Cretaceous Porphyries Associated With The Porphyry Tin Deposit In The Yanbei Area,South China:Petrogenesis And Implications For Mineralization

Porphyry tin deposts are widely distributed in the world,but their tectonic settings are quite complex.Especially,the types,petrogenesis and tectonic settings of tin mineralization-related porphyry rocks remain controversial.South China is an important base for metallic minerals in China,where the W-Sn-Nb-Ta deposits are present making it very special in the world.Two(Yanbei and Yinyan)porphyry tin deposits,which have been reported in the South China,are closely associated with Yanshanian porphyries.However,so far,the mineralization ages,rock types,petrogenesis and formation background for these porphyries remain poorly understood.This master thesis focuses on the "Cretaceous porphyries associated with the porphyry Tin deposit in the Yanbei area,South China: petrogenesis and implications for mineralization ".In this study,the Yanbei tin mineralization-related porphyries and associated magmatic rocks(e.g.,Mikengshan granites and Yanbei volcanic rocks)are investigated in detail through the methods of field geology,petrology,geochronology and element and isotope geochemistry.The rock types,formation age and geochemical characteristics were studied in detail in order to unveil the generation conditions of magma and petrogenesis.Combined with regional geology,magmatic and mineralization data,the tectonic setting of the rock formation and the source of mineralization materials are clarified and the relationship between the formation process of magmatic rocks and the tin mineralization were discussed in this study.In this dissertation,some major progresses have been made as follows:(1)LA-ICPMS zircon U-Pb age dating shows that the magmatic rocks in the Mikenshan-Yanbei area were generated in the Early Cretaceous(143 ~ 136Ma).The ore-barren granites,the volcanic rocks,the ore-rich porphyries and the the mafic dikes in the Mikengshan area were generated in ~138 Ma,143~136 Ma,~138 Ma,and 108 Ma,repectively.(2)The Mikengshan granites are classified as highly defferentiated,aluminous A-type granites with mantle components involving in the rock formation process or their source region.They mainly consist of biotite granites,and are geochemically characterized by high SiO2(74.42-76.69 wt.%);high potassium(K2O/Na2 O = 1.37-1.94);low Al2O3(12.39-13.49 wt)and Mg #(11-19);very weakly aluminous compositions(ACNK = 1.03-1.1);negative Eu,Sr and Ba anomalies;enrichment in high field strength elements;high Ga/Al ratio(3.46-4.96),and slightly high zircon saturation temperature(807 ~ 817 ?);similar to aluminous A-type granites.But they have high Rb(842-1295 ppm)and Rb/Sr(90-255)values,and rare earth element tetrad effect,similar to highly fractionated granites.Therefore,we suggest that the Mikengshan granites should be highly fractionated,aluminous A-type granites.Compared to other granites in South China,they have slightly high ?Nd(t)values(-3.4 ~-4.6)values,indicating the mantle component contribution during their formation.(3)The Early Cretaceous volcanic rocks in the Yanbei area were mainly generated through partial melting of ancient crustal rocks.The volcanic rocks in the Yanbei area mainly consist of rhyolites and breccia-bearing tuffic lavas with the main phenocrysts of quartz + feldspar/plagioclase.The potassic feldspar phenocrysts of rhyolites are made up of microclines.They are geochemically characterized by high SiO2(70.41-77.94 wt.%),peraluminous compositions(A/NKC > 1.2)and low 10000 × Ga / Al ratio(< 2.6)with wide aluminum base index ranges(AKI = 0.44-0.84)and higher differentiation index(DI = 82.26-95.21).They have relatively low ?Nd(t)value(-9.26),indicating that they were mainly generated from ancient crustal rocks.(4)The Early Cretaceous ore-bearing porphyries were mainly produced via partial melting of ancient crustal rocks.They are mainly composed of quartz,feldspar,sodium-oligoclase,biotite and cassiterite.They are geochemically characterized by high SiO2(60.78-74.99 wt.%),peraluminous compositions(A/NKC = 1.64 ~ 12.8),and high 10000×Ga / Al ratio(> 2.6)with low AKI(0.02~0.15)and DI(61.86 ~ 72.98)values.Their ?Nd(t)(-8.9 ~-9.3)values are similar to those of ancient crustal rocks in South China,indicating that they were mainly generated via the remelting of ancient crustal rocks.Both ore-bearing porphyries and volcanic rocks have similar Nd isotope compositions,implying they have similar crustal source.The early ore-barren volcanic rocks erupted directly on the surface of the earth,the late ore-rich porphyries intruded in the shallow position of crust,and the enrichment of metallogenic materials caused the mineralization at last.(5)The Early Cretaceous magmatic rocks in the Mikengshan-Yanbei area were generated through partial melting of different crustal rocks in the extension setting due to the roll-back of subudcted Paleo-Pacific Oceanic Plate.During the roll-back of subudcting Paleo-Pacific Oceanic Plate,the upwelling asthenosphere resulted in the extension of overlying lithosphere and partial melting of ovelying mantle,the underplating of mantle-derived magmas heated ancient crust or old crust reworked by mantle-derived magma or new crust formed by mantle-derived magma mixing and caused relatively high temperature crustal melting.Ancient crust-derived magmas rised,and erupted directly on the surface of the earth and intruded in the shallow position of crust,which formed the volcanic rocks and ore-rich porphyries in the Mikengshan-Yanbei area.Magmas,which were derived from ancient crustal rocks reworked by mantle-derived magmas or new crust formed by mantle-derived magma mixing,differentiated and emplaced in the crust.This caused the formation of the highly differentiated A-type granites in the Mikengshan area.Parital melting of slab-derived fluid and sediment-metasomatized mantle produced the late Early Cretaceous(~ 108 Ma)diorite porphyries.