5.1 Sources of ore-forming materials
REY concentration variations are commonly used to trace the origins of Fe and SiO2 in BIFs (Li et al., 2010; Li et al., 2014). Based on our REY study of the BIF, we found a deficit in light rare earths and an enrichment in heavy rare earths, as well as distinct Eu anomalies. This is believed to be related to its mineralization originating from seafloor hydrothermal fluids (Rao and Naqvi, 1995; Klein, 2005). After PAAS standardization, the REY of the BIF in the study area (McLennan, 1989) showed the same partitioning pattern characteristics. Additionally, the mean (La/Yb) SN is 0.57. La, Eu, and Y are essentially positively anomalous, with a slight negative Ce anomaly (Fig. 6). This phenomenon is similar to the chemical characterization of BIFs in China and abroad (Bolhar et al., 2004). The above phenomena further reveal that they all belong to Early Cambrian marine chemical sediments, which are typical chemical sedimentary rocks.
Studies have shown that positive Eu anomalies also characterize seafloor hydrothermal fluids (Danielson et al., 1992). The Eu/Eu* in this study is > 1 (Fig. 7). This implies that the BIF mineralized material in the study area originated from seafloor hydrothermal fluids and formed in a volcanic environment on the seafloor. In addition, BIF of this study have Co/Zn ratios of 0.02-0.59 and Ni/Zn ratios of 0.04-0.48. These ratios have a lot of similarity to hydrothermal-originated BIF (Co/Zn is 0.03-0.15, Ni/Zn is 0.08-0.78) (Sugitani, 1992).
Fig. 7 is about here.
Some geochemical maps also support this viewpoint. Fig. 8a, Fig. 8b and Fig. 9 indicate the samples are mostly located in the hydrothermal field. In modern seawater, after PAAS normalization, REY exhibits characteristics of La, Gd, Y positive, and Ce negative anomalies (Bolhar et al., 2004). We can see that the study samples also exhibit seawater characteristics (Fig. 6). Meanwhile, LREE is in depletion, suggesting that it is a product of deep seawater (Bolhar et al., 2004). The positive Y anomalies indicate a contribution of seawater to the formation of the BIF (Fig. 7).
Fig. 8 is about here.
Fig. 9 is about here.
The Y/Ho has been reported as 44–74 in modern seawater, while the Y/Ho is 26–28 from chondrite, which also represents hydrothermal fluids (Bau and Dulski, 1996, 1999). In contrast, the Y/Ho ratio in the study area averages 36.00 (range 33–41), which is higher than that of globular meteorites and is close to the seawater Y/Ho value. We hypothesize that the BIF in the study area probably inherited seawater and hydrothermal features. A (La/Yb)SN value > 1 represents hydrothermal fluids, while (La/Yb)SN < 1 represents seawater and hydrothermal mixing phenomena (Sugitani, 1992). The (La/Yb)SN is 0.30-1.44(average 0.57), which further indicates seawater and hydrothermal mixing. This phenomenon can be reflected by Fig. 10a, Fig. 10b (Alexander et al., 2008). They demonstrated Lanling BIF were precipitated from < 1% seawater mixed with high-temperature hydrothermal fluid.
Fig. 10 is about here.
In Fig. 10, some samples deviate from the mixing line, which is likely due to the inconsistent contribution ratios of seawater and hydrothermal fluid to the BIF formation process.
5.2 Detrital materials and their impact on BIF
It has been suggested that the Precambrian sediments are likely to have been subjected to diagenesis and metamorphism following deposition and syn-sedimentary contamination, and that REY characterization may have been affected as a result (Sun et al., 2014b; Tang et al., 2013).
Due to the stability of Al and Ti in geological processes such as hydrothermal metasomatism and diagenesis (Krandiotis and Maclean, 1987), they can be used as alternative indicators to express contamination (Moon et al., 2017). Al2O3 and TiO2 contents are low in all samples, with averages of 2.02% and TiO2 0.04%, respectively, suggesting that the BIF is relatively unaffected by contamination. In addition, Fig. 11 shows the ratio of hydrothermal materials to land-sourced sediments and determine their precipitation environments.
According to this figure, it shows that the source of the samples is close to hydrothermal sediments and far from terrestrial sediments, further revealing that the BIF in the study area had little influence from crustal material during the precipitation process.
Fig. 11 is about here.
5.3 Oxidation-reduction environment
It has been reported that Ce3+ in seawater is oxidized to Ce4+ under oxidizing conditions (Sholkovitz et al., 1994). In contrast, under reducing conditions, it cannot be separated from rare earths. Thus, Ce anomalies, when observed via the standardization of rare earth elements by PAAS, can reveal the palaeo-oceanic redox state (Sun et al., 2014a). As a result, it is generally believed that seawater lacks truly negative Ce anomalies under hypoxic or anoxic states (Bekker et al., 2010). Research suggests that Ce/Ce * vs. Pr/Pr * can reveal anomalies for La and Ce in BIF (Fig. 12; Bau and Dulski, 1996). According to this plot, we can see that the great masses of samples are located in areas without anomalies, with a few areas of positive La anomalies. It is speculated that the formation of BIF in Lanling occurred in a reducing environment. This further indicates an anoxic environment in the Precambrian.
Fig. 12 is about here.
Based on the above results, it further reveals the volcanic eruption and sedimentary events during the evolution of the North China Craton. And considering the environment in which the aforementioned BIF was formed, it is highly likely that it occurred before the Great Oxygenation Event.
The above phenomenon further confirms that an important tectonic magmatic thermal event occurred in the North China Craton at approximately 2.5 Ga (Zhang et al., 2012). This distinguishes it from other typical cratons around the world.