ORIGIN OF HIGH-Ti MAGNETITE IN MAGMATIC-HYDROTHERMAL SYSTEMS: EVIDENCE FROM IRON OXIDE-APATITE (IOA) DEPOSITS OF EASTERN CHINA
ORIGIN OF HIGH-Ti MAGNETITE IN MAGMATIC-HYDROTHERMAL SYSTEMS: EVIDENCE FROM IRON OXIDE-APATITE (IOA) DEPOSITS OF EASTERN CHINA
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DOI：
10.5382/econgeo.4901;20
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发表时间：
2022-06-01
期刊：
影响因子：
5.8
通讯作者：
Hu, Yi
中科院分区：
文献类型：
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作者：
Zeng, Li-Ping;Zhao, Xin-Fu;Hu, Yi
磁铁矿化学已被广泛用于区分一系列含磁铁矿矿床的火成岩和热液成因。然而，氧化铁-磷灰石（IOA）矿床和其他岩浆-热液系统中钛矿片状富钛磁铁矿的成因仍存在争议。本文通过对中国东部华山和桃村IOA矿床中浸染状、角砾状、块状和脉状矿石中磁铁矿的岩石学、结构和元素数据的研究，对岩浆-热液体系中磁铁矿形成和演化的物化参数（即温度、氧逸度和共存流体成分）进行了约束。在不同类型的矿石中广泛发现了原生高钛磁铁矿（1.49 ~ 4.89 wt % Ti）和次生低钛磁铁矿（多数为1 wt % Ti）。原生高钛磁铁矿含有丰富、发育良好的钛铁矿片层，形成于550℃的氧溶过程中。现场和显微证据表明，该磁铁矿与钠长石、氟磷灰石和放光石的热液矿物组合同期或稍晚。这种类型的磁铁矿具有少量/微量元素（例如，Ti, Al， V, Cr， Ni和Co）组成，与典型的火成岩磁铁矿不同。我们提出原生高钛磁铁矿是在高温含铁盐水中结晶形成的，并提出了基于Cr、Co + Ni和Ti + V + Al系统（通过激光烧蚀-电感耦合等离子体质谱[LA-ICP-MS]测定）区分高钛热液磁铁矿和火成岩磁铁矿的新标准。高钛磁铁矿颗粒随后发生不同程度的溶蚀-再沉淀耦合作用，生成缺微量元素、低钛磁铁矿和新形成的钛矿和金红石。研究表明，高钛磁铁矿可从高温水盐液中析出，这对IOA矿床和其他岩浆-热液系统的演化具有重要意义。
Magnetite chemistry has been widely used to distinguish igneous versus hydrothermal origins for a range of magnetite-bearing mineral deposits. However, the origin of Ti-rich magnetite with ilmenite lamellae from iron oxide-apatite (IOA) deposits and other magmatic-hydrothermal systems remains highly debated. In this study, we present petrographic, textural, and elemental data for magnetite in disseminated, brecciated, massive, and vein ores from the Washan and Taocun IOA deposits of Eastern China to constrain the physicochemical parameters (i.e., temperature, oxygen fugacity, and coexisting fluid compositions) of magnetite formation and evolution in magmatic-hydrothermal systems. Two types of magnetite, primary high-Ti magnetite (1.49-4.89 wt % Ti) and secondary low-Ti magnetite (mostly 1 wt % Ti), have been widely identified in different types of ores. Primary high-Ti magnetite contains abundant, well-developed ilmenite lamellae that formed during oxy-exsolution processes at temperatures 550 degrees C. Field and microscopic evidence suggest that this magnetite was coeval with or slightly later than a hydrothermal mineral assemblage of albite, fluorapatite, and actinolite. This type of magnetite has a minor/ trace element (e.g., Ti, Al, V, Cr, Ni, and Co) composition that is distinct from typical igneous magnetite. We propose that the primary high-Ti magnetite crystallized from high-temperature, Fe-bearing hydrosaline liquids and present new criteria for discriminating high-Ti hydrothermal magnetite from igneous magnetite based on Cr, Co + Ni, and Ti + V + Al systematics (as determined by laser ablation-inductively coupled plasma-mass spectrometry [LA-ICP-MS]). The high-Ti magnetite grains have subsequently undergone variable degrees of coupled dissolution-reprecipitation to produce trace element-deficient, low-Ti magnetite and newly formed titanite and rutile. This study highlights that high-Ti magnetite can precipitate from high-temperature hydrosaline liquids, which has implications in the evolution of IOA deposits and other magmatic-hydrothermal systems.