The origins of magnetite-apatite deposits are controversial, and the crux of the debate is what types of fluids form these rocks. We present evidence from 20 magnetite-apatite deposits worldwide showing ubiquitous involvement of molten salts. The studied deposits are distributed globally, from various tectonic settings, and from Precambrian to Quaternary in age. In every case, water-poor polycrystalline melt inclusions in ore-stage minerals are dominated by sulfate, chloride, and carbonate components plus variable proportions of calc-silicates, phosphates, and iron ± titanium oxides that re-melt between 285 °C and 1100 °C. These fluids are very different from what is generally expected in most geologic settings, but their ubiquitous presence in magnetite-apatite rocks indicates that molten salts are widespread and essential to the formation of these deposits.Kiruna-type magnetite-apatite rocks are important sources of iron, phosphorus, rare earth elements (REEs), and other critical elements. They are also highly contentious in terms of genesis. Some models invoke hydrothermal processes, such as precipitation from hot water or metasomatic replacement (Sullivan et al., 2023). Others argue for magmatic origins such as crystallization from Fe-rich silicate or oxide-phosphate melts (Velasco et al., 2016). Yet others argue for hybrid processes (Reich et al., 2022). At its core, the debate boils down to a basic question: what types of fluids form these enigmatic rocks?We examined the fluids involved in forming magnetite-apatite rocks through an extensive survey of fluid and melt inclusions from 20 deposits worldwide (Fig. 1; Table 1). The study localities include deposits from different geologic settings and with ages from ca. 2.5 Ga to ca. 2 Ma (Fig. 1; see the Supplemental Material1 for geologic details and references for each locality). We report detailed evidence that formation of every studied deposit involved molten salts composed of sulfate, chloride, and carbonate components. These fluids are radically different from those that have been previously invoked, but our results show that salt melts are ubiquitous in magnetite-apatite rocks.Polycrystalline melt inclusions are abundant in the primary minerals of all deposits studied (Figs. 1 and 2; Table 1; see the Supplemental Material). The inclusions are mostly or entirely filled with polymineralic crystals, commonly together with deformed vapor bubbles that occupy up to 40% of the inclusion volume. Aqueous liquid is seldom observed and occupies <1% of the inclusion volume. Within any given assemblage, properties of the inclusions are consistent in terms of mineral phases, volume fractions, and microthermometry. Hence, the inclusions show no evidence of post-entrapment modification such as water loss.The compositions of the inclusions vary between deposits but are consistent within each deposit, including between different host minerals (Table 1). At room temperature, the inclusions always contain crystalline salts—primarily alkali-calcic sulfate, chloride, and carbonate minerals, in relative proportions that show a modest correlation to the lithologies of the surrounding wall rocks (Figs. 1 and 2; Table 1). Most deposits show significant proportions of two or three salt types (Fig. 1; Table 1), suggesting a continuum of compositional types. Silicate minerals, especially calc-silicates like those found in skarns (Xu et al., 2023), are common in the inclusions (Figs. 1 and 2; Table 1). Inclusions from most deposits also contain significant amounts of iron ± titanium oxides (i.e., hematite, magnetite, and ilmenite; Fig. 2B), even up to 80 vol% in some cases. Phosphate minerals, especially monazite, are present in inclusions from half of the studied deposits (Fig. 2D). Most of the minerals in the inclusions are anhydrous (Table 1), implying that the trapped melts were water-poor (<1 wt% H2O).Anhydrite is the most common sulfate in the inclusions, along with variable amounts of baryte, cesanite, gypsum, glauberite, and celestine (Table 1; Fig. 2; Table S2 in the Supplemental Material). Chloride salts include sylvite, halite, hibbingite, and rinneite (Table 1; Fig. 2E; Table S2). Inclusions rich in chlorides differ starkly from aqueous brine inclusions in that they contain little or no liquid water. Carbonates, especially calcite and dolomite (along with lesser magnesite, natrite, and trona; Table 1; Table S2), are less common in the inclusions compared to sulfates and chlorides, but sometimes dominate (Figs. 1 and 2). Notably, where carbonates are hosted in inclusions in silicate minerals, they are always accompanied by calc-silicates, suggesting chemical exchange between the melt and the host mineral.Silicate phases in the inclusions are primarily calc-silicates such as andradite and diopside. Calc-silicates even dominate in some deposits, but are always accompanied by sulfates, chlorides, and/or carbonates (Fig. 1). Silicate minerals that make up the metasomatic haloes surrounding the orebodies—albite, scapolite, epidote, and actinolite—are common in the inclusions (Table 1). Quartz, uncommon in most magnetite-apatite rocks, sometimes appears in the inclusions but is subordinate to carbonates (Fig. S21A), indicating silica-undersaturated and calc-silicate normative bulk compositions.Heating experiments (detailed in the Supplemental Material) reveal that the polycrystalline inclusions begin to re-melt at temperature (T) as low as 285 °C and are fully molten at 615 °C to >1100 °C (Table 1; Fig. 3). Chloride-rich inclusions generally show the lowest first-melting T (Tf), as low as 285 °C. Carbonates and sulfates melt mostly in the range of 650–900 °C, consistent with eutectic relationships of multicomponent systems. Oxides, phosphates, and calc-silicates show the highest melting T > 1000 °C (Figs. 3B and 3C). Immiscible separation between calc-silicate–rich and salt-rich melt is common in the inclusions at high T, especially between 900 °C and 1100 °C for inclusions rich in chlorides and >1100 °C for the inclusions rich in calc-silicates. In some cases, the inclusions decrepitated prior to complete melting, suggesting high internal pressures. None of the inclusions could be quenched to glass even at cooling rates of >200 °C/s, suggesting that these are low-viscosity ionic liquids.The occurrence of aqueous liquid or vapor inclusions in the studied deposits is highly variable (Table 1), and samples from some deposits host no aqueous fluid inclusions at all. When present, aqueous inclusions are sometimes only vapor-rich, in other cases vapor-rich plus brines, and in yet other cases only brines (Table 1; see details in the Supplemental Material).The core implication of these results is that molten salts are ubiquitous in magnetite-apatite deposits. In contrast to recent suggestions that such melts are unusual, local phenomena (Reich et al., 2022), we find evidence for their involvement worldwide, from Archean through to recent times, and at different tectonic settings and formation depths. Similar melt inclusions also occur in other magnetite- and apatite-rich rocks (Table 1), including phoscorite (magnetite-apatite-olivine) in carbonatite pipes (Palmer, 1998; Solovova et al., 1998) and ultramafic alkaline complexes (Veksler et al., 1998; Nikolenko et al., 2020); magnetite-apatite dikes in alkalic porphyries (Kamenetsky et al., 1999); nelsonite (ilmenite-magnetite-apatite) in anorthosite complexes (Frost and Touret, 1989); and diopside-titanite-magnetite dikes associated with iron oxide veins (Bakker and Elburg, 2006). These rocks represent diverse geologic settings, but each shares a similar mineral assemblage and hosts abundant salt melt inclusions (Table 1). The most straightforward conclusion from these data is that molten salts are integral to the formation of magnetite- and apatite-rich rocks wherever they occur.In light of the evidence for molten salts, the next obvious questions are: where do they come from, and what role do they play in ore formation? Previous studies have invoked saline aqueous fluids of either evaporitic origin (Barton and Johnson, 1996; Li et al., 2015; Yan and Liu, 2022) or magmatic-hydrothermal origin (Knipping et al., 2015; Hu et al., 2020). We acknowledge the potential involvement of such fluids, but we stress that the essentially dry salt melts reported here are quite distinct from both. Inclusions similar to those described here were previously interpreted as “hydrous saline melt” formed by phase separation of a magmatic-hydrothermal fluid (Broman et al., 1999), but the absence of water in the inclusions, along with the abundance of silicate, oxide, and phosphate minerals within, suggest a non-hydrothermal origin. The great formation depth of some studied deposits (Table S1) further discounts low-pressure condensation from a hydrothermal fluid. In our view, the most likely principal sources of these salt melts are hiding in plain sight, arising from interaction between mantle-derived silicate magmas and surface-derived chemical sedimentary rocks.The most straightforward process to generate carbonate-, sulfate- and chloride-rich melts is by melting of limestones and evaporites. Many magnetite-apatite deposits are emplaced into limestones (Fig. 1), and magnetite-apatite rocks show both spatial correlations and isotopic similarities with evaporite sequences (Barton and Johnson, 1996; Li et al., 2015; Tornos et al., 2017; Bain et al., 2020, 2021; Peters et al., 2020). We contend that evaporite-derived aqueous brines are not the key factor, as such brines circulate in numerous hydrothermal settings, from orogenic belts (Morrissey and Tomkins, 2020) to the roots of many porphyry copper deposits (Runyon et al., 2019), without generating magnetite-apatite rock. The key difference is whether the evaporite package melts. Similarly, in the case of carbonate rocks, we can make an analogy to calc-silicate skarns, which are thought to be primarily hydrothermal-metasomatic in origin. The key difference, again, is whether limestone melting generates carbonate-rich melts. Such melts are potent metasomatic agents that can generate calc-silicates (Vasyukova and Williams-Jones, 2022) and crystallize to antiskarn (Anenburg and Mavrogenes, 2018) and endoskarns, the latter representing transitional cases between skarn and magnetite-apatite systems (Xu et al., 2023).Melting of limestones and evaporites should be suspected wherever hot magmas intrude such rocks. Whereas the solidus T of monomineralic salt is generally high (for example, 1460 °C and 1339 °C for anhydrite and calcite, respectively), eutectic relationships of multicomponent systems dramatically lower the melting T. The eutectic between CaSO4 and CaCO3 is at 977 °C (Treiman, 1995) and that between CaCl2 and CaSO4 is at 635 °C (Freidina and Fray, 2000). By analogy to CaCl2-CaSO4-CaF2 (Arbukhanova et al., 2009), the eutectic between CaCl2, CaSO4, and CaCO3 is likely <600 °C. Addition of alkali chlorides lowers the eutectic even further (Walter et al., 2020), as do phosphate and fluoride (Treiman, 1995), and even small amounts of water (Durand et al., 2015). Hence, halide-bearing carbonate melts persist to ~200 °C (Anenburg et al., 2020), and our results show that melts dominated by sulfates, chlorides, and carbonates are stable down to <300 °C (Table 1).While melting of chemical sediments supplies molten salts, a related industrial process—namely, smelting—provides insight into how these ingredients influence ore formation. Smelting is the process whereby metals are extracted from a rock by first melting it, then prompting separation of the melt into two immiscible liquids (liquid “matte” enriched in the metal of interest, and liquid “slag” composed of the unwanted impurities) through strategic addition of suitable fluxes (Moore, 1981). The fluxes serve multiple roles: lowering the melting T and melt viscosity, triggering immiscible phase separation, and sequestering alkalis, SiO2, Al2O3, and other impurities in the buoyant calc-silicate slag. Sulfate, chloride, and especially carbonate salts are among the chief fluxes used in industrial smelting; especially limestone in the case of traditional iron smelting (Moore, 1981).In magnetite-apatite deposits, assimilation of sulfate, chloride, and carbonate salts plays much the same role as they do in smelting: triggering immiscible separation of iron-rich liquids and promoting selective removal of silicate impurities. A process analogous to industrial smelting unfolds when hot, mantle-derived magmas encounter and assimilate fluxes—the same ones used in industrial furnaces—in the form of chemical sedimentary rocks. Just like in an industrial furnace, addition of carbonate, sulfate, and chloride fluxes triggers immiscible separation of two or more liquids while also lowering the solidus T and viscosity. Molten salts then assist in both forming and refining the nascent ores by accumulating iron, phosphate, and REEs (Anenburg et al., 2020; Pietruszka et al., 2023), and by removing calc-silicate impurities (Tornos et al., 2024). Snapshots of the early stages of such immiscible separation are recorded in melt inclusions in the andesites from El Laco, Chile (Pietruszka et al., 2023, 2024). As phase separation proceeds, significant volume fractions of iron oxides and REE phosphate in the inclusions record concentration of iron and phosphorous in the “matte” (Fig. 2B; Fig. S16A). Other components are sequestered in the low-viscosity and chemically reactive residual salt melt, stripping away impurities and generating haloes of calc-silicate rock analogous to industrial slag. The calc-silicate rich inclusions represent aliquots of contaminated melt trapped amid this zone-refining process, whereby most of the principal impurities are removed.For decades, magnetite-apatite deposits have defied attempts to define a coherent genetic model. We suggest that this is largely because one of the key ingredients—molten salts—has been overlooked. Many previous studies seem to have assumed that the fluids must be similar to those in other well-known ore deposit types, such as porphyry-copper deposits, even though the ores themselves are markedly different. Widespread evidence for molten salts in magnetite-apatite deposits offers a new perspective on what these systems represent and reconciles why they never fit neatly into the traditional “magmatic” or “hydrothermal” categories. Our interpretations of the sources and roles of these fluids remain speculative (they are working hypotheses, and need further investigation), but evidence for their widespread involvement is concrete and direct. Hence, models for how these rocks form must explicitly address the role of molten salts, not as an occasional or anomalous factor, but as a basic commonality across this entire category of deposits. More broadly, this means that our view of crustal geologic fluids must expand to include molten salts as an essential type that gives rise to distinctive mineralization and metasomatism.We thank Michael Anenburg, Dan Harlov, and an anonymous reviewer for constructive comments that helped us improve the paper. This study was supported by Natural Sciences and Engineering Research Council of Canada Discovery Grants to M. Steele-MacInnis (RG-PIN/2018-04370) and J.M. Hanchar (RG-PIN/004649-2015), and by the Spanish Grant NANOMET PID2022-138768OB-I00 funded by MCIN/AEI/10.13039/50110001133 to F. Tornos. R.S. Bottrill publishes with the permission of the Director of Mines, Tasmania.

中文翻译：

---

磁铁矿-磷灰石矿石记录了熔盐的广泛参与


磁铁矿-磷灰石矿床的起源存在争议，争论的关键是这些岩石是由什么类型的流体形成的。我们提供了来自全球 20 个磁铁矿-磷灰石矿床的证据，表明熔盐的参与无处不在。研究的矿床分布在全球各地，具有不同的构造环境，年龄从前寒武纪到第四纪。在每种情况下，成矿阶段矿物中的贫水多晶熔融包裹体主要由硫酸盐、氯化物和碳酸盐成分以及不同比例的硅酸钙、磷酸盐和铁±氧化钛组成，这些成分在 285 °C 至 1100 °C 之间重新熔化。 °C。这些流体与大多数地质环境中通常预期的非常不同，但它们在磁铁矿-磷灰石岩石中的普遍存在表明熔盐分布广泛，并且对于这些矿床的形成至关重要。基律纳型磁铁矿-磷灰石岩石是重要的来源。铁、磷、稀土元素 (REE) 和其他关键元素。它们在起源方面也存在很大争议。一些模型引用了热液过程，例如热水沉淀或交代置换（Sullivan et al., 2023）。其他人则认为岩浆起源，例如从富含铁的硅酸盐或氧化物磷酸盐熔体中结晶出来（Velasco 等，2016）。还有一些人主张混合流程（Reich 等人，2022）。争论的核心归结为一个基本问题：这些神秘岩石是由什么类型的流体形成的？我们通过对全球 20 个矿床的流体和熔体包裹体进行广泛调查，研究了参与形成磁铁矿-磷灰石岩石的流体（图 1） ; 表格1）。研究地点包括来自不同地质环境的矿床，其年龄可追溯至约 100 年。 2.5 Ga 至约。 2毫安（图二） 1;请参阅补充材料1，了解每个地区的地质详细信息和参考资料）。我们报告了详细的证据，表明每个研究的矿床的形成都涉及由硫酸盐、氯化物和碳酸盐成分组成的熔盐。这些流体与之前提到的流体截然不同，但我们的结果表明，盐熔体在磁铁矿-磷灰石岩石中普遍存在。在所有研究的矿床的原生矿物中都存在丰富的多晶熔体包裹体（图 1 和 2；表 1） ；参见补充材料）。包裹体大部分或全部充满了聚合矿物晶体，通常还含有变形的蒸气泡，占据了包裹体体积的 40%。水性液体很少被观察到并且占包涵体体积的<1%。在任何给定的组合内，包裹体的特性在矿物相、体积分数和显微测温方面都是一致的。因此，包裹体没有显示出水流失等包埋后改变的证据。包裹体的成分因矿床而异，但在每个矿床内是一致的，包括不同宿主矿物之间的成分（表 1）。在室温下，包裹体始终含有结晶盐，主要是碱钙硫酸盐、氯化物和碳酸盐矿物，其相对比例与周围围岩的岩性具有适度的相关性（图 1 和 2；表 1）。大多数沉积物显示出两种或三种盐类型的显着比例（图 1；表 1），表明成分类型的连续体。硅酸盐矿物，特别是钙硅酸盐，如在夕卡岩中发现的硅酸盐（Xu 等人，2023），在包裹体中很常见（图 1 和 2；表 1）。 大多数矿床的包裹体还含有大量的铁±钛氧化物（即赤铁矿、磁铁矿和钛铁矿；图 2B），在某些情况下甚至高达 80 vol%。磷酸盐矿物，尤其是独居石，存在于一半研究矿床的包裹体中（图 2D）。包裹体中的大多数矿物都是无水的（表 1），这意味着被捕获的熔体是贫水的（<1 wt% H2O）。硬石膏是包裹体中最常见的硫酸盐，还有不同数量的重晶石、铍石英、石膏、钙芒硝和天青石（表 1；图 2；补充材料中的表 S2）。氯化物盐包括钾盐、石盐、锂盐和锂盐（表1；图2E；表S2）。富含氯化物的包裹体与含水盐水包裹体截然不同，因为它们含有很少或不含液态水。与硫酸盐和氯化物相比，碳酸盐，特别是方解石和白云石（以及少量的菱镁矿、钠铁矿和天然碱；表 1；表 S2）在包裹体中较少见，但有时占主导地位（图 1 和图 2）。值得注意的是，当碳酸盐存在于硅酸盐矿物的包裹体中时，它们总是伴随着钙硅酸盐，这表明熔体和基质矿物之间存在化学交换。包裹体中的硅酸盐相主要是钙硅酸盐，例如钙铁矿和透辉石。硅酸钙甚至在某些矿床中占主导地位，但总是伴有硫酸盐、氯化物和/或碳酸盐（图 1）。构成矿体周围交代晕的硅酸盐矿物（钠长石、方柱石、绿帘石和阳起石）在包裹体中很常见（表 1）。石英在大多数磁铁矿-磷灰石岩石中并不常见，有时出现在包裹体中，但其地位次于碳酸盐（图 1）。 S21A），表示二氧化硅不饱和和钙硅酸盐标准散装成分。加热实验（在补充材料中详细介绍）表明，多晶夹杂物在低至 285 °C 的温度 (T) 下开始重新熔化，并在615 °C 至 >1100 °C（表 1；图 3）。富含氯化物的包裹体通常表现出最低的首次熔化 T (Tf)，低至 285 °C。碳酸盐和硫酸盐大多在 650–900 °C 范围内熔化，与多组分体系的共晶关系一致。氧化物、磷酸盐和硅酸钙的最高熔化温度 > 1000 °C（图 3B 和 3C）。富含硅酸钙熔体和富含盐熔体之间的不混溶分离在高 T 下的包裹体中很常见，尤其是在 900 °C 至 1100 °C 之间，对于富含氯化物的包裹体，以及 >1100 °C 对于富含钙硅酸盐的包裹体。在某些情况下，包裹体在完全熔化之前就爆裂了，这表明内部压力很高。即使在 >200 °C/s 的冷却速率下，也没有任何夹杂物能够淬火成玻璃，这表明这些是低粘度离子液体。所研究的沉积物中水性液体或蒸气夹杂物的出现情况变化很大（表 1） ，并且一些沉积物的样品根本不含有水性流体包裹体。当存在时，含水包裹体有时仅富含蒸气，在其他情况下富含蒸气加盐水，而在其他情况下仅含有盐水（表 1；请参阅补充材料中的详细信息）。这些结果的核心含义是熔盐普遍存在于磁铁矿-磷灰石矿床中。与最近的建议相反，这种融化是不寻常的、局部现象（Reich 等人，2017）。，2022），我们找到了它们在世界范围内参与的证据，从太古宙到近代，以及不同的构造环境和地层深度。类似的熔体包裹体也出现在其他富含磁铁矿和磷灰石的岩石中（表 1），包括碳酸盐岩管中的磷磷矿（磁铁矿-磷灰石-橄榄石）（Palmer，1998；Solovova 等，1998）和超镁铁碱性复合物（Veksler 等）等人，1998；Nikolenko 等人，2020）；碱性斑岩中的磁铁矿-磷灰石岩脉（Kamenetsky 等，1999）；斜长石复合体中的钛铁矿（钛铁矿-磁铁矿-磷灰石）（Frost 和 Touret，1989）；以及与氧化铁矿脉相关的透辉石-钛矿-磁铁矿岩脉（Bakker 和 Elburg，2006 年）。这些岩石代表了不同的地质环境，但每种岩石都具有相似的矿物组合，并含有丰富的盐熔体包裹体（表 1）。这些数据最直接的结论是，熔盐是富含磁铁矿和磷灰石岩石的形成不可或缺的一部分，无论它们发生在哪里。根据熔盐的证据，下一个明显的问题是：它们来自哪里，以及它们在矿石形成中起什么作用？先前的研究援引了蒸发成因（Barton和Johnson，1996；Li等，2015；Yan和Liu，2022）或岩浆-热液成因（Knipping等，2015；Hu等，2020）的含盐水流体。 ）。我们承认此类流体的潜在参与，但我们强调，这里报道的基本上干燥的盐熔体与两者都截然不同。与此处描述的类似的包裹体以前被解释为由岩浆热液流体相分离形成的“含水盐水熔体”（Broman 等人，2017）。，1999），但是包裹体中不存在水，并且内部含有丰富的硅酸盐、氧化物和磷酸盐矿物，表明其非热液成因。一些研究矿床的巨大地层深度（表 S1）进一步削弱了热液的低压凝结作用。在我们看来，这些盐熔体最有可能的主要来源隐藏在显而易见的地方，是由地幔衍生的硅酸盐岩浆与地表衍生的化学沉积岩之间的相互作用产生的。产生碳酸盐、硫酸盐和氯化物的最直接的过程-丰富的熔体是通过石灰石和蒸发岩的熔化而形成的。许多磁铁矿-磷灰石矿床侵位于石灰岩中（图1），磁铁矿-磷灰石岩石与蒸发岩序列表现出空间相关性和同位素相似性（Barton和Johnson，1996；Li等，2015；Tornos等，2017） ；贝恩等人，2020，2021；彼得斯等人，2020）。我们认为，蒸发岩衍生的卤水不是关键因素，因为此类卤水在许多热液环境中循环，从造山带（Morrissey 和 Tomkins，2020）到许多斑岩铜矿床的根部（Runyon 等，2019），不生成磁铁矿-磷灰石岩石。主要区别在于蒸发包是否熔化。同样，就碳酸盐岩而言，我们可以与钙硅酸盐夕卡岩进行类比，人们认为其起源主要是热液交代作用。关键的区别在于石灰石熔化是否会产生富含碳酸盐的熔体。 这种熔体是有效的交代剂，可以生成钙硅酸盐（Vasyukova 和 Williams-Jones，2022）并结晶成反矽卡岩（Anenburg 和 Mavrogenes，2018）和内皮卡岩，后者代表矽卡岩和磁铁矿-磷灰石系统之间的过渡情况（Xu 等人） al., 2023)。只要热岩浆侵入石灰岩和蒸发岩的岩石，就应该怀疑这些岩石的熔化。虽然单矿物盐的固相线 T 通常较高（例如，硬石膏和方解石分别为 1460 °C 和 1339 °C），但多组分体系的共晶关系显着降低了熔化 T。CaSO4 和 CaCO3 之间的共晶温度为 977 ° C (Treiman, 1995) 而 CaCl2 和 CaSO4 之间的温度为 635 °C (Freidina 和 Fray, 2000)。与 CaCl2-CaSO4-CaF2 类比（Arbukhanova 等，2009），CaCl2、CaSO4 和 CaCO3 之间的共晶可能 <600 °C。添加碱金属氯化物会进一步降低共晶（Walter 等人，2020），磷酸盐和氟化物也是如此（Treiman，1995），甚至少量的水（Durand 等人，2015）也是如此。因此，含卤化物碳酸盐熔体持续到约 200 °C（Anenburg 等人，2020），我们的结果表明，以硫酸盐、氯化物和碳酸盐为主的熔体在低至 <300 °C 的温度下保持稳定（表 1）。化学沉积物的熔化提供熔盐，这是一个相关的工业过程（即熔炼），可以深入了解这些成分如何影响矿石的形成。熔炼是从岩石中提取金属的过程，首先将其熔化，然后通过以下方式促使熔体分离成两种不混溶的液体（富含目标金属的液体“冰铜”和由不需要的杂质组成的液体“炉渣”）战略性地添加合适的助熔剂（Moore，1981）。 该助熔剂具有多种作用：降低熔化温度和熔体粘度，引发不混溶相分离，以及隔离浮力硅酸钙渣中的碱金属、SiO2、Al2O3 和其他杂质。硫酸盐、氯化物，尤其是碳酸盐是工业冶炼中使用的主要熔剂；尤其是传统炼铁中的石灰石（Moore，1981）。在磁铁矿-磷灰石矿床中，硫酸盐、氯化物和碳酸盐的同化作用与它们在冶炼中的作用大致相同：引发富铁液体和铁盐的不混溶分离。促进选择性去除硅酸盐杂质。当来自地幔的炽热岩浆遇到并同化化学沉积岩形式的熔剂（与工业熔炉中使用的熔剂相同）时，就会出现类似于工业熔炼的过程。就像在工业炉中一样，添加碳酸盐、硫酸盐和氯化物助熔剂会引发两种或多种液体的不混溶分离，同时也会降低固相线 T 和粘度。然后，熔盐通过积累铁、磷酸盐和稀土元素（Anenburg 等人，2020；Pietruszka 等人，2023）以及去除硅酸钙杂质（Tornos 等人，2024）来帮助形成和精炼新生矿石。 ）。这种不混溶分离的早期阶段的快照记录在智利埃尔拉科安山岩的熔体包裹体中（Pietruszka 等，2023，2024）。随着相分离的进行，夹杂物中氧化铁和稀土磷酸盐的显着体积分数记录了“冰面”中铁和磷的浓度（图2B；图S16A）。 其他成分被隔离在低粘度和化学反应性的残留盐熔体中，去除杂质并产生类似于工业炉渣的硅酸钙岩石晕。富含钙硅酸盐的包裹体代表了在该区域精炼过程中被捕获的受污染熔体的等分部分，从而去除了大部分主要杂质。几十年来，磁铁矿-磷灰石矿床一直无法定义一个连贯的遗传模型。我们认为这主要是因为关键成分之一——熔盐——被忽视了。之前的许多研究似乎都假设这些流体必须与其他众所周知的矿床类型（例如斑岩铜矿床）中的流体相似，尽管矿石本身明显不同。磁铁矿-磷灰石矿床中熔盐的广泛证据为这些系统所代表的内容提供了新的视角，并解释了为什么它们从未完全符合传统的“岩浆”或“热液”类别。我们对这些液体的来源和作用的解释仍然是推测性的（它们是可行的假设，需要进一步调查），但它们广泛参与的证据是具体和直接的。因此，这些岩石如何形成的模型必须明确解决熔盐的作用，不是作为偶然或异常因素，而是作为整个矿床类别的基本共性。更广泛地说，这意味着我们对地壳地质流体的看法必须扩大到包括熔盐作为一种基本类型，它会产生独特的矿化和交代作用。我们感谢 Michael Anenburg、Dan Harlov 和一位匿名审稿人提供的建设性意见，帮助我们改进纸。 这项研究得到了加拿大自然科学和工程研究委员会对 M. Steele-MacInnis (RG-PIN/2018-04370) 和 J.M. Hanchar (RG-PIN/004649-2015) 的发现补助金以及西班牙补助金 NANOMET PID2022 的支持-138768OB-I00 由 MCIN/AEI/10.13039/50110001133 资助给 F. Tornos。 R.S. Bottrill 经塔斯马尼亚矿业总监许可出版。