The Porphyry Copper-polymetallic Deposit In Zhongdian，West Yunnan: Magmatism And Mineraliztion

Zhongdian mining area has been a good example of Indosinian porphyriticcopper deposit in the southwestern Sanjiang region on where there are only a fewYanshanian-aged research been reported. This study has been doing research on threelate Cretaceous porphyritic copper deposits including Xiuwacu deposit, Relin depositand Hongshan deposit, and we report following results through field work andlaboratory work.(1) The dating results indicates, that the biotitic granitic porphyry from Xiuwacudeposit yields an age between86-88Ma; the monzonitic granitic porphyry from Relindeposit yields an age between80-82Ma, the monzonitic porphyry from Hongshandeposit yields an age between76-79Ma; the formation of Xiuwacu W-Mo depositdated back to85.9±3.3Ma; the formation of Relin Mo-W-Mo deposit dated back to82.4±3.03Ma; Hongshan porphyry and related copper mineralization were formed at80±1.8Ma. Those ages are identical, within error, suggests consistent ore-formingprocesses.(2) Geochemically, the late Cretaceous porphyries yield consistent characteristicsof relative high SiO2content, high Al2O3content, high K2O content, and high ratio ofK2O and Na2O, showing alkali features. They also have relative high Sr content, highSr/Y and (La/Yb)N,indicating adakitic geochemical features.(3) This study presents, that geochemical characteristics indicate Zhongdianporphyries deriving from partial melting of crustal thickening; epsilon Hf valuesindicate crustal sources; high Mg number reflects mantle material involved; Sr-Nd-Pbisotopic compositions indicate subduction-related material. Therefore, the intrusion ofthe late-Cretaceous porphyries in Zhongdian area is likely driven by the collision of ofLhasa block and Qiangtang block.(4) Study of fluid inclusions and isotopes provided evidences that the origin ofthe ore-forming fluids in the porphyritic copper deposits were coming from theadakitic porphyritic pluton. First stage adakitic magma travelling upward through thenorthwestern fractures making surrounding stratum thermal metamorphosed andalternated. Volatiles and high salinity fluids were extracted and then leading to thecrack at the top of the pluton, which transferred the lithostatic pressure to thehydrostatic pressure, thus dropping off metal elements and causing mineralization. Wefound skarn-type mineralization in Hongshan deposit because of the enrichment ofmarble in the wallrock and hence the metasomatism, whereas porphyriticmineralization were found in Xiuwacu deposit and Relin deposit due to the stablehornstone in the wallrock.