西昆仑北带喀依孜斑岩型钼矿床地质地球化学特征及年代学研究

Geological and geochemical features and geochronology of the Kayizi porphyry molybdenum deposit in the northern belt of western Kunlun,NW China

喀依孜钼矿床是近年来西昆仑地区新发现的具规模的斑岩型矿床。钼矿化赋存在花岗闪长岩体边部,由南、北两个矿化带组成,矿石呈浸染状,矿石矿物由辉钼矿及少量黄铁矿和黄铜矿组成。矿石Mo品位0.04%～1.53%,局部达10%～15%。围岩蚀变有钾化、黄铁绢英岩化和青磐岩化等斑岩型矿床蚀变组合。含矿岩体为花岗闪长岩,岩石地球化学具高SiO2、Al2O3,富K2O、Na2O,低CaO、TiO2等特点,属于高钾钙碱性系列,微量元素富集大离子亲石元素(LILE),亏损高场强元素(HFSE)和重稀土元素,显示Nb和Ta的负异常,与俯冲带岩浆地球化学特征类似,岩石成因可能与俯冲作用有关。含矿岩体锆石LA-ICPMS定年获得250.7±4.7Ma(MSWD=1.6),3件辉钼矿Re-Os模式年龄分别为254.4±1.9Ma、257.0±3.4Ma、258.5±2.0Ma,成岩成矿发生在晚二叠世末-早三叠世初。综合区域演化特征,本文认为喀依孜斑岩型钼矿床成因与古特斯洋向塔里木板块俯冲有关,形成于塔里木大陆边缘弧环境。

The Kayizi Mo deposit is a typical porphyry deposit in the northern belt of western Kunlun. The Mo mineralization occurs in the edge of a granodiorite intrusion,and consists of two mineral belts in the southern and northern sides,in which the biggest ore body (No. I) is 400m long and 140m wide. The ore is dominated by disseminated and spot-like molybdenite,minor pyrite and chalcopyrite,with Mo grade of 0.04%~1.53%,and some up to 10%~15%. The country rock alteration displays distinctive porphyry deposit alteration assemblages,including K-feldspathisation,pyrite-sericitization,and propylition. The petrochemical analyses suggest that the Mo-bearing granodiorites contain high SiO2,Al2O3,K2O and Na2O,and low CaO and TiO2,belonging to high-K calc-alkaline series. The trace elements show enrichment in Large Ion Lithophile Element (LILE),and a depletion in High Field Strength Element (HFSE) and heavy rare earth element (HREE),with Nb and Ta negative anomalies,compatible with the geochemical features of subduction magma,suggesting the petrogenesis associated with subduction. The zircon U-Pb (LA-ICPMS) dating yields the 206Pb/238U ages of 250.7±4.7Ma (MSWD=1.6). The Re-Os model ages of 254.4±1.9Ma,257.0±3.4Ma,and 258.5±2.0Ma,yielded from three molybdenites,support that the diagenesis and mineralization occurred in the Late Permian to early Triassic. Combining regional geological evolution,we considered in this paper that the Kayizi porphyry Mo deposit may have been formed as a result of the paleo Tethys subduction to the Tarim plate in the continental margin arc.