岩基后成矿作用:来自小兴安岭鹿鸣超大型钼矿的证据

Post-batholith metallogenesis:Evidence from Luming super large molybdenite deposit in Lesser Xing'an Range

小兴安岭鹿鸣钼矿是新近发现的斑岩型超大型钼矿。尽管近年有一些新年龄和新资料发表,但是关于矿区的成岩、成矿事件的时代和成因仍有很大争议。本文采用LA ICP-MS锆石U-Pb、辉钼矿Re-Os以及黑云母40Ar-39Ar等测年方法分别对矿区的花岗斑岩、辉钼矿、以及二长花岗岩（下文称鹿鸣花岗岩）中的黑云母开展年代学研究。结果显示矿区花岗斑岩形成于174.0±2Ma（MSWD=3.2）;辉钼矿等时线年龄为177.8±2.3Ma（MSWD=0.078）,辉钼矿模式年龄加权平均值为177.5±1.2Ma（MSWD=0.058）。黑云母40Ar-39Ar 900~1400℃坪年龄为175.9±1.1Ma,表明鹿鸣花岗岩形成于约176Ma（之前）。因此结合野外、岩相学、前人结果等,认为鹿鸣花岗岩岩基成岩在前（〉176Ma）,花岗斑岩成岩在后（约174Ma左右）,成矿应当在花岗斑岩成岩近同时或稍后,为早侏罗世末期。花岗斑岩含有浸染状硫化物,表明花岗斑岩体是致矿侵入体,鹿鸣（二长）花岗岩岩基仅仅是钼矿的围岩。岩石地球化学特征,尤其是Mg O含量较高,高Sr低Y等特征,以及构造环境判别显示鹿鸣花岗岩岩基和花岗斑岩形成于与俯冲有关的火山弧环境。在早侏罗世早-中期,该区在北部蒙古-鄂霍茨克海和东部的饶河、伊佐那崎洋联合汇聚下形成俯冲带之上的加厚地壳,此时与地幔楔发生过反应的幔源岩浆底侵产生广泛的壳幔相互作用,形成鹿鸣花岗岩的岩基。随后加厚下地壳拆沉导致鹿鸣花岗岩岩基快速隆升,在地壳浅部,与来自于深部的花岗斑岩岩浆（＋钼矿和深部流体）相遇,后者侵入到鹿鸣花岗岩岩基中,形成了斑岩及辉钼矿矿床。据此,提出鹿鸣钼矿属于岩基后成矿作用的产物。

The Luming super large molybdenite deposit is a newly discovered Mo deposit with more than 890 thousand tons resource at an average grade of 0. 084% in the Lesser Xing ’an Range. Here we resolved the debate of the magmatism and associated mineralization of the Luming deposit. This paper reports LA-ICP-MS U-Pb zircon,40Ar-39 Ar biotite, and Re-Os molybdenite geochronology of Luming granite porphyry（ LGP）,and Luming granite（ LG）. Field relationships show that the emplacement of the LG is succeeded by the LGP. Quartz vein and disseminated molybdenite mineralization occurs in the LGP. The geochronology shows that the LGP was formed at 174. 0 ± 2Ma（ MSWD = 3. 2）. The five molybdenite samples from Luming deposit yield a Re-Os isochron age of177. 8 ± 2. 3Ma（ MSWD = 0. 08） and a Re-Os weighted average age of 177. 5 ± 1. 2Ma（ MSWD = 0. 06）. Ar-Ar systematics indicate LG biotite has an age of 175. 9 ± 1. 1Ma,this indicates the LG was probably formed at ~ 176 Ma. Considering field geology,petrography,and previous studies,we show the batholiths of LG（ 〉 176Ma） was formed prior to the LGP（ about 174Ma）,and the oreforming intrusive body of Luming metallogenesis of molybdenite is the LGP, but not the batholiths of LG, even though the metallogenesis of molybdenite is coeval or slightly later than LG and LGP. Geochemical characteristics,especially the highly abundance of Mg O,high Sr and low Y and diagrams of texture of LG and LGP show the LG and LGP were formed in VAG of subduction setting.We suggests,during early-middle Early-Jurassic,the studied area developed thickened crust and extensive mantle-crust interaction corresponding to the convergence of Monglia-Okhotsk Sea in the west-north,Rohe and Izanaqi Ocean in the east. After that,the batholiths of LG uplifted rapidly because of delamination and the magma of LGP（ including molybdenite and magmatic fluids） coming from deeper crust was intrusived into the batholiths of LG in the shallow crust,and then formed the LGP and the Luming molybdenum deposit. Thus,the authors suggest the Luming molybdenite deposit was formed by post-batholith metallogenesis.