How carbonate dissolution facilitates sediment-hosted Zn-Pb mineralization

Abstract

Most of the world’s Zn and Pb is extracted from sediment-hosted Zn-Pb deposits. The Zn-Pb deposits hosted in carbonate rocks are hypothesized to form by mixing of acidic metal-bearing brines with reduced sulfur-bearing fluids while dissolving sedimentary carbonate. To test the role of carbonate in this process, we conducted hydrothermal experiments simulating ore formation by reacting Zn ± Pb ± Ba–bearing brines with H2S and SO42– produced by native sulfur, with and without carbonate minerals (calcite or dolomite crystals), at 200 °C and water-saturated pressure. Sphalerite, galena, and barite (or anhydrite) crystals formed only when carbonate was present in the experiment, accompanied by carbonate dissolution. The textures of sphalerite clusters are similar to those observed in ancient and modern hydrothermal deposits. Thermodynamic modeling at 150 °C and 250 °C demonstrates that mixing of metal-rich brines and H2S causes most of the Zn in solution to precipitate as sphalerite only when carbonate dissolution occurs to buffer the pH, consistent with the experimental observations. The need for a pH buffer increases with increasing temperature, and different pH buffers may play a role for different deposit types. We propose that carbonate-buffered fluid mixing is a critical process for forming post-sedimentary Zn ± Pb ± Ba deposits in sedimentary carbonate rocks.

An EBSD – EPMA Investigation of Synthetically Produced Sulfide Ores

“…The sphalerite also grows in globular clusters, examples of which are shown in Figure 2(a) in an unpublished image from previous research by the current authors [2]. That research found that the centre of the globular clusters was nanocrystalline and porous and, with respect to EBSD, did not diffract, whereas the crystals around the outer edge of the clusters were fully dense and diffracted well.…”

“…These valuable metals can be recovered as a by-product of refining the Zn-Pb ore. To improve and optimise this process, a greater understanding of the precise mechanism by which Zn-Pb minerals incorporate trace elements is needed. To this end, Liu et al [2] recently developed a novel laboratory method of hydrothermally synthesising the formation of Zn-Pb minerals via a carbonate replacement mechanism that simulates their formation in real sediment hosted systems. Liu [2] demonstrated the role carbonate plays in carbonate-replacement models of ore genesis previously proposed in the literature [3].…”

“…To this end, Liu et al [2] recently developed a novel laboratory method of hydrothermally synthesising the formation of Zn-Pb minerals via a carbonate replacement mechanism that simulates their formation in real sediment hosted systems. Liu [2] demonstrated the role carbonate plays in carbonate-replacement models of ore genesis previously proposed in the literature [3]. This technique can also be applied to the study of speciation and trace element incorporation into sediment hosted Zn-Pb deposits.…”

“…Here, we seek to compare the trace element distribution, phase morphology and mineralogical associations of Zn-Pb sulphide minerals hydrothermally produced with and without trace element additions of Fe and Ba in a process that closely simulates a real carbonate-replacement, sedimentary hosted system. Fe and Ba have been introduced into Zn-Pb sulphide minerals using the synthesis technique of Liu, the experimental details of which are reported in [2]. The carbonate used for the experiment was calcite.…”

Stratiform Host-Rock Replacement via Self-Sustaining Reactions in a Clastic-Dominated (CD-type) Zn Deposit

“…It has been suggested that acidity (pH ≈ 2) was an intrinsic feature of the hydrothermal fluids responsible for CD-type mineralization in the McArthur basin (e.g., Fig. 3, green arrow; Liu et al, 2021;Spinks et al, 2021). Most acidic ore-forming fluids are associated with the disproportionation of magmatic volatiles and result in acid sulfate mineral assemblages (e.g., Seewald et al, 2019); however, there is no evidence of coeval magmatism or of the development of widespread acid sulfate assemblages in the McArthur basin.…”