滇西北普上含矿斑岩体的岩石地球化学及蚀变特征研究

Petrographic geochemistry and alteration characterization of ore-bearing porphyry bodies in Pushang,Northwest Yunnan,China

滇西北中甸斑岩铜多金属成矿带是我国重要的铜资源基地,普上为该成矿带新发现的斑岩型铜点,含矿复式岩体主要由石英闪长玢岩和石英二长斑岩组成。为探讨该岩体成因、构造背景、物质迁移过程及成矿潜力等问题,本文对新鲜石英闪长玢岩和蚀变石英闪长玢岩分别开展了岩石化学和蚀变组分特征研究。结果显示,蚀变石英闪长玢岩的SiO_(2)(61.16%)和K_(2)O(4.20%)含量较新鲜石英闪长玢岩(分别为58.24%和3.06%)有所升高,而Al_(2)O_(3)和Na_(2)O的含量明显降低,但都具有大离子亲石元素(Rb、Ba、K、Sr)富集、高场强元素(Zr、Nb、Ta、Ti)亏损的特征,且具有较高的Ba/Nb值(分别为75.7和93.2)和较低的Ce/Pb值(分别为3.16和5.70),显示典型弧环境花岗岩的特征。普上新鲜石英闪长玢岩的(La/Sm)N值较高(平均8.18),无明显的Eu负异常,SiO_(2)与Rb、Zr/Sm值之间表现出一定的正相关性,表明其经历了角闪石的分离结晶作用;Ba/Rb(<50)和Nb/La(<1)值较低,Th/La值(>0.25)相对较高,Rb/Sr值(0.09)介于上地幔(0.034)与地壳(0.35)之间,表明岩浆源区与俯冲作用有关,岩浆来源为壳幔混源。蚀变石英闪长玢岩中大量迁入的组分包括Fe_(2)O_(3)^(T)、CaO、K_(2)O、Th、Ta、HREE和Cu、Mo、W、Ag等成矿元素,迁出的组分有Na_(2)O、MgO、MnO、Sr、Ba、U和LREE等,表明普上蚀变石英闪长玢岩受到了以钾交代为主的碱性蚀变作用的影响,引起矿物蚀变的成矿热液富含Cu、Mo等成矿金属。普上石英闪长玢岩中普遍发育角闪石斑晶且Sr/Y和V/Sc值较高,表明原始岩浆具有富水和高氧逸度的特征;蚀变岩中绢云母化广泛发育,Cu巨量迁入,在岩石化学组成与金属成矿专属性判别图解中,所有样品均在Cu-Au成矿区域。综合以上特征,认为普上石英闪长玢岩体具有形成Cu-Au矿床的潜力。

The Zhongdian porphyry copper polymetallic metallogenic belt in northwestern Yunnan is an important copper resource base in China.Pushang is a newly discovered porphyry copper spot in the metallogenic belt.The ore-bearing composite rock mass is mainly composed of quartz diorite porphyrite and quartz monzonite porphyry.In order to explore the genesis,tectonic setting,material migration process and metallogenic potential of the rock mass,this paper studies the petrochemistry and alteration component characteristics of fresh quartz diorite porphyrite and altered quartz diorite porphyrite respectively.The results show that the contents of SiO_(2)(61.16%)and K_(2)O(4.20%)in the altered quartz diorite porphyrite are higher than those in the fresh quartz diorite porphyrite(58.24%and 3.06%,respectively),while the contents of Al_(2)O_(3) and Na_(2)O are significantly lower.However,they all have the characteristics of enrichment of large ion lithophile elements(Rb,Ba,K,Sr)and depletion of high field strength elements(Zr,Nb,Ta,Ti),and have higher Ba/Nb values(75.7 and 93.2,respectively)and lower Ce/Pb values(3.16 and 5.70,respectively).It shows the characteristics of granite in typical arc environment.The(La/Sm)N of the fresh quartz diorite porphyrite is high(average value is 8.18),and there is no obvious negative Eu anomaly.There is a positive correlation between SiO_(2) and Rb,Zr/Sm ratios,indicating that it has experienced the fractional crystallization of hornblende.The ratios of Ba/Rb(<50)and Nb/La(<1)are low,the ratio of Th/La(>0.25)is relatively high,and the ratio of Rb/Sr(0.09)is between the upper mantle(0.034)and the crust(0.35),indicating that the magma source area is related to subduction,and the magma source is crust-mantle mixed source.A large number of components in the altered quartz diorite porphyrite include Fe_(2)O_(3)^(T),CaO,K_(2)O,Th,Ta,HREE,Cu,Mo,W,Ag and other ore-forming elements,and the migrated components include Na_(2)O,MgO,MnO,Sr,Ba,U and LREE,indicating that the Pushang altered quartz diorite porphyrite was affected by alkaline alteration dominated by potassium metasomatism,and the ore-forming hydrothermal fluid causing mineral alteration was rich in Cu,Mo and other ore-forming metals.The hornblende phenocrysts with high Sr/Y and V/Sc ratios are widely developed in the quartz diorite porphyrite,indicating that the original magma is characterized by rich water and high oxygen fugacity.Sericitization is widely developed in altered rocks,and Cu migrates in a large amount.All samples are in the Cu-Au mineralized area in the lithochemical composition and metal mineralization exclusivity discrimination illustration.Based on the above characteristics,the Pushang quartz diorite porphyrite has the potential to form Cu-Au deposits.