Sedimentary exhalative nickel-molybdenum ores in south China

Unique bedded Ni-Mo ores hosted by black shales were discovered in localize d paleobasins along the Yangzte platform of southern China in 1971. Textura l evidence and radiometric dates imply ore formation during sedimentation o f black shales that grade into readily combustible beds, termed stone coals , which contain 10 to 15 percent organic carbon. Studies of 427 fluid inclu sions indicate extreme variation in hydrothermal brine salinities that were contained by Proterozoic dolostones underlying the ore zone in Hunan and G uizhou. Variations of fluid inclusion salinities, which range from 0.1 to 2 1.6 wt percent NaCl equiv, are attributed to differences in the composition s of brines in strata underlying the ore bed, complicated by the presence o f seawater and dilute fluids that represent condensates of vapors generated by boiling of mineralizing fluids or Cambrian meteoric water. The complex processes of ore deposition led to scattered homogenization temperatures ra nging from 100 degrees to 187 degrees C within the Hunan ore zone and from 65 degrees to 183 degrees C within the Guizhou ore zone. While living organ isms probably did not directly accumulate metals in situ in sufficient amou nts to explain the unusually high grades of the deposits, sulfur isotope ra tios indicate that bacteria, now preserved as abundant microfossils, provid ed sufficient sulfide for the ores by reduction of seawater sulfate. Such m icrobiota may have depended on vent fluids and transported organic matter f or key nutrients and are consistent with a sedex origin for the ores. Vent fluids interacted with organic remains, including rounded fragments of micr obial mats were likely transported to the site of ore deposition by the act ion of waves and bottom currents prior to replacement by ore minerals.