柴北缘西端盐场北山铜镍硫化物矿床镁铁-超镁铁质岩体岩浆演化过程及含矿性分析

Magma evolution and Ni-Cu sulfide mineralization potentiality of the Yanchangbeishan mafic-ultramafic intrusion in the western North Qaidam orogenic belt

盐场北山岩体位于柴北缘造山带西段,是区内近年来新发现的具有良好铜镍矿化的镁铁-超镁铁质岩体,由橄榄岩相、辉石岩相和辉长岩相岩石组成。锆石U-Pb年代学研究表明,岩体形成时代为258.5±1.6Ma,为晚二叠世岩浆作用的产物。岩石中橄榄石Fo分子为82.8~86.7,均为贵橄榄石,辉石由斜方辉石和单斜辉石组成,斜方辉石种属为古铜辉石,单斜辉石为透辉石、普通辉石和顽透辉石。岩石地球化学特征显示,岩石属于亚碱性岩系列,主要氧化物SiO_2、Al_2O_3、CaO、Ti O_2、Na_2O+K_2O等与MgO呈规律性变化趋势,显示不同岩相岩石为同源岩浆演化的产物;岩石相对富集大离子亲石元素(Rb、Ba、Sr),而强烈亏损高场强元素(Nb、Ta、Zr、Hf、Ti),具有岛弧岩浆岩特征;锆石Hf同位素显示εHf(t)均为正值,变化范围为0.1～6.7之间,具有幔源岩浆成因特征;岩石原生岩浆为MgO含量略大于13.21%的高镁拉斑玄武质岩浆,岩浆源区为受俯冲洋壳析出流体交代地幔楔而成的富集地幔。岩石结晶过程中发生了强烈的结晶分异作用和同化混染作用,结晶分异作用在岩浆演化的初期使岩浆中硫发生初步富集,地壳混染作用是岩浆中硫化物达到饱和并发生熔离的主要因素。

The Yanchangbeishan mafic-ultramafic intrusion with Ni-Cu mineralization,newly discovered in the western North Qaidam orogenic belt,consists of peridotite,pyroxenite,and gabbro. Zircon chronology shows that the mean U-Pb age of the zircons in these mafic-ultramafic rocks is 258. 5 ± 1. 6 Ma,suggesting that these rocks formed in Late Permian. In these rocks,the olivines are chrysolites（ Fo = 82. 8 ~ 86. 7）,the orthopyroxenes are bronzites,while the clinopyroxenes consist of diopside,augites,and endiopsides. The petrochemistry shows that these rocks belong to sub-alkaline series,and their main oxides（ Si O2,Al2 O3,Ca O,Ti O2,Na2 O ＋ K2 O） have a regular variation with Mg O,indicating that the different lithofacies have same magmatic source and evolution. All these rocks are rich in large ion lithophile elements（ Rb,Ba,Sr） and depleted in high strength field elements（ Nb,Ta,Zr,Hf,and Ti）,showing the geochemical characteristics of the arc magma. The zircon Hf isotopes show positive εHf（t） values（0. 1 ~ 6. 7）,indicating that the magma originated from the mantle source. The primary magma of these rocks belongs to high-Mg tholeiitic basaltic series,with Mg O contents of more than 13. 21%,which were derived from the enriched mantle that formed by the precipitated fluids replaced the mantle wedges. In the process of rock crystallization,strong crystallization fractionation,assimilation,and contamination have occurred. The crystallization fractionation promoted sulfur element in the magma during the initial stage of magmatic evolution,the crustal contamination is the main factor for causing the sulfides saturation in the magma.