Microscopic evidence for nanoparticle-mediated growth of native gold in sulfide deposits at the Higashi-Aogashima Knoll Caldera hydrothermal field

Abstract

Gold (or electrum) in hydrothermal fluid precipitates directly from gold sulfide complex and/or partly via suspended nanoparticles. The hydrothermal fluid contains "invisible gold" that is atomically dispersed in sulfide minerals or as nanoparticles with a size of less than 10 nm. However, the contribution of these gold nanoparticles to the formation of native gold and its alloy with silver (electrum) remains unclear. The Higashi-Aogashima Knoll Caldera hydrothermal field, south of Tokyo, Japan, is an area of significant seafloor hydrothermal activity that is known for high-grade gold-containing minerals in sulfide-rich rocks. In this study, dry-polished thin sections were created to minimize sample damage and scanning and transmission electron microscopy were used to investigated the cross-sectional and three-dimensional morphologies of native gold grains in a sulfide-rich mound rock from the Central Cone site of the caldera. The surfaces of the gold grains comprised nanoparticles with sizes of 5-50 nm that were also attached to their periphery, which suggests that gold nanoparticles in deep-sea hydrothermal fluid were involved in the mineralization of the gold. In addition, the distribution of silver was uneven within the gold grains, which suggests that the gold precipitation comprised multiple stages at different temperatures that resulted in the post-deposition or secondary remobilization of silver.

Fig 1. Sample collection.

(A) Locations of the CC, SE, and E sites at the HAKC HF. Bathymetric data were obtained by R/V Yokosuka during cruise YK21-10 [26] using a Kongsberg EM122 multi-beam echo sounder at a frequency of 12 kHz. The map was drawn by using the open-source software Generic Mapping Tools Version 6 [27]. (B) Photo of a chimney swarm at the CC site taken during a dive. (C) Photo of the northern flank of a sulfide mound at the CC site taken during a dive. The red circle indicates the sample used in this study.

Fig 2. Polished surface of the sulfide samples.

(A) Reflected-light photomicrograph, (B) BSE-SEM image, and (C) expanded view of the wet-polished section (red box in B). (D) Reflected-light photomicrograph, (E) BSE-SEM image, and (E) expanded view of the dry-polished section (red box in E). Red triangles indicate gold grains. SEM images were taken at (B, C) 2 kV and (E, F) 5 kV. Abbreviations: Brt, barite; Ccp, chalcopyrite; Gn, galena; Opl, opal; Py, pyrite; Sp, sphalerite.

Fig 3. FIB cross-sections of a gold grain obtained from a dry-polished thin section.

(A) Cross-sectional image of Fig 2E. Red dashed lines indicate the boundaries of clear contrast differences within the grain. Red triangles indicate gold NPs on the grain. (B) EDS elemental maps of the same region shown in A. Yellow arrows indicate the relative positions of Ag-rich and Ag-poor domains. (C) Au content of the blue dotted square area in (A). (D) Ag L EDS intensity map divided by Au L intensity map. SEM image at 5 kV and EDS analysis at 20 kV. Mineral abbreviations are as in Fig 2 (Au: gold; V: void).

Fig 4. Three-dimensional reconstruction of serial FIB sections.

(A) Gold grain shown in Fig 3. (B) Slice of A showing galena surrounded by gold and opal. (C) Gold grain from a dry-polished section. (D) Slice from the red box in C. The red triangles indicate a thin layer of chalcopyrite. (E) Gold grain from a wet-polished section (largest one at the top) and NPs with high BSE contrast that is possibly gold (other than the largest one). (F) Slice from the red box in E showing NPs with a high BSE contrast. Colors: yellow, Au and putative Au (in E); green, Ccp; blue, void on/in gold; purple, Opl; brown, Gn; white, void outside gold. Sphalerite in all images and voids in C are not shown for clarity. Mineral abbreviations are as in Figs 2 and 3. See S1–S3 Movies for further details on Fig 4A, 4C, and 4E, respectively.

Fig 5. TEM images of a FIB-milled lamella containing an gold grain adjacent to and/or enclosed in opal.

(A) SEM/EDS of a lamella picked up using FIB onto a carbon grid before milling. (B) BSE-SEM image of the lamella after milling with a Ga ion beam at 5 kV. Orange triangles indicate the same position as those in A. (C) Low-magnification TEM image of the lamella around the area marked by a red triangle in B. (D) Magnified TEM image of the red squared area in C. (E) Magnification of the red square in D. The red lines indicate a lattice spacing of 2.3–2.4 Å. (F) STEM image of D. (G) STEM/EDS spectra of red circles 1–4 in F. Copper, gallium, and carbon were found as artificial signals originating from the copper grid, ion injection during FIB milling, and residual gas adsorbents, respectively. Note the absence of Pt from welding. (H) Elemental composition (wt%) of points in F. Elements from artificial signals were removed from the analysis. Scale bars: 50 nm in D and F, 10 nm in E. Mineral abbreviations are the same as in Figs 2 and 3 (Ele: electrum).