Study On Petrological Geochemistry Characteristics Of Porphyry Body In The Periphery Of Pulang Porphyry Copper Deposit

The Pulang porphyry copper deposit is located in the Southwest Sanjiang metallogenic belt,hosted in the southeast of the geza arc of south yidun island arc belt,which is a product of Ganzi-Litang oceanic crust westward subduction.The porphyry of Pulang porphyry copper deposit can be divided by No.I composite rock body and peripheral rock body in the region?Eastern periphery rock body,No.II rock body in the northern,No.III rock body in the northern periphery,Southern periphery rock body?.The former geological work was mostly based on No.I composite rock body,but the research of porphyry body in the periphery of Pulang is extremely weak.So in this paper,the porphyry body in the periphery of Pulang are studied.Based on the detailed petrology and petrography study,petrological geochemistry?major,trace,rare earth elements?,zircon U-Pb geochronology and Lu-Hf isotope are developed,which is detailed devision the rock type of porphyry in the study area,determined the exact age of diagenesis,explored the petrogenesis and tectonic setting,compared with the Pulang first mining area.The main reseach results are obtained as follows:?1?Detailed devision the rock type of porphyry in the study area.Eastern periphery is mainly composed of quartz diorite porphyrite and lack of quartz-monzonite porphyry.No.II rock body in the northern is mainly composed of quartz diorite porphyrite and quartz-monzonite porphyry.The main body of No.III rock body in the northern periphery is quartz diorite porphyrite,which is intrusived by the quartz-monzonite porphyry as irregular dike.The quartz-monzonite porphyry is the mainly ore-forming rock body.There are various rocks in the Southern periphery rock body,mainly such as quartz diorite porphyrite and structural altered rock.?2?Detailed analysis over the petrological geochemistry characteristics of the porphyry body in the periphery of Pulang.Quartz diorite porphyrite and quartz-monzonite porphyry have typical characteristics of intermediate-acid rock,which show the characteristics of high potassium and low sodium and belong to calc-alkaline quasi-aluminous rock.Trace element characteristics display that the rocks are enriched with lithophile element and are relatively depleted in high field strength elements and significant“TNT”negative anomaly,and weak Eu or without negative anomaly.It is shown that the source region magma with the crust-mantle mixing characteristics whose formation is relevant with the subduction environment.The characteristics of trace rare earth elements show that they have similar chondrite-normalized REE patterns,which the quartz diorite porphyrite and quartz-monzonite porphyry are enriched with LREE and are relatively depleted in HREE,and the LREE and LREE differentiationare relatively obvious.This is also shows the source region magma has the crust-mantle mixing characteristics.Eastern periphery rock body,No.II rock body in the northern,No.III rock body in the northern periphery and Southern periphery rock body have similar trace and rare earth elements characteristics and chondrite-normalized REE patterns,which is the product of comagmatic evolution.With relatively high degree of magma differentiation,magma evolution process is controlled by partial melting and fractional crystallization at the same time,accompanied by a certain degree of assimilation.?3?Used the ziron U-Pb method to determine the diagenetic age of porphyry in the study area.The rock-forming age of quartz diorite porphyrite in Eastern periphery rock body,quartz diorite porphyrite and quartz-monzonite porphyry in No.II rock body in the northern and No.III rock body in the northern periphery and quartz diorite porphyrite in the southern periphery rock body,which are 221.3±0.3Ma,210.5±0.3Ma,210.0±0.3Ma,230.6±0.9Ma,216.7±0.9Ma,220.0±0.33Ma respectively,are obtained.U-Pb datas of zircon shows the porphyry body in the periphery of Pulang was formed at the Late Triassic and tectonic-magmatic activity occurred in the late Indosinian and early Yanshanian.?4?Applied the Lu-Hf isotope to define the composition of the porphyry source region of the study area.Analysis of zircon Lu-Hf isotopes shows that the Lu-Hf isotope composition of porphyry body in the periphery of Pulang was similar,with the characteristics of negative values of?_Hf?t?and corresponding to the ancient crust model ages of two stage?976¹494Ma?.At the same time,the values of ¹⁷⁶Yb/¹⁷⁷Hf with all less than 1.2 indicate that there is also a mixture of mantle derived materials besides crust derived materials.It is indicated that the source magma of porphyry body in the periphery of Pulang are partly melted from the Middle Proterozoic crust,with the addition of mantle derived materials and the crust mantle mixing characteristics.In combination with various indicators of the petrology and petrological geochemistry,the porphyry body in the periphery of Pulang shall be the I-type granite with adakitic geochemistry features.?5?Compared with No.I composite rock body in the Pulang first mining area,tectonic setting and diagenetic pattern of the porphyry body in the periphery of Pulang are similar to those of the Pulang first mining area,which are the product of isogenesis magmatism.In the deep part quartz diorite porphyrite in the Pulang first mining area and in the porphyry body in the periphery of Pulang should be the same rock mass,which is only exposed at different locations on the surface.No.I composite rock body in Pulang first mining area is the most main metallogenic centers which is the epitaxial part of the Pulang first mining area in the eastern periphery rock body.Another secondary hydrothermal-alteration-mineralization center may be found in the No.II rock body and No.III rock body.Therefore the deep prospecting potential of eastern periphery is small and No.II rock body,No.III rock body are bigger.Southern periphery rock body is found gold mineralized body which is closely related to structural altered rock,has the potential to search for"structural altered rock type"gold deposits in mineralization center outer zone.