How carbonate dissolution facilitates sediment-hosted Zn-Pb mineralization

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.

Simultaneous replacement of plagioclase by albite and K-feldspar: natural evidence and hydrothermal experiments

<p>Replacement of feldspars occurs ubiquitously during fluid-rock interaction in crusts, and the formation of micro- to nano- pores along with the replacement potentially provides significant impacts on hydrological properties within the crust (e.g. Plümper et al., 2017; Yuguchi et al., 2019). In this contribution, we report the novel texture of the plagioclase replacement by K-feldspar and albite and showed the conditions of such replacement. The mafic schists near the pegmatitic quartz diorite within the Kinkasan Island, NE Japan show extensive feldspar alteration at various stages, involving Na-rich and K-rich fluids, respectively. Interestingly, during the later K-rich fluid infiltration at 400-570 ˚C at 0.3–0.45 GPa, plagioclase (An35-60) was replaced by K-feldspar (An0Ab1Or99) and albite (An4Ab94Or2) intergrowth, meaning that simultaneous K-feldspathization and albitization, and nano- to microscale pore network developed preferentially along with albite, resulting in an increase of the bulk rock porosity up to 1.34±0.14%.</p><p>To understand the relationship between K-feldspar and albite formations within the same plagioclase grain, we conducted the hydrothermal experiments on the feldspar replacement by using different pairs of starting minerals (anorthite, An96Ab4; labradorite, An66Ab33Or1; albite, An1Ab99) and fluid compositions (2M KCl and/or NaCl aqueous solutions) for 4-8 days. AIn all runs, the replacement processes of feldspars developed the distinct reaction front and pores formation close to the reaction front with porosity up to ~7%. In the experiments with KCl solution, the reaction front migrated twice faster than those with the mixture of KCl and NaCl. The most intense replacement occurred in the run of Labradorite-KCl solution, where large cavities were formed in the center of the labradorite grain with developing albite exsolution, and homogenous rim of K-feldspar precipitation. Such occurrences are similar to the replacement texture observed in the mafic schist within the Kinkasan Island and suggest the preferential removal of Ca and the fixed Na during K-feldspar formation. Our experimental results indicate the primary controls of the fluid composition on the replacement texture, pore formation, and the reaction rate.</p><p>Keywords feldspar replacement, micropores, fluid transport, hydrothermal experiment, Kinkasan</p>

A simple and effective capsule sealing technique for hydrothermal experiments

Capsule sealing has always been a key procedure in hydrothermal experiments to explore the composition and properties of geo-fluids and their influence on various geological processes. Previously reported capsule sealing techniques have primarily focused on either weld-sealing or cold-sealing methods, which have some disadvantages and limitations. Here, we report on a newly developed, simple, and effective capsule sealing technique incorporating operations from the cold-sealing and weld-sealing techniques. The technique includes three steps: first, preparing inner and outer tubes, both with a flat bottom at one end; subsequently, reverse-buckling the tubes to form a preliminary seal; and finally, welding shut the tiny slit at one end of the tubes. The new capsule sealing technique was tested in experiments for fluid inclusion synthesis. Fluid inclusions were successfully synthesized in 10 runs over a range of conditions (800~900 °C, 1~1.5 GPa). Considering the insignificant mass changes recorded and the occurrence of free fluid from the recovered capsules, the new capsule sealing technique was proven to be reliable. The simple and effective capsule sealing technique has the following advantages over the previous techniques. First, the capsule sealing technique is simple, effective, and easy to operate. The technique does not require a capsule body and lid with a complex structure, nor does it require dies or special tools. The critical weld-sealing operation is easier to complete due to the narrow and uniform slit surrounded by more metal, during which loss of volatilization is prevented by the preliminary seal. Second, the capsules can be sealed with uniform thickness and regular shape, prechecked for leakage in an oven, and annealed under high temperature and high pressure with less deformation, which could improve the success rate of experiments. Third, the theoretically required capsule materials can be changed (such as precious metals, alloys, etc.), as can the dimensions required to construct a capsule with the desired size and wall thickness (large volume or thick wall). Thus, sealed capsules are suitable not only for piston cylinders but also for multi-anvil presses and other gas-media or hydrothermal-media apparatus, such as autoclaves and pressure vessels, which means a wider range of temperatures and pressures are accessible and thus more fields of application.