# Chemical Signatures of Ge in Magnetic of Wugang BIF, China
## 3 Results and Discussion
### Controlling Factor of Ge in Magnetite in the Hydrothermal System  
The Wugang BIF is appropriate for reconstructing its origin because (1) the alternating banding texture (Figure 2a,b,d) indicated that the Wugang BIF precipitated under stable depositional conditions [23]; (2) the preserved fine-grained magnetite (euhedral to sub-hedral) indicates the preservation of the original signatures [11]; and (3) the relatively narrow range of trace element composition of magnetite can provide insights into the original signature of magnetite-precipitation conditions. Non-altered magnetite and altered magnetite show different compositional trends, with the former having a homogenous compositional trend and the latter having a wide variation [11]. Therefore, according to the relict banding texture, euhedral to subhedral magnetite, and homogenous magnetite trace element composition, the Wugang BIF and magnetite can be used as proxies for paleo-depositional reconstruction, even though they have experienced post-depositional metamorphism and deformation. Figure 3: Plots of Ge versus selected trace elements of (**a**) Zn, (**b**) Ni, (**c**) Co, (**d**) Al, (**e**) Mg, (**f**) Ti, (**g**) Cr, and (**h**) V of magnetite from Wugang BIF (red dot). The chemical behavior and correlation are given by the dashed line. The Ge concentration of magnetite from the Wugang BIF was not high and can be considered its chemical signature. The primary sources of iron in the BIFs were hydrothermal fluid and seawater, and magnetite in the BIFs was shown to be of hydrothermal origin [14]. BIFs are mainly composed of oxide, silicate, carbonate, and sulfide sedimentary facies [5,6]. Among these, the oxide facies contained higher concentrations of Ge than the other facies [55]. The (ferro-) ferric oxyhydroxides, including ferrihydrite (Fe(OH)\({}_{3}\)), greeniale ((Fe)\({}_{3}\)Si\({}_{2}\)O\({}_{5}\)(OH)\({}_{4}\)), siderite (FeCO\({}_{3}\)), and amorphous silica (SiO\({}_{2}\) - nH\({}_{2}\)O) are the precursor mineral phases of magnetite. Ge is incorporated into (ferro) ferric oxyhydroxide during its precipitation from aqueous solutions [20]. The concentrations of Ge in the magnetites in BIFs are relatively higher than those in other ore deposits and are 10-100 times higher than those in the bulk continental crust [22,51]. It is generally accepted that the absorption of Ge in BIFs results from its incorporation into iron hydroxides during precipitation from aqueous solutions [20]. Ge in hydrothermal fluids is mainly controlled by two sources: (a) concentrated in evolved magmatic-hydrothermal fluids through fractional crystallization, or (b) country rock through migrating fluids. Therefore, the concentration of Ge in magnetite is essentially controlled by the composition of the hydrothermal fluids and the degree of the fluid-rock reaction [54]. Ge enrichment processes are closely linked to Earth's evolutionary processes [21]. The great oxidation event (GOE) at 2.3 to 2.4 Ga has led to an increase in /O\({}_{2}\) and influenced the Fe oxidation process (ferrous iron to ferric iron) [5]. Fe\({}^{3+}\) hydroxide was formed, and the chemisorptive concentration of Ge is promoted [21]. Ge has a high affinity for iron hydroxides or oxides and can coprecipitate into iron hydroxides or oxides through sorption processes at pH \(>\) 6 [21,56]. In addition, the \"missing Ge sink\" in the ocean was due to the sequestration of Ge into authigenic Fe-oxyhydroxides in marine sediments [57], which might be one of the reasons for the absorption of Ge in the magnetite in the Wugang BIF. Considering the environmental conditions of the Precambrian (during the Neoarchean-Paleoproterozoic) and  
Figure 4: Plots of Ti + V versus Al + Mn of magnetite from Wugang BIF [12]. The decreasing temperature trend is indicative of the estimated formation temperature of each ore deposit. Ti + V and Al + Mn concentrations of magnetite are proportional to temperature [14].