Geochemistry and iron isotope systematics of coexisting Fe-bearing minerals in magmatic Fe-Ti deposits: A case study of the Damiao titanomagnetite ore deposit, North China Craton
Geochemistry and iron isotope systematics of coexisting Fe-bearing minerals in magmatic Fe-Ti deposits: A case study of the Damiao titanomagnetite ore deposit, North China Craton
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岩浆铁钛矿床共生含铁矿物地球化学及铁同位素系统学研究——以华北克拉通大庙钛磁铁矿矿床为例
DOI：
10.1016/j.gr.2019.12.001
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发表时间：
2020
影响因子：
6.1
通讯作者：
Sun Pu
中科院分区：
文献类型：
--
作者：
Wei Youqing;Niu Yaoling;Gong Hongmei;Duan Meng;Chen Shuo;Guo Pengyuan;Sun Pu
为探讨钛磁铁矿的亚固相冷却历史，对大庙斜长岩型铁单矿型铁单矿型氧化钛矿床中分离出的磁铁矿、钛铁矿和黄铁矿的地球化学和铁同位素组成进行了分析。FeSingle键钛氧化物形成两个系列的固溶体，即ULVö尖晶石-磁铁矿(USP-mTSS)和赤铁矿-钛铁矿(Hem-Ilmss)固溶体。磁铁矿的尖晶石含量为14-27摩尔%，钛铁矿的含量为5-8摩尔%赤铁矿。矿物分离的主要元素组成表明，钛铁矿主要是在~820-600℃温度范围内通过超细尖晶石的氧化作用从USP-MTS中析出的，并与磁铁矿经历了氧化物间的再平衡。与出溶有关的是大量的矿物间铁同位素分馏作用。磁铁矿矿物具有高δ57Fe(+0.27−+0.65‰)的特征，而钛铁矿矿物具有较低的δ57Fe(−0.65~−0.28‰)。在岩石学上观察到两种类型的黄铁矿，每一种都有独特的铁同位素指纹。具有较高δ57Fe(δ57Fe=0.63−+0.95‰)的I型黄铁矿(黄铁矿I)与岩浆成因一致，而具有较低δ57Fe(δ57Fe=−0.90~−0.11‰)的II型黄铁矿(黄铁矿II)可能是从流体中沉淀出来的。黄铁矿的铁同位素指纹可能反映了氧逸度的变化，而黄铁矿II的铁同位素指纹可能是流体活动的结果。磁铁矿和钛铁矿之间的铁同位素分馏是亚固相线过程(包括ULVö尖晶石氧化和氧化物间重新平衡)的净结果，尽管存在，但不需要改变氧逸度。尽管磁铁矿-钛铁矿对的不同组成反映了氧逸度的变化，但氧化铁同位素分馏并不反映这种变化。
Geochemical and iron isotopic compositions of magnetite, ilmenite and pyrite separates from the Fesingle bondTi oxide ores hosted in the Damiao anorthosite-type Fesingle bondTi ore deposit were analyzed to investigate sub-solidus cooling history of the titanomagnetite. The Fesingle bondTi oxides form two series of solid solutions, namely, ulvöspinel-magnetite (Usp-Mtss) and hematite-ilmenite (Hem-Ilmss) solid solutions. The magnetite separates have 14–27 mol% ulvöspinel, while the ilmenite separates have 5–8 mol% hematite. Major element compositions of the mineral separates suggest that the ilmenites were mainly exsolved from the Usp-Mtssby oxidation of ulvöspinel in the temperature range of ~820–600 °C and experienced inter-oxide re-equilibration with the magnetites. Associated with the exsolution is the substantial inter-mineral iron isotope fractionation. The magnetite separates are characterized by high δ57Fe (+0.27 − +0.65‰), whereas the ilmenite separates have lower δ57Fe (−0.65 to −0.28‰). Two types of pyrite are petrographically observed, each of which has a distinctive iron isotope fingerprint. Type I pyrite (pyriteI) with higher δ57Fe (δ57Fe = +0.63 − +0.95‰) is consistent with magmatic origin, and type II pyrite (pyriteII) with lower δ57Fe (δ57Fe = −0.90 to −0.11‰) was likely to have precipitated from fluids. Iron isotopic fingerprints of the pyriteIprobably indicate variations of oxygen fugacity, whereas those of the pyriteIImay result from fluid activities. The iron isotopic fractionation between the magnetite and ilmenite is the net result of sub-solidus processes (including ulvöspinel oxidation and inter-oxide re-equilibration) without needing varying oxygen fugacity albeit its presence. Although varying composition of magnetite-ilmenite pairs reflects variations of oxygen fugacity, inter-oxide iron isotopic fractionation does not.