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 Extensión 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.

中文翻译：

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智利El Laco氧化铁-磷灰石矿床磁铁矿和磷灰石的地球化学：对成矿的意义

智利北部上新世更新世El Laco氧化铁-磷灰石（IOA）矿床的块状磁铁矿体（> 90 vol％磁铁矿）的露头和近地表暴露构造与玄武岩熔岩流相似且组成重叠高温和低温热液磁铁矿。现有模型-液体不溶混和安山岩熔岩流的完全交代替代-试图通过完全火成岩或完全热液过程来解释矿体的成因。重要的是，这些模型是通过仅研究近地表和露头样品而开发的。在这里，我们介绍了对露头和钻芯样品进行全面研究的结果，这些样品需要用于El Laco矿床演化的新模型。背散射电子（BSE）成像，电子探针微分析（EPMA），激光消融电感耦合等离子体质谱（LA-ICP-MS）用于研究表层和钻芯样品中磁铁矿和磷灰石的组织和组成变化，以便从侧面和垂直方向全面了解组织的质地和组成矿体。从五个矿体（Cristales Grandes，Rodados Negros，San Vicente Alto，Laco Norte和Laco Sur）的39个表面样品和三个矿体（Laco Norte，Laco Sur和ExtensiónLaco Sur）的47个钻芯样品中分析了磁铁矿。研究了来自三个矿体（Cristales Grandes，Rodados Negros和Laco Sur）的八个表面样品的磷灰石的地球化学。这些样品中磁铁矿的微量和微量元素组成与火成岩和岩浆热液系统中的磁铁矿相似。来自矿体较深区域的磁铁矿晶粒含有> 1 wt％的钛，以及钛铁矿氧化析出薄片和间隙钛铁矿。来自矿体深部的钛磁铁矿中钛铁矿的氧析出层，间隙钛铁矿和类似火成岩的痕量元素浓度与钛酸盐磁铁矿从硅酸盐熔体或高温岩浆热液中的原始结晶相一致。磁铁矿中痕量元素浓度从中间深度到浅深度的系统降低与冷却岩浆热液流体中磁铁矿的逐步生长相一致。来自表面露头的磷灰石晶粒富含F（通常> 3 wt％），其成分与火成岩和岩浆热液磷灰石重叠。磁铁矿和氟磷灰石晶粒包含矿物质夹杂物（例如独居石和方解石），这些矿物包裹体表明矿化后或矿化后的交代变化。磁铁矿晶粒通常在三重交界处相遇，这为矿化期间或之后用热液使矿石矿物重新平衡提供了证据。此处提供的数据与El Laco矿体的成因是一致的，这是由于减压作用引起的火山口塌陷而动员的含磁铁矿岩浆热液悬浮液的浅埋和喷发。形成矿石的磁铁矿流体悬浮液的流变特性与玄武岩熔岩流相似，这解释了地表露头的构造以及空洞和瓦斯逸出管的存在。独石和post石），表明矿化后或矿化后的交代改变。磁铁矿晶粒通常在三重交界处相遇，这为矿化期间或之后用热液使矿石矿物重新平衡提供了证据。此处提供的数据与El Laco矿体的成因是一致的，这是由于减压作用引起的火山口塌陷而动员的含磁铁矿岩浆热液悬浮液的浅埋和喷发。形成矿石的磁铁矿流体悬浮液的流变特性与玄武岩熔岩流相似，这解释了地表露头的构造以及空洞和瓦斯逸出管的存在。独联体或矿化后的矿山或石）。磁铁矿晶粒通常在三重交界处相遇，这为矿化期间或之后用热液使矿石矿物重新平衡提供了证据。此处提供的数据与El Laco矿体的成因是一致的，这是由于减压作用引起的火山口塌陷而动员的含磁铁矿岩浆热液悬浮液的浅埋和喷发。形成矿石的磁铁矿流体悬浮液的流变特性与玄武岩熔岩流相似，这解释了地表露头的构造以及空洞和瓦斯逸出管的存在。保留了在矿化期间或之后用热液使矿石矿物重新平衡的证据。此处提供的数据与El Laco矿体的成因是一致的，这是由于减压作用引起的火山口塌陷而动员的含磁铁矿岩浆热液悬浮液的浅埋和喷发。形成矿石的磁铁矿流体悬浮液的流变特性与玄武岩熔岩流相似，这解释了地表露头的构造以及空洞和瓦斯逸出管的存在。保留了在矿化期间或之后用热液使矿石矿物重新平衡的证据。此处提供的数据与El Laco矿体的成因是一致的，这是由于减压作用引起的火山口塌陷而动员的含磁铁矿岩浆热液悬浮液的浅埋和喷发。形成矿石的磁铁矿流体悬浮液的流变特性与玄武岩熔岩流相似，这解释了地表露头的构造以及空洞和瓦斯逸出管的存在。此处提供的数据与El Laco矿体的成因是一致的，这是由于减压作用引起的火山口塌陷而动员的含磁铁矿岩浆热液悬浮液的浅埋和喷发。形成矿石的磁铁矿-流体悬浮液具有类似于玄武岩熔岩流的流变性质，这解释了地表露头的构造，空洞和瓦斯逸出管的存在。此处提供的数据与El Laco矿体的成因是一致的，这是由于减压作用引起的火山口塌陷而动员的含磁铁矿岩浆热液悬浮液的浅埋和喷发。形成矿石的磁铁矿流体悬浮液的流变特性与玄武岩熔岩流相似，这解释了地表露头的构造以及空洞和瓦斯逸出管的存在。