Biogenic origin of intergrown Mo-sulphide- and carbonaceous matter in Lower Cambrian black shales (Zunyi Formation, southern China)

The Lower Cambrian Zunyi Formation in southern China hosts complex intergrown phases of Mo-sulphides and anthracite-like carbonaceous matter and graphite (MoSC), pyrite and Ni-sulphides. These sulphides occur in a few centimeters thick polymetallic stratiform layer in black shales above phosphorites, cherts, dolomites and barite. The ore horizon contains 7 wt.% Mo, 2 wt.% Ni, 19 wt.% S and 13 wt.% Corg and Ctot (CO2: 58 wt.%), 14 wt.% CaO, 1.2 wt.% As, 2000 ppm Se, 5400 ppm Zn, 450 ppm U and V, and up to 1 ppm platinum group elements. Three types of MoSC phases, always including framboidal pyrite, are distinguished: (1) rounded structures, few hundred microns large, (2) relict inclusions in diagenetic pyrite (3) grains interstitial to sulphurised and phosphatised organic matter. At the nanoscale, MoSC is represented by nanometric porous rounded structures of MoS2 finely intergrown with carbonaceous matter of typical nano-“onion” structures. MoSC also contains nanocrysts of quartz, clays, Ca-sulphate, carbonate and Ni-sulphides. Micro-Raman spectroscopy shows that the carbon phase has a maturation grade of anthracite indicating temperatures below 300 °C. MoSC is mainly composed of 25 wt.% Mo, 23 wt.% S, 5.6 wt.% Fe, 2.5 wt.% Ni, 33 wt.% C, 1.5 wt.% O, 0.6 wt.% N and several hundreds of ppm of Cu, As, Se, Sb, Pd, Zn, Co and Sn. Electron energy loss spectra show that N is related to nanosized carbonaceous matter. The mean C/N ratio (∼ 57) of MoSC is similar to those observed in marine organic matter, and those reported from zooplankton at hydrothermal vents. The mineralized shales are crosscut by hydrothermal veins composed of carbonate or barite, always associated with quartz, apatite, and Fe and Zn sulphides, a paragenesis also observed as nanocrysts in MoSC. Although Mo originated from seawater, its extreme concentration is due to an enhanced growth of bacterial consortia in a diffuse hydrothermal vent system. These consortia may be dominated by nitrogen fixing and Mo dependant hyperthermophilic or mesophilic species containing nitrogenase, composed also of Mo, S, Fe. Organic matter decay led thus to the formation of MoSC and the included framboidal pyrite.