Irish-type zinc-lead deposits represent a distinctive sub-class of the carbonate-hosted zinc-lead deposit family, having geological features and genetic models that are hybrids between sedimentary exhalative (SEDEX; also known as clastic zinc-lead deposits) and Mississippi Valley-type (MVT) deposits. They are important sources of global zinc and lead production, and the Navan Irish-type deposit (also known as the Tara deposit) has been an important contributor to global metal supply (e.g., ~105 Mt @8.1% Zn and 2%Pb; Ashton et al., 2010).
The paper by Fallick et al. (2001) in Economic Geology argued that the size of the Navan deposit was due to bacteria. Their study utilized sulfur and lead isotopes to understand the sources of sulfur and lead in the deposit. While there had been previous studies on sulfur isotopes in the Navan deposit (e.g., Anderson et al., 1998), these studies were focused on a limited number of samples that were texturally very coarse, and representative to only part of the ores being mined at Navan. The study by Fallick et al. utilized concentrates from large, metallurgical bulk samples from the mine and argued that the concentrates were more statistically representative of the ore deposit than previous samples were. The lead isotopic data for the concentrates was relatively straightforward and suggested the lead (and likely other metals) were derived from hydrothermal fluid leaching of metals from the basement rocks. The sulfur isotopic story, in contrast, was much more interesting.
Previous sulfur isotopic work had illustrated that there were two main populations of sulfur: one population with δ34S that was negative and derived from bacterial sulfate reduction of seawater sulfate (BSR); and a second population with δ34S that was positive (hydrothermal) and derived from thermochemical sulfate reduction of seawater sulfate (TSR)(Anderson et al., 1998). While the previous work illustrated there were two sources of sulfur in the deposits, Fallick et al. quantified the proportions of bacterial versus hydrothermal sulfur in the deposit. Their work illustrated that nearly 90% of the ores had biological sulfur signatures and that the enhanced biological activity within the Navan sedimentary basin was critical to forming the large size of the deposit.
So where do these various sulfur and lead sources fit into a model of how the deposit formed (Figure 1)? The authors build on previous models for the generation of Irish-type (and SEDEX) deposits (e.g., Russell et al., 1981), but add very important additional constraints on deposit forming processes (Figure 1). Hydrothermal circulation cells in an extensional environment resulted in basinal brines (bittern brines) descending and recharging through basement rocks (Figure 1A). During this process the brines leached metals from the basement rocks (Figure 1A). Additionally, seawater sulfate in the brine was reduced via thermochemical sulfate reduction as the brines interacted with basement rocks (Figure 1). These processes resulted in a hydrothermal fluid that was saline, warm (90o-270oC; Wilkinson et al., 2010), metal-rich, with reduced hydrothermal sulfur (i.e., H2S; Figure 1). Coincident with hydrothermal fluid generation was ongoing bacterial sulfate reduction at the sediment-water interface within the Navan sedimentary basin, which resulted in abundant reduced sulfur (i.e., bacterial H2S) and cool seawater (<25oC?) to be present in pore spaces in host carbonate rocks (Figure 1). The upwelling of the saline hydrothermal fluids along synsedimentary basement faults resulted in the mixing of the warm, metal- and hydrothermal H2S-bearing hydrothermal fluids with the cooler, bacterial H2S-bearing near-surface pore fluids (Figure 1B). The mixing of these two fluids resulted in rapid cooling and dilution of the metal-bearing brine (Figure 1B), and the complexing of metals with both hydrothermal and bacterial H2S to form the ore minerals sphalerite (ZnS) and galena (PbS):
PbCl2(aq) + H2S(aq) = PbS(s) + 2HCl(aq) (galena formation); and
ZnCl2(aq) + H2S(aq) = ZnS(s) + 2HCl(aq) (sphalerite formation).
Figure 1. Generalized model for the generation of mineral deposits in the Irish Midlands. A) Schematic cross section illustrating the circulation of hydrothermal fluids through basement rocks. This circulation of fluids resulted in leaching of metals and the reduction of seawater sulfate the sulfide (H2S) by thermochemical sulfate redution (i.e., generation of hydrothermal sulfur). B) Three-dimensional cartoon representation of the ore forming environment with basement faults, seafloor topography, and potential mixing processes between various fluids. C. Representation of paleogeography in the Irish Midlands with deposits (red areas) associated with fault controlled shelves/islands where strongly evaporated bittern brines could be generated. From Wilkinson et al. (2011).
So how did bacteria result in the Navan deposit being so large? The answer for this comes from the nature of seafloor hydrothermal vents, which the Irish-type deposits clearly were (e.g., Boyce et al., 1983). Hydrothermal vents are extremely inefficient at forming sulfide mineralization. In fact, the majority of metals present in hydrothermal vents go “up in smoke” and are not precipitated as sulfides within the sulfide chimneys (e.g., Converse et al., 1984). One of the major reasons for this is that the hydrothermal vent fluids are generally deficient in the H2S required to precipitate the metals as sulfide minerals that form sulphide chimneys (e.g., sphalerite, galena, pyrite, chalcopyrite). In the case of Navan, there was abundant bacterial H2S present at the site of deposition; therefore, much of the metal that would normally go “up in smoke” complexed with the bacterial H2S and formed sulfide mineralization. Therefore, without bacterial sulfate reduction and bacteria present in the Navan sedimentary basin it is likely the Navan deposit would have been a much smaller deposit!
The paper is also a great example of how a title can have great impact on the reader. Furthermore, this paper has one of the best lines at the end of the abstract that explains the essence of the manuscript: “…..no bacteria, no giant ore deposit.” A great example of impact-oriented writing.