Petrogenesis And Metallogenic Potential Of Niangniangshan Complex In The Xiaoqinling Area

The Xiaoqinling area is the second largest gold producing area in our country.More than 50gold deposits and over 1,200 gold-bearing quartz veins have been explored in this area with the proven gold reserves of about 400 tons,which possess great economic value.Early Cretaceous granites are widely distributed in the Xiaoqinling area.These granites are temporally and spatially associated with the gold deposits in the area.However,whether they have a genetic link is still controversial.The Niangniangshan pluton is a representative complex intrusion in the Xiaoqinling area,and its emplacement age is close to the gold metallogenic age.Therefore,this article took the Niangniangshan complex as the object,carried out research on petrology,geochronology,geochemistry and zircon mineralogy,and analyzed the temperature and redox state of magma,and discussed the petrogenesis,geodynamics and mineralization potential.The Niangniangshan complex is composed of three phases.The lithologies are porphyritic medium-grained biotite monzogranite,porphyritic medium-fine-grained biotite monzogranite,and fine-grained monzongranite.Zircon U-Pb ages are 135.0 Ma,133.2 Ma and 122.6 Ma,respectively.Combined with their relationship in the field,it is found that the Niangniangshan complex was produced by two episodes of magmatic activity(?134 Ma and?123 Ma)in the Early Cretaceous.The?134 Ma rocks(the first and second phases)have higher Sr(563–1114ppm)and Ba(1455–1958)contents,and higher Sr/Y ratios(57–164).The?123 Ma rocks(the third phase)have low Sr(88–124ppm),Ba(60–117ppm)content and Sr/Y ratio(16–52).In the normalized trace element diagram,the?134 Ma granites have no significant Eu anomalies(0.75–1.06)and show depletion of Zr and Hf.The?123 Ma rocks have more obviously negative Eu anomalies(0.73–0.84)and show enrichment in Zr and Hf.These geochemical and geochronological characteristics indicate that the Niangniangshan complex was formed by a two-episode magmatism,with the time of?134 Ma and?123 Ma.The two group rocks both have low Mg O,Cr,Co and Ni content,higher ⁸⁷Sr/⁸⁶Sr(t)(0.7072–0.7108),lower?Nd(t)(-16.97to-14.49)and?Hf(t)(-29.62 to-11.38).These features indicate that the two group rocks are produced by partial melting of the lower crust.The?134 Ma granites were generated at high pressure where plagioclase is unstable,while the?123 Ma granites were formed at a shallower level where plagioclase can keep stable.The difference in melting depth also indicate that the transformation of the lithosphere from thickening to thinning in the Xiaoqinling area probably occurred between?134 Ma and?123 Ma.Zircon trace element indicate that zircon crystallization temperature of the?134 Ma granite range from 605 to 740?,and the oxygen fugacity(?FMQ)range from-5.30 to-1.5.The?123Ma granite has a higher zircon crystallization temperature(660 to 835°C)and higher oxygen fugacity(?FMQ is from-1.41 to-0.29).It also shows that?FMQ values are positively correlated with temperature in the?134 Ma granites,while are negative correlated with temperature in the?123 Ma granites.This indicates that?134 Ma magma will become more reduced with evolution,but?123 Ma magma will be more oxidized.These features,combined with the redox state of typical magmatic hydrothermal Au(Mo)deposits in the world,suggest that?123 Ma granites have a greater potential to form magmatic hydrothermal Au(Mo)deposits compared to ?134 Ma granites.