The Geochemistry of Magnetite and Apatite from the El Laco Iron Oxide-Apatite Deposit, Chile: Implications for Ore Genesis
The Geochemistry of Magnetite and Apatite from the El Laco Iron Oxide-Apatite Deposit, Chile: Implications for Ore Genesis
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DOI：
10.5382/econgeo.4753
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发表时间：
2020-11-01
期刊：
影响因子：
5.8
通讯作者：
Childress, Tristan M.
中科院分区：
文献类型：
--
作者：
La Cruz, Nikita L.;Ovalle, J. Tomas;Childress, Tristan M.
智利北方上新世-更新世El Laco氧化铁-磷灰石（IOA）存款的块状磁铁矿矿体（磁铁矿体积百分比> 90%）的露头和近地表暴露的结构类似于玄武岩熔岩流，其成分与高低温热液磁铁矿重叠。现有的模式-安山质熔岩流的液体不可渗透性和完全交代交代-试图解释矿体的成因完全是火成或完全热液过程。重要的是，这些模型是通过仅研究近地表和露头样本开发的。在这里，我们提出了一个全面的研究结果，从露头和钻孔岩芯，需要一个新的模式的埃尔拉科矿石存款的演变样品。背散射电子（BSE）成像，电子探针显微分析（EPMA）和激光烧蚀电感耦合等离子体质谱（LA-ICP-MS）被用来调查的磁铁矿和磷灰石的纹理和成分的变化，从表面和钻芯样品，以获得一个整体的了解横向和纵向通过矿体的纹理和成分。对5个矿体（Cristales Grandes、Rodados Nehran、圣维森特Alto、Laco Norte和Laco Sur）的39个地表样品和3个矿体（Laco Norte、Laco Sur和Extension Laco Sur）的47个钻孔岩芯样品进行了磁铁矿分析。本文研究了三个矿体（Cristales Grandes、Rodados Nehran和Laco Sur）8个地表样品中磷灰石的地球化学。这些样品中磁铁矿的微量元素组成与岩浆岩和岩浆热液系统中的磁铁矿相似。来自矿体深部的磁铁矿颗粒含钛>1 wt %，以及钛铁矿氧化出溶片晶和间隙钛铁矿。深部钛磁铁矿中的钛铁矿氧出溶纹层、间隙钛铁矿和类火成微量元素的富集与钛磁铁矿的硅酸盐熔体或高温岩浆热液的原始结晶相一致。磁铁矿中微量元素浓度从中间到浅深度的系统性降低与冷却岩浆热液流体中磁铁矿的逐步生长是一致的。地表露头的磷灰石颗粒富含F（通常> 3wt%），其成分与火成岩和岩浆热液磷灰石重叠。磁铁矿和氟磷灰石颗粒含有矿物夹杂物（例如，独居石和钍石），表明同矿化或矿化后交代蚀变。磁铁矿颗粒通常在三联点处相遇，这保存了成矿过程中或成矿后矿石矿物与热液流体再平衡的证据。这里提供的数据与埃尔拉科矿体的成因一致，这些矿体是通过含磁铁矿的岩浆热液悬浮液的浅侵位和喷发而形成的，这些悬浮液是由减压引起的火山大厦塌陷而动员起来的。成矿磁铁矿流体悬浮液具有类似于玄武岩熔岩流的流变学性质，这解释了地表露头中的结构和空腔和气体逸出管的存在。
The textures of outcrop and near-surface exposures of the massive magnetite orebodies (>90 vol % magnetite) at the Plio-Pleistocene El Laco iron oxide-apatite (IOA) deposit in northern Chile are similar to basaltic lava flows and have compositions that overlap high- and low-temperature hydrothermal magnetite. Existing models-liquid immiscibility and complete metasomatic replacement of andesitic lava flows-attempt to explain the genesis of the orebodies by entirely igneous or entirely hydrothermal processes. Importantly, those models were developed by studying only near-surface and outcrop samples. Here, we present the results of a comprehensive study of samples from outcrop and drill core that require a new model for the evolution of the El Laco ore deposit. Backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to investigate the textural and compositional variability of magnetite and apatite from surface and drill core samples in order to obtain a holistic understanding of textures and compositions laterally and vertically through the orebodies. Magnetite was analyzed from 39 surface samples from five orebodies (Cristales Grandes, Rodados Negros, San Vicente Alto, Laco Norte, and Laco Sur) and 47 drill core samples from three orebodies (Laco Norte, Laco Sur, and Extension Laco Sur). The geochemistry of apatite from eight surface samples from three orebodies (Cristales Grandes, Rodados Negros, and Laco Sur) was investigated. Minor and trace element compositions of magnetite in these samples are similar to magnetite from igneous rocks and magmatic-hydrothermal systems. Magnetite grains from deeper zones of the orebodies contain >1 wt % titanium, as well as ilmenite oxyexsolution lamellae and interstitial ilmenite. The ilmenite oxyexsolution lamellae, interstitial ilmenite, and igneous-like trace element concentrations in titanomagnetite from the deeper parts of the orebodies are consistent with original crystallization of titanomagnetite from silicate melt or high-temperature magmatic-hydrothermal fluid. The systematic decrease of trace element concentrations in magnetite from intermediate to shallow depths is consistent with progressive growth of magnetite from a cooling magmatic-hydrothermal fluid. Apatite grains from surface outcrops are F rich (typically >3 wt %) and have compositions that overlap igneous and magmatic-hydrothermal apatite. Magnetite and fluorapatite grains contain mineral inclusions (e.g., monazite and thorite) that evince syn- or postmineralization metasomatic alteration. Magnetite grains commonly meet at triple junctions, which preserve evidence for reequilibration of the ore minerals with hydrothermal fluid during or after mineralization. The data presented here are consistent with genesis of the El Laco orebodies via shallow emplacement and eruption of magnetite-bearing magmatic-hydrothermal fluid suspensions that were mobilized by decompression-induced collapse of the volcanic edifice. The ore-forming magnetite-fluid suspension would have rheological properties similar to basaltic lava flows, which explains the textures and presence of cavities and gas escape tubes in surface outcrops.