CYCLIC VARIATIONS OF SULFUR ISOTOPES IN CAMBRIAN STRATABOUND NI-MO-(PGE-AU) ORES OF SOUTHERN CHINA

Cyclic variations of delta S-34 values over a range of at least 48 par ts per thousand in pyrite nodules from stratabound Ni-Mo-PGE(Au) ores of southern China are attributed to biogenic reduction of seawater sul fate in an anoxic, phosphogenic, and metallogenic basin. Cyclic introd uction and mixing of normal seawater into typically stagnant basin wat ers led to extreme variations in delta S-34 values of aqueous sulfide species present at different times. Intermittent venting of metal-lade n hydrothermal fluids into such a bacteriogenic sulfide-rich environme nt resulted in precipitation of metal sulfides as pseudomorphous repla cements of organic debris and as sulfide sediments that record large d eltaS-34(CDT) variations from -26 to + 22 parts per thousand. Apatite and silica dominated the replacement of the organic debris when metals were not being introduced into the basin. The combination of abundant organic debris, localized topographic basins for accumulation of the debris, bacterial production of sulfide species, and introduction of m etal-bearing hydrothermal fluids provided the environment necessary to form these unusually rich Ni-Mo ores.