北京密云沙厂BIF型铁矿地质地球化学特征与成矿时代

北京密云沙厂铁矿为产于太古宙密云岩群变质岩系中的鞍山式铁矿,具有条带状铁建造(BIF)特征。矿石主要呈条带状构造,致密块状结构;矿石类型以磁铁石英岩型铁矿石为主,w(Fe)=25%~30%。围岩主要有黑云母(角闪)斜长片麻岩、辉石变粒岩和斜长角闪岩,局部见英云闪长质片麻岩。对选自黑云斜长片麻岩中的锆石进行U-Pb定年,具有核边结构、Th/U0.4的锆石其核部2组年龄为2 619Ma±25Ma和2 544Ma±16Ma。锆石的2 619Ma±25Ma年龄代表原岩为基性火山岩的黑云斜长片麻岩的形成年龄,同时也代表火山喷发和沙厂铁矿BIF沉积年龄;2 544 Ma±16 Ma锆石年龄可能代表后期TTG特征的花岗质片麻岩的侵位、叠加改造年龄。岩石的主、微量元素特征表明黑云斜长片麻岩富集Rb、Ba、Sr,亏损Th、U,表明沙厂BIF沉积时可能存在新太古代(约2 550 Ma)地壳增生事件。认为沙厂铁矿属Algoma型BIF,是新太古代末华北陆块初步克拉通化和大规模岩浆热事件的代表之一。

Geochemical characteristics and metallogenic age of BIF type Fe deposit at Shachang in Miyun County, Beijing

Shachang Fe deposit at Miyun county,Beijing is an Anshan-style iron deposit in Archean metamorphic rock series of the Miyun group characterized by BIF.The ore is banded and densely massive and dominated by magnetite quartzite type with w(Fe)=25％00～30％.The country rock includes mainly biotite-plagioclase gneiss,pyroxene granulite and plagioclase amphibolite and local tonalitic gneiss.The UPb age of the core of rim-core zircon (Th/U ratio＞0.4) from biotite plagioclase gneiss is dated at 2 619±25Ma and 2 544±16Ma respectively.The former represent the age of the protolith (basic volcanic rock) of biotite plagioclase gneiss as well as those of the volcanic eruption and deposition of BIF at Shachang,the latter the age of TTG emplacement and the reworking events.Major and trace element analysis of the biotite plagioclase gneiss is characterized by Rb、Ba、Sr enrichment and Th depletion suggesting that during deposition of BIF may occur a crustal accretion event at about 2 550 Ma.Therefore,we think that the Shachang iron deposit belongs to Algoma type and is one of the representatives of preliminary cratonization of the North China continental block and large-scale magmatic thermal events at end of Neoarchean Era.