Studies On The Geochemical Characteristics And Forming Processes Of The Chongjiang Ore-bearing Porphyry In The Gangdese Copper Belt, Southern Tibet,China

The Gangdese porphyry copper belt is a new discovered ore belt in the South Tibet, China. A lot of small ore-bearing intrusions, such as the Chongjiang, the Qulong, the Nanmu, the Tinggong, the Dongga, the Lakang'e, have been found in this ore belt. The Chongjiang porphyric intrusion occurred in the middle of the Gangdese porphyry copper belt in the Southern Tibet plateau.In this paper, our work focuses on the petrology, petrochemistry, trace element and REE geochemistry and isotope geochemistry of the Chongjiang ore-bearing porphyry. The rock series and rock type, the soruce characteristics, tectonics setting and the forming age of the Chongjiang ore-bearing porphyry have been discussed.From the above studies, the following achievements have been made:1. The Chongjiang ore-bearing porphyry could be divided into eary-stage, main-stage and late-stage. The early-stage rock and the main-stage rock are diorite, granodioritic porphyry and quartz-monzonitic porphyry, respectively, and belong to the high-alkaline series. The main stage granodioritic porphyry and quartz-monzonitic porphyry display massive, porphyritic texture, comprising plegioclase, k-feldspar, minor mica and quartz as phenocrysts. The late-stage is granitic porphyry, locating between the high-alkaline series and the schoshonitic series.2. Compositons of trace element of the Chongjiang porphyry shows the subduction-related affinity. It is concluded that the Chongjiang porphyry is formed in the extend environments of the Himalayan-Tibetan continent-continent collision orogenic zone. The rocks of different stages have the identical trace element and REE patterns and similar isotopic compositons. The trace element characteristics suggests that the Chongjiang porphyries derive from the mixing source of mantle and crust.3. The rock type of the Qulong ore-bearing porphyry is granodioritic porphyry and the rock series of porphyry is calc-alkaline series. The Qulong and the Chongjiang porphyry display different trace element concentration. The Qulong has higher contents of the trace elements such as Rb, La, Th, U and lower Sr than thoseof the Chongjiang porphyry The trace element patterns of the Chongjiang porphyry and the Qulong porphyry is similar, which indicates they derive from the mixing source of mantle and crust. The Qulong porphyry has the lower ratios of 87Sr/86Sr and higher ratios of 143Nd/144Nd than those of the Chongjiang porphyry, suggesting that the Qulong and the Chongjiang porphyries come from different source region. The Qulong porphyry may contain much more mantle materials.4. Zircon SHRIMP dating of quartz-monzonitic porphyry for the Chongjiang porphyry is 12.9±0.3Ma. It represents the formation age of the ore-bearing porphyry. Based our new data, together with previous work, we have concluded that the Chongjiang ore-bearing porphyry emplaced in two periods: 12-13.8Ma and 14~15.6Ma, respectively. It is suggested that magmatic activity of the Chongjiang ore-bearing porphyry is mutiperiod. The first stage is 14~15.6Ma, and the second stage is 12~13.8Ma. Based on our new age date, together with the previous work, it is concluded that the diagenetic and metallogenic epoch of the ore-bearing porphyries in the Gangdese porphyry copper belt is 18Ma to 12 Ma. Lifespan of magmatic activity of the Gangdese ore-bearing porphyries is about 6Ma.5. The ore-bearing porphyries and the barren porphyries in the Gangdese copper belt are different in the characteristics of the trace element patterns and the Sr-Nd-Pb isotopic composition. It implies they have the different formation mechanism and derive from the different source region.6. Based on the characteristics of petrochemistry, the trace elements and the isotopic geochemistry, it has been suggested that the Chongjiang porphyry is formed as follows: in the extend environments (10~18Ma), mantle-derived fluids and heat flow upwelling and lower crust began partial melting. Mantle-derived fluids and lower crust interact and formed the primary magma. Then, the magma emplaced into the crust and formed the ore-bearing porphyries of the Gangdese copper belt (12-18Ma). The early ore-bearing porphyries emplaced around 17 Ma, such as the Qulong ore-beraring porphyry, and the late ore-bearing porphyries emplaced around 13 Ma, such as the Chongjiang ore-bearing porphyry.