Anhydrite‐Assisted Hydrothermal Metal Transport to the Ocean Floor—Insights From Thermo‐Hydro‐Chemical Modeling

Abstract

High‐temperature hydrothermal venting has been discovered on all modern mid‐ocean ridges at all spreading rates. Although significant strides have been made in understanding the underlying processes that shape such systems, several first‐order discrepancies between model predictions and observations remain. One key paradox is that numerical experiments consistently show entrainment of cold ambient seawater in shallow high permeability ocean crust causing a temperature drop that is difficult to reconcile with high vent temperatures. We investigate this conundrum using a thermo‐hydro‐chemical model that couples hydrothermal fluid flow with anhydrite‐ and pyrite‐forming reactions in the shallow subseafloor. The models show that precipitation of anhydrite in warming seawater and in cooling hydrothermal fluids during mixing results in the formation of a chimney‐like subseafloor structure around the upwelling, high‐temperature plume. The establishment of such anhydrite‐sealed zones reduces mixing between the hydrothermal fluid and seawater and results in an increase in vent temperature. Pyrite subsequently precipitates close to the seafloor within the anhydrite chimney. Although anhydrite thus formed may be dissolved when colder seawater circulates through the crust away from the spreading axis, the inside pyrite walls would be preserved as veins in present‐day metal deposits, thereby preserving the history of hydrothermal circulation through shallow oceanic crust.

Numerical Simulation of the Self‐Organizational Origin of Concentrically Zoned Aggregates of Siderite and Pyrite in Sediment‐Hosted Massive Sulfide Deposits

“…The enrichment of the redox sensitive elements, Mo and V, in CZPA is indicative of an euxinic diagenetic environment that can concentrate these elements either from oxidized hydrothermal fluids or pore fluids (seawater) (Figure S3 in Supporting Information S1) (Tribovillard et al, 2006). In addition, the high concentrations of low temperature chalcophile elements, such as Pb, Bi, Ag, Cu and Sb, in CZPA is consistent with their origin from a low-temperature seafloor hydrothermal fluid that mixed with pore fluids in a diagenetic environment (Figure S3 in Supporting Information S1) (Gu et al, 2007;Guo et al, 2020;Ruhlin & Owen, 1986) (Figure 15). Although the origin of SHMS deposits has long been ).…”

Rock Magnetic Signatures of Hydrothermal Mineralization in the Trans‐Atlantic Geotraverse (TAG) Hydrothermal Field

“…One of the major aftermaths of drilling is the entrainment of seawater into the stockwork (Humphris et al., 1996). At temperatures between ∼150 and 300°C (considering the presence of anhydrite and its role in hydrothermal mineralization; Guo et al., 2020), the addition of sulfate from seawater will oxidize Fe 2+ to Fe 3+ (Shen & Buick, 2004): $$$8{\text{Fe}}^{2+}+10{\mathrm{H}}^{+}+{\text{SO}}_{4}^{2-}\to 8{\text{Fe}}^{3+}+{\mathrm{H}}_{2}\mathrm{S}+4{\mathrm{H}}_{2}\mathrm{O}$$$ …”

“…One of the major aftermaths of drilling is the entrainment of seawater into the stockwork (Humphris et al, 1996). At temperatures between ∼150 and 300°C (considering the presence of anhydrite and its role in hydrothermal mineralization; Guo et al, 2020), the addition of sulfate from seawater will oxidize Fe 2+ to Fe 3+ (Shen & Buick, 2004):…”

Detachment-parallel recharge explains high discharge fluxes at the TAG hydrothermal field

“…Thermodynamic properties of fluids, i.e. water, are calculated using the IAPWS-IF97 formulation 52,53 , that provides the fluid properties as nonlinear functions of temperature and pressure. All the symbols and their physical meanings and typical values can be found in Supplementary Table 1.…”