冈底斯西段朱诺斑岩铜矿床角闪石成分特征及其地质意义

MINERAL COMPOSITION OF AMPHIBOLE IN ZHUNUO PORPHYRY CU DEPOSIT IN THE WESTERN SEGMENT OF GANGDESE BELT AND ITS GEOLOGICAL SIGNIFICANCE

朱诺矿床为冈底斯西段一典型的形成于后碰撞环境的斑岩型铜矿,二长花岗斑岩及其中的包体为成矿岩体,本次研究通过对岩体中的角闪石进行矿物学研究,讨论岩浆演化和成矿作用过程。结果表明,包体和二长花岗斑岩中的角闪石均为镁角闪石。包体中第一类角闪石w(MgO)为11.9%~14.61%,w(Al2O3)为8.83%~9.03%,w(TiO2)为1.63%~1.66%,为幔源岩浆成因。包体中第二类角闪石w(MgO)为15.09%~16.65%,w(Al2O3)为3.92%~5.73%,w(TiO2)为0.19%~0.52%;二长花岗斑岩中角闪石w(MgO)为15.09%~17.8%,w(Al2O3)为3.24%~5.73%,w(TiO2)为0.11%~0.66%,二者为壳源变质成因。包体中第一类角闪石结晶深度为9.1~10.1 km,lgfO2为-11.9^-11.9,水含量为3.8%~4.4%;包体中第二类角闪石结晶深度为2.0~2.1 km,lgfO2为-13.2^-12.8,水含量为4.0%~4.2%;二长花岗斑岩中角闪石结晶深度为1.5~3.0 km,lgfO2为-13.3^-12.0,水含量为3.2%~3.9%。因此,朱诺矿床包体的角闪石可能形成于两个岩浆房,幔源岩浆在深部岩浆房形成包体中第一类角闪石,此时的岩浆具有高氧逸度和水含量,幔源残余岩浆继续上升,携带已形成的角闪石向上运移到地壳浅部,该过程中幔源残余岩浆与壳源岩浆发生混合作用,幔源岩浆补给大量的水、S及金属元素给壳源岩浆。当岩浆上升至浅部岩浆房时,二长花岗斑岩和包体中的第二类角闪石开始结晶形成,此时岩浆仍具有高氧逸度和水含量,在今后的勘查过程中应注意寻找含包体的二长花岗斑岩。冈底斯西段矿床的氧逸度和水含量较东段矿床更低,暗示西段遭受更多印度陆壳物质的混入,但这种改变是有限的,冈底斯西段的岩体仍满足成斑岩型铜矿的氧逸度和水含量,具备成矿潜力。

The Zhunuo deposit is a typical porphyry copper deposit formed in the post-collision environment in the western segment of the Gangdese in the southern Tibet,and the granitic porphyry and its inclusions are metallogenic rocks.Mineralogical characteristics of amphibole in the porphyry are studied in order to discuss its magmatic evolution and and its relationship with Cu mineralization process.It shows that the amphibole in inclusion and monolith porphyry is magnesium amphibole.The geochemical analysis of the first type amphibole in the inclusions shows w(MgO)=11.9%-14.61%,w(Al2O3)=8.83%-9.03%,and w(TiO2)=1.63%-1.66%,belong to mantle source magma genesis,and the geochemical analysis of second type amphibole in the inclusions shows w(MgO)=15.09%-16.65%,w(Al2O3)=3.92%-5.73%,w(TiO2)=0.19%-0.52%.The amphibole in monzonitic granite porphyry shows w(MgO)=15.09%-17.8%,w(Al2O3)=3.24%-5.73%,w(TiO2)=0.11%-0.66%,belong to crustal metamorphic genesis.Depth of the crystallization of the amphibole in the first type inclusions is 9.1-10.1 km,lgfO2=-11.9 to-11.9,water content=3.8%-4.4%;while the depth of the crystallization of the amphibole in the second type inclusions is 2.0 to 2.1 km,lgfO2=-13.2 to-12.8,water content=4.0%-4.2%.Depth of the crystallization of the amphibole in the monzonitic granite porphyry is 1.5-3.0 km,lgfO2=-13.3 to-12.0,water content=3.2%-3.9%.Thereby,it is likely that the amphibole formed in two different magma chambers at different depths.Mantle-derived magma migrated from depth into the deep magma chamber and formed the first type amphibole in the inclusions.The magma has a high oxygen fugacity and water content during this time.The residual magma of mantle source continues to rise,carrying the formed amphiboles upward to the shallow part of the crust.Mantle-derived residual magma and crust-derived magma mixed up during the process and the mantle-derived residual magma supplied largr amount of water,S and metal elements to the crust-derived magma.When the magma moved to shallow magma chmber,the second type amphibole in the monzonitic granite porphyry and inclusions started crystallization,with high content of water and oxygen fugacity.In the future exploration process,attention should be paid to search the monzonite porphyry with hosting inclusions.The oxygen fugacity and water content of the western Gangdeses are relatively low,suggesting that more Indian continental crust materials were added to the western segment with limited degree.Therefore,the western segment of Gangdese Porphyry Copper Belt has potential for metallogenesis.