矿浆型铁矿的氧同位素判别标志:以宁芜玢岩铁矿为例  

Oxygen isotopic discriminant marker of magmatic iron deposits: Ningwu porphyrite iron ore as an example

作　　者：李延河[1] 段超[1] 韩丹[1] 刘锋[1] 万德芳[1] 王成玉[1] 

机构地区：[1]国土资源部成矿作用与资源评价重点实验室中国地质科学院矿产资源研究所,北京100037

出　　处：《岩石学报》2017年第11期3411-3421,共11页Acta Petrologica Sinica

基　　金：国家重点研发计划(2016YFC06002006);国家自然科学基金项目(41473014);基本业务费项目(YYWF2016)联合资助

摘　　要：自然界是否存在矿浆型铁矿以及如何判别矿浆型铁矿是地质学家争论探索了几十年的问题。大量地质现象和实验研究证实,中酸性岩浆在高氧逸度、富磷等挥发分和助熔剂的条件下,硅酸盐熔体与铁氧化物熔体之间可以发生液态不混熔,熔离出富铁氧化物熔体或富铁岩浆。但有些学者认为实验无法直接一次性熔离出高品位铁矿浆,因此不存在矿浆型铁矿。实际上,高品位富铁矿浆可能是经过多次熔离富集形成的,而非一次简单熔离完成;磁铁矿中钛含量偏低,可能与矿浆型铁矿遭受后期热液改造、钛大量丢失有关,最典型的例子莫过于智利拉科铁矿。长江中下游玢岩铁矿是我国重要铁矿资源类型,其中是否发育矿浆型铁矿也一直存在激烈争论。为了避免不必要的争议,本文将铁矿浆限定为由岩浆熔离形成的铁氧化物浓度>30%的富铁熔体,由铁矿浆演化形成的铁矿床称为矿浆型铁矿。根据宁芜成矿岩体中锆石的钛温度计确定了岩浆的温度,根据锆石的氧同位素组成及磁铁矿与锆石之间的氧同位素分馏方程,计算出岩浆温度下直接从熔体中熔离出来的磁铁矿的δ^(18)O_(Mt)值为4.2‰。据此建立了宁芜玢岩铁矿中矿浆型铁矿的氧同位素判别标志,如果矿体中磁铁矿的δ^(18)O_(Mt)≥4.0‰,则为矿浆型铁矿,否则为热液型或浆-液过渡型铁矿。判别结果表明,钟姑山矿田矿浆型和热液型矿体同时存在,梅山铁矿介于矿浆型-热液型之间,而凹山矿田铁矿则为热液型,与野外观测及前人研究结果一致。The existence of magma-type iron ore in nature and its identification has been the subject of contentious debate among geologists since decades. Many geological phenomena and experimental studies have confirmed that acidic magma can facilitate immiscibility between a silicate melt and iron oxide melt, under high oxygen fugacity, phosphorus-rich, volatile, and flux conditions. However, some scientists believe that experiments cannot enable the direct melting of high-grade iron magma ore; hence, it is not possible to obtain the ore magma. Nevertheless, high-grade iron-rich magma ore may be formed by multiple melting and enrichment processes. The low content of titanium magnetite from the magma ore is subsequently emplaced by hydrothermal alteration, which leads to the migration of titanium, the most typical example of which is seen in the El Laco iron deposit in Chile. The porphyry iron deposit located in the Middle-Lower Yangtze River is an important iron ore resource in China. The confirmed existence of magma type ore is highly disputed. In this study, iron magma ore is defined as an iron rich melt formed by magma liquation whose concentration of iron oxide is greater than 30%. The iron deposit formed by the evolution of the iron-rich magma is called a magma iron deposit. Based on the magma crystallization temperature of intrusions in the Ningwu area obtained by a magmatic zircon titanium thermometer, oxygen isotopic compositions of zircon of metallogenic-associated intrusions, and the oxygen isotope fractionation equation between magnetite and zircon, the oxygen isotope of the magnetite formed by direct liquation in magmatic temperature has been calculated （δ18OMt=4.2‰）. By using the oxygen isotope discrimination technique for magnetite, the value of δ18OMt for magnetite from the magma ore is determined to be more than 4.0‰, while that for the hydrothermal genesis magnetite is lower than 4.0‰. Thus, there are both magmatic as well as hydrothermal-type ores in the Gushan and Yangzhuang iron deposits.

关 键 词：玢岩铁矿 铁矿浆 氧同位素 判别标志 

分 类 号：P597.2[天文地球—地球化学] P618.31[天文地球—地质学]