Carbonate Replacement as the Principal Ore Formation Process in the Proterozoic McArthur River (HYC) Sediment-Hosted Zn-Pb Deposit, Australia

Abstract

The McArthur River (HYC) Zn-Pb-Ag deposit in the Carpentaria Zn belt, northern Australia, is one of the world’s largest and most studied sediment-hosted base metal deposits, owing to its lack of deformation and preservation of sedimentary and ore textures. However, the ore formation process (syngenetic vs. epigenetic) is still a subject of controversy. In this paper we focus on key characteristics of the HYC deposit that remain unexplained: preservation of sedimentary carbonate (dolomite) and its association with Zn, and the role of thallium (Tl) and manganese (Mn) distribution in the orebody. Our findings demonstrate a sequence of events during ore formation: Tl is hosted almost exclusively within euhedral pyritic overgrowths around early diagenetic pyrite; sphalerite mineralization occurred after Tl-bearing pyrite overgrowths, in association with acid dissolution (replacement) of laminated and nodular dolomite across the subbasin; and outer rims are enriched in Mn on preserved dolomite at the dissolution reaction front in contact with sphalerite. New thermodynamic fluid chemistry modeling demonstrates the metal distribution and paragenesis can be explained by acidic, oxidized ore fluids entering the pyrite-dolomite host lithology, allowing reduction and pH buffering by acid carbonate dissolution, resulting in stepwise metal deposition in an evolving fluid. We argue this represents strong evidence for epigenetic ore formation at HYC. Furthermore, the primary control on ore deposition is not synsedimentary faulting in the subbasin; rather, the chemical potential of sedimentary carbonate within reduced, sulfidic lithologies appears to be of critical importance to precipitation of sphalerite.

How carbonate dissolution facilitates sediment-hosted Zn-Pb mineralization

“…The modeling results show that a simple mixing of Zn-bearing (∼1000 ppm Zn) brines with H 2 S of as much as 0.2 m under acid or near-neutral pH conditions would not effectively precipitate sphalerite at 150-250 °C. This is because the partial precipitation of sphalerite decreases the fluid pH and stops the ore deposition due to the high solubility of sphalerite in acidic solutions, as shown in Cooke et al (2000) and Spinks et al (2021). The much higher degree of galena precipitation in the carbonate-free experiments is due to the lower solubility of galena compared to sphalerite (e.g., Etschmann et al, 2018).…”

“…Sediment-hosted Zn-Pb deposits, such as Mississippi Valley-type (MVT), Irish-type, and sedimentary exhalative (SEDEX) (e.g., Large et al, 2005;Leach et al, 2005;Wilkinson, 2014), are the most important Zn and Pb resources globally. Different ore-genesis models have been proposed for different classes of these deposits: Large et al (1998Large et al ( , 2005 proposed a syn-sedimentary-exhalative origin for the giant HYC deposits (McArthur River, Australia), while Perkins and Bell (1998) and Spinks et al (2021) identified key features of post-sedimentary mineralization and carbonate replacement by metal sulfide minerals. A carbonate replacement model is proposed for the Irish-type (e.g., Wilkinson, 2014;Wilkinson and Hitzman, 2015) and MVT-type deposits (e.g., Corbella et al, 2004).…”

The Teena Zn-Pb Deposit (McArthur Basin, Australia). Part II: Carbonate Replacement Sulfide Mineralization During Burial Diagenesis—Implications for Mineral Exploration

“…There has been considerable debate over the sulfide paragenesis at the McArthur River (HYC) deposit, and specifically the relative timing of fine-grained diagenetic pyrite and sphalerite mineralization (summarized by Large et al, 1998). For example, previous researchers either favor models in which the bulk of the Zn mineralization is syn-py1, consistent with a SEDEX model (e.g., Large et al, 1998;Ireland et al, 2004) or post-py1, which is consistent with subseafloor replacement (Williams, 1978;Eldridge et al, 1993;Spinks et al, 2021).…”

“…Using this reference value there is no evidence of carbonate loss, yet it does not preclude the possibility that the interbedded siltstones are unmineralized precisely because of a lack of preexisting nodular carbonate. Indeed, recent work on the ore zone at the McArthur River (HYC) deposit has now provided more convincing petrographic evidence of widespread sulfide replacement of carbonate (Spinks et al, 2021).…”

“…Contrary to the SEDEX model, many researchers have made petrographic arguments that CD-type deposits formed via replacement processes during diagenesis (Grondijs and Schouten, 1937;Williams, 1978;Magnall et al, 2020a;Spinks et al, 2021). Furthermore, recent studies have shown that the protolith of CD-type deposits accumulated under a range of depositional redox conditions, albeit with minimal evidence of prolonged euxinia (Reynolds et al, 2015;Magnall et al, 2018Magnall et al, , 2020b.…”