Abstract:

The Tongchang deposit is the most typical copper deposit in Mianxian-Lueyang-Ningqiang (Mian-Lue-Ning) area in southern Shaanxi, China. By studying on trace elements and REEs geochemistry of chalcopyrite (pyrite) as well as geological bodies in the deposit, we can draw some conclusions as followings. Firstly, the chalcopyrite is characterized by enrichment of Ni and Zn, depletion of Co, which is similar to the spilite of Late Proterozoic Guojiagou Formaion, while different from diorite and albite rocks in the mine. There are two kinds of chalcopyrites with different REE patterns, one is enriched in LREE and the other has relatively flat REE pattern. The pyrite grains hosted in internal and external contact zones of the diorite intrusion have different Co/Ni ratios, suggesting multiple sources of ore-forming materials. Analyzing on the background values of those oreforming elements in various geological bodies indicates that ore-forming material is mainly from the spilite and minor from the diorite. Secondly, the chalcopyrite is characterized by obvious negative Eu anomalies and its δEu value is much lower than those of the diorite and spilite. The depletion of Eu is likely related to greisenization and albitization caused by volatile fluids during mineralization. Furthermore, Y/Ho ratios of the chalcopyrite are similar to those of albite rocks, implying the close relationship between copper mineralization and Na replacement. Thirdly, due to the inheritance of the trace elements feature from source of ore-forming mineralizing materials, the Ni and Co content in the chalcopyrite is n-n×10 times than in the pyrite, corresponding to that in Cu-Ni sulfide type deposit and different from that in skarn-type, porphyry-type, volcanic-subvolcanic hydrothermal type as well as VMS Cu deposit. The metallogenic model of Tongchang copper deposit can be listed as following. During the Hercynian period, with the closure, subduction and collision of Mian-Lue ancient oceanic basin, the dutile-brittle thrusting nappe structure and strike-slip fault were formed in Mian-Lue-Ning area. The metasomatism of mixed hydrothermal by regional metamorphic fluid and rainwater, which rich in Na⁺, K⁺ and CO₂, resulted in the translation of ore-forming materials (e.g. Cu and Ni) from strata of Guojiagou Group and spilite (and diorite) to form the metallogenic hydrothermal fluid. The hydrothermal fluid was characterized by low temperature and salinity, and enriched in ore-forming materials. The Tongchang deposit was resulted from filling and metasomatism of the hydrothermal fluid in the EW-and NE-trending dutile strike-slip faults in this area.

Key words: Mian-Lue-Ning area, Tongchang, copper deposits, trace elements, source of ore-forming materials, ore genesis