Genesis of Qujiashan manganese deposit, Shaanxi Province: constraints from geological, geochemical, and carbon and oxygen isotopic evidences

The Qujiashan manganese deposit is located in the Longmen-Daba fold belt along the northern margin of the Yangtze Block. The layered ore bodies are distributed within the purple-red calcareous shale. Qujiashan is a high-grade w(MnO)=8.92% to 18.76%) manganese deposit with low-phosphorus w(P2O5)=0.08% to 0.16%) content. It also has a low total REEs contents (with an average of 101.3x10(-6)), and has inconspicuous Ce (0.81 to 1.29) and Eu (1.00 to 1.25) anomalies. lg(Ce/Ce*) values are from -0.02 to 0.11. The ores have high SiO2/Al2O3 and Al/(Al + Fe + Mn) ratios. In figures of Fe-Mn-[(Ni+Cu+Co)x10] and lgU-lgTh, all samples show that hydrothermal exhalative fluids played an important role during mineralisation. The delta C-13(PDB) and delta O-18(SMOW) values of eight ore samples are from -20.7 parts per thousand to -8.2 parts per thousand (with an average of -12.4 parts per thousand) and from 14.3 parts per thousand to 18.7 parts per thousand (with an average of 17.0 parts per thousand), respectively. These carbon and oxygen isotopic features indicate that hydrothermal fluids derived from deep earth are participation in the metallogenic process, which is also supported by high paleo-seawater temperatures varying from 47.08 to 73.98 degrees C. Therefore, the geological and geochemical evidences show that the Qujiashan deposit formed from submarine exhalative hydrothermal sedimentation.