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

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