Sulphate-reducing bacteria-mediated pyrite formation in the Dachang Tongkeng tin polymetallic deposit, Guangxi, China

Abstract

Mediation by sulphate-reducing bacteria (SRB) is responsible for pyrite (FeS₂) formation. The origin of the Dachang tin polymetallic ore field is related to the mineralisation of submarine hydrothermal vent sediments. Here, we investigated SRB in these ores via morphological, chemical, and isotopic analyses. Polarised and scanning electron microscopy indicated that trace SRB fossils in the metal sulphide ore were present in the form of tubular, beaded, and coccoidal bodies comprising FeS₂ and were enclosed within a pyrrhotite (FeS) matrix in the vicinity of micro-hydrothermal vents. The carbon (C), nitrogen (N), and oxygen (O) contents in the FeS₂ synthesised by SRB were high, and a clear biological Raman signal was detected. No such signals were discerned in the peripheral FeS. This co-occurrence of FeS, FeS₂, and the remains of bacteria (probably chemoautotrophic bacteria) was interpreted as the coprecipitation process of SRB-mediated FeS₂ formation, which has, to the best of our knowledge, not been reported before. Our study also illustrates that combined energy-dispersive X-ray spectroscopy, Raman spectroscopy, and isotopic analysis can be used as a novel methodology to document microbial-mediated processes of mineral deposition in submarine hydrothermal vent ecology on geological time scales.

Figure 1

Geotectonic location and ore deposit profile. (a) Geotectonic location of the Dachang Sn polymetallic ore field in Guangxi (Modified from ref.). (b) Geological profile of the Tongkeng Sn polymetallic deposit (Modified from ref.). This map was created using Adobe Illustrator 2020.

Figure 2

Microtextural characteristics of pyrite (FeS₂) and related mineralogy. Tubular, beaded, and spherical SRB traces composed of FeS₂ were distributed within pyrrhotite (FeS). (a) Micro-hydrothermal vents, filamentous bodies, and FeS. (b) Tubular and spherical bodies, and FeS. The FeS₂-containing filamentous and tubular SRB in (c) and (d) were metasomatised with arsenopyrite, sphalerite, and cassiterite to form a residual structure. (e‒h) Backscattered electron (BSE) image of FeS₂ and related mineralogy. po = pyrrhotite (FeS); py = pyrite (FeS₂); apy = arsenopyrite; sp = sphalerite; cst = cassiterite. The white arrows in (a) and (c) indicate micro-hydrothermal vents.

Figure 3

Plane polarised light photomicrograph, SEM‒EDX backscatter images, and EDX elemental maps of SRB traces. The analysed elements included S, Fe, C, N, and O. (a) Plane polarised light photomicrograph. (b) and (c) SEM‒EDX backscatter images. (d‒h) EDX elemental maps of Fe, S, C, N, and O, respectively.

Figure 4

Electron images of SRB microstructure. (a) and (b) No nitric acid (HNO₃)–etched FeS₂. (c–h) 70% HNO₃ etched FeS₂. (a) Micro-hydrothermal vents. (b) Layered structures at the edges of the FeS₂. (c) HNO₃-etched layered structures of the FeS₂ edges. (d) and (e) Backscattered electron image of spheroidal SRB fossils. (f) and (h) Backscattered electron image of filamentous SRB.

Figure 5

Representative results from Raman spectroscopy analyses on major minerals and putative microbial features. (a and b) Pyrrhotite. (c‒g) Pyrite. Measurement point locations are shown in Fig. 3b and c.

Figure 6

Sulphate-reducing bacteria growth modes. Sulphate-reducing bacteria (green dots) growing at a micro-hydrothermal vent (grey); chemoautotrophic SRB proliferate, branch, and expand around the micro-hydrothermal vent in tubular and spherical shapes. Yellow represents pyrite (FeS₂), orange represents pyrrhotite (FeS), and blue represents seawater.