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[SCG53-02] Gold precipitation mechanism via nano-colloidal grain at the Higashi-Aogashima Knoll Cardella hydrothermal field
Keywords:invisible gold, seafloor hydrothermal deposit, Higashi-Aogashima Knoll Cardella, Izu-Bonin Arc, dry polishing, FIB-SEM
Gold is usually found in quartz or sulfide minerals included in hydrothermal deposits and placer deposits as either native gold or electrum, the latter being an alloy of gold and more than 20% of silver. The gold and/or electrum grain is thought to be formed by reduction of atomically dissolved form such as [Au(SH)2]– and [AuCl2]– into colloidal nanoparticles by boiling of hydrothermal fluid. The colloidal particles, whose size ranging from 2 nm to several hundreds of nm and called as “invisible gold”, are found as inclusions in carbon, quartz, and arsenian pyrite, or as nanoparticles in hydrothermal fluid itself. However, it has been still unclear how these “invisible gold” contribute to formation of micrometer-sized gold grains. Here we investigated a Au-rich mound rock of Central Cone (CC) Site, Higashi-Aogashima Knoll Cardella (HAKC) hydrothermal filed, which is known to have 102 and 432 ppm of Au and Ag, respectively by previous study, so as to find out the evidence of gold growth mediated by colloidal electrum particles with a partial loss of Ag.
Aogashima is the southmost island in Izu Islands, Tokyo Prefecture, Japan, and HAKC is located at ca. 12 km east offshore. There have been found four hydrothermal sites within this caldera, and the CC Site is the richest in Au and Ag. The mound rock was collected by remotely operated vehicle Hyper-Dolphin, and thin sections were prepared on a glass slide.
The mound rock is primarily comprised by sulfide and sulfate minerals such as sphalerite (Sp), barite (Brt), galena (Gn), chalcopyrite (Ccp), pyrite (Py) together with gangue amorphous silica with very weak birefringence (opal, Opl) and unidentified clay minerals. Typical size of gold was around 10 μm and most frequently found at the marginal part of sphalerite or at the interface between sphalerite and opal. The mineralogical information was identified by optical microscopy, while the size of gold is ambiguous because gold is highly ductile and elongated during water-assisted high-rate polishing process. Thus, we made a polished section without any lubricant and water with minimized vertical force to suppress elongation of gold grains to observe the original texture of gold. Voids (V) were found preferentially around the opal side of the gold grains in the dry-polished sections, which would be the path of hydrothermal fluid. The Au content in the gold grains were more than 90% by EPMA and SEM-EDS analyses.
Three-dimensional morphology was investigated using serial sectioning by focused ion beam (FIB)-SEM (see Fig. A, B). We found an automorphic galena particle within gold, a thin layer of chalcopyrite covering void-faced gold, and clay minerals on void-faced gold, indicating that the gold growth took place at relatively low temperature, and that the temperature of hydrothermal fluid was not constant/stable. We also found a nanoparticulate gold of ca. 20 nm attached on a gold grain. The inhomogeneous distribution of Ag was revealed by SEM-EDS measurements, where Ag is localized at opal or void-faced side (Fig. B). The structure of the edge of gold grain was further investigated by fabricating TEM lamella in FIB-SEM and analyzed by TEM and STEM-EDS. Nanoparticles of ca. 10 nm in size were attached at the periphery of gold grains forming ca. 50 nm layer, and the nanoparticles contained up to 40% of Ag with Au. On the other hand, the Ag content decreases below 20% below the electrum layer. These results indicate that the colloidal gold initially formed as electrum nanoparticles agglomerate to form gold grains, and Ag was removed possibly as sulfides during gold growth due to higher solubility of Ag than Au in sulfidic solution. Ag was also found as acanthite (Ag2S) within opal, indicating that Ag precipitate separately from gold.
This study was supported by JSPS KAKENHI JP23K13211, JP23H03812 and JP23H01914, JSPS Bilateral Program JPJSBP120229945, and Canon Research Foundation.
Figure. (A) Three-dimensional reconstruction of a gold grain and surrounding minerals. (B) A slice of (A) and EDS analysis. Red triangles indicate attached colloidal particles, and yellow arrows are for the aid of positioning between images. Scale bars, 2 μm.
Aogashima is the southmost island in Izu Islands, Tokyo Prefecture, Japan, and HAKC is located at ca. 12 km east offshore. There have been found four hydrothermal sites within this caldera, and the CC Site is the richest in Au and Ag. The mound rock was collected by remotely operated vehicle Hyper-Dolphin, and thin sections were prepared on a glass slide.
The mound rock is primarily comprised by sulfide and sulfate minerals such as sphalerite (Sp), barite (Brt), galena (Gn), chalcopyrite (Ccp), pyrite (Py) together with gangue amorphous silica with very weak birefringence (opal, Opl) and unidentified clay minerals. Typical size of gold was around 10 μm and most frequently found at the marginal part of sphalerite or at the interface between sphalerite and opal. The mineralogical information was identified by optical microscopy, while the size of gold is ambiguous because gold is highly ductile and elongated during water-assisted high-rate polishing process. Thus, we made a polished section without any lubricant and water with minimized vertical force to suppress elongation of gold grains to observe the original texture of gold. Voids (V) were found preferentially around the opal side of the gold grains in the dry-polished sections, which would be the path of hydrothermal fluid. The Au content in the gold grains were more than 90% by EPMA and SEM-EDS analyses.
Three-dimensional morphology was investigated using serial sectioning by focused ion beam (FIB)-SEM (see Fig. A, B). We found an automorphic galena particle within gold, a thin layer of chalcopyrite covering void-faced gold, and clay minerals on void-faced gold, indicating that the gold growth took place at relatively low temperature, and that the temperature of hydrothermal fluid was not constant/stable. We also found a nanoparticulate gold of ca. 20 nm attached on a gold grain. The inhomogeneous distribution of Ag was revealed by SEM-EDS measurements, where Ag is localized at opal or void-faced side (Fig. B). The structure of the edge of gold grain was further investigated by fabricating TEM lamella in FIB-SEM and analyzed by TEM and STEM-EDS. Nanoparticles of ca. 10 nm in size were attached at the periphery of gold grains forming ca. 50 nm layer, and the nanoparticles contained up to 40% of Ag with Au. On the other hand, the Ag content decreases below 20% below the electrum layer. These results indicate that the colloidal gold initially formed as electrum nanoparticles agglomerate to form gold grains, and Ag was removed possibly as sulfides during gold growth due to higher solubility of Ag than Au in sulfidic solution. Ag was also found as acanthite (Ag2S) within opal, indicating that Ag precipitate separately from gold.
This study was supported by JSPS KAKENHI JP23K13211, JP23H03812 and JP23H01914, JSPS Bilateral Program JPJSBP120229945, and Canon Research Foundation.
Figure. (A) Three-dimensional reconstruction of a gold grain and surrounding minerals. (B) A slice of (A) and EDS analysis. Red triangles indicate attached colloidal particles, and yellow arrows are for the aid of positioning between images. Scale bars, 2 μm.