The IOCG deposit at Olympic Dam (South Australia) is hosted within the Roxby Downs Granite, which displays a weakly mineralised contact to the orebody (hereafter ‘outer shell’). In a mineralogical-geochemical characterisation of Fe-oxides from the outer shell, we show silician magnetite (Si-magnetite) and HFSE-bearing hematite define the early stages of alkali-calcic alteration. This association forms in the presence of hydrothermal K-feldspar and calc-silicates via overprinting of magmatic magnetite and ilmenite breakdown. Geochemical modelling, at ≥ 400 °C, shows such reactions occur at pH- f O 2 conditions coinciding with shifts from K-feldspar to sericite, and ilmenite to rutile stability. The subsequent Si-magnetite+siderite association forms down-T in the absence of K-feldspar. Transition from granular to bladed morphologies in Si-magnetite is part of a series of Fe-oxide interconversions, followed by formation of zoned, U-W-Sn-Mo-bearing hematite. Enrichment in REE, Y and U in Si-magnetite and the prevalence of U-W-Sn-Mo-bearing hematite support a granite-derived fluid. Combined, petrographic and geochemical evidence show a transition among Fe-oxides from the outer shell to the orebody attributable to the evolution of the same fluid. Unusual massive magnetite intervals and Fe-oxide nodules in granite are considered due to either the presence of inherited lithologies, metasomatic products, or the result of magnetite-rich, crystal mush forming in the melt. We propose a model, corroborated by recently published data including high-precision U-Pb dating of magmatic zircon and hydrothermal hematite, in which an ‘outer shell’ is initiated at the 6–8 km depth of granite emplacement during volatile release from fluids ponding at intrusion margins. Granite cupola collapse at shallower levels (2–3 km?) follows via uplift along faults, facilitating intense brecciation and ore formation.

中文翻译：

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定义 IOCG 系统的早期阶段：来自南澳大利亚奥林匹克坝矿床外壳中氧化铁的证据

奥林匹克坝（南澳大利亚）的 IOCG 矿床位于 Roxby Downs 花岗岩内，与矿体（以下简称“外壳”）呈弱矿化接触。在外层铁氧化物的矿物地球化学表征中，我们显示硅磁铁矿（Si-磁铁矿）和含 HFSE 的赤铁矿定义了碱钙蚀变的早期阶段。这种结合在热液钾长石和钙硅酸盐存在下通过岩浆磁铁矿和钛铁矿分解的叠印形成。≥ 400 °C 时的地球化学模型显示，这种反应发生在 pH-f O 2 条件下，同时发生从钾长石到绢云母的转变，以及钛铁矿到金红石的稳定性转变。在没有钾长石的情况下，随后的 Si-磁铁矿 + 菱铁矿组合形成 down-T。Si-磁铁矿从粒状到叶片形貌的转变是一系列 Fe-氧化物相互转化的一部分，随后形成带 UW-Sn-Mo 的带状赤铁矿。硅磁铁矿中 REE、Y 和 U 的富集以及含 UW-Sn-Mo 的赤铁矿的普遍存在支持花岗岩衍生的流体。结合岩相学和地球化学证据表明，铁氧化物从外壳到矿体的转变归因于同一流体的演化。花岗岩中不寻常的块状磁铁矿间隔和铁氧化物结核被认为是由于存在继承的岩性、交代产物，或者是在熔体中形成富含磁铁矿的结晶糊状物的结果。我们提出了一个模型，最近发表的数据证实了这一点，包括岩浆锆石和热液赤铁矿的高精度 U-Pb 测年，其中，在侵入边缘积聚的流体释放挥发性物质期间，在 6-8 公里深度的花岗岩就位处开始形成“外壳”。较浅层（2-3 公里？）的花岗岩冲天炉随着沿断层的隆起而坍塌，促进了强烈的角砾化和矿石形成。