Comparing Geochemistry Study of Porphyry Copper-gold Deposits in Different Tectonic Environments

Author:Zuo Hua Sheng

Supervisor:yang xiao yong

Database:Doctor

Degree Year:2019

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Pages:158

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In this thesis,we focused on comparing geochemistry study of adakitic intrusive rocks and Cu-Au mineralization related to porphyry copper-gold deposits in two different tectonic settings.The study consists of two parts:1)The main Grasberg adakitic rocks and Cu-Au mineralization in Grasberg porphyry Cu-Au deposit,Papua,Indonesia;2)Chating ore-bearing adakitic rocks and diorite porphyry as ore-barren in LYRB.Through the systematic geochemical analysis of adakitic rocks related to ore-bearing rocks,the rock series,genetic mechanism and source area composition are studied.We attempted to constrain the relationship of magma and mineralization,and dynamic setting of porphyry copper-gold deposits under different tectonic settings.Through the comparison of geochemical characteristics of adakitic rocks related to mineralization,magmatic source area and ore-forming physicochemical conditions are discussed.The followings at the details.1)The main Grasberg adakitic rocks and Cu-Au mineralization in Grasberg porphyry Cu-Au deposit,Papua,IndonesiaThe Cenozoic porphyries in the Grasberg area host a supergiant Cu-Au porphyry deposits in West Papua,Indonesia.We mainly focuses on the relationship of magma and mineralization,and dynamic setting of adakitic rocks in the main metallogenic period of the Grasberg copper-gold deposit.The LA ICP-MS zircon U-Pb age analysis of three samples shows that the main metallogenic intrusive rocks formed in 3.06-2.86 Ma,belong to Pliocene.They show the geochemical characteristics of adakitic rocks,such as high Sr,low Y,Yb content,and high Sr/Y(48.8-65.5)and(La/Yb)N.They also show high contents of Al2O3 and K2O and high ratio of K2O/Na2O(0.94 to 1.33),which belong to potassic adakitic rocks.They are also enriched in light rare earth and large ion lipophilic(LILEs)elements,depleted with high field strength(HFSEs)and heavy rare earth elements,and have no Eu negative anomaly(Eu/Eu*=0.97-1.28,av.1.04),Nb-Ta-Ti was obviously negative abnormal,Pb was positive abnormal.They also show enriched(87Sr/86Sr)i=0.7052-0.7060,negative whole-rock εNd(t)=-11.8 to-14.6,and high radiogenic Pb isotopic compositions with(206Pb/204Pb)i=17.88-18.19,(207Pb/204Pb)i=15.54~15.58,(208Pb/204Pb)i=38.49~38.59.Combined with the discovery of ancient inherited zircon cores,negative εHf(t)=-6.7 to-20 and old two-stage model age 1.6-2.3 Ga,it indicates that there are a large number of ancient lower crusts in the source area of Grasberg adakitic rocks.Simultaneously,the moderate content of MgO and Mg#(38-52,av.42)and isotopic characteristics indicating the participation of mantle materials.The Th/Yb-Ba/Th and Nb/Y-Ba/La diagram also proves that the mantle end element was an enriched lithospheric mantle metasomatism by fluid.The zircon Ce4+/Ce3+and Eu/Eu*combined with S isotopes show that the Grasberg ore-bearing adakitic rocks have high oxygen fugacity,high water content and deep sulfur.Therefore,we believe that the Grasberg ore-bearing adakitic rocks were formed by the partial melting of the ancient lower crust and mixed with the enriched lithospheric mantle melt metasomatism,which experienced the separation and crystallization of clinopyroxene and hornblende,followed by assimilation and contamination of the lower crust.The copper-gold mineralization is the product of the interaction between the ancient lower crust and the enriched lithospheric mantle metasomatism.Based on the characteristics of isotopes and trace elements,combined with the Cenozoic tectonic evolution in this area,we concluded that the Grasberg adakitic rocks were in a post-collision setting.Before 25 Ma,the passive Australian northern margin oceanic crust commenced subducting into a north-dipping subduction zone beneath the Pacific Plate.In the early stage of subduction,the wetter oceanic crust and overlying sediments were subducted,eclogization of the subducted slab resulted in dewatering of the subducted basalts and sediments,with those released fluids metasomatizing the mantle wedge,forming an arc lithosphere mantle.As the Australian Plate continued subducting,collisional orogeny begans.At 6 Ma,collisional delamination of the Australian continental lithosphere in a NE-dipping subduction zone starting.Due to upwelling of hot asthenosphere and decompression of stretched lithospheric mantle occurred from ca.6 to 3 Ma,which created partial melting of ancient lower continental crust.Simultaneously,fluid metasomatized lithospheric mantle was partially melted due to the heat provided by the upwelling of the asthenosphere.Mixing of melting of enriched lithospheric mantle and parting of ancient lower crustal formed the adakitic magmas,followed by the fractional crystallization of clinopyroxene and hornblende with assimilated with lower crust.The collisional jamming of the subduction zone ended with a short-lived period(ca.4~2 Ma)of strike-slip faulting in the core of the highlands of west New Guinea.This strike-slip event created pathways first for the ascent of magma and later for the focused flow of the magmatic volatiles that generated the Cu-Au deposits at shallow levels.2)Chating ore-bearing adakitic rocks and diorite porphyry as ore-barren in LYRBThe newly discovered super-large Chating porphyry Cu-Au deposit is associated with early Cretaceous intrusive rocks in the Lower Yangtze River Belt(LYRB),eastern China.Two types of intrusive rocks in the Chating area have been identified,i.e.,the quartz-diorite porphyry(adakitic rocks),related the Cu-Au mineralization and diorite porphyry as ore-barren.Zircon LA-ICP-MS U-Pb dating yields 145~136 Ma of the Chating intrusions.They both belong to the high-K calc-alkaline series,with enrichments in large-ion lithophile elements(LILEs)and light rare earth elements(LREE),depletions in high field-strength elements(HFSEs)and heavy rare earth elements(HREE),showing slightly negative Eu anomalies.The quartz-diorite porphyry has geochemical characteristics with adakitic affinity with high Sr contents and Sr/Y ratios,low MgO(1.31-2.09 wt.%)contents,medium(La/Yb)N and Mg#(34.6-54.6),and low K2O/Na2O(<1)values,decoupling of Sr/Y and(La/Yb)N,low Th/U(3.49-6.08)values.Having slightly enriched Sr-Nd-Hf isotopes((87Sr/86Sr)i=0.70655-0.70717;εNd(t)=-6.9 to-8.8;εHf(t)=-7.6 to-10.6)and high radiogenic Pb isotopes((206Pb/204Pb)i=18.22-18.70,(207Pb/204Pb)i=15.63-15.66 and(208Pb/204Pb)i=38.49-38.97),the Chating adakitic rocks have been identified to be formed from slab-derived crust mixed with metasomatized mantle-derived magma by partial degree of crustal assimilation.As to the barren diorite porphyry,since it has relatively lower SiO2 contents,high contents of MgO(3.89-4.06 wt.%),Cr and Ni(avg.47.6 ppm and avg.15.01 ppm)and high Mg#(51.2-69.8),with similarity of Sr-Nd-Pb-Hf isotope features to those of quartz-diorite porphyry(adakitic rocks),which implies that this barren diorite porphyry could be generated from partial melting of enriched LYRB SCLM assimilated with continental crust material.Zircon Ce4+/Ce3+and Eu/Eu*indicate that the ore-bearing Qtz-diorite porphyry adakitic rocks has a higher oxygen fugacity than the barren diorite porphyry,closely related with the partial melting of the subducted oceanic slab in an oxidizing environment,thus favorable for Cu-Au mineralization.Combined with the Mesozoic tectonic evolution in this area,the Chating area became an active continental margin environment related with the subduction of the Palaeo-Pacific Plate.Before ca.148 Ma,the paleo-Pacific Plate was being subducted beneath the continental lithosphere in the Yangtze Block,dehydration or fluid of subducted oceanic crust metasomatized the lithospheric mantle,forming an enriched mantle.At about 145 Ma,partial melting of the subducted oceanic slab and sediments occurred beneath the Chating area,generating primary adakitic melts.The adakitic melts could not have been ascended directly to the shallow crust but rather underwent mixing with melts from the metasomatized mantle-derived magma.The mixed high oxygen fugacity magma experienced a partial degree of crustal assimilation during the process of magma ascent and was emplaced in the shallow crust to form the ore-bearing Qtz-diorite porphyry intrusions,as well as their related Cu-Au deposits,in the Chating region.A few million years later,a small amount of partial melting of enriched mantle-derived magma with a partial degree of lower continental crust material assimilation could have been emplaced in the shallow crust to form the ore-barren diorite porphyry intrusions.Based on the study of two porphyry copper-gold deposits in Grasberg,Papua,Indonesia and Chating in LRYB,we found that the formation of porphyry copper-gold deposits in different tectonic backgrounds is closely related to the composition of their source areas and their tectonic dynamic setting.