Figure 3

Palaeoenvironmental signals for queluzite and graphitic schist from the Morro da Mina Mn-ore deposit. (a) Positive correlation between δ⁹⁸Mo values and whole-rock Fe/Mn ratios from Morro da Mina in comparison with Archaean and Palaeoproterozoic banded iron formations – BIF¹³. The data for Morro da Mina define the high-Mn end of the correlation, suggesting that the Mo-isotope signals of Mn oxidation (δ⁹⁸Mo < 0) were transferred to reduced, sulfide-bearing sediments that gave rise to the queluzite Mn ore mined at Morro da Mina. (b) Molybdenum-isotope values are negatively correlated with Ce anomalies (Ce/Ce*, calculated using the equation of Lawrence and Kamber⁴¹; PAAS refers to Post-Archaean Australian Shale⁴²). This correlation connects the Mo-isotope signals of Mn oxidation with Ce recycling due to reductive dissolution of Mn-oxide particles across the contact between oxic and euxinic waters – i.e., redoxcline. (c–d). The positive Ce anomaly of most queluzite samples is mirrored in the negative Ce anomaly of graphitic schist.