Reaction-driven magmatic crystallisation at the Maoniuping carbonatite

Domain: earth_science
Published: 2025-08-04
Editorial outcome: major_revision

Abstract
Igneous rocks form by solidification of magmas through cooling or volatile degassing following decompression. Expelled H2O is thought to trigger alteration around intrusions, leading to formation of metasomatic halos. This mechanism is often invoked to explain many magmatic–hydrothermal rock associations, some of them economically mineralised. Maoniuping in China is one of the four largest operating rare earth element (REE) mines globally, whose origin has been attributed to such hydrothermal exsolution. However, no direct evidence links hydrothermal fluids to the formation of Maoniuping and its associated REE mineralisation. Here we show that the REE deposit at Maoniuping formed magmatically from a carbonatitic brine-melt. Textural and chemical evidence reveals extensive interaction with its quartz syenite host, producing albitised fenites. Coupled metasomatism with these fenites led to silica contamination of the carbonatite melt, triggering crystallisation of refractory alkali–ferromagnesian silicates—an antiskarn. This solidified the melt due to removal of the fluxing elements Na and K. Thus, carbonatite melts can crystallise by element assimilation from their environments, precipitating alkali liquid fluxes into solid minerals. Temperature decrease and volatile degassing merely play a secondary role in this igneous rock-forming process. Solidification driven by coupled antiskarnisation and fenitisation affects both the mineral assemblage and ore fabric, and likely operated in most carbonatite-hosted REE deposits elsewhere.

Introduction
Most light REE (the rare earth elements La to Sm) are sourced from carbonatites, igneous rocks composed primarily of magmatic carbonate minerals such as calcite and dolomite1–3. The fundamental mechanisms that lead to the ubiquitous occurrence of REE deposits within these rocks are mostly understood4: Carbonatite rocks contain late-stage magmatic zones with Mg–Fe-rich carbonate assemblages of dolomite, ankerite, and siderite. These zones contain high concentrations of REE, which are typically incompatible in crystallising minerals during earlier stages of carbonatite melt differentiation (primarily calcite)5,6. Evolved carbonatite melts, now depleted of much of their initial CaCO3, resemble solute-rich brines and are often referred to as brine-melts to differentiate them from aqueous hydrothermal fluids as distinct coeval phases from these evolved carbonatite melts4,7,8. Fractionation of CaCO3 and MgCO3 leads to strong enrichment in other incompatible chemical components, most commonly H2O, Na, K, Ba, fluorine, sulfate, and, vitally, the REE5,8,9. This late stage, REE-rich, magmatic brine-melt then forms the mineralisation inside the dolomite–ankerite portion of the carbonatite10, often with associated voluminous barren calcite carbonatites surrounding it. Subsequent hydrothermal activity commonly redistributes REE from fluid-soluble primary igneous minerals (e.g., burbankite) to secondary alteration minerals (e.g., bastnäsite), and may remobilise the REE on a local scale of centimetres to several metres4,11–13. However, this remobilisation does not materially change the spatial distribution or grade of the REE (although it can change the mineral hosts, to the detriment or benefit to economic processing). Indeed, in carbonatite complexes where the entire calcite-to-ankerite sequence is observed, strong REE mineralisation is seldom found in calcite carbonatites, instead being overwhelmingly hosted by dolomite, ankerite, ferrocarbonatites, and their weathered products13–16.

Four carbonatite-associated REE deposits currently provide the majority of the world’s REE supply. Three of them contain substantial REE hosted by lithologies that fit into the aforementioned framework with REE mineralisation primarily hosted in iron-rich parts of carbonatites: The dolomite±calcite Sulphide Queen orebody at Mountain Pass (USA)2,17, the magnesio- and ferrocarbonatites at Mount Weld (Australia)13,18, and the banded dolomite at the Eastern Orebody of Bayan Obo (China)19–24. However, the fourth (Maoniuping, China) is unusual because its bastnäsite-dominated REE mineralisation is associated with calcite, lacking any dolomite or ankerite25. Moreover, most ore veins contain less than 30% calcite, placing them outside the carbonatite definition of the IUGS nomenclature guide or community conventions26,27. This led previous workers to conclude that the Maoniuping ore is not hosted by a carbonatite per se, but rather formed as a local hydrothermal offshoot of various fluid-exsolving alkaline carbonatite or silicate melts21,28–35.

As with most carbonatites, many currently observed minerals at Maoniuping contain abundant fluid inclusions, which trap hydrothermal fluids present during mineralisation30,31,33,35,36 (i.e., a syn-magmatic fluid, if these fluids coexisted with a carbonatite melt or brine-melt3). The Maoniuping fluid inclusions are mostly hosted in quartz and fluorite, and they are rich in alkali sulfates, which have been suggested to facilitate REE mobility and mineralisation in carbonatite systems21,28,30,31,35,36. However, most Maoniuping fluid inclusion studies have found no ore or notable REE contents inside these inclusions28,33,36. Previous suggestions that a fluid in Maoniuping carried substantial REE are based solely on a single fluorite-hosted fluid inclusion31,37. This is problematic because fluorite itself is REE-rich, such that distinguishing fluid-hosted and mineral-hosted REE is challenging. Gangue-hosted fluid inclusions in hydrothermal ore-forming systems elsewhere typically contain the ore itself. For example, chalcopyrite is a ubiquitous daughter mineral in quartz-hosted fluid inclusions found in porphyry copper deposits38. The lack of ore-bearing fluid inclusions at Maoniuping raises the question of whether an ore-forming fluid, as a distinct phase from the carbonatite melt or its evolved brine-melt equivalent, existed at all. Some of the confusion might arise from ambiguity regarding fluids in carbonatite systems. Whereas the distinction is clear in silicate magmas that solidify at roughly 600–700°C and coexist with an exsolved H2O-dominated fluid phase, relationships between fluids, brines, and melts in carbonatites prove somewhat inscrutable8,39. Recent work7,9 revealed that high-temperature carbonatite melts undergo continuous evolution to a lower temperature brine-melt, with temperatures as low as 300°C, all the while remaining immiscible from the true H2O-dominated fluid5. The REE remain in this low-temperature magmatic brine-melt as opposed to the coexisting hydrothermal fluid9. Additional experimental work demonstrated high REE solubility in concentrated alkaline brines (i.e., brine-melts) at these temperatures40. Preliminary results indicate that REE might partition to a hydrothermal fluid in subvolcanic conditions (while remembering the melt–fluid distinction in carbonatite systems is not clear cut)41, but Maoniuping formed at depths where REE have been demonstrated to partition away from hydrothermal fluids (2–2.4 kbar28,30), remaining in the magmatic brine-melt instead7,9,42. Nevertheless, the discovery that low-temperature brine-melts exist as liquid phases that continuously differentiate from high-temperature carbonatite melts, all while being distinct from aqueous hydrothermal fluids, is relatively new3,4,8. Consequently, previous studies have likely misclassified magmatic low-temperature brine-melt inclusions as hydrothermal fluid inclusions43. The distinction can be particularly elusive if both brine-melt and fluid phases are entrapped in the same inclusion. In this case, a REE–carbonate-rich brine-melt might be diluted with H2O, NaCl, and Na2SO4. This obscures the magmatic character of the brine-melt and leads to erroneous interpretations that a REE-bearing hydrothermal fluid phase have existed, with the spurious conclusion that a magmatically-exsolved fluid is crucial for mineralisation44. The distinction between melts, brine-melts, and fluids is not only a matter of nomenclature—it has important implications for understanding spatial REE mobility within carbonatite systems, the timing of formation, changes in REE mineral hosts during alteration, and ultimately our capacity for effective REE exploration and exploitation.

The concept of an ore-forming fluid is predicated on the assumption that any vein deposit within an igneous complex, but without a clear magmatic mineral assemblage, formed from magmatic-derived hydrothermal fluids exsolved out of a crystallising magma, either local or adjacent45. This assumption is largely true for silicate igneous complexes38, but remains unsubstantiated in carbonatite systems. The presence of a separate synmagmatic hydrothermal fluid phase in equilibrium with actively forming magmatic mineralisation does not necessarily mean the fluid had any REE carrying capacity. Taken together with the extreme immobility of REE in calcite+fluorite saturated fluids46–48 and the preferential partitioning of REE to melts rather than fluids9,42, exsolved-hydrothermal formation models for the mineralisation at Maoniuping are tenuous, and a magmatic model should be considered. Nonetheless, mineral textures and relationships in Maoniuping are perplexingly different to other magmatically mineralised carbonatites. Here, we examine textures and mineral compositions from the Dagudao mining camp of Maoniuping and suggest a magmatic solidification process to explain its formation.

Results
The Dagudao open pit contains three main ore-bearing rock units, from bottom to top: (1) stockworks, (2) veinlets, and (3) coarse ore veins. These rock units have been described in detail elsewhere25,29. All rocks described here have been photographed and sampled from the top ore vein unit around coordinates 101°58'57“E–28°26'59“N.

Textural relationships
Maoniuping comprises sub-vertical carbonatite and ore veins of various thicknesses, from the metre (Fig. 1a) to centimetre scale (Fig. 1b). These veins intrude into quartz syenite (often referred to as nordmarkite28,34,49) that forms the country rock. Pyroxene of aegirine-augite and aegirine compositions is ubiquitous (albeit not always present) at vein–syenite contacts in Dagudao (Fig. 2a–d and Supplementary Data 1). These pyroxene-rich zones contain minor amphibole, baryte, phlogopite, and K-feldspar (Fig. 3). Xenoliths of quartz syenite inside carbonatite retain their angular shapes and are coated by pyroxene overgrowths. This indicates that while vein interiors were liquid, pyroxene zones were forming outwards from the syenite contact, rather than replacing the syenite (Fig. 2a–d). These pyroxene coats are well-known from Maoniuping25,32,35,36,50. Quartz syenite rims at the contact with ore vein-hosted pyroxene are altered to bleached albite layers with almost no quartz (Figs. 2a–c, 3). The bleached alteration rim thickness roughly correlates with the thickness of the pyroxene layer that overgrows quartz syenite at the vein contacts. Phlogopite is occasionally concentrated in a thin layer between albite and pyroxene (Fig. 3). Large centimetre-scale K-feldspar crystals occasionally form in and around pyroxene layers, particularly where these layers are relatively thick (Fig. 2c).

Ore vein interiors consist primarily of baryte, fluorite, calcite, bastnäsite, phlogopite, and amphibole (Fig. 2e–j). Pyroxene also occasionally occurs in vein interiors. Quartz, when present, is always interstitial and likely to be secondary. The minerals are often coarse-grained and pegmatitic in appearance, and local metre-scale modal mineral compositions vary greatly (Figs. 1, 2). Calcite shows appreciable modal variability, ranging from >90% to <10%. This led previous researchers to define carbonatites sensu stricto, and other non-carbonatite ore veins, as two distinct lithological units on maps or cross sections of Maoniuping51–53. Despite this variability, calcite-dominated rocks are rare on the surface and more abundant in deep drill holes25. Both rock types—carbonatites and ore veins—are petrological cumulates (as distinct from mechanical cumulates), forming from carbonatite melts and differentiated brine-melts, respectively. Therefore, both liquids can react with surrounding silicate rocks and form antiskarn reaction zones (see below for more details).

Veins at Maoniuping are commonly anastomosing without clear cross-cutting relationships (Fig. 1b). Instead, veins often bifurcate, indicating contemporaneous formation. Within veins, amphibole formed first, shown by its euhedral shape, inclusion in later minerals, and lack of truncation by other minerals (Figs. 2d, e, h–j, 4b). Apart from amphibole, no subsequent mineral crystallisation sequence is texturally apparent. Most calcite, baryte, fluorite, phlogopite, and bastnäsite crystals appear euhedral or nearly so, with occasional interpenetrating textures (Fig. 2e–j). Fractures inside one mineral are often filled with another, and vice versa, suggesting co-crystallisation. Crystal-lined cavities indicate open space crystallisation in miarolitic cavities (Fig. 2e, f).

Mineral compositions
Amphibole crystals are commonly zoned, with moderately sodic cores of richterite–ferro-richterite compositions, and highly sodic rims of arfvedsonite–magnesio-arfvedsonite compositions (Fig. 4a, b and Supplementary Data 1). Pyroxene compositions likewise show a trend from moderately sodic aegirine-augite cores to highly sodic aegirine rims (Fig. 4c–f, h and Supplementary Data 1). These zoning patterns are not readily apparent in BSE imaging and require elemental maps for characterisation (Fig. 4g). Pyroxene and amphibole compositional trends indicate that Na-bearing ferromagnesian silicate formation was characterised by increasing Na and Fe3+, concurrent with decreasing Ca and Mg contents54. Rare earth element patterns for both pyroxene and amphibole show a clear sinusoidal pattern with two inflection points, one at Pr–Nd, the other at Tm–Er (Fig. 5a). There is no substantial Eu anomaly. Similar patterns were reported before, demonstrating their widespread presence at Maoniuping34. These patterns are characteristic of igneous pyroxene and amphibole crystallised directly from carbonatite melts55, or of igneous pyroxene and amphibole that crystallised within silicate rocks in thermodynamic equilibrium with a carbonatite melt56,57, where high CaO/MgO activity ratios are implicated in triggering sinusoidality58. They differ from REE patterns of pyroxene and amphibole typical to other silicate rocks, in which pyroxene is often heavy REE-enriched, whereas amphibole is middle REE-enriched relative to both light REE and heavy REE.

Partitioning of REE between calcite and fluorite is also indicative of crystallisation from a carbonatite melt. Often, fluorite forms by neutralisation of acidic fluids, boiling of highly saline fluids, or fluid mixing of contrasting compositions. In all such cases, fluorite is unlikely to form in equilibrium with calcite. Specifically for carbonatites, hydrothermal fluorite forms by replacement of calcite (e.g., Okorusu, Namibia59). In contrast, the calcite–fluorite assemblages at Maoniuping appear to be in equilibrium. A pattern of calcite and fluorite REE partitioning in an experimental carbonatite magma system60 is given in Fig. 5b. REE ratio patterns observed in some calcite–fluorite pairs from Maoniuping are essentially identical (within a factor of up to two) to the experimental pattern (Fig. 5b). The order of magnitude difference probably results from the Maoniuping crystals forming at a lower temperature (<700°C, see below) than the experimental runs (710°C60), or from different activities of chemical components that permit REE substitution in calcite and fluorite, such as monovalent charge-balancing cations61. Nevertheless, the parallel patterns indicate simultaneous equilibrium crystallisation of calcite and fluorite. There is no other plausible mechanism to generate calcite/fluorite REE ratios this close to experimental partitioning. Neutralisation of acidic fluids cannot exist in a carbonatite system because a calcite-saturated system could not exsolve acidic fluids to begin with, and other forms of hydrothermal fluorite deposition often require high NaCl or CaCl2 contents62, which were not observed in any Maoniuping fluid inclusion studies. Whilst fluorite and calcite REE patterns at Maoniuping vary63, these equilibrium pairs establish an evolutionary stage in which both co-crystallised, while later light REE-depleted fluorites64,65 record a syn- or post-solidification time in which most light REE were sequestered in bastnäsite.

Maoniuping ore veins as magmatic carbonatites
The above results indicate that textural and chemical observations of shallow mineralised zones at the Dagudao mining camp of Maoniuping are consistent with magmatic rather than hydrothermal formation. Recent discussion of unusually low-temperature carbonatite melts such as brine-melts, and silicate–carbonatite reaction antiskarn processes3, may better explain Maoniuping. Recent drilling into Maoniuping revealed abundant unevolved and barren (relative to the upper ore veins) calcite carbonatites at depth25, supporting the view that early calcite fractionation is key, and any magmas forming the currently observed REE deposit on the surface had a strongly evolved brine-melt character. Nonetheless, both are of the same continuous magmatic differentiation series, consistent with previous Sr–Nd–Pb–C isotope studies showing a match between ore veins and more obvious magmatic carbonatites at Maoniuping66.

Crystallisation via contamination
Key observations at Maoniuping are the pyroxene reaction zones. These zones have previously been interpreted as a result of fenitisation28,67—a process in which carbonatite-derived alkali fluids metasomatise surrounding silicate rocks, forming an assemblage of alkali silicates (including the currently observed K-feldspar and alkali amphiboles)68. However, the pyroxene zones clearly show sharp contacts with the quartz syenites, with pyroxene growth proceeding away from contact zones into veins (Figs. 2, 3). This indicates that the pyroxene zones crystallised inside the carbonatite melt conduit, and did not replace quartz syenite. Pyroxene preferentially nucleated at the syenite–carbonatite melt interface, but did not replace the syenite or any other rock. As such, the pyroxene zones are not fenites. Instead, these textures are identical to antiskarns revealed by experimental studies in which silica-free carbonatite melts reacted with solid silicate rocks46, and to natural systems where similar carbonatite–silicate reactions have been documented58,69–73. Alternatively, the Maoniuping pyroxene zones have previously been suggested to form by simple fractionation of silica-bearing carbonatite magmas74. However, as carbonatite melts below about 1000°C contain negligible dissolved silica75–78, this is unlikely. Instead, we propose that silica was supplied by the host quartz syenite, supported by the bleached zones, which essentially consist of the original mineral assemblage of the quartz syenite, minus quartz and K-feldspar (as observed elsewhere10,71). Whereas pyroxene was experimentally shown to form at the contact itself, other ferromagnesian minerals form farther inside the carbonatite melt46. In addition, occasional layers of phlogopite on the syenite–antiskarn contact (Fig. 3b) are consistent with previous studies of silica contamination in carbonatites10,70,79, with SiO2 and Al2O3 sourced from quartz syenite xenoliths, MgO from the carbonatite melt, and K2O from both, producing decreased K-feldspar contents in the albitised zones. Formation reactions of early pyroxene can be written considering the Ca–Mg endmember

CaMg(CO3)2(melt) + 2SiO2(rock) = CaMgSi2O6(diopside) + 2CO2(gas) (1)

and formation reactions of late pyroxene can be written considering the Na–Fe3+ endmember

Na2CO3(melt) + Fe2O3(melt) + 4SiO2(rock) = 2NaFeSi2O6(aegirine) + CO2(gas) (2)

The calcic cores and sodic rims of pyroxene (Fig. 3h) and amphibole are consistent with thermodynamic modelling of silica contamination progression of a carbonatite melt54, and are consistent with the compositional range of natural magmatic carbonatite-hosted pyroxene55,80,81. Importantly, reaction 2 consumes Fe3+, consistent with the lack of magnetite at Maoniuping, a mineral typical of calcite carbonatites elsewhere. Carbonatites crystallising from brine-melts, such as dolomite and ankerite carbonatites, are expected to have very little to no magnetite, since Fe3+ has been fractionated away. As the Maoniuping ore veins likely formed from such a brine-melt, Fe3+ was present in very small amounts, or not at all. In the absence of available Fe3+ within the carbonatite melt, Fe3+ can be generated by reduction of CO2 to methane54,82:

4Na2CO3(melt) + 8Fe2+CO3(melt) + 16SiO2(rock) + 2H2O(melt) = 8NaFe3+Si2O6(aegirine) + CH4(gas) + 11CO2(gas) (3)

Evidence for this process is gleaned from fluid inclusion studies of Maoniuping reporting enigmatic CH4-bearing carbonic inclusions30, which would otherwise be unexpected in a typically oxidised, sulfate-rich, carbonatite system83. Similar reactions can be written for amphibole, and when considering K2O in syenite or K2CO3 in the carbonatite melt, also for phlogopite54. This sequestration of Mg and Fe in ferromagnesian silicates prevents the formation of dolomite and ankerite, leaving calcite as the only carbonate mineral forming from the melt54,79,84.

The tabular K-feldspar grains forming in the antiskarn, adjacent to the syenite xenoliths (Figs. 2c, 3e), provide additional evidence for silica transport into the carbonatite melt. Based on previous thermodynamic modelling, two distinct feldspars indicate typical formation temperatures of up to about 650°C54. Rare alkali feldspar perthites (Fig. 2) concordantly suggest the process started at <700°C54, since higher temperatures would stabilise nepheline73 or kalsilite, which are not observed at Maoniuping. This temperature range agrees with both published melt inclusion studies30. Antiskarn formation and associated decarbonation reactions are highly pressure dependent54, with silicate–carbonatite disequilibrium preferred at shallower settings. This is consistent with the abundant antiskarn textures observed near the surface and described above. In contrast, evidence for these reactions is much less common in rocks obtained from drill holes that reached almost 1000 metres depth25. Similar textures were observed elsewhere in the Mianning-Dechang belt, including at Muluozhai44 and Dalucao34.

Fenitisation–antiskarnisation by reaction with carbonatite melts
Fenites are traditionally understood to form via aqueous alkali-rich fluids expelled from carbonatite melts68. In contrast, here fenites form directly by reaction with the carbonatite melt or its more evolved derivative brine-melt with no involvement of any aqueous fluids. That is, the carbonatite melt itself is the metasomatising agent. Formation of fenites occurs alongside the transfer of silica from the silicate rocks to the carbonatite melt, in which refractory silicate minerals form. These feldspar-dominated reaction rims formed by replacement of the original quartz syenite after extraction of silica by carbonatite melts; therefore, they are classified as fenites (whereas pyroxene does not, in fact, belong to the fenites). In this case, the formation of ferromagnesian silicates as part of the antiskarn assemblage and the albitisation of the host silicate rocks to fenites is a coupled reaction, and one cannot happen without the other.

We have no reservations regarding fenites forming via alkaline aqueous fluids, which undoubtedly exist as a separate liquid phase in carbonatite systems. However, we suggest that many traditionally recognised fenites may actually be antiskarn–fenite pairs in which the carbonatite melt has been completely reacted out, resulting in no carbonate minerals that indicate the former existence of this melt.

Metasomatic loss of flux elements and solidification of the residual brine-melt
It is well known that Na and K are efficient fluxing elements that strongly reduce the solidus of carbonatite melts, demonstrated by both experimental studies85 and natural observations from the only erupting natrocarbonatite at the Oldoinyo Lengai volcano in Tanzania86. Melt inclusion studies of Maoniuping show an upper limit of about 850°C30,36,87. Most of these inclusions were observed in quartz, but quartz is not an equilibrium high-temperature phase in carbonatite melts. Quartz can only be stable in a carbonatite system at temperatures below the calcite + quartz = wollastonite curve at about 500–600°C (depending on pressure and CO2 activity)54. This indicates that quartz melt inclusion contents have been made refractory by the antiskarn-forming reactions, leading to spuriously high apparent trapping temperatures8. For a carbonatitic brine-melt to be liquid at 600°C and lower, it requires abundant Na or K5,8,85.

An inevitable consequence of alkali-rich ferromagnesian mineral formation is the removal of Na and K from the carbonatite melt. A 10% decrease in alkali carbonate components is equivalent to a ~100°C increase in the carbonatite melting point85. Once alkalis are progressively sequestered in silicate minerals, fewer fluxes remain to maintain the liquid state, and this process advances crystallisation of all other components, some of them also acting as fluxes, notably with baryte sequestering sulfate. This process likely occurs in a non-isothermal environment, since hot brine-melts are encountering presumably cooler host rocks, excess vapour is advecting heat upwards, and mineral formation releases latent heat of crystallisation. However, any temperature variations are negligible in this solidification mechanism.

When considering the driving forces for known magmatic solidification processes in temperature–pressure–composition space, most systems are explained by either temperature decrease or decompression-led volatile loss. Here, the cause for solidification is composition change. Any changes in temperature or pressure, although likely to occur, do not play an important role.

The formation of the silicate antiskarn assemblage is accompanied by the generation of CO2 gas derived from decarbonation reactions (e.g., Reactions 1, 2 and 3). Carbon isotopes of calcite from Maoniuping record this decarbonation process25. Alkali consumption to silicates also removes cations from alkali sulfates, leading to degassing of SO2 (evident by previous S isotope studies50,88). Crucially, ore-forming chemical components are primarily carried by the magmatic brine-melt, and not dilute gas nor any hydrothermal fluids8,42. Gas release is also likely responsible for the brecciate textures observed at Maoniuping (e.g., Fig. 2a–d), as has been demonstrated elsewhere for antiskarn-bearing subvolcanic carbonatite systems89. Once solidification is complete, cooling occurs, and barren residual hydrothermal fluids are eventually captured as fluid inclusions39, primarily in late-stage, post-mineralisation, hydrothermal quartz. We also note that many fluid inclusions are observed in the readily cleaved fluorite and bastnäsite. In the violent environment of brecciation during antiskarn formation, many crystals can fracture with late fluid inclusions trapped inside cleavage planes. At Maoniuping, this is demonstrated by observation of, among other textures, sheared amphibole, bent calcite, and fractured fluorite and bastnäsite with baryte and quartz infilling, respectively25. After healing, they can be easily mistaken for primary inclusions trapped during crystal growth. The character of these fluid inclusions contributed to the erroneous interpretation that Maoniuping formed hydrothermally. The fluid inclusions often contain soluble alkali sulfates and chlorides, but lacks some of the crucial chemical components that make up Maoniuping’s solid mineral assemblage: Ba in baryte, F in fluorite, and the REE. These elements are only substantially soluble and mobile in acidic fluids47,48, but such low-pH fluids are not attainable in a calcite-bearing carbonatite system, as they could not form in the first place, and they will be immediately neutralised if somehow formed or existed. This Ba-F-REE immobility indicates that all mineralisation formed magmatically, and later hydrothermal fluids were unreactive with the already solidified mineral assemblage.

Preservation of soluble brine-melts
A major challenge with the study of brine-melts lies in their ephemerality, since they are dominated by soluble salts and not preserved in nature. An instructive example is found in the natrocarbonatite lavas of Oldoinyo Lengai, which dissolve and alter within hours after eruption, leaving almost no record of their existence for preservation in geological history90,91. Likewise, experimental studies that reached evolved carbonatite melt compositions and brine-melts show that they are dominated by soluble components, and requires special preparation and analytical techniques even in a laboratory setting5,7,9,85,92. Thus, any expectation for solidified brine-melts to be preserved in the geological record is futile93,94. This seriously hampers the development of magmatic models for carbonatite formation and associated REE mineralisation. Conversely, solidified silicate magmas are invariably retained in the geological record. Other than haphazardly trapped melt inclusions, for which interpretation is often contentious, there is no clear evidence for the former existence of brine-melts in most carbonatites8. This leads to the widespread opinion that hydrothermal fluids must have been implicated in ore formation. Here, Maoniuping provides a unique opportunity to observe preserved brine-melts in the form of refractory silicate minerals.

The zoning from Ca–Mg-dominated amphiboles and pyroxenes to their sodic equivalent traces brine-melt formation. In a non-contaminated system, a brine-melt forms after fractionation of calcite and dolomite3. As CaCO3 and MgCO3 are the most abundant components of primitive carbonatite melts, their removal requires enrichment of all other components. Thus, formation of a brine-melt is inevitable as a carbonatite melt differentiates4,5. At Maoniuping, although abundant calcite cumulates are present in deep drilling and occasional surface exposure (Fig. 1a), no dolomite cumulates are present, and the presumably inevitable formation of a brine-melt is not attained. However, the removal of CaCO3 and MgCO3 as calcite and dolomite is only one way to achieve this differentiation. Alternatively, in silica-contaminated systems, these components can be sequestered to refractory silicates such as amphibole and pyroxene and removed from the carbonatite melt85, with the excess CO2 degassed instead of forming carbonates. In this way, the inevitable brine-melt formation is realised, and recorded by the continuous zoning from the initial Ca–Mg silicate cores to their alkaline equivalents at their rims.

In non-contaminated systems, brine-melts crystallise to an assortment of non-silicate minerals4. Alkali carbonates such as nyerereite and gregoryite are the primary hosts of Na and K, while REE and Ba form burbankite and carbocernaite95,96. Dolomite, ankerite, magnesite, and siderite often sequester most Mg and Fe. A variety of other alkaline minerals host additional anions such as fluoride, sulfate, and phosphate. For instance, experiments revealed the formation of eitelite, bonshtedtite, moraskoite, rouvilleite, neighborite, and parascandolaite5, with some of these or other related minerals observed in fluid or melt inclusions within natural carbonatite complexes8,94,97–102. These minerals are soluble, with the alkalis Na and K readily removed by later fluids. The preserved mineral assemblage is almost entirely devoid of alkalis26, with non-soluble elements redistributed to secondary carbonates or phosphates, leading to the false appearance of primary hydrothermal deposition4.

In Maoniuping, the non-silicate phase assemblage is not substantially different to the ubiquitous secondary assemblages found in most carbonatites, with abundant bastnäsite, baryte, and fluorite. However, their conspicuous phenocrystic morphology (Fig. 2) indicates primary formation instead of in situ replacement of soluble alkali-rich minerals. This texture was previously inferred as hydrothermal formation, but we suggest their presence is indicative of antiskarnisation. Silica contamination triggered the formation of alkali silicates (aegirine, phlogopite, and arfvedsonite)54, preventing Na and K from forming any soluble minerals and causing other elements (e.g., REE, Ba, F) to crystallise into insoluble minerals. These insoluble minerals (bastnäsite, baryte, and fluorite, respectively) are the same minerals that would have formed had soluble minerals (e.g., burbankite) magmatically crystallised in the first place, and then hydrothermally dissolved9. The difference is textural, with these minerals now appearing coarser and well-formed, and even pegmatitic in some cases. The alkalis themselves were retained in the system, sequestered in silicate minerals89. In essence, the externally introduced silica served as the glue which kept most components of the brine-melt within the system, instead of permitting them to be washed away as soluble carbonates and other minor salts9. Even components which were not strictly bonded to silica, such as carbonate, fluoride and sulfate, lost their alkali cation counter-part, which initially kept them in the melt. Instead, these components directly formed insoluble and well-faceted bastnäsite, fluorite, and baryte, respectively. Using REE as an example, bastnäsite is the principal ore mineral at Maoniuping, and it exhibits clear pegmatitic textures (Fig. 2e, g–j). The igneous formation of the Maoniuping bastnäsite stands in stark contrast to its typical occurrence in most carbonatites elsewhere, where bastnäsite forms as a late postmagmatic hydrothermal replacement of ephemeral alkali REE carbonates such as burbankite or carbocernaite4,11,12.

Experimental work demonstrated that magmatic bastnäsite does not form in alkali-rich carbonatite melts96. Instead, it can only form when no alkali elements are present in the melt5, when the melt reaches exceptionally high contents of dissolved bastnäsite95. In nature, bastnäsite typically forms by dissolution of the Na2CO3 component in burbankite. At Maoniuping, bastnäsite formation cannot be attributed to hydrothermal burbankite replacement, as alkalis were sequestered in ferromagnesian silicates. This prevented burbankite from crystallising in the first place. This explains the previously-suggested magmatic origin for bastnäsite33 and further supports our genetic model in which alkalis are removed to refractory silicates (Fig. 4a, b), followed by subsequent solidification of the residual melt in an actively alkali-depleting melt system.

Assimilation-induced crystallisation of carbonatitic brine-melts
Maoniuping is relatively young having formed during the Oligocene33,49,66. It is spectacularly preserved with deep, unweathered sections exposed due to active quarrying. The Maoniuping veins are long and thin, allowing ample contact with the host quartz syenites. Therefore, the formation of its shallow REE-rich portion was entirely dominated by the antiskarn-forming reactions and resulting reaction-driven solidification, and it serves as the type locality for this end-member process. In both surface exposure and deep drilling, the lack of surrounding distal fenites (indicating a lack of excess Na and K) and lack of dolomite or ankerite (indicating a lack of available Mg and Fe) patently demonstrate that alkali-consuming reactions progressed to completion. In contrast, other carbonatite complexes often appear as semi-circular ring intrusions, with sizes from many tens to a few hundred of metres across. In these circular complexes, reaction-driven solidification would only occur at the edges, close to the contact with the siliceous country rocks. Formation of aegirine and other silicates would then put a barrier, armouring the internal carbonatite melt from reacting any further and permitting the residual melt to fractionate according to the predictable dolomite and ankerite evolutionary path103. The outer rings of many carbonatite complexes are dominated by silicate minerals with ubiquitous pyroxene of carbonatite trend compositions (e.g., Fig. 4h). These silicate rings were previously explained as the products of silicate-carbonatite melt immiscibility, or alternatively as early fractionated cumulates from a single carbonated silicate melt57,104. However, these silicate-rich zones of carbonatite complexes occasionally contain REE carbonates57, or Ba-Sr minerals56, which would be exceptionally unusual in early-stage rocks, because these are some of the most soluble elements in carbonatite melts and only become substantially enriched in late-stage brine-melts8,95,105. Their characteristic niobium enrichment also supports reaction-driven formation and antiskarnisation106.

The world’s largest deposit, Bayan Obo, primarily contains REE in the so-called banded dolomite ores, but contains additional substantial mineralisation within calcite carbonatite dykes (Wu dykes107) surrounded by silicate rocks containing mostly aegirine and riebeckite (an alkali amphibole), occasionally in contact with quartz conglomerate country rocks. In these calcite carbonatite dykes, the mineralisation often occurs as phenocrystic grains of bastnäsite, parisite and monazite, with associated baryte and fluorite107–109. Although previously interpreted as fenites, the similarity to Maoniuping is striking, such that a likely interpretation is that the silicate-rich rocks are antiskarns, and the mineralised calcite carbonatites are metasomatically contaminated alkaline carbonatite brine-melts. Although a clear separation between antiskarns (ferromagnesian minerals) and fenites (albitised silicate host rock) is not yet clearly reported from Bayan Obo109, albite is an abundant mineral in the reaction zones110, and we predict that further investigation will reveal increasing albite contents towards the host silicate rocks. Likewise, Mountain Pass contains several calcite carbonatite dykes with bastnäsite and baryte phenocrysts, which intrude into surrounding siliceous country rocks very much like observed in Maoniuping2,17.

The exact mineral assemblage composition of the antiskarn–fenite pair need not necessarily be like in Maoniuping. Here, the carbonatite was mostly sodic and intruded into SiO2 and Al2O3 rich quartz syenite. This manifested itself as albite in fenite and aegirine in the juxtaposed antiskarn. Other cases might be different. For example, a more potassic carbonatite intruding into pelitic sediments or their metamorphic equivalent might result in abundant phlogopite or biotite (as often occurs around many carbonatites, with glimmerites in Mount Weld being a notable example13). Even in the deeper sections of Maoniuping itself, one can find small antiskarn–fenite pairs that represent a more potassic composition with K-feldspar and phlogopite as the main products, in contrast to albite and aegirine in the shallower sections shown here25. Equivalent mica-dominated reaction zones are observed elsewhere in the Mianning-Dechang belt (e.g., Lizhuang and Dalucao34). Similarly, carbonatite melts of varying compositions intruding an even greater chemical variety of host rocks can lead to a plethora of fenitisation and antiskarnisation styles. Focus should be put on the mechanism of silica transfer and alkali sequestration, rather than the presence of any individual mineral (such as albite, phlogopite or aegirine).

REE deposits typically form in the shallower levels of mostly vertically-zoned carbonatite intrusions111. It is in these depths and low pressures that antiskarn formation reactions are favoured54. Given the diversity of near-surface host rocks, we propose a crucial role for their composition—and particularly their silica contents—that control mineralisation style in the carbonatites that intrude into them. We suggest that all carbonatite-hosted REE deposits form along a continuum between negligible interaction with the country rocks (e.g., Kankgankunde, Chilwa Island15 and Saint-Honoré14), where the country rocks are silica poor and rarely lead to antiskarn formation, to complete interaction (the REE deposit at Maoniuping). The location of a single carbonatite complex along the continuum has important implications for its hosted REE mineralisation. Ideally, the economics of a deposit are improved when the ore is concentrated in a small volume. As reaction-driven solidification extracts REE from the melt into the solid phase, limited antiskarn formation (such as in ring complexes) will deplete some REE from the carbonatite melt, leaving less ore of value in the residual melt, which would then fractionate to non-ideally mineralised ferrocarbonatites. In this case, the total REE budget of the carbonatite system would be detrimentally spread over the large volume of both the early calcite carbonatites and later ferrocarbonatites. Alternatively, when the carbonatite melt intrudes as thin veins with abundant reaction surfaces (as in Maoniuping25), the REE are deposited within the antiskarn in their entirety, because the evolved carbonatite and brine-melts are completely consumed and never advance to ferrocarbonatite formation. The above discussion demonstrates that ideal REE concentration would occur at the two ends of the continuum: either full reaction or none, whereas partial reaction would act to lower REE grades.

Methods
Electron microscopy and analysis
Back-scattered electron (BSE) images and major-element analyses of minerals were acquired using a JXA-8230 electron probe microanalyser (EPMA) at the Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS), in Beijing, China. The EPMA was operated at an accelerating voltage of 15 kV, a beam current of 20 nA, and a beam size of 5 μm. The following natural and synthetic standards, analytical lines and detector types were used for EPMA analysis: jadeite (Na-Kα and Al-Kα on TAP, Si-Kα on PET); forsterite (Mg-Kα on TAP); topaz (F-Kα on TAP); K-feldspar (K-Kα on PET); wollastonite (Ca-Kα on PET), hematite (Fe-Kα on LIF); rutile (Ti-Kα on LIF); MnO (Mn-Kα on LIF); NaCl (Cl-Kα on PET). Matrix corrections were carried out using the ZAF correction programme supplied by the manufacturer. Element maps were acquired using a beam current of 100 nA, a beam size and pixel spacing of 1 µm, and 50 ms dwell time per pixel.

Mineral maps were obtained using a TESCAN Integrated Mineral Analyser (TIMA) system at the Xi’an Kuangpu Geological Exploration Technology Co., Ltd. The TIMA system comprises a TESCAN MIRA3 Schottky field emission SEM and four high flux EDS detectors (EDAX Element 30) arranged at 90° intervals around the chamber. We used a spatial resolution of 9 μm and 1200 X-ray counts per pixel. Operating conditions were an acceleration voltage of 25 kV and a beam current of 9 nA. Measured BSE and EDS data were matched with a phase database for rapid mineral identification.

In situ trace element analysis
In situ trace element compositions of both pyroxene and amphibole were determined by an excimer 193 nm ArF Analyte Excite Laser ablation inductively coupled plasma–mass spectrometry (ICP-MS) system, coupled to an Agilent 7700x at the Nanjing FocuMS Technology Co., Ltd., Nanjing. This was carried out on the same spots which had been analysed by EPMA and analysed trace elements include Rb, Sr, Ba, Th, U, Nb, Ta, Zr, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu, Hf. The analyses condition involved a 7 Hz repetition rate and a beam diameter of 25−40 μm. In addition, BCR-2, BHVO-2, AVG-2, and RGM-2 glasses were used as external calibration standards, and Chinese Geological Standard Glasses (CGSG)-1, -2, -4, and -5 (prepared by National Research Centre for Geoanalysis, Beijing, China) were treated as quality control. Raw data reduction was performed offline by ICPMSDataCal using 100% normalisation, without applying an internal standard.

Bulk trace element analysis
Representative mineral samples were prepared using conventional crushing, sieving, and heavy liquid separation. Trace element analyses of both calcite and fluorite were performed at the National Research Centre of Geoanalysis, CAGS, Beijing, China. For analyses of trace elements and REEs, whole-rock powder samples (50 mg) were dissolved in distilled 1 mL HF and 0.5 mL HNO3 in 15 mL Savillex Teflon screw-cap capsules at 190°C for one day, dried, digested with 0.5 mL HNO3, and then dried again. The capsule content was digested with 0.5 mL HNO3 and dried again to ensure complete digestion. Then, the sample was digested with 5 mL HNO3 and sealed at 130°C in an oven for 3 h. After cooling, the solution was transferred to a plastic bottle and diluted to 50 ml before analysis. The sample solutions were analysed for trace elements by ICP-MS. According to the procedure of State Standard of the Peoples Republic of China (GB): Methods for chemical analysis of silicate rocks-Part 30: Determination of 44 elements. The analytical precision for most elements was better than 5% according to the lab work procedure, repeatability and reproducibility of standard samples measurement results. To verify the procedure’s accuracy and precision, several standard samples of GBW07120, GBW07103, GBW07105 and GBW07187 were analysed together with other samples.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The submitted manuscript represents a strongly revised version of a manuscript, which I previously reviewed. Most of the concerns I raised back then (March 2022) are well addressed now and apart from few minor things I do not have any major objectives anymore. All of my comments are given in the attached manuscript file and I hope that they are of some use. I recommend publication of this manuscript after minor revisions.", "reviewer_2": "Review report on ‘Magma solidification via reaction freezing at the Maoniuping carbonatite’\nPlease note all line numbers are based on the annotated Word document opened on Mac. Minor changes may occur in a Window system.\n(1) Significance and focus of this study\nThe goal of this study is to demonstrate that the pyroxene-amphibole zones (antiskarns) are linked to the vein-type REE mineralisation style at Maoniuping. Instead of a hydrothermal origin, the authors propose that the mineralisation formed magmatically from low-temperature brine-melts that metasomatized the country rock. While the formation of antiskarns via metasomatic exchanges between a brine-melt and the country rock seems reasonable, the (potential) genetic link between the antiskarns and the REE mineralisation has not been thoroughly investigated. The simultaneous antiskarn formation and ore-vein crystallisation should first be confirmed, before concluding that the two are inherently related. Additional geological/petrographic/geochemical evidence is required to support this hypothesis.\nIn addition, the authors suggest that the REE mineralisation formed by isothermal ‘’metasomatic freezing’’. This is a novel petrological process that, according to the authors, results in the isothermal solidification of a melt. Although the general mechanism (removal of alkali fluxes from the melt) is plausible, the thermodynamic evidence has not been provided. Primary crystallisation temperatures for the antiskarns and the REE ore-veins should also be determined to support this claim. Furthermore, the authors appear to contradict themselves on the isothermal nature of this process.\nAll in all, the authors address some important and relevant points that definitely deserve attention. However, the authors draw some major conclusions from rather limited lines of evidence, and particularly lack reliable geology information derived from their own study. For example, the ‘’entire formation’’ of Maoniuping has been suggested to be the result of ‘’antiskarn forming reactions and resulting reaction freezing’’, while only a few outcrops have been studied. The authors also state themselves that ‘’evidence for these reactions is much less common in rocks obtained from drill cores that reached 1000 metres depth.’’ The authors should be more careful in phrasing the implications of their work.\nSince Maoniuping is an unusual case of carbonatite system, as the authors themselves suggest, the significance of this study perhaps needs to be boasted less, at least the authors should not generalise the significance to the entire carbonatite system, but more towards presenting a fuller picture of carbonatite system that contains odd examples (line 99-103).\n(2) The argument against a hydrothermal genesis of the REE mineralisation at Maoniuping is built on a shaky ground.\nThe authors mis-interpret the references on previous inclusion work (line 73) and use the misinterpretation to doubt the validity of previous study. See comment on point (4)-misinterpretation of references.\nFluid inclusions in many hydrothermal fluids do not always contain ore minerals (Line 73-74). For example, gold rarely occurs as daughter minerals in the quartz fluid inclusions in the quartz-vein type. The lack of REE daughter minerals in inclusions is not sufficient to argue against a hydrothermal origin. If, according to the authors, the fluid inclusions are actually low T brine-melt inclusions (line 95), the fact that those inclusions are absent in ore minerals still argues against the validity of a brine-melt model for REE mineralisation.\nThen the authors concern about a mixture of brine-melt and fluid phases in the sample inclusions, which can lead to erroneous interpretation (line 97). What would you expect to observe if the brine-melt and fluid phases are trapped in the same inclusions? Which mineral would be an ideal host mineral? And what phases would you expect in such an inclusion; H2O plus abundant halite and REE minerals? And, has any Raman work suggested a hint of this speculation?\n(3) The authors try to introduce a theory to explain the formation of Maoniuping, which needs substantial revision and clarification.\nReaction freezing and isothermal reactions\nWhat does ‘’freezing’’ exactly mean? The authors will first need to introduce the term before using it. This seems to be a discussion point, not something for the result section (line 216). Reaction freezing is brought up again in the discussion (line 421) but still not clearly explained. Do the authors mean the prevention of forming Na-K soluble minerals? According to their antiskarn theory, although formation of Na-K soluble minerals is restrained, reactions to form bastnäsite, baryte, and fluorite are actively going on (and simultaneously!), so reactions in the system do not seem to freeze.\nThe authors also advocate isothermal reactions, a theory with many flaws. The reactions presented in the Discussion section occur over decreasing temperature as the authors noted themselves (line 260-261), so not isothermal. The zonation of the pyroxenes is consistent with thermodynamic modelling of a cooling system (see line 259), so not isothermal. Removal of Na and K from the carbonatite melt is supposed to reduce the temperature (line 331-332), so not isothermal. Other evidence arguing against an isothermal or magma freezing process is presented in the comment to figure 6.\nBrine-melts and antiskarn-albitisation\nThe formation of brine-melts should be briefly discussed in the Discussion section to give clarity. They could have formed due to the fractionation of calcite at depth (as the authors suggest). Another possibility is that the brine-melts only formed near the contact of the carbonatite with the syenite. Due to the metasomatism (albitisation + antiskarn formation), Ca and Mg are lost to the pyroxenes (Ca-Mg-rich cores). The alkalis are removed from the melt and incorporated into albite + pyroxene and amphibole (alkali-rich rims). What is left is a ‘’brine-melt’’ that eventually crystallised as ore veins.\nThe process of antiskarn formation also needs more elaboration and revision at places.\nLine 207-208: A crystallisation temperature for the Maoniuping crystals is not given in the text. Provide a temperature range.\nLine 219-220: The observations only focus on the ore veins associated with a px-amp zone. Therefore, a magmatic formation can only be suggested for these zones/veins.\nLine 223-225: It seems obvious that these low-temperature brine melts exist at shallower depths, near the edge of the intrusion. The ‘’barren’’ carbonatites could just be the result of high-temperature crystallisation of the less evolved melt. They are barren because the REE are distributed over a large volume.\nLine 245: The bleached zones also lack K-feldspar. Did the potassium end up in the phlogopite? And maybe some minor K partitioned into the albite/anorthoclase?\nLine 279-281: This indicates that the formation of antiskarns is most likely a local phenomenon at shallower depths, instead of being responsible for the crystallisation of the entire carbonatite intrusion (as stated in line 415-416).\nLine 292-296: The consistent presence of antiskarn-albitisation pair is also not a convincing theory. According to the authors, the pyroxene antiskarn is always observed together with an albitised zone. Some of the figures in this manuscript and references (e.g. 81) do not actually support this. The authors will need to present a realistic estimate of the prevalence of the antiskarn and albitised zones, respectively, and their spatial relationship at a deposit scale, if they wish to continue using this theory to explain the formation of entire Maoniuping.\nLine 320: Do you suggest that the antiskarn-forming reaction occurred at 850°C or around/below? I don’t think you have reliable temperature information for the anti-skarn reaction. The melt inclusion T in reference 29 seems to be wrong; while the feldspar pairs are actually hosted in the granite, instead of carbonatite or ore vein.\nLine 332-336: The authors suggest the existence of a triggering point, however, the process is more likely to be gradual. The removal of alkalis leads to the gradual crystallisation of the carbonatite, with alkalis being depleted from the outer rim (forming antiskarns) inward. As a result, the carbonatite intrusion would solidify progressively, starting from the outer boundaries and moving toward the core. This process doesn’t start when alkalis are ‘’wholly sequestered’’, it starts the moment the first alkalis are removed.\nThe point made in the manuscript is clear:\n-The alkalis (Na, K) are removed from the melt as they react with the silicate host rock.\n- Consequently, the melt becomes richer in the (Ca, Mg)CO3 component (more towards the right of the diagram in Fig. 6a).\n- A carbonatite melt enriched in the (Ca, Mg)CO3 component (depleted in alkalis) has a higher liquidus, thus starts to crystallise.\nAs this process occurs when the hot melt interacts with the cold(er) host rock, heat would be transferred to the country rock (i.e. cooling). The process would only be isothermal if the heat released by crystallisation equals the heat loss to the country rock. Any evidence for that?\nLine 330-332: Another quantitative estimate to be made is the volume of ferromagnesian minerals formed from the amount of alkali carbonate removal. Is the scale close to the reality in Maoniuping?\nLine 400-402: The authors suggest that the introduced silica glues all components in the system. This contradicts to what the authors proposed in Lines 253-255, where mass exchanges including Na, K, Al, Fe have occurred between the syenite and brine-melt.\nLine 413-415: How does Fig. 4a (composition of Amp) support the ‘’solidification of the residual melt in an actively alkali-depleting melt system’’? Instead, Fig. 4a only supports the first part of this sentence; the removal of alkalis to the refractory silicates (e.g. Amp + Px)\nAntiskarn and REE mineralisation\nLine 341-344: The authors attribute pegmatitic crystal growth (line 343) to the gas-dominated volatile phase (hydrothermal fluids). The REE are thought to remain in the brine-melt (line 340). However, the ore veins, suggested to have crystallised from the REE-enriched brine-melt, contain pegmatitic REE-bearing crystals such as bastnäsite (line 154 and line 393-394). Did the pegmatitic bastnäsite form from the brine-melt or from the ‘’low-density gas-dominated volatile phase’’? The composition (REE-bearing) implies the former, while the crystal habit (pegmatitic) and texture (breccia) suggests the latter. This is contradicting.\nLine 349: The authors suggest that the residual hydrothermal fluids are captured as fluid inclusions in minerals. That sounds unrealistic. In what minerals? If the minerals have already crystallised during the (early) magmatic phase, they can’t capture late-stage hydrothermal fluids. Moreover, if part of the hydrothermal fluids remained uncaptured, what would occur to those escaped fluids? Fluid-mineral reactions? Any dissolution and replacement textures? Or formation of hydrothermal minerals?\nLine 353-354: Please elaborate on this process and also discuss the role of brine-melt in this scenario.\nLine 349-350: If all mineralisation formed magmatically, how did the minerals incorporate fluid inclusions? Fluid/melt inclusions are entrained during crystal growth? Furthermore, this contradicts with the authors’ previous statement: Line 94 - 95: ‘’Consequently, previous studies have likely misclassified magmatic low-temperature brine-melt inclusions as “fluid inclusions.’’\nIf the fluid inclusions are indeed brine-melt inclusions, this would suggest that they are enriched in REE? As stated in line 345: ‘’Crucially, ore-forming chemical components are primarily carried by the magmatic brine-melt, and not the dilute gas (i.e., the hydrothermal fluid).’’\nLine 419: ‘Maoniuping veins’- Maoniuping shows a variety of mineralisation styles, including thin and thick veins and stockwork. Be rigorous when describing the Maoniuping geology . Also, a lack of surface exposure of ‘’surrounding distal fenites’’ does not necessarily mean that these are not present at deeper parts of the system.\nLine 420: Entirely? The authors will need to present a quantitative estimate of the occurrence of antiskarn and their spatial link to the REE mineralisation before drawing this conclusion.\nLine 425-427: Reaction freezing near the edges is also the case for the pyroxene zones at Maoniuping. The pyroxene zones (‘’antiskarns’’) are only found at the surface, in close association with albitised (fenitised) syenite. Is there evidence to suggest the ‘’metasomatic freezing’’ occurred far away from the contact zone with the syenite?\nLine 438-449: Is the silica-rich host rock also albitised at these other localities? Or is there any other evidence that suggest the introduction of silica into the carbonatite? As stated in line 292, the formation of antiskarns is apparently coupled to albitisation.\nLine 453: Maoniuping may not represent an end member of complete interaction. The antiskarn formation at Maoniuping is only observed at the surface. Furthermore, the reactions that drive antiskarn formation are highly pressure-dependent (line 277). This seems to be continuum from deep emplacement (no antiskarns) to shallower emplacement depths (antiskarns). A single carbonatite intrusion could exhibit antiskarns at shallower depths, while the deep-seated core was not affected by the process. Depending on the present-day erosional level, you would either find antiskarns (limited erosion) or not at all (highly eroded; only deeper parts are exposed).\nLine 455: The reaction degree of the brine-melt with silica rich rocks may also play a key role in determining how much mineralisation can be produced, instead of only a volume thing.\nLine 461-462: What causes the carbonatite melt to intrude as thin veins? The proposed continuum seems to be one of erosional level. As shown in Vasyukova & Williams-Jones (2022). Deeper => massive carbonatite intrusions (ring complexes) Shallower => thin veins that originate from a deeper carbonatite body\nLine 464-467: Rephrase this sentence to better summarize the conclusion. Ideally with additional information of geological indications, such as the host rock, possibly structural control, brine-melt and wall rock interaction degrees, and etc\n(4) The authors need to check their references more carefully and critically. At many places the authors mis-interpret referred articles for their own favour and tend to misguide readers.\nLine 68: Reference 30 actually also shows melt and melt-fluid inclusions, in addition to fluid inclusions. The authors of reference 30 suggest the melt and melt-fluid inclusions are magmatic, whereas the fluid inclusions are hydrothermal products. The authors of the manuscript simply ascribe the fluid inclusion to the product of synmagmatic fluids without reasoning.\nLine 73: Simply wrong reference of original work. The fluid inclusion study in reference 30 actually was based on data from measuring fluorite, quartz, bastnaesite, calcite, barite.\nLine 91: Couldn’t find the key information of ‘2-2.4 kbar’ in the reference paper.\nLine 140: Please add which figures in each of the reference papers show the well-known pyroxene coats. Fig. 2b in reference 35 actually shows the absence of a pyroxene reaction zone between syenite and carbonatite. The feature is also not clearly noted in reference 34. In reference 24, the pyroxene coats are not obvious; fig. 3d indeed shows some kind of zonation, which is distinct from the pyroxene coats feature in this work and has distinct mineral assemblages (pyrite and K-feldspar).\nLine 276: In reference 24, the perthite occur in the alkaline granite, instead of the carbonatite or the reaction zone. Misuse of references.\nLine 279: In reference 29, the methodology section states that ‘The measurable temperature range was −196 to +600°C’, but in their result section, temperature higher than 600 oC was reported.\nLine 326-329: In reference 43, Muluozhai is quite different from this studied deposit in terms of mineral assemblages and wallrock type. The authors will need to give more detailed information of the melt inclusions in Muluozhai, including the host mineral, from what unit (from pyroxene-dominated zones or in ore veins?), the composition and homogenisation T.\nLine 293: In reference 81, the figures actually show either the presence of albitisation or the antiskarn, instead of the presence of both always.\nLine 339: In reference 24, the measured calcite is actually from the calcite carbonatite. The authors of this manuscript use the data of this calcite from the carbonatite magma, which is an earlier phase than the brine-melt, to discuss about the brine-melt with syenite, and the antiskarn formation and the degassing process. The authors will first need to establish a reasonable link of their own work to previous work before using their results for discussion.\nLine 444: In all references 102-104, the authors suggest the REE mineralization is minor in the Wu dyke, and REE minerals occur in aggregates, suggesting they were primary minerals crystallizing directly from carbonatite magma. In reference 104, Fig.3d is are used for showing the texture and crystal size of bastnäsite, parisite and monazite, with associated baryte and fluorite, which is very different from the Maoniuping. If, according to the theory of this manuscirpt, those mineralisation is also formed from brine-melt, we would expect albitisation, breccia type mineralisation (degassing), mineral assemblage zoning, coarse and well crystalised REE minerals and abundant bastnäsite, baryte, and fluorite. Instead, the REE mineralisation in Wu dyke has abundant bastnäsite in aggregates and in association with quartz, calcite. It is a pre-mature conclusion that the authors try to conclude by comparing the Wu dyke and Maoniuping.\n(5) The geology, petrology and mineralogy work of Maoniuping still requires a significant improvement.\nThe temporal and spatial relationships of the carbonatite magma, brine-melt and hydrothermal fluids at Maoniuping need clarification first and foremost. It is often not clear what rock unit the authors discuss about.\nLine 131: Carbonatite only occurs at a very small outcrop in the Guangdoushan open pit. The authors need to note the location of samples in the figures of this study. Throughout the context, the authors should not generalise their work to the entire Maoniuping, because it contains two open pits which have varied geology, and the mineralisation style varies at depth.\nLine 131-132: What is the spatial relationship of the carbonatite and ore vein? And what are their mineralogical differences/similarities?\nLine 134: The Maoniuping seems to have > 20 ore veins in contact with syenite (and granite). Figure 2a-d can hardly explain the ubiquities of pyroxene of aegirine-augite and aegirine. Can you provide an estimate of the ubiquities of the occurrence? Furthermore, Fig. 2a actually shows the pyroxene develops at the carbonatite-syenite contact, instead of the vein-syenite contact. For the real ore vein-syenite contact, the pyroxene zone is either not obvious or missing in Figs. 2b and 2c.\nLine 137-139: Detailed field photos or thin section micrographs need to be provided to show the growth direction.\nLine 158: The authors also make this ‘’ore vein-carbonatite’’ distinction in their figures and text. They should clarify what they define as ‘’ore veins’’ before demonstrating the textural relationships in the figures.\nLine 163-163: Please add thin section petrography work as supplement to support this statement. Hardly see mineral textures from Fig.2.\nLine 166-168: Crystal shapes cannot be determined from these photos.\n(6) Figures are poorly illustrated or organised. Suggestions for improvement are listed below.\nFigure 1: Define where and which open pits these outcrops are located.\nFigure 1b: Quantify how common this is? And do the ore veins also contain cpx? Or only around the edges? What are the spatial relationships of the pyroxene band with the ore veins? Are the ore veins always associated with a pyroxene band?\nFigure 2: This figure is used as evidence that pyroxene grows outwards (within the carbonatite conduit), a feature which is not well visible from this figure. The authors need to provide more detailed images. Add a table of the mineralogical composition and proportions of the different units.\nFigure 2: In Fig. 2a, pyroxene-amphibole band forms, but there seems to be no REE mineralisation in the calcite carbonatite. Whereas in Fig. 2d there is REE mineralisation in the so called ‘ore vein’ in association with pyroxene band. What causes this difference?\nFigure 3: Provide a thin section to show the transition from antiskarn to Ca-rich carbonatite. A gradual change would be expected if the antiskarn formed from the carbonatite melt. What does white represent in figure 3b and 3c? There is a yellow rectangle on the right bottom in figure 3c- delete it. Rotate either fig. 3a or 3b or 3c or make the orientation aligned.\nFigure 4: Please add a scan of the thin section either in figure 4 or as a supplement, which shows the co-occurrence relationship of REE minerals and those silicate minerals being measured. Please consider adding electron probe data for a line across the pyroxene crystal in the elemental maps, to have more quantitative assurance of the Na/K/Fe/Mg variation (line 184). Need to re-organise the order of plates. Fig. 4g goes before fig. 4c. Also need to state where this sample is from, i.e. ore vein. Be consistent with naming cpx or aeg-aug in the plates. What colour scale for the WDS elemental maps? Pinkish = higher concentration?\nFigure 5: Please list all the elements on the X-axis. The REE element pattern is more like a humped sinusoidal pattern, instead of sinusoidal. Also note that in reference 54, the REE element pattern in fig.6 is primitive mantle normalised, while your data is Chondrite normalised.\nFig. 5a: Where are these samples from? What mineral assemblage? Add literature data on Cpx-Amp REE patterns from carbonatite and non-carbonatite (i.e. silicate) magmas.\nFig. 5b: Where do these fluorite-calcite pairs originate from? What rock, lithological unit, location? Please consider providing a thin section to show the mineral assemblage and the textural relationship to demonstrate the co-genetic relationship between fluorite and calcite. Hard to judge what this data represents. The authors also need to explain more explicitly in the context what element activities might have caused the order of difference (line 209) and how the experimental set-up compares to Maoniuping.\nFigure 6: This figure aims to illustrate a new crystallisation process. However, the metasomatic freezing mechanism involves the removal of alkalis by the introduction of new components into the system (predominantly Si). As the composition of the melt is externally changed, this defies the point of using a eutectic (homogeneous) system to explain the process? This is not a simple crystallisation process; it involves changing the melt composition through metasomatism. I don’t think you can visualise this on such a binary phase diagram. The authors can consider using a different figure, maybe experimental figures would be better, to show how the temperature remains stable while antiskarns are forming.\nOther minor comments are listed below and added in the annotated manuscript. A couple of places are missing key references and flagged out in the annotated manuscript.\nTitle: The title doesn’t reflect the discussion points of this manuscript. Either magma solidification or reaction freezing is sufficiently explained in the context.\nAbstract: Add how REE mineralisation is related to the antiskarn formation and wall rock contamination.\nIntroduction: Need more introduction on magmatic metasomatism, antiskarn and the genetic relationship of carbonatite magma, brine-melt and hydrothermal fluids in carbonatite REE system in general.\nLine 11: The authors are mixing things up. Magmas do not solidify as a result of degassing. The solidification process is only temperature driven. The volatile degassing is an effect of exsolution by decompression.\nLine 12 - 13: Not accurate. Magmatic metasomatism involves a silicate melt phase reacting with host rock. It is not only about H2O: see page 282-283: https://link.springer.com/article/10.1007/BF00307287\nH2O is emphasised here for its role in forming metasomatic fluids, but in the later discussion section, H2O is barely discussed but instead SO2 and CO2 were discussed.\nLine 26-27: I don’t think this manuscript has provided sufficient evidence to conclude the relative importance of wall rock contamination and T decrease/volatile degassing. Modelling may provide some insight.\nLine 34-35: This is not a description of process. This is a description of fact (if that is true).\nLine 41: What about FeCO3? Since ankerite is mentioned in Line 44.\nLine 66: It is an odd turning. The end of last paragraph presents a controversy on whether the ore-host rock is a carbonatite or hydrothermal rock. The following paragraph then starts with ‘as with most carbonatite..’ Abundant fluid inclusions can also exist in hydrothermal rocks. Don’t misguide readers.\nResults:\nLine 179: ‘Commonly’ is overstated. Only one amphibole crystal was measured by EPMA spot, and shows difference in chemical composition. In the WDS element maps, the two Amp crystals show moderate enrichment of Ca at the rim, but for other elements, the zoning is not obvious.\nLine 184: Please consider adding electron probe data for a line across the pyroxene crystal in the elemental maps, to have more quantitative assurance.\nDiscussion:\nPlease provide decent petrographic evidence to show the minerals growth pattern and paragenetic sequence, especially the growth direction of pyroxene. Contradicting points were made in terms of the paragenesis. The authors suggest ‘amphibole and phlogopite most likely formed at the same time (line 247)’, which seems to contradict with line 163: ‘’Within veins, amphibole formed first..’’\nThe authors will need to present a mass balance estimate to support the theory based on reaction 2 (line 262-265). The pyroxene reaction zone doesn’t seem to be prevalent geologically and cannot consume a huge amount of Fe to explain the lack of magnetite.\nThe authors will also need to provide a larger area of micro-XRF scan than figure 3 to demonstrate a transitional zone from the pyroxene reaction zone to calcite carbonate (or calcite abundant area in the ore vein). The petrographic evidence will be essential to support the reaction series and conclusions in line 272-274.\nWhat is the discussion on feldspar all about? To suggest their formation in a hot environment? And what does it have to do with the antiskarn formation? If the authors want to use the feldspar discussion to support their point, they need to provide their own petrographic evidence of co-genesis of the two distinct feldspar and their co-genesis with the REE minerals.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to reviewers\nReviewer comments in black and serif font.\nOur response in orange and sans serif font.\nReviewer #1 (Remarks to the Author):\nThe submitted manuscript represents a strongly revised version of a manuscript, which I previously\nreviewed. Most of the concerns I raised back then (March 2022) are well addressed now and apart from\nfew minor things I do not have any major objectives anymore. All of my comments are given in the\nattached manuscript file and I hope that they are of some use. I recommend publication of this\nmanuscript after minor revisions.\nThank you. Answers to issues raised in the annotated version of the manuscript for\nReviewer 1 and reviewer 3 are directly added to Word files.\nReviewer #2 (Remarks to the Author):\nReview report on ‘Magma solidification via reaction freezing at the Maoniuping carbonatite’\nPlease note all line numbers are based on the annotated Word document opened on Mac. Minor changes\nmay occur in a Window system.\n(1) Significance and focus of this study\nThe goal of this study is to demonstrate that the pyroxene-amphibole zones (antiskarns) are linked to the\nvein-type REE mineralisation style at Maoniuping.\nNot exactly. We use this observation to demonstrate a novel magmatic process. This\npoint will be repeated several times here, and also explained in the cover letter.\nInstead of a hydrothermal origin, the authors propose that the mineralisation formed magmatically from\nlow-temperature brine-melts that metasomatized the country rock. While the formation of antiskarns\nvia metasomatic exchanges between a brine-melt and the country rock seems reasonable, the (potential)\ngenetic link between the antiskarns and the REE mineralisation has not been thoroughly investigated.\nThe simultaneous antiskarn formation and ore-vein crystallisation should first be confirmed, before\nconcluding that the two are inherently related. Additional geological/petrographic/geochemical evidence\nis required to support this hypothesis.\nAdditional evidence provided, see below and in the revised manuscript for specific\ndetails.\nIn addition, the authors suggest that the REE mineralisation formed by isothermal ‘’metasomatic\nfreezing’’. This is a novel petrological process that, according to the authors, results in the isothermal\nsolidification of a melt. Although the general mechanism (removal of alkali fluxes from the melt) is\nplausible, the thermodynamic evidence has not been provided. Primary crystallisation temperatures for\nthe antiskarns and the REE ore-veins should also be determined to support this claim. Furthermore, the\nauthors appear to contradict themselves on the isothermal nature of this process.\nThis is now clarified in the manuscript. We removed the reference to “freezing” as this\nimplies non-isothermal process, and can be interpreted differently by other people.\nAlthough “freezing” sounds good, we now just talk about “solidification” to remove any\nambiguities. It’s important to note that in reality, the process is unlikely to be isothermal\nas this is an evolving magmatic system with crystallisation-driven latent heat release,\nnon-equilibrium reactions between different thermal bodies, etc. The point is that the\nprocess does not require any temperature change (or pressure, for that matter) in\ncontrast to most commonly accepted magmatic models. In order words, in the P–T–X\nthermodynamic space, the previously known crystallisation models are driven by changes\nin P and T, but here we show a model driven by changes in X. This is the big story here,\nand the manuscript has been revised throughout to clarify this.\nAll in all, the authors address some important and relevant points that definitely deserve attention.\nHowever, the authors draw some major conclusions from rather limited lines of evidence, and\nparticularly lack reliable geology information derived from their own study. For example, the ‘’entire\nformation’’ of Maoniuping has been suggested to be the result of ‘’antiskarn forming reactions and\nresulting reaction freezing’’, while only a few outcrops have been studied. The authors also state\nthemselves that ‘’evidence for these reactions is much less common in rocks obtained from drill cores\nthat reached 1000 metres depth.’’ The authors should be more careful in phrasing the implications of\ntheir work.\nThat is a fair comment – we should not talk about “entire formation of Maoniuping” as a\nblanket statement. We now focus this to the shallow mineralised REE-rich zones.\nHowever, we emphasise that REE mineralisation without any antiskarns is extremely\nuncommon at Maoniuping.\nSince Maoniuping is an unusual case of carbonatite system, as the authors themselves suggest, the\nsignificance of this study perhaps needs to be boasted less, at least the authors should not generalise the\nsignificance to the entire carbonatite system, but more towards presenting a fuller picture of carbonatite\nsystem that contains odd examples (line 99-103).\nWe disagree. Maoniuping is an endmember case, and is therefore informative. From our\nexperience, almost every other carbonatite in the world has elements in it which are like\nMaoniuping. Several salient examples are given in the paper, but it is not our role or in\nout scope to point out every single petrographic observation in carbonatites that should\nbe reinterpreted based on our model. We will leave that to future workers to do. That\nsaid, we again emphasise that the point of this manuscript is not to explain the formation\nof Maoniuping. It is to use Maoniuping as an example to demonstrate a novel magmatic\ncrystallisation process. Claims that we do not fully explain Maoniuping, and there are\nplaces (deeper, elsewhere, some veins in a different mining pit) that do not fully conform\nto our model are missing the point of the manuscript. Of course that not every single\nrock in Maoniuping will be explained by our model. However, our model leads to a\nfundamental change in the understanding of many rocks at Maoniuping, and by\nextension, elsewhere in the world.\n(2) The argument against a hydrothermal genesis of the REE mineralisation at Maoniuping is built on a\nshaky ground.\nThe authors mis-interpret the references on previous inclusion work (line 73) and use the\nmisinterpretation to doubt the validity of previous study. See comment on point (4)-misinterpretation of\nreferences.\nNo, there is no misinterpretation. We have discussed this point repeatedly in our previous\nsubmission of this manuscript, reviewed by one of the authors of that very same paper.\nWe include our response here,\nto the right (reviewer in orange,\nour reply in dark blue). We\nstand by our statement that the\noriginal Xie 2015 “finding” of a\nREE-rich fluid is incorrect.\nMaoniuping is one of the best\nstudied carbonatite localities in\nthe world, and since that paper\nnot a single report of fluid\ninclusions has managed to\nreplicate the finding of a REE-\nrich fluid inclusion (as distinct\nfrom brine-melt). If the reviewer\nor other believe that REE\nmobility in Maoniuping was\ndriven by hydrothermal fluids,\nwe invite them to publish a\npaper demonstrating that,\ninstead of relying on a single\nerroneous and nonreproducible\ndecade-old report. We would\nalso add that REE-solubility in fluids have been studied experimentally quite extensively\nin the past decade, and no study* had found any notable REE-solubility in the\nhydrothermal fluids like the ones in Maoniuping (*except from studies based on the\nerroneous Xie 2015 report). As this point is already made very clear in our manuscript, no\nrevision is required here. We did, however, add some general statements on the overall\n(un)suitability of fluid inclusions studies to resolve these issues.\nFluid inclusions in many hydrothermal fluids do not always contain ore minerals (Line 73-74). For\nexample, gold rarely occurs as daughter minerals in the quartz fluid inclusions in the quartz-vein type.\nThe lack of REE daughter minerals in inclusions is not sufficient to argue against a hydrothermal origin.\nIf, according to the authors, the fluid inclusions are actually low T brine-melt inclusions (line 95), the fact\nthat those inclusions are absent in ore minerals still argues against the validity of a brine-melt model for\nREE mineralisation.\nUsing gold is not the best example, because the solubility of gold is so small that even\nwhen present in a fluid inclusion, it is not in sufficient abundance to form observable\ndaughter minerals. The claim of absence of brine-melt inclusions as evidence against is\nagain not ideal, because we know from thermodynamics and experimental studies which\nfluids and which brine-melts can dissolve REE in carbonatite systems. REE-deposits don’t\nmagically appear – the REE has to come from somewhere. Even when inclusions are not\npresent, residual minerals and experimentally-founded thermodynamic evidence can\nform a basis for an interpretation (as is thoroughly discussed in the second half of our\nDiscussion). We know it cannot be just a “hydrothermal fluid”, and the only other model\nthat exists as of 2025 is brine-melts. As above, this information is already in the\nmanuscript, and no revision is made. If the reviewers (or someone else) wish to claim\nthat hydrothermal fluids can mobilise REE, please present the thermodynamic data that\nit is possible in the chemical system buffered by the minerals present at Maoniuping.\nThen the authors concern about a mixture of brine-melt and fluid phases in the sample inclusions, which\ncan lead to erroneous interpretation (line 97). What would you expect to observe if the brine-melt and\nfluid phases are trapped in the same inclusions? Which mineral would be an ideal host mineral? And\nwhat phases would you expect in such an inclusion; H2O plus abundant halite and REE minerals? And,\nhas any Raman work suggested a hint of this speculation?\nGood point – this has been clarified in the revised manuscript. Regarding an ideal host\nmineral – nothing really. The best host is quartz, but quartz does not form in the\npresence of a brine-melt. All other options are either REE-rich themselves (bastnasite,\nfluorite), obscuring the REE-rich nature of the inclusions, or poor containers of inclusions\n(calcite, baryte). Ideally, the silicates amphibole or pyroxene would work great, but they\nform at non-equilibrium from a contaminated (thus compromised brine-melt) and the\ncomposition would be biased. Note that brine-melts aren’t necessarily halite-dominated.\nThe most common anion would be carbonate, then followed by sulfate, chloride, etc. We\nare not sure how Raman would be useful in this case, as the reviewer didn’t state exactly\nwhich question should be answered by Raman, and it is unclear what “this speculation”\nactually is. Nevertheless, this issue is now better explained in the manuscript.\n(3) The authors try to introduce a theory to explain the formation of Maoniuping, which needs\nsubstantial revision and clarification.\nReaction freezing and isothermal reactions\nWhat does ‘’freezing’’ exactly mean? The authors will first need to introduce the term before using it.\nThis seems to be a discussion point, not something for the result section (line 216). Reaction freezing is\nbrought up again in the discussion (line 421) but still not clearly explained. Do the authors mean the\nprevention of forming Na-K soluble minerals? According to their antiskarn theory, although formation of\nNa-K soluble minerals is restrained, reactions to form bastnäsite, baryte, and fluorite are actively going\non (and simultaneously!), so reactions in the system do not seem to freeze.\nAs above, mentions of “freezing” are removed to avoid ambiguity. The reviewers have\ncorrectly understood our claim, so no revision is made according to the second half of\nthis comment.\nThe authors also advocate isothermal reactions, a theory with many flaws. The reactions presented in the\nDiscussion section occur over decreasing temperature as the authors noted themselves (line 260-261), so\nnot isothermal. The zonation of the pyroxenes is consistent with thermodynamic modelling of a cooling\nsystem (see line 259), so not isothermal. Removal of Na and K from the carbonatite melt is supposed to\nreduce the temperature (line 331-332), so not isothermal. Other evidence arguing against an isothermal or\nmagma freezing process is presented in the comment to figure 6.\nWe added a paragraph clarifying what do we mean by “isothermal”. We note that the\nremoval of Na and K from the melt to refractory minerals does not reduce temperature,\nit increases the melting point thus triggering a phase transition from liquid to solid. This is\nalready clear enough in the manuscript, no revision done on this.\nBrine-melts and antiskarn-albitisation\nThe formation of brine-melts should be briefly discussed in the Discussion section to give clarity. They\ncould have formed due to the fractionation of calcite at depth (as the authors suggest). Another\npossibility is that the brine-melts only formed near the contact of the carbonatite with the syenite. Due\nto the metasomatism (albitisation + antiskarn formation), Ca and Mg are lost to the pyroxenes (Ca-Mg-\nrich cores). The alkalis are removed from the melt and incorporated into albite + pyroxene and\namphibole (alkali-rich rims). What is left is a ‘’brine-melt’’ that eventually crystallised as ore veins.\nYes, absolutely. This is very correct, and we did not want to get into too much depth\ninitially, but you are absolutely right and it is useful to include this information in the\npaper. Manuscript is revised accordingly.\nThe process of antiskarn formation also needs more elaboration and revision at places.\nLine 207-208: A crystallisation temperature for the Maoniuping crystals is not given in the text. Provide a\ntemperature range.\nRange of < 700 °C added to the text, based on feldspars (as opposed to feldspathoids)\nand literature melt inclusion studies.\nLine 219-220: The observations only focus on the ore veins associated with a px-amp zone. Therefore, a\nmagmatic formation can only be suggested for these zones/veins.\nYes, but this includes most ore veins as both pyroxene and amphiboles are ubiquitous\nthe shallow parts of Maoniuping, particularly Dagudao. This is now clarified. Again, we\nnote that REE without px zones are very rare at Maoniuping, so although this is not a\nunique crystallisation model, it is the most common one.\nLine 223-225: It seems obvious that these low-temperature brine melts exist at shallower depths, near the\nedge of the intrusion. The ‘’barren’’ carbonatites could just be the result of high-temperature\ncrystallisation of the less evolved melt. They are barren because the REE are distributed over a large\nvolume.\nYes, indeed, as is the case for almost all carbonatites in the world that contain mostly\nbarren calcite carbonatites and mineralised portions are shallow and much more Mg-Fe\nrich. These shallow zones are exactly the parts that are Ba–REE–Sr-rich. This is already in\nthe introduction and later on in the discussion, so no revision made.\nLine 245: The bleached zones also lack K-feldspar. Did the potassium end up in the phlogopite?\nAnd maybe some minor K partitioned into the albite/anorthoclase?\nYes, this exact thing is explained a few sentences down.\nLine 279-281: This indicates that the formation of antiskarns is most likely a local phenomenon at\nshallower depths, instead of being responsible for the crystallisation of the entire carbonatite intrusion\n(as stated in line 415-416).\nCorrect, the manuscript has been revised accordingly.\nLine 292-296: The consistent presence of antiskarn-albitisation pair is also not a convincing theory.\nAccording to the authors, the pyroxene antiskarn is always observed together with an albitised zone.\nSome of the figures in this manuscript and references (e.g. 81) do not actually support this. The authors\nwill need to present a realistic estimate of the prevalence of the antiskarn and albitised zones,\nrespectively, and their spatial relationship at a deposit scale, if they wish to continue using this theory to\nexplain the formation of entire Maoniuping.\nWe no longer claim that this formed the entire Maoniuping complex. Nevertheless, we\nadded relevant references demonstrating that the shallow part is indeed dominated by\nantiskarns. Demonstrating albitisation is much harder, because it is a feature that was\nmostly overlooked in previous studies, but here we show that it is indeed the case. We\nadded annotations better pointing out bleached zones.\nLine 320: Do you suggest that the antiskarn-forming reaction occurred at 850°C or around/below? I don’t\nthink you have reliable temperature information for the anti-skarn reaction. The melt inclusion T in\nreference 29 seems to be wrong; while the feldspar pairs are actually hosted in the granite, instead of\ncarbonatite or ore vein.\nNot 850 °C, the next sentence explains why this estimate is wrong. And then immediately\nafter that we’re talking about a range of 600 °C-ish based on thermodynamic constraints.\nElsewhere in the paper the presence of perthites also constrains temperature to much\nlower than 850 °C, again supported by thermodynamics. We don’t know why the\nreviewer states that ref 29 (Z&L 2019 OGR) is about a granite, not a carbonatite. It was\nco-authored by one of us (YL), and this is very clearly from the carbonatite. At no place in\nref 29 does it say the feldspars are hosted in granite.\nLine 332-336: The authors suggest the existence of a triggering point, however, the process is more likely\nto be gradual. The removal of alkalis leads to the gradual crystallisation of the carbonatite, with alkalis\nbeing depleted from the outer rim (forming antiskarns) inward. As a result, the carbonatite intrusion\nwould solidify progressively, starting from the outer boundaries and moving toward the core. This\nprocess doesn’t start when alkalis are ‘’wholly sequestered’’, it starts the moment the first alkalis are\nremoved.\nYes, this is correct. Manuscript revised to correct and clarify this.\nThe point made in the manuscript is clear:\n-The alkalis (Na, K) are removed from the melt as they react with the silicate host rock.\n- Consequently, the melt becomes richer in the (Ca, Mg)CO3 component (more towards the right of the\ndiagram in Fig. 6a).\n- A carbonatite melt enriched in the (Ca, Mg)CO3 component (depleted in alkalis) has a higher liquidus,\nthus starts to crystallise.\nYes, you are very much correct.\nAs this process occurs when the hot melt interacts with the cold(er) host rock, heat would be transferred\nto the country rock (i.e. cooling). The process would only be isothermal if the heat released by\ncrystallisation equals the heat loss to the country rock. Any evidence for that?\nAs explained above, the isothermal process is clarified in the revised manuscript.\nLine 330-332: Another quantitative estimate to be made is the volume of ferromagnesian minerals formed\nfrom the amount of alkali carbonate removal. Is the scale close to the reality in Maoniuping?\nThis is impossible to assess without knowing exactly how much brine-melt existed, what\nwas its composition, and whether any has been lost. You’re essentially asking us to derive\na fluid-melt/rock ratio when most parameters are unknown. This can be fudged until\nsomething comes out that makes sense, with low confidence. We prefer to not to do\nthis, as this will be speculation disguised as quantification, and an astute reader will see\nthrough this easily.\nLine 400-402: The authors suggest that the introduced silica glues all components in the system. This\ncontradicts to what the authors proposed in Lines 253-255, where mass exchanges including Na, K, Al, Fe\nhave occurred between the syenite and brine-melt.\nHow does this contradict? Most mass exchange is Si and Al going from the syenite to the\ncarbonatite, where they “glue” Ca, Mg, K, and Na. Minor Na goes to albitise the syenite,\nwhere it is also glued by Si. Minor K is lost from the syenite, and is again glued by Si-Al-Mg\nto make phlogopite. This is exactly what we’re saying, so we don’t see the problem here.\nBut you’re right – some clarification is in order because Si does not glue everything. REE\nand Ba for example do not end up in silicates. The manuscript has been revised to\naddress this.\nLine 413-415: How does Fig. 4a (composition of Amp) support the ‘’solidification of the residual melt in an\nactively alkali-depleting melt system’’? Instead, Fig. 4a only supports the first part of this sentence; the\nremoval of alkalis to the refractory silicates (e.g. Amp + Px)\nCorrect, figure reference moved to a more correct and less confusing location.\nAntiskarn and REE mineralisation\nLine 341-344: The authors attribute pegmatitic crystal growth (line 343) to the gas-dominated volatile\nphase (hydrothermal fluids). The REE are thought to remain in the brine-melt (line 340). However, the\nore veins, suggested to have crystallised from the REE-enriched brine-melt, contain pegmatitic REE-\nbearing crystals such as bastnäsite (line 154 and line 393-394). Did the pegmatitic bastnäsite form from\nthe brine-melt or from the ‘’low-density gas-dominated volatile phase’’? The composition (REE-bearing)\nimplies the former, while the crystal habit (pegmatitic) and texture (breccia) suggests the latter. This is\ncontradicting.\nYou’re right, this is a bit confusing. What we meant is that minerals are crystallising from\nthe brine-melt, but the presence of a coexisting gas permits the volume into which the\ncrystals are forming. We admit this is somewhat speculative (and could even be wrong!),\nand does not contribute that much to the paper. This sentence is now removed.\nLine 349: The authors suggest that the residual hydrothermal fluids are captured as fluid inclusions in\nminerals. That sounds unrealistic. In what minerals? If the minerals have already crystallised during the\n(early) magmatic phase, they can’t capture late-stage hydrothermal fluids. Moreover, if part of the\nhydrothermal fluids remained uncaptured, what would occur to those escaped fluids? Fluid-mineral\nreactions? Any dissolution and replacement textures? Or formation of hydrothermal minerals?\nWhy unrealistic? Most hydrothermal fluid inclusions are hosted in quartz (which is a late\nhydrothermal mineral), or in the easily cleaved fluorite and bastnäsite, so these would be\nsecondary inclusions disguised as primary since they’re along cleavage planes\nmasquerading as growth zones. This was discussed at length in fluid inclusion studies\nbefore (Xie 2019, Liu&Zheng 2019), and is appropriately referenced in the manuscript.\nThe nature of fluid inclusions is now further clarified in the manuscript.\nRegarding what would be the fate of the fluids and associated mineral assemblage, there\nwould be not much happening. The last part of the manuscript is exactly about this – in\nmost carbonatites the fluid alteration is the cause for the common bastnäsite–fluorite\nmineral association. At Maoniuping, this is already the assemblage that forms\nmagmatically, already in equilibrium with any hydrothermal fluids, such that no alteration\nwould occur. This is already rather clear in the manuscript, so no revisions done.\nLine 353-354: Please elaborate on this process and also discuss the role of brine-melt in this scenario.\nAssuming line numbers are correct, this part is talking about pH and acidic fluids. There\nisn’t much more to say – the cited references talk about REE mobility in acidic fluids.\nMaoniuping is a calcite-buffered, alkaline and non-acidic system, hence REE are not\nmobile. The role of the brine-melt is that it’s alkaline. Added a few words to clarify.\nLine 349-350: If all mineralisation formed magmatically, how did the minerals incorporate fluid\ninclusions? Fluid/melt inclusions are entrained during crystal growth? Furthermore, this contradicts\nwith the authors’ previous statement: Line 94 - 95: ‘’Consequently, previous studies have likely\nmisclassified magmatic low-temperature brine-melt inclusions as “fluid inclusions.’’\nSee answer above.\nIf the fluid inclusions are indeed brine-melt inclusions, this would suggest that they are enriched in REE?\nAs stated in line 345: ‘’Crucially, ore-forming chemical components are primarily carried by the magmatic\nbrine-melt, and not the dilute gas (i.e., the hydrothermal fluid).’’\nPotentially? Unfortunately, the only study that measured REE contents in fluid inclusions\nis the Xie study above, which we have issues with (see our comment earlier). The\nproblem is that obvious melt inclusions are hosted in either bastnäsite or fluorite, both\nare REE-rich minerals. Their REE are more abundant that what would be expected in the\nmelt inclusions, such that actually measuring REE in the inclusion is problematic. This is\nthe crux of the magmatic–hydrothermal debate of Maoniuping! Usually, fluid or melt\ninclusions are very useful methods to reveal the presence of ore elements in fluids or\nmelts. However, here melt inclusions are hosted in REE-rich minerals, and often mistaken\nas fluid inclusions. Fluid inclusions are not expected to contain any (according to our\nmodel) and indeed do not contain any. Therefore, fluid or melt inclusions are not a good\ntool to decipher the REE formation in Maoniuping and an alternative approach (i.e., ours)\nis required. This is now clarified in the manuscript.\nLine 419: ‘Maoniuping veins’- Maoniuping shows a variety of mineralisation styles, including thin and\nthick veins and stockwork. Be rigorous when describing the Maoniuping geology . Also, a lack of surface\nexposure of ‘’surrounding distal fenites’’ does not necessarily mean that these are not present at deeper\nparts of the system.\nClarified that this is not only on the surface, but also in deep drilling. Added some\nexplanation on the various type of veins and relevant references. Regarding distal\nfenites, yes, absence of evidence is not evidence for absence, but at some point, if you’re\nnot finding something where you might expect it to be, then the sensible conclusion is\nthat it’s not there. Maoniuping is an active quarry with very active brownfield exploration\naround it. If distal fenite veins existed, they would have already been found.\nLine 420: Entirely? The authors will need to present a quantitative estimate of the occurrence of\nantiskarn and their spatial link to the REE mineralisation before drawing this conclusion.\nYes, corrected.\nLine 425-427: Reaction freezing near the edges is also the case for the pyroxene zones at Maoniuping. The\npyroxene zones (‘’antiskarns’’) are only found at the surface, in close association with albitised (fenitised)\nsyenite. Is there evidence to suggest the ‘’metasomatic freezing’’ occurred far away from the contact zone\nwith the syenite?\nAt Maoniuping, there is no “far away”. Most, if not all, dykes and veins are very thin, up\nto several metres. What this section describes is large circular carbonatite complexes\nwhere the intrusion is many tens to hundreds of metres across. An important point is\nthat REE mineralisation is always immediately adjacent or mere centimetres away from\nantiskarn-related silicates. Now clarified in the manuscript.\nLine 438-449: Is the silica-rich host rock also albitised at these other localities? Or is there any other\nevidence that suggest the introduction of silica into the carbonatite? As stated in line 292, the formation\nof antiskarns is apparently coupled to albitisation.\nExcellent question. No one ever looks at the host rock, because people erroneously think\nthat alteration ends at the “fenite”, mistaking antiskarns as fenites. It is our hope that\nthis study will encourage people to look further into the silica rocks and find this\nalteration. Note that at Maoniuping we see albitisation because it’s a sodic system\nintroducing into a very felsic quartz syenite, and it might be different in other systems\nwith different chemical compositions. Similar host rocks exist all over the Mianning-\nDechang belt so it is difficult to assess intrusions into different host rocks in our case. This\nis a very good point and now added to the manuscript. To your original question, we\nadded some references mentioning albite in those rocks (albeit not proper albitisation\nlike at Maoniuping).\nLine 453: Maoniuping may not represent an end member of complete interaction. The antiskarn\nformation at Maoniuping is only observed at the surface. Furthermore, the reactions that drive antiskarn\nformation are highly pressure-dependent (line 277). This seems to be continuum from deep\nemplacement (no antiskarns) to shallower emplacement depths (antiskarns). A single carbonatite\nintrusion could exhibit antiskarns at shallower depths, while the deep-seated core was not affected by the\nprocess. Depending on the present-day erosional level, you would either find antiskarns (limited erosion)\nor not at all (highly eroded; only deeper parts are exposed).\nYes, correct. Note, however, that REE mineralisation is typically a shallow process,\ndemonstrating an important role for host rock composition. Remember that REE-\nenrichment is typically only achieved after protracted fractionation, and almost always\nappears close to the surface in carbonatites worldwide. Deep REE mineralisation is rather\nrare, anywhere. This is now clarified and referenced in the manuscript.\nLine 455: The reaction degree of the brine-melt with silica rich rocks may also play a key role in\ndetermining how much mineralisation can be produced, instead of only a volume thing.\nYes, this has been repeated several times in the manuscript.\nLine 461-462: What causes the carbonatite melt to intrude as thin veins? The proposed continuum seems\nto be one of erosional level. As shown in Vasyukova & Williams-Jones (2022). Deeper => massive\ncarbonatite intrusions (ring complexes) Shallower => thin veins that originate from a deeper carbonatite\nbody\nLocal stress fields? Pre-existing structures? Host rock type? Structural and geophysical\nassessment of intrusion dynamics is beyond the scope of this manuscript. Nonetheless, it\nis not a question of erosional level. Many other carbonatites that haven’t reached the\nendmember conditions at Maoniuping do have REE mineralisation, but it looks different.\nThis is already discussed in detail in the manuscript.\nLine 464-467: Rephrase this sentence to better summarize the conclusion. Ideally with additional\ninformation of geological indications, such as the host rock, possibly structural control, brine-melt and\nwall rock interaction degrees, and etc\nWe choose to leave this sentence as is, but we have the major improvements we made in\nthe previous paragraphs make this conclusion clearer, with a specific reference to host\nrocks and interaction degrees.\n(4) The authors need to check their references more carefully and critically. At many places the authors\nmis-interpret referred articles for their own favour and tend to misguide readers.\nLine 68: Reference 30 actually also shows melt and melt-fluid inclusions, in addition to fluid inclusions.\nThe authors of reference 30 suggest the melt and melt-fluid inclusions are magmatic, whereas the fluid\ninclusions are hydrothermal products. The authors of the manuscript simply ascribe the fluid inclusion\nto the product of synmagmatic fluids without reasoning.\nYes, they have melt inclusions and naturally those would be magmatic. But their\nconclusion is that the mineralisation is hydrothermal, and this is what we’re challenging\nhere. We clarified what we mean by synmagmatic.\nLine 73: Simply wrong reference of original work. The fluid inclusion study in reference 30 actually was\nbased on data from measuring fluorite, quartz, bastnaesite, calcite, barite.\nYes, they worked on all those minerals. But the infamous REE-bearing fluid inclusions is\nhosted in fluorite. They did not find REE in inclusions hosted by other minerals.\nLine 91: Couldn’t find the key information of ‘2-2.4 kbar’ in the reference paper.\nApologies – wrong paper cited. Changed to the correct one: abstract of Z&L 2019 clearly\nsays: “pressures of 2.0–2.4 kbar” with rationale in the paper itself. Note that Zheng 2021\nalso infers 240 MPa (=2.4 kbar).\nLine 140: Please add which figures in each of the reference papers show the well-known pyroxene coats.\nFig. 2b in reference 35 actually shows the absence of a pyroxene reaction zone between syenite and\ncarbonatite.\nYes, fig 2b does not show it, but fig 2a does show it. Note that the carbonatite in their 2b\nis not particularly mineralised. It’s just mostly carbonate, so no antiskarn is expected\nthere. As above, the point of this manuscript isn’t to show that every single rock at\nMaoniuping has an antiskarn–albite pair with associated REE mineralisation. It is to show\na process that occurs in many if not most rocks, but not all.\nThe feature is also not clearly noted in reference 34.\nGuo & Liu 2019 shows exactly this, very clearly in their fig 4D.\nIn reference 24, the pyroxene coats are not obvious; fig. 3d indeed shows some kind of zonation, which is\ndistinct from the pyroxene coats feature in this work and has distinct mineral assemblages (pyrite and K-\nfeldspar).\nFirst, this is a deeper sample from drilling so the extent of the reaction will be diminished.\nHowever, note the very clear syenite to feldspar to aegirine to carbonatite. This is very\ndistinctive, and an excellent example of compositional control, where instead of\nalbite+aegirine±phlogopite as in the shallow samples, there is now K-\nfeldspar+phlogopite±aegirine. This is also added to the conclusion section.\nLine 276: In reference 24, the perthite occur in the alkaline granite, instead of the carbonatite or the\nreaction zone. Misuse of references.\nYou are correct, ref 24 does not talk about perthite in the carbonatite. There are\nperthites in the rocks. We now added a figure to show this with a newly acquired TIMA\nmap of perthite feldspar.\nLine 279: In reference 29, the methodology section states that ‘The measurable temperature range was\n−196 to +600°C’, but in their result section, temperature higher than 600 oC was reported.\nTheir higher than 600 °C refers to the previous study by Xie 2009, which they cite\nappropriately. Their own data is just given as “> 600 °C” because that’s the temperature\nrange of their instruments. No issue here.\nLine 326-329: In reference 43, Muluozhai is quite different from this studied deposit in terms of mineral\nassemblages and wallrock type. The authors will need to give more detailed information of the melt\ninclusions in Muluozhai, including the host mineral, from what unit (from pyroxene-dominated zones or\nin ore veins?), the composition and homogenisation T.\nObservation of brine-melts in Muluozhai is very interesting, but going into the depth of\nexplaining this will make the manuscript even longer. It is already long enough and\nreference-heavy, so we decided to remove this part completely as it does not actually\ncontribute much to our manuscript.\nLine 293: In reference 81, the figures actually show either the presence of albitisation or the antiskarn,\ninstead of the presence of both always.\nTo be clear, there are no figures that show any rocks in ref 81 (Le Bas 2008). It is all based\non the description in the text. This reference does not contribute much here, and\ntherefore we removed it in order to lower the overall reference count of our manuscript.\nLine 339: In reference 24, the measured calcite is actually from the calcite carbonatite. The authors of this\nmanuscript use the data of this calcite from the carbonatite magma, which is an earlier phase than the\nbrine-melt, to discuss about the brine-melt with syenite, and the antiskarn formation and the degassing\nprocess. The authors will first need to establish a reasonable link of their own work to previous work\nbefore using their results for discussion.\nThis is incorrect. Their calcite measured for isotopes is explicitly described as being from\nthe reaction zones (see their bottom right of page 27, and data tables). The link is clear.\nLine 444: In all references 102-104, the authors suggest the REE mineralization is minor in the Wu dyke,\nand REE minerals occur in aggregates, suggesting they were primary minerals crystallizing directly from\ncarbonatite magma. In reference 104, Fig.3d is are used for showing the texture and crystal size of\nbastnäsite, parisite and monazite, with associated baryte and fluorite, which is very different from the\nMaoniuping. If, according to the theory of this manuscirpt, those mineralisation is also formed from\nbrine-melt, we would expect albitisation, breccia type mineralisation (degassing), mineral assemblage\nzoning, coarse and well crystalised REE minerals and abundant bastnäsite, baryte, and fluorite. Instead,\nthe REE mineralisation in Wu dyke has abundant bastnäsite in aggregates and in association with quartz,\ncalcite. It is a pre-mature conclusion that the authors try to conclude by comparing the Wu dyke and\nMaoniuping.\nCrystal size and textures are irrelevant. Thermodynamics (which is what we’re essentially\nproposing here) works regardless of crystal size. There is also no requirement for the\npresence of albite, or even zoning. Maoniuping shows it well, other places don’t. This is\nwhy we use Maoniuping to demonstrate the process. The Wu dyke story is included here\nas an example of additional localities that can be reinterpreted using inferences from\nMaoniuping. This is already written several times in the manuscript, at different places.\n(5) The geology, petrology and mineralogy work of Maoniuping still requires a significant improvement.\nWe will use this opportunity again to explain that the purpose of this paper is not to\nexplain Maoniuping and provide a thorough description of its geology, petrology, and\nmineralogy. The purpose is to show a novel magmatic process, that is very obvious at\nMaoniuping (if you know where to look for and what to look at), show that this process\nalso leads to magmatic REE mineralisation, and to show that it likely occurs in all\ncarbonatites globally. As such, there is no need for us to describe the geology, petrology,\nand mineralogy of Maoniuping, especially since this has been done before. There are\nmany papers doing exactly that, all of them extensively referenced in our manuscript.\nThe scope of Nature Communications is to show impactful findings in a concise manner,\nwhich is what we’re doing. Full length detailed descriptions of Maoniuping are a better fit\nfor journals such as Journal of Petrology (as has indeed been done, more than once).\nTherefore, in order to stay within reasonable length limits and scope of our current\ntarget journal, we will only add information when it is absolutely required for the\nunderstanding of our suggested process.\nThe temporal and spatial relationships of the carbonatite magma, brine-melt and hydrothermal fluids at\nMaoniuping need clarification first and foremost.\nThis is already clear- a carbonatite melt evolves to brine melt by (1) crystallising calcite,\nand (2) reacting Ca and Mg away to silicates. Hydrothermal fluids coexist with the various\nmelts at all stages. This process is present several times in the manuscript and we are not\nsure what additional clarification is required. Specific critique will be useful.\nIt is often not clear what rock unit the authors discuss about.\nLine 131: Carbonatite only occurs at a very small outcrop in the Guangdoushan open pit. The authors need\nto note the location of samples in the figures of this study. Throughout the context, the authors should\nnot generalise their work to the entire Maoniuping, because it contains two open pits which have varied\ngeology, and the mineralisation style varies at depth.\nYou are correct, this information has been overlooked in the paper. All samples and\nphotos are from Dagudao, not from Guangdoushan. The Guangdoushan part of\nMaoniuping is poorly mineralised, and accordingly consisting not many antiskarns. This is\nin the manuscript now and serves as additional indirect evidence for the importance of\nantiskarn reactions to the Maoniuping REE mineralisation.\nLine 131-132: What is the spatial relationship of the carbonatite and ore vein? And what are their\nmineralogical differences/similarities?\nThe difference between “carbonatite” and “ore vein” is already clearly explained via the\nmodal abundance of calcite in the following paragraph.\nLine 134: The Maoniuping seems to have > 20 ore veins in contact with syenite (and granite). Figure 2a-d\ncan hardly explain the ubiquities of pyroxene of aegirine-augite and aegirine. Can you provide an\nestimate of the ubiquities of the occurrence?\nMaoniuping has many more than 20 ore veins, actually closer to several hundreds. This is\na very common texture in Dagudao section of Maoniuping, as shown by many other\nstudies on Maoniuping before. Some examples from three different groups are Liu et al\nalready cited in the paper where appropriate. A quantitative or semi-quantitative survey\nof how many of the veins have antiskarns in them is beyond the scope of this manuscript,\nbut it is the majority of them at Dagudao.\nFurthermore, Fig. 2a actually shows the pyroxene develops at the carbonatite-syenite contact, instead of\nthe vein-syenite contact. For the real ore vein-syenite contact, the pyroxene zone is either not obvious or\nmissing in Figs. 2b and 2c.\nAs above, the difference between “ore vein” and “carbonatite” is mostly up to the\nrelative modal proportion of calcite. Both “ore veins” and “carbonatites” are cumulate\nrocks crystallising from a carbonatite melt or brine-melt, and as such both can react with\nthe silicate rocks to form antiskarns. This is now clarified in the manuscript.\nLine 137-139: Detailed field photos or thin section micrographs need to be provided to show the growth\ndirection.\nClarified that the pyroxene zones are forming outwards. Not necessarily that pyroxene\nhas any preferred orientation (as it probably does not). Note that the alternative\nhypothesis, that pyroxene somehow nucleated several cm away from syenite and then\ngrew towards it is unrealistic and would need quite some petrological gymnastics to\nexplain.\nLine 158: The authors also make this ‘’ore vein-carbonatite’’ distinction in their figures and text. They\nshould clarify what they define as ‘’ore veins’’ before demonstrating the textural relationships in the\nfigures.\nThe very same line you’re quoting here is the one that defines what ore veins are and\nhow they are distinct from carbonatites—we do not understand this comment.\nLine 163-163: Please add thin section petrography work as supplement to support this statement. Hardly\nsee mineral textures from Fig.2.\nFigure 2j shows this very well, we do not understand how it is “hardly”. Also added a\nreference to figure 4b which already shows exactly what you are looking for.\nLine 166-168: Crystal shapes cannot be determined from these photos.\nMaybe you had a low resolution version of the figures? Fig 2f clearly shows fluorite cubes\nand calcite rosettes. 2e shows prismatic bastnasite very well. Amphibole needles are\nobvious in almost all figure panels. 2c shows tabular K-feldspar.\n(6) Figures are poorly illustrated or organised. Suggestions for improvement are listed below.\nFigure 1: Define where and which open pits these outcrops are located.\nDone–Dagudao. Coordinates and direction added.\nFigure 1b: Quantify how common this is? And do the ore veins also contain cpx? Or only around the\nedges? What are the spatial relationships of the pyroxene band with the ore veins? Are the ore veins\nalways associated with a pyroxene band?\nAnswers to all these questions were either already answered in the paper, or added to\nthe present revision.\nFigure 2: This figure is used as evidence that pyroxene grows outwards (within the carbonatite conduit), a\nfeature which is not well visible from this figure. The authors need to provide more detailed images.\nAgain, I suspect that you had low resolution figures. Figures 2a and 2d clearly show\nangular syenite xenoliths with pyroxene mantles, within the carbonatite or ore vein\nmatrix. There is no question about it. Once you see that and know how to recognise it,\nthen it is very well visible in 2b and 2c as well. What exactly are you looking to see in\n“more detailed images” that isn’t already visible here?\nAdd a table of the mineralogical composition and proportions of the different units.\nWhich “units” and what mineralogical composition are you looking for? If this is the\ndistinction between stockworks, veinlets, and coarse veins, then we now added it to the\nrevised manuscript. Which minerals? What remaining questions do you have regarding\nthe model we are presenting in our manuscript, that mineralogical compositions that\naren’t already in the manuscript are required for?\nFigure 2: In Fig. 2a, pyroxene-amphibole band forms, but there seems to be no REE mineralisation in the\ncalcite carbonatite. Whereas in Fig. 2d there is REE mineralisation in the so called ‘ore vein’ in\nassociation with pyroxene band. What causes this difference?\nGood observation and excellent question. First of all, in all cases, there will be REE\nmineralisation very close (centimetres) to an antiskarn. This is obvious in many figures,\nadmittedly not all of them. As for the actual observation of spatial separation,\nwe do not have a clear answer for it, but two guesses: (1) The calcite-rich part formed at\nsomewhat higher temperatures before the local melt differentiated sufficiently to form a\nproper brine-melt, or (2) this is adjacent to an ore vein, not visible at this photo scale.\nBrine-melts are excellent diffusion mediums, and it could be that ore minerals nucleated\nin one spot, literally tens of cm away, depleting this particular zone from ore elements\nresulting in only calcite. This is a common feature of carbonatites.\nHowever, we choose not to explore these possibilities because, again, there is no good\nanswer and any speculation will not benefit the manuscript.\nFigure 3: Provide a thin section to show the transition from antiskarn to Ca-rich carbonatite.\nThe transition from pyroxene to calcite is rather abrupt, as is clearly evident in Fig 2, and\nin many examples in previous papers, all of which are cited here.\nA gradual change would be expected if the antiskarn formed from the carbonatite melt.\nNot at all. Experimental evidence shows the transition to be abrupt, and antiskarn\nstudies elsewhere in the world very often show this transition to be abrupt and sharp.\nWhat does white represent in figure 3b and 3c?\nEpoxy resin – now added to the caption.\nThere is a yellow rectangle on the right bottom in figure 3c- delete it.\nThis yellow rectangle serves a purpose, and it is already explained in the figure caption.\nRotate either fig. 3a or 3b or 3c or make the orientation aligned.\nThis is not possible because 3b and 3b are different orientations with respect to each\nother. There is no one single rotation that can correctly align all panels of figure 3. This is\nalso not necessary, as we believe most readers will be able to understand the orientation\non their own.\nFigure 4: Please add a scan of the thin section either in figure 4 or as a supplement, which shows the co-\noccurrence relationship of REE minerals and those silicate minerals being measured.\nFigure 3 is literally this – just showing a mineral map instead of a PPL/XPL thin section,\narguably a superior method to identify minerals. This is in the caption of both figures 3\nand 4.\nPlease consider adding electron probe data for a line across the pyroxene crystal in the elemental maps, to\nhave more quantitative assurance of the Na/K/Fe/Mg variation (line 184).\nGood suggestion – quantitative data now added to the figure, with actual data in the\nsupplementary data tables.\nNeed to re-organise the order of plates. Fig. 4g goes before fig. 4c.\nWhy? Is there a rule that BSE images must precede WDS maps? Please explain the\nrationale for this request.\nAlso need to state where this sample is from, i.e. ore vein.\nDone\nBe consistent with naming cpx or aeg-aug in the plates.\nDone\nWhat colour scale for the WDS elemental maps? Pinkish = higher concentration?\nYes – colour scale now added to the figure.\nFigure 5: Please list all the elements on the X-axis.\nAll elements will not fit, but added Ce and Nd.\nThe REE element pattern is more like a humped sinusoidal pattern, instead of sinusoidal.\nThis is a very unimportant distinction.\nAlso note that in reference 54, the REE element pattern in fig.6 is primitive mantle normalised, while\nyour data is Chondrite normalised.\nThis does not matter because the primitive mantle is essentially ×10 chondrite, equally for\nall elements. All it does it shift the pattern up or down, and does not affect shape or\ncurvature at all.\nFig. 5a: Where are these samples from? What mineral assemblage?\nThese are the same samples from the rest of the manuscript, with the same mineral\nassemblages. Note added to figure caption.\nAdd literature data on Cpx-Amp REE patterns from carbonatite and non-carbonatite (i.e. silicate)\nmagmas.\nLiterature data on cpx-amp ree patterns already exists in ref 54 (Reguir et al), which you\nyourself just mentioned two sentences above. REE patterns from silicates are very\nvariable, but typically HREE-enriched for cpx, and MREE-enriched for amph. This is now\nadded to the manuscript. It is difficult to add a citation, because unlike for carbonatites,\nthere is no review paper about “REE patterns of diopside in silicate rocks”.\nFig. 5b: Where do these fluorite-calcite pairs originate from? What rock, lithological unit, location?\nPlease consider providing a thin section to show the mineral assemblage and the textural relationship to\ndemonstrate the co-genetic relationship between fluorite and calcite. Hard to judge what this data\nrepresents.\nThis data represents magmatic co-crystallisation of calcite and fluorite from a carbonatite\nmelt, in equilibrium. There is simply no other way for their REE ratios to exactly\nreproduce experimental partitioning ratios. This is now further explained in the\nmanuscript. Data provenance added to supplementary information. We also added\nadditional calcite–fluorite pair data from Liu et al 2019 to further show this is prevalent\nthroughout Maoniuping, and not just a one-off.\nThe authors also need to explain more explicitly in the context what element activities might have caused\nthe order of difference (line 209) and how the experimental set-up compares to Maoniuping.\nWithout going too much into crystal chemistry and partitioning theory, those are\nprobably activities of monovalent elements (Na, K, etc) that can charge balance the\ntrivalent REE when they’re substituting divalent Ca in fluorite and calcite. This is now\nclarified in the manuscript. The experimental setup does not matter – experiments are\nused to derive equilibrium thermodynamics which are then applicable universally and\nneed not replicate Maoniuping or any other natural system.\nFigure 6: This figure aims to illustrate a new crystallisation process. However, the metasomatic freezing\nmechanism involves the removal of alkalis by the introduction of new components into the system\n(predominantly Si). As the composition of the melt is externally changed, this defies the point of using a\neutectic (homogeneous) system to explain the process? This is not a simple crystallisation process; it\ninvolves changing the melt composition through metasomatism. I don’t think you can visualise this on\nsuch a binary phase diagram. The authors can consider using a different figure, maybe experimental\nfigures would be better, to show how the temperature remains stable while antiskarns are forming.\nThis figure aims to illustrate the process via comparison to previously known processes.\nYes, exactly, the composition changes. It doesn’t mean the system stops being eutectic.\nIt is still very much eutectic, but the eutectic composition is never reached. The lack of\nfluxing elements (Na and K in this case) push the composition sideways. We think it is a\nvery good way (albeit admittedly simplified) to visualise the process. I am not sure which\n“experimental figures” the reviewer is referring to since no experiments were conducted\nin this study. Nonetheless, this has been clarified in the figure caption. Mentions of\n“isothermal” and “freezing” removed.\nOther minor comments are listed below and added in the annotated manuscript. A couple of places are\nmissing key references and flagged out in the annotated manuscript.\nThese issues are addressed separately in the attached manuscript file.\nTitle: The title doesn’t reflect the discussion points of this manuscript. Either magma solidification or\nreaction freezing is sufficiently explained in the context.\nTitle revised.\nAbstract: Add how REE mineralisation is related to the antiskarn formation and wall rock contamination.\nNot much leeway because of word limit, but some modifications have been made.\nIntroduction: Need more introduction on magmatic metasomatism, antiskarn and the genetic\nrelationship of carbonatite magma, brine-melt and hydrothermal fluids in carbonatite REE system in\ngeneral.\nWe believe it is already well introduced. If there are specific questions the reviewer has,\nwe will be happy to address them.\nLine 11: The authors are mixing things up. Magmas do not solidify as a result of degassing. The\nsolidification process is only temperature driven. The volatile degassing is an effect of exsolution by\ndecompression.\nWe are not mixing things up. Decompression -> exsolution and loss of H O -> higher\n2\nmelting point -> solidification.\nLine 12 - 13: Not accurate. Magmatic metasomatism involves a silicate melt phase reacting with host rock.\nIt is not only about H2O: see page 282-283: https://link.springer.com/article/10.1007/BF00307287\nWe did not say it is only H2O. You are correct, but magmatic metasomatism is\noverwhelmingly understood to form via expelled fluids. This is an abstract and there is no\nneed to explain every single exception to the rule.\nH2O is emphasised here for its role in forming metasomatic fluids, but in the later discussion section,\nH2O is barely discussed but instead SO2 and CO2 were discussed.\nExactly – this is the point. We are showing a metasomatic process that is not H O-driven.\n2 2\nduring antiskarn formation, but are not crucial to the process. However, their presence\nprovides additional evidence for antiskarnisation. Hence, it is magmatic, not\nhydrothermal. This is already clear enough in the manuscript and no revisions made.\nLine 26-27: I don’t think this manuscript has provided sufficient evidence to conclude the relative\nimportance of wall rock contamination and T decrease/volatile degassing. Modelling may provide some\ninsight.\nWe disagree. The antiskarns are chock full of aegirine, arfvedsonite, phlogopite. You\nsimply cannot make them by cooling or degassing. What would be sufficient evidence?\nWhich kind of modelling is the reviewer expecting?\nLine 34-35: This is not a description of process. This is a description of fact (if that is true).\nWe don’t understand this comment. The next sentences describe the process. What is\nthe issue?\nLine 41: What about FeCO3? Since ankerite is mentioned in Line 44.\nIt is enriched, forming ankerite. References cited in that area describe it in more detail,\nwhich is not crucial for the paper’s introduction.\nLine 66: It is an odd turning. The end of last paragraph presents a controversy on whether the ore-host\nrock is a carbonatite or hydrothermal rock. The following paragraph then starts with ‘as with most\ncarbonatite..’ Abundant fluid inclusions can also exist in hydrothermal rocks. Don’t misguide readers.\nNo one is misguiding anyone. This is clearly spelled out in the paper: Maoniuping is a\ncarbonatite. Previous work suggested that the mineralisation is hydrothermal, but that\ndoes not mean Maoniuping is not a carbonatite. This is already clear enough in the paper,\nand the whole paragraph starting in line 66 talks exactly about that. Additionally, isotope\nstudies conclusively demonstrated that the ore-veins are isotopically identical to the\nMaoniuping magmatic carbonatites. This information is now added to the manuscript.\nResults:\nLine 179: ‘Commonly’ is overstated. Only one amphibole crystal was measured by EPMA spot, and shows\ndifference in chemical composition. In the WDS element maps, the two Amp crystals show moderate\nenrichment of Ca at the rim, but for other elements, the zoning is not obvious.\nThere is obvious zoning in the two Amp crystals, with Ca decreasing and Na increasing\ntowards the rim. This is exactly what we’re claiming. We never said anything about the\nother elements, so the fact that they’re “not obvious” is not an issue. The EPMA spots\nshow clear zoning. What is overstated about “commonly”? How many amphibole BSE\nimages with line profiles along them are required to convince the reviewer that\namphiboles are commonly zoned?\nLine 184: Please consider adding electron probe data for a line across the pyroxene crystal in the\nelemental maps, to have more quantitative assurance.\nCompositions (as Na/Ca) added, with full numbers in the supplementary tables. That said,\nwe believe the zoning is very obvious and there is enough assurance that this is indeed\nthe case. We also point out that the TIMA maps show exactly this type of zoning, across\nmany hundreds of crystals.\nDiscussion:\nPlease provide decent petrographic evidence to show the minerals growth pattern and paragenetic\nsequence, especially the growth direction of pyroxene.\nThis was addressed in an earlier comment.\nContradicting points were made in terms of the paragenesis. The authors suggest ‘amphibole and\nphlogopite most likely formed at the same time (line 247)’, which seems to contradict with line 163:\n‘’Within veins, amphibole formed first..’’\nCorrect, we removed the mention of phlogopite.\nThe authors will need to present a mass balance estimate to support the theory based on reaction 2 (line\n262-265). The pyroxene reaction zone doesn’t seem to be prevalent geologically and cannot consume a\nhuge amount of Fe to explain the lack of magnetite.\nThe pyroxene zone is very prevalent geologically. Figure 1a, for example, shows a metre-\nscale chunk of pyroxene. This alone is sufficient to equivalent to many metres cubed of\nmagnetite-bearing calcite carbonatite. Remember that whilst magnetite is common in\ncarbonatites, it is only really common in early calcite carbonatites and much rarer in\nevolved brine-melt-derived carbonatites. So not much is expected anyway. This is now\nclarified in the manuscript.\nAdditionally, what is stated in this section is that Fe3+ is generated by redox coupling\nCO –H O and Fe2+. How does one convincingly conduct a mass balance for this? One does\n2 2\nnot – it is a poorly constrained game of fudging numbers and an astute reader will not fall\nfor it. Might as well not even try.\nThe authors will also need to provide a larger area of micro-XRF scan than figure 3 to demonstrate a\ntransitional zone from the pyroxene reaction zone to calcite carbonate (or calcite abundant area in the\nore vein). The petrographic evidence will be essential to support the reaction series and conclusions in\nline 272-274.\nThere is no transitional zone, as described above, and there is no expectation for such a\nzone. The conclusions in line 272 (perthites) are now supported by an additional figure\nshowing mineral maps (done by TIMA, not XRF as claimed by the reviewer)\nWhat is the discussion on feldspar all about? To suggest their formation in a hot environment?\nBased on A&W2024, feldspar perthite can be used as a temperature constraint, which is\nexactly what we’re doing.\nAnd what does it have to do with the antiskarn formation?\nFeldspar is part of the antiskarn. It is a silicate that can only form via contamination. This\nis clarified in the manuscript.\nIf the authors want to use the feldspar discussion to support their point, they need to provide their own\npetrographic evidence of co-genesis of the two distinct feldspar and their co-genesis with the REE\nminerals.\nPetrographic evidence added, there is now a whole figure showing exactly this.\nReviewers’ comments below in dark blue, with our responses in orange.\nFigure 6a is not in its appropriate use. Consider deleting it or use another phase diagram.\nThe binary eutectic phase diagram is normally used for an iso-chemical system. It is okay to\nuse the diagram to describe the change of carbonatite melt from A to B, but not from B to C.\nDuring this stage, according to the authors, alkalis are removed from the carbonatite melt or\nbrine-melt through reactions with the host rock, a process that is accompanied by the\nincorporation of Si and Al (± K) into the melt. This is no longer an iso-chemical system, nor a\npure magmatic system, and the temperature doesn’t stay the same.\nFrom B to C, the diagram gives the illusion that the (Na,K)2CO3 component of the initial melt\nis removed, hence enriching the fluid in the (Ca,Mg)CO3 component. The only way to\nachieve that in such a binary eutectic diagram is through fractional crystallisation of pure Na-\nK carbonates. But that is not what is happening. In fact, only the alkalis are removed, and\nnot in the form of (Na,K)2CO3 as the diagram suggests. Instead, the alkalis are sequestered\nin silicates. The externally added silica and aluminium were not part of the starting\ncomposition, and do not fit anywhere in this two-component carbonatite melt system. In\nother words, from A to B occurs within a closed carbonatite melt system (two-component),\nwhereas the transition from B to C takes place in an open and reactive (multi-component)\nmagmatic-metasomatic system. Hence, this not a suitable figure to visualize this process.\nWe see your point of view and indeed this is a very qualitative and unconstrained\npseudobinary. Given that it’s not too important anyway, we adopt your recommendation to\ndelete it from the paper, and update the text accordingly.\nLine 39: What do you mean by ‘proper’ hydrothermal fluids?\nProper meaning aqueous – clarified in the manuscript.\nLine 123: ‘’a purely magmatic model’’. The model that the authors propose lacks hydrous\nREE-enriched fluids, but is definitely not purely magmatic. It involves solid-melt element\nexchanges, i.e. contaminating the melt through metasomatism.\nIt is magmatic in the sense that the minerals are crystallising from the magma.\nMetasomatism may not be the best word because metasomatism typically means\nreplacement, but here we have a coupled non-hydrothermal replacement (fenite) and\nmagmatic (antiskarn). In any case, for the sake of accuracy, we adopt your recommendation\nand remove the word “purely”.\nLine 197: However, the discussion on LREE partitioning in lines 267-270 seems to assume\ncalcite-fluorite pair crystalises earlier than the fluorite-bastnasite pair.\nYes, but this part in line 197 is textural. The fact that mineral chemistry shows continuous\ncrystallisation over an interval is somewhat expected (and noted in previous studies as\nalready appropriately referenced in our work). This line is now clarified that we’re talking\nabout textures.\nLine 279-281: Awkward sentence- rephrase.\nGood catch—now corrected.\nLine 290: K-feldspar is also consumed.\nYes, corrected.\nLine 338: In addition to silica, aluminium (and possibly K) is also involved in the exchange.\nYes, but those aren’t a required part of the antiskarn process. Silica transfer is the\noverwhelming driving force. No changes made.\nLine 371: Give the quartz context. Quartz is not mentioned to the petrography of either anti-\nskarn or the vein interiors. Are the studied quartz with melt inclusions associated with late\nhydrothermal phases?\nYes, this is now added to the petrography section.\nLine 385: ‘’a non-isothermal environment’’ – but this is not what B to C in figure 6b shows.\nFigure 6 removed, so not an issue.\nLine 494: Move the sentence ‘’Elsewhere, bastnäsite … in burbankite’’ to before line 492 (‘’At\nMaoniuping..’’). First describe how bastnäsite typically forms, then discuss why this can’t be\nthe case for Maoniuping.\nDone.\nLine 533: It is not a frozen but a contaminated brine-melt.\nAdded “contaminated” for clarification and accuracy, but kept “frozen” because it conveys\nour point very well.\nLine 565: It is a local phenomenon at Maoniuping.\nYes, we’re talking only about the REE deposit, not the entire carbonatite body. This is\nalready written in the paper. No changes made.\nLaurens Tromp & Lingli Zhou"}, {"round": 2, "reviewer_comments": {"reviewer_2": "Review report\nThe revised manuscript is clearer and more readable than its previous version. While the majority of our concerns have been addressed, some issues still remain.\nMajor comment:\nFigure 6a is not in its appropriate use. Consider deleting it or use another phase diagram. The binary eutectic phase diagram is normally used for an iso-chemical system. It is okay to use the diagram to describe the change of carbonatite melt from A to B, but not from B to C. During this stage, according to the authors, alkalis are removed from the carbonatite melt or brine-melt through reactions with the host rock, a process that is accompanied by the incorporation of Si and Al (± K) into the melt. This is no longer an iso-chemical system, nor a pure magmatic system, and the temperature doesn’t stay the same.\nFrom B to C, the diagram gives the illusion that the (Na,K)2CO3 component of the initial melt is removed, hence enriching the fluid in the (Ca,Mg)CO3 component. The only way to achieve that in such a binary eutectic diagram is through fractional crystallisation of pure Na-K carbonates. But that is not what is happening. In fact, only the alkalis are removed, and not in the form of (Na,K)2CO3 as the diagram suggests. Instead, the alkalis are sequestered in silicates. The externally added silica and aluminium were not part of the starting composition, and do not fit anywhere in this two-component carbonatite melt system. In other words, from A to B occurs within a closed carbonatite melt system (two-component), whereas the transition from B to C takes place in an open and reactive (multi-component) magmatic-metasomatic system. Hence, this not a suitable figure to visualize this process.\nMinor comments:\nLine 39: What do you mean by ‘proper’ hydrothermal fluids?\nLine 123: ‘’a purely magmatic model’’. The model that the authors propose lacks hydrous REE-enriched fluids, but is definitely not purely magmatic. It involves solid-melt element exchanges, i.e. contaminating the melt through metasomatism.\nLine 197: However, the discussion on LREE partitioning in lines 267-270 seems to assume calcite-fluorite pair crystalises earlier than the fluorite-bastnasite pair.\nLine 279-281: Awkward sentence- rephrase.\nLine 290: K-feldspar is also consumed.\nLine 338: In addition to silica, aluminium (and possibly K) is also involved in the exchange.\nLine 371: Give the quartz context. Quartz is not mentioned to the petrography of either anti-skarn or the vein interiors. Are the studied quartz with melt inclusions associated with late hydrothermal phases?\nLine 385: ‘’a non-isothermal environment’’ – but this is not what B to C in figure 6b shows.\nLine 494: Move the sentence ‘’Elsewhere, bastnäsite … in burbankite’’ to before line 492 (‘’At Maoniuping..’’). First describe how bastnäsite typically forms, then discuss why this can’t be the case for Maoniuping.\nLine 533: It is not a frozen but a contaminated brine-melt.\nLine 565: It is a local phenomenon at Maoniuping.\nLaurens Tromp & Lingli Zhou", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}]

Decision letter