments. Together, these data are consistent with a an ore deposition model in which fluids originated in an evaporitic deposit lower in the basin and interacted with metals and OM in the Tawallah Group at temperatures above 250 °C and a depth of ~6 km before ascending along a flower structure associated with the Emu Fault and cooling to 200 ± 20 °C before reaching Barney Creek sediments. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Sedimentary organic matter (OM) is subject to a host of mechanisms such as thermal maturity, hydrothermal alteration and metamorphism that can alter its molecular and stable isotopic composition over geologic timescales (Peters et al., 2005). The 1639±2 Ma (Carr et al., 1996; Page and Sweet, 1998; Symons, 2007) Barney Creek Formation (Fm) in the Northern Territory of Australia contains what may be the least altered OM on Earth older than one billion years (Brocks and Schaeffer, 2008; Jackson et al., 1986). Though much of the Barney Creek Fm is relatively unaltered, the McArthur River, or “Here's Your Chance” (HYC) Pyritic Shale Member within the Barney Creek Fm contains one of the world's five largest clastic sediment-hosted Pb/Zn/Ag ore deposits (Gustafson and Williams, 1981) and is the result of the interaction of hot, metal rich brine with Barney Creek Fm sediments and OM during or after deposition. As such, the HYC ore deposit offers a rare opportunity to study the effects of hydrothermal alteration on otherwise well- preserved and extremely ancient OM. The model for ore deposition is a matter of ongoing debate, and understanding the nature of alteration of OM in the HYC may help to refine the genetic model for this deposit and thereby guide future mineral exploration efforts. Relatively little is known about the isotopic composition of individual organic compounds of Proterozoic age and older, in part due to the rarity of significant quantities of well-preserved bitumen in rocks of this age (Jackson et al., 1986; Waldbauer et al., 2009). A consistent characteristic of Precambrian OM distinguishing it from that preserved in younger sediments is that extractable hydrocarbons are typically enriched in 13 C relative to associated kerogen (Li et al., Earth and Planetary Science Letters 301 (2011) 382–392 ⁎ Corresponding authors. Tel.: +1 912 344 5677; fax: +1 608 262 0693. E-mail addresses: kenwilliford@gmail.com (K.H. Williford), k.grice@curtin.edu.au (K. Grice). 1 Present address: Department of Geoscience, University of Wisconsin, 1215 Dayton St. Madison, WI 53706, USA. 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.11.029 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl