河南省安林矽卡岩型铁矿的成岩时代和成矿物质来源探讨

位于华北克拉通中部的河南省安林铁矿是典型的邯邢式矽卡岩型铁矿,矿体产于闪长质岩石和中奥陶统灰岩的接触带。LA-ICPMS锆石U-Pb定年结果表明安林闪长岩体的侵位年龄为123.38±0.81Ma,略晚于华北克拉通东部地区的含矿岩体,形成于岩石圈大规模减薄伸展时期。但其中含有古老的锆石说明岩浆经历了地壳的混染。闪长质岩石具有相对低SiO2、高Mg#、高碱,富集Ba、Sr和LREE大离子亲石元素,亏损Nb、Ti、Ta等高场强元素的特点,暗示了其形成于岩石圈地幔。岩相学特征以及Harker图解指示了岩浆经历了较强的分离结晶作用,因此推断安林闪长岩可能是软流圈地幔上涌导致富集的岩石圈地幔发生部分熔融形成原始的辉长质岩浆在上升过程中或岩浆房中发生了以铁镁矿物为主的分离结晶作用,同时受到地壳物质的混染的结果。安林地区矿石具有和闪长岩体相似的稀土元素地球化学特征,反映铁的成矿作用与岩浆作用密切相关。矿床中闪长岩体、矽卡岩、矿石和碳酸盐围岩的主量元素对比研究表明钠交代作用引起Na、K、Fe、Si等元素在各岩类间的迁移,其中迁移出的铁为成矿提供了物质基础。安林地区初始岩浆富含水,分离结晶作用使残留岩浆水饱和而发生出溶;且岩浆在演化过程中受到区内膏盐层和碳酸盐地层的混染,促进了岩浆中流体的出溶并使出溶的流体富含Cl-,为有利于铁质活化的富Cl-岩浆流体的形成创造了条件。

Geochronology of the Anlin iron skarn deposit, Henan Province and insight into the sources of iron

The Anlin deposit, which occurs in the central North China Block (NCB), Henan Province, is a typical example of Han-Xing type iron skarn deposits. Its skarns and extensive mineralizations, preferentially occurred at the carbonate/intrusion contacts. U-Pb dating of zircon from a diorite porphyry yielded an age of 123.38±0.81Ma for Anlin deposit, which is slightly older than ore-bearing intrusions in eastern NCB. It shows Anlin deposit was formed in the timing of large-scale lithospheric thinning in NCB. The existence of ancient zircons indicates the magmas were contaminated by continental crust. The dioritic rocks in Anlin area are characterized by relatively high Mg^#, high content of alkaline, and low content of SiO₂. They are rich in LILEs (Ba, S) and LREEs, depleted in HFSEs (Nb, Ti, Ta). According to the petrography and Harker variation diagrams, we speculate that the Anlin diorite rocks originated from fractional crystallization dominated by mafic minerals of primitive gabbroic magma, which is derived by partial melting of fertile lithospheric mantle in response to the asthenosphere upwelling. Meanwhile, lower crustal contamination occurred in the same process of the fractional crystallization. Similar chondrite-normalized rare earth element distribution patterns in iron ores and diorite rocks indicate that the iron mineralization had a closely relationship with magmatism. Comparison of major elements geochemistry of diorite rocks, skarn, ore, and carbonate wall rocks suggests that sodic metasomatism makes elements such as Na, K, Fe, and Si migrate in the different rocks, and it is the iron that provides source material for basic mineralization. The primary magma of Anlin area is water-rich. The fractional crystallization accelerated the residual magma water-saturated and exsolution. In the evolution of magma, the contamination from gypsum and carbonate layers not only promoted the magma exsolution but also made the fluid enrich in Cl^-. It created the good conditions for the forming of Cl^--rich magmatic fluid, which is conducive to iron activation.