莱芜张家洼铁矿磁铁矿LA-ICP-MS微量元素特征及其对成矿过程的制约

莱芜张家洼铁矿位于华北克拉通东缘的鲁西地区，矿石成因类型为夕卡岩型铁矿。矿体赋存在早白垩世高镁闪长岩与奥陶系马家沟组灰岩及白云岩接触带附近。本文通过对莱芜岩浆和热液磁铁矿电子探针(EPMA)以及激光剥蚀电感耦合等离子体质谱(LA ICP MS)分析，探讨磁铁矿微量元素组成及变化规律对成岩和成矿作用的指示，为揭示张家洼铁矿的矿床成因及其成矿流体演化过程提供重要制约。分析结果表明，莱芜岩浆磁铁矿与热液磁铁矿相比明显富集Ti、V、Cr等亲铁元素，相对富集Nb、Ta、Zr、Hf等高场强元素以及Sn、Ga、Ge、Sc等中等相容元素，Mg、Al、Mn、Zn、Co显著富集于热液磁铁矿中。Ti、V、Cr以及Mg、Al、Mn、Zn在岩浆和热液中具有不同的地球化学行为，Ti、V、Cr从熔体中进入磁铁矿主要受温度、分配系数以及fO2控制。Mg、Al、Mn、Zn主要受控于水岩反应和后期绿泥石+碳酸盐脉的交代，这些元素通过类质同象替换富集于热液磁铁矿中。Co在热液磁铁矿中除了受水岩相互作用和后期流体交代的影响外，硫化物的出现会导致Co含量急剧降低。Si、Ca、Na及Sr、Ba在岩浆和热液磁铁矿中的地球化学行为非常一致。Ti Ni/Cr图能够用于区分岩浆和热液磁铁矿，莱芜岩浆磁铁矿中Ti含量较高且Ni/Cr比值≤1，热液磁铁矿Ti含量较低且绝大多数Ni/Cr比值≥1。张家洼热液磁铁矿可分为早、晚两个阶段：早期阶段包括(1)早期原生粒状磁铁矿和(2)早期次生磁铁矿；晚期阶段包括(3)晚期原生磁铁矿和(4)晚期次生磁铁矿。原生磁铁矿具有典型的三联点结构特征；次生磁铁矿受后期热液交代影响表现为多空隙，通常呈不规则状、树枝状、骸晶以及交代残余结构。磁铁矿微量元素生动记录了成矿流体演化过程，从早期到晚期、从原生到次生都显示Mg、Al、Mn、Zn包括Co含量持续升高，表明成矿流体可能朝着富集这些微量元素的方向演化。后期流体的交代导致绿泥石蚀变为磁铁矿，连续水岩相互作用和后期流体的交代以及绿泥石直接蚀变是导致热液磁铁矿富集Mg、Al、Mn、Zn等元素的主要原因。热液磁铁矿晚期孔隙较为发育，孔隙度的增加促使更多的流体和磁铁矿发生反应。热液磁铁矿的微量元素不仅能够反映矿床形成的物理化学条件，而且可以反映围岩性质以及水岩相互作用过程。

LA-ICP-MS trace element characteristics of magnetite from the Zhangjiawa iron deposit,Laiwu and constraints on metallogenic processes.

The Zhangjiawa skarn iron deposit is located at the Laiwu area of Luxi Block, eastern North China Craton. It is genetically associated with the Early Cretaceous high Mg diorite, and occurs in the contact zones between the Ordovician Majiagou Formation limestone/dolomite and diorite. In this paper, we carried out detailed analyses of major and trace elements in magnetite using electron microprobe and laser ablation ICP MS methods, aimed to reveal the geochemical features as well as evolutional trends from the magmatic to the hydrothermal magnetites and thus provide significant constraints on the genetic processes of the Zhangjiawa Fe deposit. The results showed that igneous and hydrothermal magnetites in the Zhangjiawa deposit have distinct geochemical features. Compared with the hydrothermal magnetite, the igneous magnetite was significantly enriched in siderophile elements such as Ti, V and Cr. High field strength elements such as Nb, Ta, Zr, and Hf, and moderate compatible elements of Sn, Ga, Ge and Sc, were also relatively enriched in the igneous magnetite. However, lithophile elements such as Mg, Al, Mn, Zn and Co were remarkably enriched in hydrothermal magnetite. Ti, V, Cr, Mg, Al, Mn, and Zn typically exhibited distinct behaviors between the magmatic and hydrothermal systems. Mineralization of magmatic Ti, Cr and V in magnetite was mainly controlled by temperature, partition coefficient and fO2; whereas Mg, Al, Mn and Zn commonly enrichment in hydrothermal magnetite through isomorphic replacement was primarily controlled by water rock interaction and later stage metasomatism of chlorite and carbonate veins. In hydrothermal magnetite, cobalt content was strongly affected by sulfide in addition to water rock interactions and post fluid metasomatism, which would decrease drastically in the presence of sulfides. Si, Ca, Na, Sr and Ba displayed highly consistent geochemical behaviors in both the magmatic and hydrothermal magnetites systems, while Ti verus Ni/Cr ratio could be used to distinguish igneous and hydrothermal magnetites. Our analysis indicate that the Zhangjiawa hydrothermal magnetites can be divided into two stages according to the petrographical evidence. The early stage magnetite includes early primary granular magnetite and early secondary magnetites; the late stage magnetite is composed of late primary and late secondary magnetites. The primary magnetite commonly has the typical triple point structure, while the secondary magnetite is characterized as porous and commonly shows irregular, dendritic, skeletal and metasomatic relict textures. Mg, Al, Mn, Zn and Co contents continued to increase from the early to late stage as well as from the primary to secondary magnetite, possibly due to water rock interactions and breaking down of chlorite during later alteration. Pores are common in the late stage hydrothermal magnetite, likely resulted from elevated fluid magnetite interaction. Therefore, the trace elements in hydrothermal magnetite not only indicate the physical and chemical conditions of the ore forming fluids, but also reflect the properties of the surrounding rocks as well as water rock interactions.