Transient non-soluble noble metal transport in hydrothermal ore systems

Domain: earth_science
Published: 2025-03-13
Editorial outcome: major_revision

Abstract
The transport of noble metals (Au, Ag) by metal-rich melts in hydrothermal ore systems is now acknowledged as a complementary mechanism to complexing ligands in solution. However, it is unclear where/when both mechanisms coexist and whether metal-rich melts can be physically transported by hydrothermal fluids. Here we show evidence for a suspension-like transport of nano- to micron-sized metal-rich sulfide-sulfosalt melts within epithermal fluids at <400 °C, forming irregular and bleb-like polymineral inclusions of Ag-Au-Cu-Pb(-Fe-Zn)-As-Sb-S-Se upon cooling. These polymineral inclusions, 5 nm to 40 µm in size, are cogenetic with fluid inclusions in quartz. Numerical modeling based on particle fluidization and settling theory shows hydrothermal fluids can mechanically transport metal-rich sulfide-sulfosalt nano-micromelts at fluid flow rates <10–1 m/s. The chemical similarity between nano- and micron-scale polymineral inclusions suggests the coalescence of nanomelt precursors during transient transport from their source(s) to deposition sites, playing a key role in noble metal mineralization.

Introduction
In hydrothermal ore systems, metal transport has traditionally been attributed solely to aqueous complexation (e.g., Cl–, HS–, OH–)1. However, updated empirical thermodynamic and experimental modeling suggest that metal-rich melts are likely present at hydrothermal conditions2,3, and can significantly upgrade the noble metal (e.g., in Au and Ag) endowment of ore deposits4–7. In agreement, analyses of micro- and nano-textures in ore minerals from a broad suite of hydrothermal deposits worldwide—including orogenic gold, skarn, intrusion-related gold, volcanogenic massive sulfide, porphyry Cu, greisen and granitic cupola, epithermal, and sub-epithermal (Supplementary Data 1, and references therein)—indirectly support the role of metal-rich melts in ore genesis. The presence of these melts in hydrothermal systems has recently been confirmed by Sousa-Guimarães et al.8 and Jian et al.9, who reported the co-occurrence of micron-sized solidified polymetallic melts and fluid inclusions within gangue quartz. They postulated that these melts formed late, near or at the deposition site, due to the inability of hydrothermal fluids to transport such “large”, high-density melt droplets. However, these studies (and others previous) have overlooked the role of nano-sized melt droplets as mineralizing agents transportable by upwelling hydrothermal fluids. Consequently, the available research has raised lingering fundamental questions: How, when, and where do metallic melts influence metal transport in hydrothermal mineralizing systems instead of aqueous complexes, or coupled with them? Can these melts exsolve from a deep-seated source (e.g., crystallizing magma) and be physically transported by hydrothermal fluids, or do they only form near or at the deposition site?

In this study, we aim to address these questions by analyzing polymineral inclusions—corresponding to solidified sulfide-sulfosalt melts—in the nanoscale realm, targeting solid micro and nanoparticles coexisting with fluid inclusion assemblages hosted by quartz from the El Hilo Au-Ag bonanza (Fig. 1a). El Hilo is part of the Natividad epithermal district in southern Mexico (longitude: 96.428°W, latitude: 17.303°N), where mineralized veins are enclosed in Miocene porphyritic dacites and Paleozoic carbonaceous metasedimentary rocks10. This ~5-cm-wide Au-Ag bonanza ore zone grades up to 2 wt% Au and 31 wt% Ag, occurs within barren quartz, and lacks any internal lamination (Fig. 1a). Our approach reveals that nanomaterials (e.g., atomic clusters, nanomelts, nanoparticles) may be the most pristine evidence of a relationship between chemical and physical transport of noble metals in hydrothermal solutions. In addition, we provide numerical modeling evidence for the mechanical transport of metal-rich nanomelt droplets by upwelling hydrothermal fluids, thus explaining noble metal enrichments upon ore deposition in a range of geological environments.

Results
Polymineral nanoinclusions track physical metal transport
Gangue quartz at the El Hilo bonanza hosts abundant nano- to micron sized polymineral inclusions (PMI) of Ag(-Cu)-Sb-As-Se sulfosalts, electrum (Au-Ag), and Ag-Cu-Fe-Pb sulfides (Figs. 1b–j, 2a–m, Supplementary Fig. 1–5). This PMI-rich (mineralized) quartz crosscuts and replaces PMI-poor (barren) quartz along very sinuous interfaces and embayments, in some cases leaving remnants or “islands” of PMI-poor quartz surrounded by PMI-rich quartz (Fig. 1b–d, Supplementary Fig. 1). These textures suggest coupled dissolution-reprecipitation reactions between barren quartz and PMI-bearing hydrothermal fluids11. Field-emission scanning electron microscopic (FE-SEM) and transmission electron microscopy (TEM) inspection of PMI-rich quartz shows that PMIs ranging from ~5 nm (Fig. 2e, g, k) to ~40 µm (Fig. 1f–j) in diameter are randomly distributed through the grains. These PMIs comprise one to five minerals (Figs. 1e–j, 2g–l), including electrum, pearceite-polybasite [(Ag9CuS4)(Ag,Cu)6(As,Sb)2S7], pyrargyrite-proustite [Ag3(Sb,As)S3], tetrahedrite-group minerals [(Cu,Ag)6[Cu4(Fe,Zn,Cu)2](Sb,As)4S13], aguilarite (Ag4SeS), acanthite (Ag2S), chalcopyrite (CuFeS2), and pyrite (FeS2).

Embayed electrum grains occur intergrown with sulfides or sulfosalts within PMIs (Fig. 1f–j), thus indicating that electrum crystallized early and was eventually resorbed during sulfide and sulfosalt crystallization. Chalcopyrite exhibits cuspate contacts with Ag±Cu-sulfosalts while galena occurs as islands within these sulfosalts (Fig. 1g–h, Supplementary Fig. 2), suggesting that sulfosalts postdated sulfides. Based on the analysis of 100 PMIs, we computed average compositions (±1σ) of 61.3 ± 17.1 wt% Ag, 2.4 ± 7.7 wt% Au, 4.6 ± 3.6 wt% Sb, 2.3 ± 1.3 wt% As, 15.5 ± 4.9 wt% S, 1.8 ± 1.9 wt% Se, 6.0 ± 6.5 wt% Cu, and 3.9 ± 6.3 wt% Fe (Supplementary Figs. 2–3; Supplementary Data 2).

Nano-PMIs are bleb-like, some are perfectly rounded, while most micro-PMIs are irregular. Regardless of the size, minerals within PMIs show curvilinear or cuspate boundaries, usually joined at triple points (Figs. 1e–j, 2b–m, Supplementary Figs. 2–5). Semi-quantitative chemical composition obtained by SEM- (Fig. 1j, Supplementary Fig. 3) and scanning transmission electron microscopy (STEM)-electron dispersive spectroscopy (EDS; Supplementary Fig. 5) analyses indicates that PMIs are in the Ag-Au-Cu-Pb(-Fe-Zn)-As-Sb-S-Se multi-component system (Supplementary Data 2). Similar microtextural and chemical features of PMIs have been explained by the sudden deposition of dissolved metals and S from solution12 or from fluid-mediated metal-rich melts9,13,14. Direct precipitation of focalized Au-Ag ores locally grading in the wt% scale as observed at El Hilo should have required episodic metal deposition from large volumes of hydrothermal fluid. However, this is inconsistent with (1) the absence of laminated textures at El Hilo (Fig. 1a), which would account for episodic fluid injections, and (2) an extent of quartz veining and alteration haloes at El Hilo that is comparable to those of neighboring low-grade or unmineralized veins10. Alternatively, precipitation of those amounts of Au and Ag from a single fluid injection would have required unrealistically high precious-metal solubilities—which, for instance, approach ~0.1 ppm Au and 50 ppm Ag in fluids at 10 wt% NaCl equiv. and 350 °C15. Rather, our observations are more consistent with deposition from fluid-mediated melts enriched in metals, which is an effective mechanism to produce focused Au-Ag over-enrichments in orogenic gold deposits6,9,16, even from fluids with minimal contents of precious metals (e.g., 0.2 ppb Au5).

To date, no experimental investigation fully elucidates the intricate relationships among cotectics, eutectics, and thermal boundaries within the multi-component system Ag-Au-Cu-Pb(-Fe-Zn)-As-Sb-S-Se, especially at the nano-realm. Nevertheless, the existence of metal-rich As-Sb-S-Se melts (referred to hereafter simply as “metal-rich melts”, since metals account for >70 wt%; Supplementary Data 2) is consistent with fluid temperatures of ~270–400 °C17 exceeding binary and ternary eutectic points of the simpler systems: Au-Pb (212 °C), As-S (310 °C), Au-Sb (360 °C), Ag-As-S (280 °C), Pb-Sb-S (240 °C), and Pb-As-S (305 °C)2,4,18. Accordingly, the stability of metal-rich melts in low-temperature (<400 °C) hydrothermal fluids is favored in (1) complex multicomponent systems containing low-melting-point chalcophile elements (LMCE), namely As, Sb, Se, Pb, Bi, etc.2,5; and (2) some metallic nanomaterials (e.g., Au-Ag alloy nanoparticles <10 nm in size), whose melting temperature decreases dramatically at the nanoscale compared with bulk counterparts19,20. Nanomaterials are considered by some to be the precursors of nucleation, growth, and, ultimately, mineral formation21,22. Therefore, nano- and micro-PMIs (Fig. 2b–m, Supplementary Fig. 5) represent solidified metal-rich melts that seem to have mediated ore mineral nucleation and growth during the formation of the El Hilo bonanza zone.

Numerical testing of nano-micromelts as metal physical carriers
Gangue quartz hosts abundant nano- to micron scale PMIs (i.e., former metal-rich melts) along with fluid inclusions (Figs. 1c, 3a) with homogenization temperatures of 273–397 °C and salinities of 14–19 wt% NaCl equiv. (Supplementary Data 3)17. Frequently, PMIs co-occur with fluid inclusions, thus suggesting that melt precursors were originally entrained alongside their carrying aqueous fluids. In other words, these are evidence for heterogeneous trapping of mineralizing fluids carrying materials in suspension23, which we interpret as complementary information that cannot be extracted from fluid inclusions alone, and thus their study is vital to understanding part of the evolution of ore-bearing fluids.

Upwelling hydrothermal fluids are able to mechanically transport materials (e.g., melt droplets or solid particles) only if the fluid-material interaction force balances the weight of the material, which is obtained for fluid velocities greater than a minimum fluidization velocity24. Minimum fluidization velocity (vf; dashed lines in Fig. 3b) for a “bed of suspended particles”—in our case, a fluid containing melt droplets (see next section)—is given by25,26:

4ρg = 150 (1-ϕ)η vf / Dp2 + 1.75 ρf vf2 / Dp (1-ϕ) / ϕ3 (1)

where Δρg is the density difference between the material (i.e., metal-rich droplet ~6,300 kg/m3) and the fluid (837 kg/m3), g is the acceleration due to gravity (9.81 m/s2), η is the fluid viscosity (Pas), Dp is the particle diameter (m), and ϕ is the porosity—proportion of fluid relative to total volume (melt droplets + fluid). Figure 3b shows that melt droplets smaller than a few micrometers could have been fluidized by fluids with ascent rates <10–2 m/s at any given porosity. In addition, for a single melt droplet suspended in a fluid to be transported upward against gravity, upward flow rates v (volumetric flow rate divided by total cross-sectional area) must also exceed the terminal settling velocity25 (vt; solid lines in Fig. 3b), as of

vt = sqrt(4 Dp g Δρ / 3 ρf CD) (2)

where CD is a drag coefficient that depends on the Reynolds number:

Re = v Dp ρf / η (3)

Fluid upwelling velocities on the order of 10–2 to 100 m/s have been calculated in some hydrothermal systems27–29. Taking ~10–1 m/s as a conservative value, our model shows that upwelling hydrothermal fluids at different fluid regimes (laminar and transitional; Re of 1, 10, and 100) would have fluidized and carried suspended metal-rich melt droplets in excess of 100 µm (Fig. 3b). This droplet size is larger than most PMIs trapped in quartz (Figs. 1–2), which average 4.5 ± 12.8 µm (2σ, n = 313; Fig. 3a), indicating that ore-bearing fluids at El Hilo would likely have been able to physically transport metal-rich melt droplets.

Flow rates higher than the fluidization velocity can be sustained, and hence the droplets remain in a fluidized condition, if the pressure loss in the hydrothermal system—due to interaction between any solids in suspension as well as friction losses due to complexities in the fracture network—is less than the initial pressure that started the flow26. Pressure loss Δp along some vertical distance L can be obtained from the Ergun equation (Eqs. 20-42 in Sissom and Pitts25):

Δp / L = 150 (1-ϕ)2 η vf / Dp2 ϕ3 + 1.75 (1-ϕ) ρf vf2 / Dp ϕ3 (4)

Setting v = vf and Dp = 5 µm (average PMI size; Fig. 3a) in Eq. 4, we computed the pressure changes across a fluidized bed of height L—here regarded as the distance to the source of melt droplets—at a given porosity (Fig. 3c). Although these calculations do not consider all the complexities inherent to these hydrothermal systems (e.g., variations in fracture aperture and rugosity), our results provide a first estimate of the pressure difference required to sustain the minimum fluidization condition over a given length. Vein-formation depths of ~1.1 km (30 MPa) calculated from fluid inclusions10 added to a distance to the causative intrusion <4 km (i.e., <110 MPa)—arguable for intermediate-sulfidation epithermal deposits with no genetic link with high-sulfidation epithermal deposits30,31—yields a pressure gradient of ~80 MPa. This calculated pressure gradient exceeds the minimum fluidization condition for particles in high-porosity (>0.7) media (Fig. 3c), as expected for a fluid-dominated flow carrying melt droplets in suspension. Therefore, these observations suggest that minimum fluidization conditions could have been sustained over long distances (e.g., 4 km) from a given melt source to the deposition site (El Hilo).

During transit, metal-rich melts might have reacted with the fluid phase, resulting in either melt partial (or complete) dissolution by the fluids32 or Au(-Ag) collection by the melts5,6. As already noted, the El Hilo bonanza is characterized by an over-enrichment of precious metals, both in solution and suspension. Such an over-enrichment implies that the fluids had met their solubility limits for precious metals—Au and Ag have low solubilities in hydrothermal solutions (on the order of ppb1,33–35)—, thus avoiding the dissolution of metal-rich melts. Instead, PMIs with Ag contents ~61.3 wt% and Au ~2.4 wt% (Supplementary Data 2) may reflect that these melts probably collected noble metals from the hydrothermal solution. This agrees with the preferential partitioning of noble metals from the aqueous solution to the melt, which can be dramatic in some chemical systems (e.g., D liquid Bi/aqueous fluid ~4×108)3,5. Further, nano-scale PMIs (Fig. 2b–m, Supplementary Fig. 5) consist of the same ore minerals as micron-sized PMIs (Fig. 1e–j, Supplementary Fig. 2), thus suggesting that nanomelt (~5 nm) droplets may have coalesced to form larger ones (~40 µm) while migrating or upon deposition.

Once onsite, boiling and conductive heat loss might have destabilized metals in solution while promoting metals partitioning into the suspended melt droplets5. Fluid turbulence due to boiling might have increased the Brownian motion of suspended (nano)melts, thus enhancing their interaction with the fluid (i.e., metal scavenging) as well as their coalescence—similar to metal deposition from colloidal suspensions17,36,37. Consequently, the concurrent deposition of suspended metal-rich melts and their subsequent solidification to PMIs as the temperature decreased, likely upgraded the “regular” mineralization process through precipitation from brines, thus leading to the Au-Ag bonanza.

Provenance of metal-rich sulfide-sulfosalt (nano)melts
El Hilo contains metal-rich nano and micromaterials (i.e., PMIs, former melts) that were probably transported within hydrothermal fluids from a certain source(s) to the deposition site (Fig. 3a–c). Similar to other intermediate-sulfidation epithermal deposits31, the causative magmatic body for the Natividad deposit is not exposed and is thus interpreted as distally located10,17. In this rationale, nano-melt droplets could have originated near the magmatic source and/or once the fluids reached the epithermal environment, before arriving at the deposition site (Fig. 4a). In fact, metal-rich droplets could have originated even deeper, in the middle–lower crust or mantle38,39, and then transported to the uppermost crust by silicate melts40.

A magmatic source for metal-rich (nano)melts is supported by sulfides and sulfosalts from El Hilo and neighboring veins showing δ34S between –3.2‰ and –0.3‰10, within the –3‰ to 3‰ range of mantellic S41. Fluid inclusion data indicate that these melts were trapped from fluids with temperatures of 273–397 °C and salinities of 14–19 wt% NaCl equiv17. In the absence of evaporites, epithermal fluids at these temperatures and salinities represent mixtures of magmatic metal-rich brines and low-salinity fluids (e.g., deeply evolved meteoric waters or late condensed magmatic vapors)31,42,43. These magmatic brines are characterized by high temperatures (up to 650 °C), salinities (26–75 wt% NaCl equiv.), and densities (>1.3 g/cm3)15. Hence, magmatic brines typically stall above crystallizing intrusions after their release44 (Fig. 4a, inset 1), which may induce rock fracturing due to the overpressure26,45 (dotted red line in Fig. 4b). The magmatic fluid release takes place near the brittle-ductile threshold (~400 °C), out of reach of external fluids, and may be parental to potassic cores of porphyry-type mineralization15,44–46. Thus, mineral assemblages of potassic cores in porphyry deposits could be regarded as a pristine archive of hydrothermal processes (e.g., fluid release) that take place in a purely magmatic environment. Nanomelt droplets of Au-Ag(-Pd-Pt)-Te-Bi-Se-S have been reported in potassic cores at Elatsite47 (Bulgaria) and Skouries48 (Greece) porphyry Cu(-Au) deposits (Supplementary Data 1). Whether the El Hilo bonanza is associated with underlying porphyry-type mineralization, this suggests that metal-rich nanomaterials can indeed be produced during magmatic fluid release from fractionating magmas49,50, and then be stored at depth in hyper-saline magmatic fluids (Fig. 4a, inset 1). Consistently, recent numerical simulations51 and textural observations32 have shown that Cu-Fe-rich (+Pd+Au) sulfide droplets—and other dense metallic materials52—can float alongside vapor bubbles within andesitic–dacitic melts, and then transfer metals to magmatic brines. Over time, isotherms migrate downward as crystallization of the magma body proceeds53 and magmatic brines can mix with low-salinity fluids, thus resulting in a low–moderate-salinity and metal-rich fluid that migrates along fracture networks (Fig. 4a, b) and feeds overlying epithermal systems (Fig. 4a, inset 2)31,42,43; this was probably the case for the Natividad epithermal deposit10. In agreement with our model (Fig. 3b–c), this suggests that if metal-rich nanomelts were present in magmatic brines at depth (e.g., 4 km), they could have been fluidized by upwelling low–moderate-salinity fluids and subsequently well up to the epithermal environment.

Metal-rich magmatic-hydrothermal fluids must transit fracture-controlled pathways while rising through the epithermal environment, where fluids are prone to boiling, fluid buffering by interaction with wall rocks, and mixing with external waters31,42,45. At El Hilo, Au-Ag ores are associated with the occurrence of (1) coexisting fluid-rich and vapor-rich fluid inclusions, which indicates boiling; (2) methane in some fluid inclusion assemblages, suggesting the assimilation of carbonaceous materials; and (3) fluids with δ18O from 1.7‰ to 4.4‰ and δ13C from –6.0‰ to –4.4‰, showing mixtures of magmatic fluids and meteoric waters10,17. Some authors have found that H2S (plus other volatiles) loss due to boiling1,31,42 and fluid buffering by carbonaceous metasedimentites54 are key to concentrating Au(-Ag) in hydrothermal deposits. Thus, fluid inclusion and C–O isotopic data from El Hilo suggest that boiling, fluid buffering, and/or mixing could have produced precious metal-rich nanomelt droplets in the epithermal setting (Fig. 4a, inset 3). Consistently, authors have found that these processes promote the formation of nano-sized metallic materials in hydrothermal fluids12,36,50, which may constitute colloidal suspensions that can be transported within the fluids and generate bonanzas49,55,56. Indeed, our numerical models (Fig. 3b, c) support this mechanism of mechanical transport to explain the formation of the El Hilo bonanza.

Nevertheless, this scenario does not exclude the further formation of nanomelts upon precipitation from solution, given that some Au and Ag were probably transported (alongside melts) by complexing ligands (e.g., AuHS2– and AgCl2–) that are readily destabilized by boiling and conductive heat loss, among other phenomena15.

In summary, our geological, fluid inclusion, mineralogical, and numerical modeling results suggest that metal-rich sulfide-sulfosalt melt droplets were produced at a certain distance from their ultimate deposition site, and were then transported in suspension by upwelling hydrothermal fluids. However, it remains unclear how far melt droplets were produced (i.e., near causative intrusion vs. along the way). Figure 4b summarizes examples of hydrothermal ore deposits where metal-rich melts contributed to Au±Ag mineralization (Supplementary Data 1, and references therein). Au-Ag-LMCE(-S) melts are commonly associated with ore-forming fluids with low–moderate temperatures (~100–400 °C) and salinities (<20 wt% NaCl equiv.), which are parental to various types of magmatic-hydrothermal ore deposits15,57,58 (Fig. 4b). These deposits are produced by mixtures of magmatic metal-rich brines and low-salinity fluids and may potentially undergo comparable evolutionary processes to those described for El Hilo31,42,43,57,59 (Fig. 4a). We thus postulate that these processes represent a complementary mechanism of metal transport (alongside other metals in solution) to explain the enrichment of noble metals in hydrothermal fluids, playing a key role in the formation of high-grade mineralization across diverse geological environments.

Methods
Scanning electron microscopy and focused-ion beam
Polymineral inclusions (PMI) hosted by quartz were first identified with a petrographic microscope, before using electron microscopy. SEM analyses were performed with a field emission QemScan 650 (Thermo Fisher-FEI) SEM and a JSM-IT800 super hybrid lens version (JEOL) SEM, both at the Centro de Instrumentación Científica (Universidad de Granada), Spain. SEM images are backscattered electron images acquired with solid-state detectors installed in both SEMs. The instruments are equipped with energy dispersive spectrometer (EDS) silicon drift detectors. Accelerating voltage and beam current were optimized depending on the size of PMIs to ensure high-quality images and an adequate number of counts for EDS measurements.

After selecting the best areas for thin-foil extraction, we used an FIB-SEM to prepare three thin foils of quartz containing nano- and micron-scale PMIs (Supplementary Fig. 4). FIB-SEM analyses were carried out with a Dual Beam FEI Thermo-Fisher Scientific, model Helios 650 at the Laboratorio de Microscopías Avanzadas (LMA; Instituto de Nanociencia de Aragón–University of Zaragoza), Spain. The selected regions of interest were covered with a first layer of C (~300 nm) and a second layer of Pt (~1 μm), which functioned as protection during the milling, polishing, and extraction of the thin foil. The bulk material was first removed on both sides of the lamellae with a Ga+ ion milling at 30 kV and 2.5 nA current, and the subsequent polishing with a 30 kV current at 0.23 nA. The final polishing step was completed by milling the thin foils with a 5 kV current at 68 pA. The electron transparency was monitored by an Everhart-Thornley SE detector and using a 5 kV electron beam. After achieving the electron transparency, the thin foils were rapidly polished using a low energy 5 kV current at 10 pA to reduce the amorphization until a final thickness of ~80 nm was attained. Subsequently, the thin foils were undercut with a 30 kV at 2.5 nA current, lifted out, and transferred from the sample to a TEM Cu grid using an OmniProbe nanomanipulator with a tungsten tip. To weld the thin foils to the tungsten tip and the TEM grid, an ion-beam-assisted Pt deposition was performed.

High-resolution transmission electron microscopy
A probe-corrected Titan (Thermo-Fisher-FEI) TEM equipped with a XFEG field emission gun was used to analyze the thin foils at the LMA (Zaragoza, Spain). This microscope is equipped with a high-brightness X-FEG and a spherical aberration Cs-corrector at the condenser system (probe-corrected). Areas of interest within the thin foil were imaged using (1) high-angle annular dark-field (HAADF) to obtain Z high-contrast images by scanning transmission electron microscopy (STEM), and (2) HRTEM images to describe textural features of PMIs and to index minerals. The Titan was running at 300 kV while HRTEM images were acquired using the Gatan CCD Camera. The composition of PMIs was obtained with EDS analyses using an Ultim Max detector (Oxford Instruments). All these data were treated using the AZtecTEM software package (Oxford Instruments).

Estimation of polymineral inclusion composition
The composition of PMIs was calculated for 100 PMIs ranging in area between 0.5 and 103.2 µm2. Minerals within PMIs were first identified using EDS spectra. Then, we estimated the areal proportion of each mineral within each PMI using the ImageJ-based Fiji software60. Finally, we computed the chemistry of every PMI using electron probe microanalyzer data for electrum, pearceite-polybasite, pyrargyrite-proustite, tetrahedrite-group minerals, and aguilarite from Cano et al.10, as well as ideal compositions for chalcopyrite, galena, and pyrite. All these calculations are presented in Supplementary Data 2.

Fluidization velocity, settling velocity, and pressure gradients
We estimated the fluidization velocity using Eq. 1, settling velocity using Eqs. 2 and 3, and pressure gradients to maintain minimum fluidization conditions using Eq. 4. These equations contemplate several variables, including (1) fluid and PMI density difference (Δρ), (2) fluid viscosity (η) and density (ρ) at a given temperature, pressure, and salinity; (3) the gravitational acceleration (g), (4) the particle diameter (Dp), (5) the porosity (φ), (6) the drag coefficient (CD), and (7) the Reynolds number (Re). (1) A fluid density of 837 kg/m3 was estimated using fluid inclusion data from Cano et al.10,17 for the El Hilo bonanza zone in the HokieFlincs_H2O-NaCl software61 (Supplementary Data 3). A PMI density of 6300 kg/m3 was computed using mineral proportions for each PMI combined with mineral densities from Mindat.org62 (Supplementary Data 2). This density represents a maximum estimation, considering that molten PMIs (i.e., metal-rich sulfide-sulfosalt melts) should have a slightly lower density. (2) Fluid viscosity was computed using experimental data by Kestin et al.63 for H2O-NaCl solutions at variable temperature, pressure, and salinity. We calculated fluid viscosity at 0 (0 mol/kg), 10 (2 mol/kg), and 19 (4 mol/kg) wt% NaCl considering an average temperature of 340 °C and pressure of 300 bar (~1.1 km), as indicated by fluid inclusion data from Cano et al.10,17 (Supplementary Data 3). (3) The gravitational acceleration was set at 9.81 m/s2. (4) The particle diameter was estimated for 313 PMIs from Assemblages 1 to 8 (Supplementary Fig. 2) by applying the Thresholding option of the Fiji software, which was possible due to the large difference in brightness between PMIs and the hosting quartz in back-scattered electron images. This diameter corresponds to the Feret’s diameter, which is the longest distance between any two points in the PMI. (5) The porosity was set at 0.3, 0.5, 0.7, and 0.9 to reflect variable fluid (φ)/melt droplets + particles (1–φ) ratios within the fractures, in order to provide a more comprehensive representation of the hydrothermal system; (6) the drag coefficient was calculated according to Sissom and Pitts25 for spherical particles (Fig. 20–2); non-spherical particles (cylinders, prisms, disks, etc.) would lead to smaller settling velocities (vt)25. (7) The Reynolds number was set at 1, 10, and 100 to comprise laminar and transitional flows. See Supplementary Data 3 for more information on calculations.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Manuscript: A transient non-soluble noble metal transport in hydrothermal ore systems\nThe manuscript presents an intriguing study on the transport of noble metals by polymetallic melts in hydrothermal ore\nsystems, proposing this as a complementary mechanism to metal-bearing ligands in solution. The authors provide evidence\nfor suspension-like transport of nano-to-micron sized polymetallic melts within epithermal fluids, leading to the formation of\npolymetallic inclusions in quartz. While the manuscript introduces exciting ideas, there are some areas that require further\nclarification and discussion.\nMajor Issues\nData Interpretation:\nThe authors interpret the polymetallic inclusions in quartz as indicative of deposition from a melt (line 110). Since this\ntextural interpretation forms the core of their model, it is essential that it is robust and well-supported. It is possible that these\ntextures could also result from the sudden deposition of dissolved metals and sulphur from hydrothermal fluids during a\nboiling event. To strengthen their interpretation, the authors should provide a more thorough discussion, demonstrating that\nno other deposition processes could account for the observed textures. Upon reviewing the provided figures (Figure 1.f, g, h,\ni, j, and Figure 2.a and b), the polymetallic inclusions do not appear particularly blebby. More evidence and detailed\nexplanation are needed to substantiate the claim that these textures are indeed a result of deposition from a melt.\nComposition Discrepancy:\nThe abstract states that nano-to-micron sized polymetallic melts within epithermal fluids form bleb-like polymetallic\ninclusions of Au-Ag(-As-Se-Sb) (line 24). However, the data presented in the manuscript indicate that the polymetallic\ninclusions are composed of \"Ag(-Cu)-Sb-As-Se sulfosalts, electrum, and Ag-Cu-Fe-Pb sulfides.\" Furthermore, line 108\ndescribes a “Ag-Au-Cu-Pb(-Fe-Zn)-As-Sb-S-Se multi-component system.” The presence of sulphur is significant as it\ninfluences the behaviour of the polymetallic melt. This inconsistency between the abstract and the actual data presented\nneeds to be addressed to maintain the reliability of the conclusions drawn.\nTransport of PM Droplets:\nThe manuscript suggests that polymetallic (sulfosalt and sulfides) material is mechanically transported by rising\nhydrothermal fluids as melt droplets (lines 182-184, 191-193) from a “distally located” magmatic intrusion to the surface.\nWhile the authors demonstrate that velocity calculations make such transport possible, it is not discussed how such material\nwould remain in a chemically stable suspension within the hydrothermal fluid. Existing experimental studies show that\nsulfide melts are not easily transported (Rose and Brenan, 2001). The manuscript does not sufficiently address the chemical\nprocesses by which the melt droplet could be kept in a stable suspension in the aqueous epithermal fluid (“LMCE-bearing\nmetallic (nano)melts could be kept suspended,” line 233). The potential for the melt to react with the aqueous fluid, rather\nthan remaining inert and stable, needs more discussion.\nAdditionally, how the nano-to-micro size droplets would percolate through kilometres of rock without reacting with the wall\nrock or being mechanically stopped by limited porosity requires further elaboration.\nTo demonstrate the transport of the melt, the authors should explain how it can be in both mechanical and chemical\nequilibrium within the fluid.\nSpecific Comments\nLine 24: The statement about the composition of polymetallic inclusions does not match the data presented. The manuscript\nneeds to clarify that the inclusions are primarily sulfide melts, which alters the interpretation of the findings.\nLine 34: The references cited (Econ. Geol. 99, 1063–1084 (2004); Toothe 2008 and 2011; Frost 2002) are used to support\nthe hypothesis of polymetallic melts in hydrothermal fluids. However, these studies primarily discuss local migration within\nhost rocks or low-melting point chalcophile elements, not the long-distance transport in aqueous fluids proposed by the\nauthors. The manuscript should critically assess these references and clarify their relevance to the current study.\nLine 78: There is a critical need for a more detailed discussion on the phases referred to as PMI by the authors: Ag(-Cu)-Sb-\nAs-Se sulfosalts, electrum, and Ag-Cu-Fe-Pb sulfides. The manuscript must differentiate between polymetallic melts and\nsulfide melts and address their transport mechanisms individually.\nLine 105: More detailed descriptions of the textures are needed to convince the reader that the textures are indeed the\nremnant of melts.\nLine 122: Can you be more specific here regarding what early segregation refers to? Do you mean early segregation from a\nparental magma?\nLine 131: This statement is slightly confusing. It does not appear that your data shows sulfides PMI within a fluid inclusion.\nRather, they seem to be occurring within the same quartz crystal.\nLine 169-170: This sentence needs clarification. It is unclear what is interpreted to happen to metals in suspension and how\nit relates to polymetallic melt.\nLine 191-193: This part needs more explanation. Are the polymetallic melts at that stage sulfide melts? (It should be,\nconsidering your data.) More discussion is needed to explain how the sulfide melts/droplets can be segregated from the\nmagma into the aqueous fluid.\nLine 232-233: There is a discrepancy between the literature cited and the interpretation made by the authors. The metal-rich\nbrines referred to in line 232 are brines in which metals are dissolved as chloro-complexes and not suspended as\nnanomelts. There is no data to support the claim that nanomelts are present in the brines. A discussion is needed to address\nhow such nanomelts would be stable in this environment and how they might have been extracted from the magmatic\nprecursor.\nFigure 1: The manuscript lacks a discussion on quartz dissolution textures presented in Figure 1, which could provide\ninsights into the deposition mechanisms and the environment of formation.\nFigure 4: The composition of the rising fluid and its ability to stabilise polymetallic inclusions during transport must be\naddressed. This includes discussing the role of fluid-rock interactions and the impact of wall rock porosity.\nIn conclusion, the ideas proposed are novel and exciting and could help explain ultra-high-grade enrichment, which remains\nto be understood. However, I believe that more work needs to be done on this manuscript to convincingly explain how the\nproposed transport is chemically and physically possible.\nReference:\nRose, L. A., and Brenan, J. M., 2001, Wetting properties of Fe-Ni-Co-Cu-OS melts against olivine: Implications for sulfide\nmelt mobility: Economic Geology, v. 96, no. 1, p. 145-157.", "reviewer_2": "See attached review comments and edited PDF", "reviewer_3": "This manuscript introduced a new mechanism of enrichment of noble metals in magmatic-hydrothermal ore forming systems.\nThe authors propose that nano-micro sized melts may be present and suspend in hydrothermal fluids and be transported by\nfluids to form mineralization. Evidence provided includes SEM, EDS, TEM and EPMA analyses, etc. The idea reads\ninteresting with relevant data and observations. However, I have some concerns related to whether the melts could be stable\nin the fluids at the T-P-salinity conditions as discussed, as well as if the calculation of the fluids’ capability to transport and\nmaintain melts suspension is accurate enough. Please see below:\n1. It is important to consider the chemical stability of the noble metal melts in the hydrothermal fluids with relatively high T\nand salinity. At conditions of El Hilo fluids, e.g., 100-400ºC, up to 20wt% NaCl, the fluids may dissolve significant amounts of\nmetals, in particular when considering that chloride complexes are thought to be important species for Au, Ag, and PGEs in\nCl-dominated fluids and contribute to their hydrothermal transportation. Hence, it is important to provide constraints on if\nthe nano-micro metal melts can be chemically stable as well in such fluids. I suggest the authors to provide thermodynamic\ncalculations of the relevant metals and if possible, the fluid composition measurement of the fluid inclusions to demonstrate\nwhat the solubility of metals is.\n2. The calculation of the capability of upwelling fluids to mechanically transport particles is based on the assumption of the\nfluids of a fixed density of 840kg/m3 at 340ºC and 300 bar. This was then applied to calculate the settling velocity of fluids\nwith various salinities. However, it is important to note that the salinity has a significant influence on the fluid density as well.\nI have done some calculations myself: for fluid at 340ºC, 300bar, the density of fluids for 0m, 2m, and 4m NaCl would be\n670, 793.7, and 879.14kg/m3 respectively. There is a big variation there, and what is the influence of such on the settling\nvelocity? The authors state that the salinity has an insignificant impact on the settling velocity because they ignored salinity’s\nimpact on density. I suggest the authors to revise their calculations and provide a more accurate estimate.\nIn summary, although the proposed idea is new and interesting, I suggest some of the key information described above is\nmissing. These are essential to demonstrate that the metals can really be stable and transported by the upwelling fluids.\nThis information should be provided to further evaluate the proposed model before it can be accepted.", "reviewer_4": "Review of: “A transient non-soluble noble metal transport in hydrothermal ore\nsystems”\nBy: Néstor Cano, José M. González-Jiménez, Antoni Camprubí, Eduardo González-\nPartida\nSubmitted to: Nature Communications\nManuscript Number: NCOMMS-24-33396\nReviewer: Andrew Tomkins, Monash University\nSummary\nThis paper presents an interesting discussion of how epithermal gold deposits,\nexemplified by very gold-rich samples from the El Hilo deposit, might form by\ntransport and coalescence of nano-sized polymetallic melt droplets. I generally find the\npaper very interesting and the combination of ideas is plausible. I have published a bit\non Au-rich polymetallic melts myself, so my tendency would be to support publication\nof work on this topic. However, the problem is that there is not any strong\nevidence presented to support their model of long-distance droplet transport. The work showing\nthat there were droplets of Au-rich polymetallic melt present in the studied samples is\nwell done and the figures are nice. However, that sort of work has been published before\nand by itself is not new enough to be publishable in Nature Comms. The new idea,\nwhich could bring the paper to point of being suitable for Nature Comms, focus on\ntransport of nanoparticulate polymetallic melt droplets. I find that sort of new idea to\nbe interesting and creative and my tendency is to encourage editors to publish this sort\nof thing. One problem is that the Stokes Law modelling is too simplistic to be a passably\nreliable way of representing the system at hand. The discussion that follows is\nspeculative and not supported by any evidence that I consider to be strong enough to\nsupport what they are saying. The ideas are plausible, but there really isn’t enough basis\nfor putting them forward (see my more detailed comments below). I think that I have\nto recommend rejection. If the authors could find stronger evidence and undertake more\nsuitable modelling it would be appropriate to resubmit. I have made some suggestions\nfor editing the text on the PDF file and my specific comments are listed below.\nSpecific Comments\nTable S1: There are a lot more than those listed in this table. If you’re aiming for\na comprehensive list, you have a bit of work to do (e.g., do a quick search on the Au-\nBi association). If you’re aiming for a list of some examples, this is probably OK, but\nyou should indicate that the list is just an exemplar of a bigger set of known deposits.\nLine 46: Careful here. Many people have recently postulated that nano-particulate\ngold is important in ore formation, so your wording here is inaccurate. It would be\nbetter to say that few people have investigated the role of nano-sized melt droplets in\nore formation, which is what you’re actually doing. Also, be aware that work similar\nto yours has been published already, so you should be citing that and indicating how\nyour work is different, which it is (you should do a comprehensive search for more,\nsuch as amongst the 42 papers that cite the one I’m listing here):\nHastie, E.C.G., Schindler, M., Kontak, D.J. et al. (2021) Transport and coarsening of\ngold nanoparticles in an orogenic deposit by dissolution–reprecipitation and Ostwald\nripening. Commun Earth Environ 2, 57. https://doi.org/10.1038/s43247-021-00126-6\nLine 59: If you’re looking at inclusions trapped in mineralised quartz veins in\nan ore deposit, you cannot glean insights into the early stages of metal transport. The\ninclusions are trapped as quartz crystallises, so the thermo-chemical conditions at the\ntime of fluid infiltration are not even preserved well (the conditions have to change in\norder for quartz to form).\nLine 61: “…mechanic[al] transport of metallic nanomelt precursors…” What is this\nsupposed to mean? The precursor to a nanomelt droplet would be an aqueous complex\nlike AuHS- for example, and talking about mechanical transport of that makes no\nsense. Perhaps just delete the word “precursors”? Or use language that explains what\nyou mean.\nLine 119: Yes, but be a little careful here; it applies for nanoparticles < ~5 nm.\nSee:\nDick et al., 2002, Size-dependent melting of silica-encapsulated gold nanoparticles. J.\nAm. Chem. Soc. 124, 10, 2312–2317.\nLines 120-123: I have no problem with the idea that nanos can sometimes serve\nas nuclei for growth of bigger things, whether they be minerals or melt droplets.\nHowever, I don’t think their existence in a mineralised quartz vein, amongst some\nlarger polymetallic melt droplets, is evidence that they formed early. It’s just evidence\nthat they existed at the same place and time. It is plausible that they could have\nformed a long way away and been carried to the site of deposition, but it’s probably\nmore likely that they formed as the quartz hosting them crystallised, given that this\nwould have been in response to changing T, P and/or X conditions (X being\nchemistry). You could delete the over interpretation from here and leave the overall\npoint til later after you’ve discussed physical carriage.\nFig 3a, b: In a, move the numbers down onto the x-axis and perhaps find a better\nway to indicate the size bins (why not use whole numbers to save space?). In b, why\nis the critical size in the middle of the fluid ascent rate range? Would be more\nconservative to be at the bottom – that’s also similar to the size of many of your\nparticles, which might explain why they’re there.\nLine 185: “…beyond which they would have sank and been lost.” Hydrothermal\nplumbing systems are not continuous vertical pipes through which things can be\ncarried or sink. They are tortured pathways, commonly with a significant horizontal\ncomponent, that are activated by fluid-mediated deformation. Yes, small particles can\nbe carried when the flow rate is high enough, but no, particles will not sink; they just\nwill be too heavy to be carried along. See next comment.\nGeneral point about the usage of Stokes Law: Using the simple version of\nStokes Law for this exercise is a major simplification of all hydrothermal systems,\nwhich inherently have complex structurally controlled fluid flow pathways.\nCommonly the structures that allow fluid transit are activated by addition of fluid\nfrom below in a dynamic environment (increased pore fluid pressure causes the rocks\nto break, allowing fluid transmission). The simple version of Stokes Law applies to a\ncontinuous body of fluid, such as a vertical pipe with frictionless walls, that dense\nparticles sink through; there is no tortuosity in the migration path. Those working on\nparticle sinking through melt networks in a solid framework of crystals have derived\nmodifications of Stokes Law to deal with the tortuosity problem. But this still\nwouldn’t apply directly to your case. Perhaps somebody has already attempted to\nnumerically model carriage of dense particles in a hydrothermal fluid system?\nCurrently, Figure 3b and the discussion surrounding it are largely irrelevant\nbecause of this over simplification. The discussion on whether a given size particle\ncan be transported is not based on appropriate modelling. Are the fluid ascent rates\ngiven an average of the ascent rate through a broad section of crust, or are they local\nascent rates applicable to a particular structure?\nOne possible way to deal with this problem is to acknowledge the difficulty of\nmodelling a complex system, show that the simple Stokes Law modelling finds that\nsub-micron particles settle at rates 3 orders of magnitude slower than estimated fluid\nascent rates (Fig 3b), and therefore suggest that nanodroplets are likely to be easily\ntransported long distances in hydrothermal systems (not the cautious tone of the text).\nThe other way to deal with the problem is to numerically model the complex system,\nwhich is perhaps what’s needed for a Nature Comms paper.\nLines 185-189: “Recent studies have shown that shallow boiling, cooling, and\n8,9,15,27\nmixing with external fluids (such as meteoric water or seawater) promote the\ncoalescence of nano-sized materials in hydrothermal fluids. These processes are\ncharacteristic of the shallow epithermal environment , suggesting that some\n28,29\npolymetallic melts from El Hilo should have originated outside this environment.”\nSure, coalescence is likely promoted in that environment, but this doesn’t require that\nthe particles formed outside that environment. Given the dramatic changes in P, T and\nX, it’s more likely that they formed when the fluids experienced boiling, cooling,\nmixing. I agree that it’s plausible that some of them formed prior to reaching the site\nof deposition."}, "author_response": "Response to the comments on the manuscript:\nA transient non-soluble noble metal transport in hydrothermal ore systems\nNéstor Cano, José M. González-Jiménez, Antoni Camprubí, Eric Morales-Casique, and Eduardo\nGonzález-Partida\nFor practicality, the lines referenced throughout this document correspond to those in the\nManuscript_no tracked changes file, which is included as a related manuscript file.\nResponse to the comments by Reviewer 1\n(Reviewer’s comments are italicized and our responses are in blue)\nWe thank Reviewer 1 for taking the time to revise our manuscript and for their overall positive\nappraisal. Their comments have considerably improved the article. We have responded to the\nsuggested changes point-by-point below.\nThe manuscript presents an intriguing study on the transport of noble metals by polymetallic melts in\nhydrothermal ore systems, proposing this as a complementary mechanism to metal-bearing ligands\nin solution. The authors provide evidence for suspension-like transport of nano-to-micron sized\npolymetallic melts within epithermal fluids, leading to the formation of polymetallic inclusions in\nquartz. While the manuscript introduces exciting ideas, there are some areas that require further\nclarification and discussion.\nMajor Issues\nData Interpretation:\nThe authors interpret the polymetallic inclusions in quartz as indicative of deposition from a melt\n(line 110). Since this textural interpretation forms the core of their model, it is essential that it is\nrobust and well-supported. It is possible that these textures could also result from the sudden\ndeposition of dissolved metals and sulphur from hydrothermal fluids during a boiling event. To\nstrengthen their interpretation, the authors should provide a more thorough discussion,\ndemonstrating that no other deposition processes could account for the observed textures.\nWe agree with this comment and have included a more complete discussion in lines 110–128. Indeed,\nboiling (cooling, neutralization, etc.) effectively triggers deposition of metals in solution, probably\nforming similar textures as those shown in Figs. 1 and 2. However, El Hilo is a ~5 cm-wide bonanza\nore with very high Au and Ag contents (2 kg/t Au and 31 kg/t Ag), which lacks any laminated\narrangement suggestive of repeated fluid injections. In addition, the extent of hydrothermal veining\nand alteration haloes at El Hilo are comparable to those of neighboring less mineralized veins,\nsuggesting similar amounts of circulating fluids. We thus interpret that El Hilo records a single\nhydrothermal pulse that was over-enriched in Au and Ag.\nSuch a precious metal focus is difficult to reconcile with in-situ precipitation from dissolved species\nalone, considering the solubility limits for these metals in hydrothermal solutions (hundreds of ppb;\ne.g., Pokrovski et al., 2014; Pearce et al., 2015; Wagner et al., 2016; Liu et al., 2019). To explain this\ninconsistency, ours and previous studies have proposed complementary mechanisms, including the\ninvolvement of Au-Ag colloidal suspensions (e.g., Saunders and Burke, 2017; McLeish et al., 2021,\n2024; Petrella et al., 2022) or (S-bearing) metallic melts rich in low-melting-point chalcophile\nelements (LMCE, As, Sb, Se; e.g., Ciobanu et al., 2006; Jian et al., 2022, 2024; Vikent’eva and\nBortnikov, 2023). The occurrence of LMCE + precious metal-bearing mineral assemblages in\npolymineral inclusions suggests that metallic melts are a more plausible mechanism than Au-Ag\ncolloidal suspensions to explain the El Hilo mineralization.\nUpon reviewing the provided figures (Figure 1.f, g, h, i, j, and Figure 2.a and b), the polymetallic\ninclusions do not appear particularly blebby. More evidence and detailed explanation are needed to\nsubstantiate the claim that these textures are indeed a result of deposition from a melt.\nWe agree that some inclusions are irregular instead of blebby. However, this is particularly evident in\nthe largest inclusions from Figs. 1f–j and Supplementary Fig. S2. In contrast, most smaller inclusions\n(e.g., Figs. 1e, 2b–m, S2) appear blebby at the micron and nanoscale, some even being perfectly\nrounded (e.g., Fig. 2e and g). We believe this results from the coalescence and growth of metallic\nmelts upon migration (or deposition), which agrees with the notable compositional and textural\nsimilarities between micro- and nano-sized inclusions (lines 220–223). It is worth noting that\npolymineral inclusions (PMI) with comparable shapes and textures—although larger than those at El\nHilo—have been recently proved by Jian et al. (2024) to be former metallic melts. These authors\nperformed heating experiments and induced the partial melting of PMIs at hydrothermal temperatures\n(<400 °C).\nComposition Discrepancy:\nThe abstract states that nano-to-micron sized polymetallic melts within epithermal fluids form bleb-\nlike polymetallic inclusions of Au-Ag(-As-Se-Sb) (line 24). However, the data presented in the\nmanuscript indicate that the polymetallic inclusions are composed of \"Ag(-Cu)-Sb-As-Se sulfosalts,\nelectrum, and Ag-Cu-Fe-Pb sulfides.\" Furthermore, line 108 describes a “Ag-Au-Cu-Pb(-Fe-Zn)-As-\nSb-S-Se multi-component system.” The presence of sulphur is significant as it influences the\nbehaviour of the polymetallic melt. This inconsistency between the abstract and the actual data\npresented needs to be addressed to maintain the reliability of the conclusions drawn.\nAgreed. We have corrected these issues in lines 23, 80, 115, and 130–131.\nTransport of PM Droplets:\nThe manuscript suggests that polymetallic (sulfosalt and sulfides) material is mechanically\ntransported by rising hydrothermal fluids as melt droplets (lines 182-184, 191-193) from a “distally\nlocated” magmatic intrusion to the surface. While the authors demonstrate that velocity calculations\nmake such transport possible, it is not discussed how such material would remain in a chemically\nstable suspension within the hydrothermal fluid. Existing experimental studies show that sulfide melts\nare not easily transported (Rose and Brenan, 2001).\nAgreed. Previous workers have found that sulfide and polymetallic melts are able to migrate short\ndistances (centimeters to meters) during prograde metamorphism (Frost et al., 2002; Tomkins et al.,\n2007) or magmatic crystallization (Rose and Brenan, 2001). However, these workers have focused\non metallic melt mobility occurring along grain boundaries, in the absence of an external transporting\nagent (e.g., fluid or magma). In our case study, we assess the mobility of nanomelt droplets by\nupwelling hydrothermal fluids, which would permit greater migration distances than those achieved\nby metallic melts alone. Thus, it would be inadequate to compare both scenarios. Indeed, some\nauthors have found that sulfide melts can be physically transported across crustal-scale distances by\nsilicate magmas in some tectonic settings (e.g., Schettino et al., 2023).\nThe manuscript does not sufficiently address the chemical processes by which the melt droplet could\nbe kept in a stable suspension in the aqueous epithermal fluid (“LMCE-bearing metallic (nano)melts\ncould be kept suspended,” line 233). The potential for the melt to react with the aqueous fluid, rather\nthan remaining inert and stable, needs more discussion.\nWe agree with this comment and have included lines 210–223 to support our ideas. We think that\nmetallic melts did not remain inert but, instead, could have reacted with the fluid by (1) scavenging\nmetals from dissolved species, and/or (2) being partly dissolved by the fluid. On the one hand,\nprecious metal scavenging by metallic melts could have occurred during migration (or upon\ndeposition) and agrees with the very high Au and Ag contents of PMIs (2 wt% Au and 61 wt% Ag)\nand, naturally, of the El Hilo bonanza (2 wt% Au and 31 wt% Ag). This is explained by the preferential\npartitioning of metals from aqueous solutions to metal-rich sulfide melts (Tooth et al., 2008, 2011).\nOn the other hand, available solubility models for hydrothermal fluids at 10 wt% NaCl equiv., pH ~5,\nand 350 °C—which are conditions similar to those of El Hilo fluids—yield solubility limits at ~0.1\nppm Au and 50 ppm Ag S (Kouzmanov and Pokrovski, 2012). These data align with calculations by\n2\nother authors, which suggest Au solubilities on the order of ppbs (e.g., Seward et al., 2014; Pearce et\nal., 2015; Wagner et al., 2016; Liu et al., 2019). Considering this, dissolution of Au and Ag amounts\nas high as those found at El Hilo (in the wt% scale) would have required large volumes of\nhydrothermal fluids, which are not reflected in the extent of quartz veining and/or alteration haloes at\nEl Hilo when compared to neighboring low-grade or unmineralized stages of mineralization (lines\n120–128). Thus, it is likely that ore-bearing fluids of El Hilo had met their solubility limits and were\nunable to dissolve any more precious metals from suspended material without attaining their\nsaturation and precipitation. Consequently, we think that an at-large dissolution of metallic melts by\nthe fluids has no logical support (lines 212–216).\nAdditionally, how the nano-to-micro size droplets would percolate through kilometres of rock without\nreacting with the wall rock or being mechanically stopped by limited porosity requires further\nelaboration. To demonstrate the transport of the melt, the authors should explain how it can be in\nboth mechanical and chemical equilibrium within the fluid.\nWe agree with this comment. Although the location/depth of the magmatic source at Natividad is\nunknown, intermediate-sulfidation epithermal deposits typically occur <4 km away from their\ncausative intrusion (Camprubí and Albinson, 2007; Sillitoe, 2010; Wilkinson et al., 2013). Veins at\nNatividad are structurally controlled by NE-trending fault systems, which produce fracture networks\nthat constitute efficient fluid paleo-conduits (Cano et al., 2024). It is likely that these paleo-conduits\nfacilitated the upwelling of hydrothermal fluids by reducing the chance of being stopped by limited\nporosity lithologies. Although we cannot assure that the fluid flow was continuous (and rapid) from\nthe source to the deposition site, our model (Fig. 3b–c) shows that metal-rich sulfide nanomelts could\nhave been transported even at very low flow rates (<0.01 m/s). If the fluids were indeed stopped at\nsome point during ascent, so that nanoPMIs settled, Fig. 3b shows that the addition of more fluid\nfrom below at a given ascent rate (<10–5 m/s) would have readily fluidized the particles again.\nAs for possible reactions with the wall rocks, we have found no explanations or cues from the existing\nliterature as to how these would have been possible. Therefore, we chose not to speculate on the\nmatter and, instead, referred to the plausibility of reactions with the hydrothermal fluid itself, either\nby partial solubilization by the fluid or by elemental scavenging by the droplets (lines 210–223).\nMoreover, we agree that melt droplets near the walls of the fractures could have been physically\ntrapped (e.g., sorption or obstruction) by the wall rocks. However, those along the center of the flow\nwere more likely to have been fluidized and transported, aided by the connectivity of the fracture\nnetwork.\nSpecific Comments\nLine 24: The statement about the composition of polymetallic inclusions does not match the data\npresented. The manuscript needs to clarify that the inclusions are primarily sulfide melts, which alters\nthe interpretation of the findings.\nAgreed. We have replaced “polymetallic” with “metal-rich sulfide melts” throughout the text.\nLine 34: The references cited (Econ. Geol. 99, 1063–1084 (2004); Toothe 2008 and 2011; Frost 2002)\nare used to support the hypothesis of polymetallic melts in hydrothermal fluids. However, these studies\nprimarily discuss local migration within host rocks or low-melting point chalcophile elements, not\nthe long-distance transport in aqueous fluids proposed by the authors. The manuscript should\ncritically assess these references and clarify their relevance to the current study.\nAlthough we agree with this comment, we used the references (Frost et al., 2002; Tomkins et al.,\n2004; Tooth et al., 2008, 2011) in the sense that their research proved that metal-rich sulfide and\npolymetallic melts could exist at hydrothermal conditions (temperature, fS , fO ) and can even\n2 2\nexsolve directly from hydrothermal fluids. We do not use them to discuss melt transport within the\nfluids. We have rephrased lines 33–36 for clarity. Also, we refer to thermodynamic calculations of\nFrost et al. (2002) and Tomkins et al. (2004) to show that metal-rich sulfide melts were stable at fluid\ntemperature ranges established for the El Hilo bonanza ores (i.e., fluid temperatures up to 400 °C).\nLong-distance transport as such is addressed by means of our calculations and the works of Oliver et\nal. (2006) and Okamoto and Tsuchiya (2009).\nLine 78: There is a critical need for a more detailed discussion on the phases referred to as PMI by\nthe authors: Ag(-Cu)-Sb-As-Se sulfosalts, electrum, and Ag-Cu-Fe-Pb sulfides. The manuscript must\ndifferentiate between polymetallic melts and sulfide melts and address their transport mechanisms\nindividually.\nAgreed. We have replaced “polymetallic inclusion” with “polymineral inclusions” to be more precise.\nWe did not find any S-free inclusion (except for some electrum inclusions), as PMIs average 15.5 ±\n4.9 wt% S (1σ, Table S2).\nLine 105: More detailed descriptions of the textures are needed to convince the reader that the\ntextures are indeed the remnant of melts.\nAgreed. We have included some additional information on textures in lines 110–117. Nearly identical\ntextures and compositions of PMIs have been interpreted by several authors (cf. Ciobanu et al., 2006;\nSousa-Guimarães et al., 2019; Jian et al., 2022, 2024; Vikent’eva and Bortnikov, 2023) as evidence\nof deposition from a melt in a range of ore deposits.\nLine 122: Can you be more specific here regarding what early segregation refers to? Do you mean\nearly segregation from a parental magma?\nWe meant segregation from the source(s), which could have been the parental magma and/or\nhydrothermal fluids themselves. We deleted “segregation” and rephrased this line to be more precise.\nLine 131: This statement is slightly confusing. It does not appear that your data shows sulfides PMI\nwithin a fluid inclusion. Rather, they seem to be occurring within the same quartz crystal.\nAgreed. We did not observe any polymineral inclusions (PMI) inside individual fluid inclusions.\nInstead, PMI inclusions occur with fluid inclusion assemblages (FIA) in quartz. That is, PMIs are\nconsidered as solid inclusions (as is the specific term in the common lore of fluid-inclusionists) with\nwhich fluid inclusions were co-trapped. Such is the most logical interpretation as per the evidence\nprovided through the specific petrographic studies that are addressed to fluid inclusions. We have\nrephrased lines 147–150 for clarity.\nLine 169-170: This sentence needs clarification. It is unclear what is interpreted to happen to metals\nin suspension and how it relates to polymetallic melt.\nAgreed, we have reformulated lines 224–227. We propose that fluid turbulency caused by boiling\nincreased the Brownian motion of suspended melts, thus enhancing their reaction (i.e., scavenging of\nmetals) with the ore-bearing fluid and their coalescence.\nLine 191-193: This part needs more explanation. Are the polymetallic melts at that stage sulfide\nmelts? (It should be, considering your data.)\nWe agree with this comment. Assuming that sulfide melts were formed in the causative intrusion\n(this is one possibility; lines 242–256 and 271–286), we do think that they should have been metal-rich\nsulfide melts at the stage of magmatic fluid release. This hypothesis agrees with δ34S between –\nVCDT\n3.2‰ and –0.3‰ in sulfides and sulfosalts from El Hilo (Cano et al., 2024), matching the range of\nmantellic S (Chaussidon and Lorand, 1990; line 242-244). It also agrees with previous reports of\nmetal-rich sulfide melts in porphyry(-epithermal) systems (e.g., Elatsite, Bulgaria; González-Jiménez\net al., 2021), supporting the idea that metal-rich sulfide+LMCE melts can be produced during deep\nmagmatic fluid release (lines 271–286).\nMore discussion is needed to explain how the sulfide melts/droplets can be segregated from the\nmagma into the aqueous fluid.\nWe agree with this comment and have added lines 273–283 accordingly. Metals behave incompatibly\nduring magmatic differentiation and are thus concentrated alongside volatiles until the late stages of\ncrystallization. Once volatile saturation is achieved, the magmatic fluid phase (+ metals) exsolves and\nmigrates to produce (or not) mineralization (Becker et al., 2019; and references therein). Considering\nthe metal endowment of the exsolved fluids and their temperature ranges (up to 650 °C; Kouzmanov\nand Pokrovski, 2012)—which are above the eutectic point of several LMCE-bearing chemical\nsystems—, metal-rich melts could potentially segregate directly from these fluids (e.g., Tooth et al.,\n2011).\nIn our view, sulfide melt droplets did not necessarily coexist with the magma before being segregated\ninto the fluid phase. For instance, the results of Schettino et al. (2023) suggest that sulfide melts can\noccur in magmas and be transported (mechanically) by them from the mantle to shallow magma\nreservoirs. Once in shallow reservoirs, Jeanvoine et al. (2024) found that sulfide melt droplets can\nwell up across magmas and eventually transfer metals to the hydrothermal fluid via flotation (lines\n276–279). These authors showed that Cu-rich (+Pd+Au) sulfide droplets <65 µm in diameter could\nbe transported upward by water bubbles, and subsequently supply metals to form Cu-Au-rich seafloor\nsulfide deposits in subduction settings. Although this combination of processes could have occurred\nnear the magmatic source of El Hilo, we think that it was not a necessary prerequisite for the formation\nof bonanza-type ores because metal-rich sulfide melts could have also been produced directly from\nthe fluids (Tooth et al., 2011) along the way to the deposition site (this is the other possibility; lines\n296–298).\nLine 232-233: There is a discrepancy between the literature cited and the interpretation made by the\nauthors.\nAgreed. We have rephrased lines 273–276.\nThe metal-rich brines referred to in line 232 are brines in which metals are dissolved as chloro-\ncomplexes and not suspended as nanomelts. There is no data to support the claim that nanomelts are\npresent in the brines.\nIt is arguable that the entire metal load may come dissolved as chloride complexes. Firstly, it is\nbroadly known that gold chloride complexes would not be stable in this environment, and it would\nbe likely transported as thiosulfide complexes, rather than chlorides. Such complexes are assumed to\nbe subordinate in intermediate sulfidation brines, provided that most intermediate sulfidation\nepithermal deposits worldwide tend to be depleted in gold, compared to their low and high sulfidation\ncounterparts. Therefore, the exceptionally high gold grades in this mineralized section may\nadditionally underpin the nanomelt hypothesis as an essential contributor of this metal. We do not\ndiscuss this issue in the manuscript because it stems as speculative and, however reasonable, there is\nno way to prove it. As for the capability of relatively high salinity brines (as determined from fluid\ninclusion data) for transporting silver and base metals, it is beyond doubt, as is common lore from\nepithermal deposits already.\nSecondly, the geological, geochemical, and textural evidence obtained at El Hilo does not support\nthat the metal load was entirely provided by soluble metal-bearing complexes. This bonanza zone is\na few cm wide and contains weight percents of Au and Ag, most of which occur as micro- to nano-\nsized polymineral inclusions within quartz. As discussed earlier, the extent of hydrothermal veining\nat El Hilo does not justify the amount of fluid required to explain these grades by precipitation from\ndissolved species alone (e.g., AgCl –, AuCl –, and/or AuHS –)—considering precious metal solubility\n2 2 2\nlimitations (Pokrovski et al., 2014; Pearce et al., 2015; Wagner et al., 2016; Liu et al., 2019). These\nobservations support a transient suspension-like (i.e., colloidal) transport of nanomaterials within the\nfluids, which agrees with our numerical models (Fig. 3b–c). Of course, direct evidence of nanomelts\nin inclusion fluids, which would be an iron-clad proof for their occurrence, was out of our reach.\nHowever, in our view, and in accordance with preexisting literature, we have provided evidence that\nis credible enough as to account for the occurrence of such nanomelts.\nA discussion is needed to address how such nanomelts would be stable in this environment and how\nthey might have been extracted from the magmatic precursor.\nAgreed. Constraints on nanomelt stability and extraction from the magmatic precursor are provided\nin lines 271–283.\nFigure 1: The manuscript lacks a discussion on quartz dissolution textures presented in Figure 1,\nwhich could provide insights into the deposition mechanisms and the environment of formation.\nAgreed. We have double-checked our samples and expanded the discussion on quartz textures in lines\n81–85. In addition, we have included some photomicrographs of quartz dissolution-replacement\ntextures in the new Supplemental Fig. S1.\nFigure 4: The composition of the rising fluid and its ability to stabilise polymetallic inclusions during\ntransport must be addressed.\nWe agree with this comment and have included lines 210–223 accordingly. In addition, we have\nmodified Fig. 4a to provide a more complete description of our model. It would be impossible to\nreliably constrain the composition of rising ore-bearing fluids prior to their trapping in fluid\ninclusions. However, our fluid inclusion data show that mineralizing fluids have temperatures up to\n~400 °C and salinities up to ~19 wt% NaCl equiv. (Cano et al., 2023). In the absence of evaporites\naround Natividad (and regionally), these salinity ranges suggest a remarkable component of magmatic\nbrines in the fluids. Consequently, it is likely that the upwelling ore-bearing fluid had a similar (or\nsomewhat less diluted) composition. Regardless of the composition of the fluid input, it could have\nnot dissolved all Au and Ag contained in suspended materials to form the 5 cm-wide bonanza zone,\ngiven the metal solubility limitations that have been explained earlier. We argue that this could have\nfavored the stabilization of metal nanomaterials instead of their dissolution.\nThis includes discussing the role of fluid-rock interactions and the impact of wall rock porosity.\nWe agree with this comment and have included lines 287–305 to discuss these factors. Hydrothermal\nveins at the Natividad deposit are hosted by porphyritic dacites and/or carbonaceous metasedimentites\n(Fig. 4a). However, the highest Au grades occur in those veins (including El Hilo) hosted by the latter.\nThus, it is likely that redox reactions between oxidized mineralizing fluids and reduced carbonaceous\nmaterials could have been an ingredient in focusing precious metals (lines 287–298). In agreement,\nwe previously found methane in some fluid inclusions from El Hilo (Cano et al., 2024)—probably\nformed via thermogenesis of organic matter present in wall rocks—, which provides evidence for\nfluid-rock interaction to some extent. Although we cannot assert to what extent these reactions\ncontributed to ore formation, we can hypothesize their role as observed by authors in other\nhydrothermal ore deposits (e.g., Petrella et al., 2021).\nAs for the impact of wall-rock porosity, veins at Natividad are structurally controlled by faults that\nconstitute fluid paleo-conduits (Cano et al., 2024). These paleo-conduits could have enhanced the\nupwelling of hydrothermal fluids by reducing the chance of being hindered or stopped by limited\nporosity lithologies. Nevertheless, if the fluids were indeed stopped at some point during ascent so\nthat the nanoPMIs settled, Fig. 3b shows that the addition of more fluid from below at a given ascent\nrate (<10–5 m/s) would have readily fluidized the particles again. As additional information (not\nincluded in this manuscript or in earlier publications), we may also state that we found neither\nlithologies that came to disperse fluid flow laterally nor any broadening of hydrothermal alteration\nhalos that would be suggestive of such a case. In fact, alteration halos around the mineralized\nstructures in metasedimentites are so narrow that they are barely visible at a macroscopic (and even\nmicroscopic; Cano et al., 2024). Therefore, although we cannot know the extent of water-rock\ninteraction below the known mineralized area, it seems unlikely that such a mechanism can be\nregarded as a ruling factor for chemical reactions other than very locally-driven redox reactions.\nCITED REFERENCES\nBecker, S.P., Bodnar, R.J., and Reynolds, T.J., 2019, Temporal and spatial variations in characteristics\nof fluid inclusions in epizonal magmatic-hydrothermal systems: Applications in exploration\nfor porphyry copper deposits: Journal of Geochemical Exploration, v. 204, p. 240–255,\ndoi:10.1016/j.gexplo.2019.06.002.\nCamprubí, A., and Albinson, T., 2007, Epithermal deposits in México - Update of current knowledge\nand an empirical reclassification: Special Paper of the Geological Society of America, v. 422,\np. 377–415, doi:10.1130/2007.2422(14).\nCano, N., Camprubí, A., Proenza, J.A., and González-Partida, E., 2024, Genesis and evolution of the\nNatividad Au-Ag(-Ge) epithermal deposit in southern Mexico: Unravelling the final chapters\nof the Sierra Madre del Sur metallogeny: Ore Geology Reviews,\ndoi:https://doi.org/10.1016/j.oregeorev.2024.105979.\nCano, N., González-Jiménez, J.M., Camprubí, A., Domínguez-Carretero, D., González-Partida, E.,\nand Proenza, J.A., 2023, Nanomaterial accumulation in boiling brines enhances epithermal\nbonanzas: Scientific Reports, v. 13, p. 1–7, doi:10.1038/s41598-023-41756-4.\nChaussidon, M., and Lorand, J.P., 1990, Sulphur isotope composition of orogenic spinel lherzolite\nmassifs from Ariege (North-Eastern Pyrenees, France): An ion microprobe study:\nGeochimica et Cosmochimica Acta, v. 54, p. 2835–2846, doi:10.1016/0016-7037(90)90018-\nG.\nCiobanu, C.L., Cook, N.J., Damian, F., and Damian, G., 2006, Gold scavenged by bismuth melts: An\nexample from Alpine shear-remobilizates in the Highiş Massif, Romania: Mineralogy and\nPetrology, v. 87, p. 351–384, doi:10.1007/s00710-006-0125-9.\nFrost, B.R., Mavrogenes, J.A., and Tomkins, A.G., 2002, Partial melting of sulfide ore deposits during\nmedium- and high-grade metamorphism: Canadian Mineralogist, v. 40, p. 1–18,\ndoi:10.2113/gscanmin.40.1.1.\nGonzález-Jiménez, J.M., Piña, R., Kerestedjian, T.N., Gervilla, F., Borrajo, I., Pablo, J.F. de, Proenza,\nJ.A., Tornos, F., Roqué, J., and Nieto, F., 2021, Mechanisms for Pd–Au enrichment in\nporphyry-epithermal ores of the Elatsite deposit, Bulgaria: Journal of Geochemical\nExploration, v. 220, p. 106664, doi:10.1016/j.gexplo.2020.106664.\nJeanvoine, A., Park, J.W., Pelleter, E., Bézos, A., Chazot, G., Hwang, J., and Fouquet, Y., 2024, The\nflotation of magmatic sulfides transfers Cu-Au from magmas to seafloor massive sulfide\ndeposits: Communications Earth and Environment, v. 5, p. 1–16, doi:10.1038/s43247-024-\n01571-9.\nJian, W., Mao, J., Cook, N.J., Chen, L., Xie, G., Xu, J., Song, S., Hao, J., Li, R., and Liu, J., 2022,\nIntracrystalline migration of polymetallic Au-rich melts in multistage hydrothermal systems:\nexample from the Xiaoqinling lode gold district, central China: Mineralium Deposita, v. 57,\np. 147–154, doi:10.1007/s00126-021-01090-z.\nJian, W., Mao, J., Lehmann, B., Cook, N.J., Li, J., Song, S., and Zhu, L., 2024, Hyper-enrichment of\ngold via quartz fracturing and growth of polymetallic melt droplets: Geology, v. 52, p. 411–\n416, doi:10.1130/G51875.1/6275576/g51875.pdf.\nKouzmanov, K., and Pokrovski, G.S., 2012, Hydrothermal Controls on Metal Distribution in\nPorphyry Cu (-Mo-Au) Systems: Society of Economic Geologists, Special publication, v. 16,\np. 573–618, doi:10.5382/sp.16.22.\nLiu, W., Chen, M., Yang, Y., Mei, Y., Etschmann, B., Brugger, J., and Johannessen, B., 2019, Colloidal\ngold in sulphur and citrate-bearing hydrothermal fluids: An experimental study: Ore Geology\nReviews, v. 114, p. 103142, doi:10.1016/j.oregeorev.2019.103142.\nMcLeish, D.F., Williams-Jones, A.E., Clark, J.R., and Stern, R., 2024, Extreme shifts in pyrite sulfur\nisotope compositions reveal the path to bonanza gold: Proceedings of the National Academy\nof Sciences, v. 121, doi:https://doi.org/10.1073/pnas.2402116121.\nMcLeish, D.F., Williams-Jones, A.E., Vasyukova, O. 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V., and Bortnikov, N.S., 2023, Evidence of Mineral Melting in Ores of the Svetlinsk\nGold Deposit, South Urals, Russia: Doklady Earth Sciences, v. 513, p. 1321–1325,\ndoi:10.1134/S1028334X2360233X.\nWagner, T., Fusswinkel, T., Wälle, M., and Heinrich, C., 2016, Microanalysis of fluid inclusions in\ncrustal hydrothermal systems using laser ablation methods: Elements, v. 12, p. 323–328.\nWilkinson, J.J., Simmons, S.F., and Stoffell, B., 2013, How metalliferous brines line Mexican\nepithermal veins with silver: Scientific Reports, v. 3, p. 1–7, doi:10.1038/srep02057.\nResponse to the comments by Reviewer 2 (Dr. Andrew Tomkins)\n(Reviewer’s comments are italicized and our responses are in blue)\nThis paper presents an interesting discussion of how epithermal gold deposits, exemplified by very\ngold-rich samples from the El Hilo deposit, might form by transport and coalescence of nano-sized\npolymetallic melt droplets. I generally find the paper very interesting and the combination of ideas is\nplausible. I have published a bit on Au-rich polymetallic melts myself, so my tendency would be to\nsupport publication of work on this topic. However, the problem is that there is not any strong\nevidence presented to support their model of long-distance droplet transport. The work showing that\nthere were droplets of Au-rich polymetallic melt present in the studied samples is well done and the\nfigures are nice. However, that sort of work has been published before and by itself is not new enough\nto be publishable in Nature Comms. The new idea, which could bring the paper to point of being\nsuitable for Nature Comms, focus on transport of nanoparticulate polymetallic melt droplets. I find\nthat sort of new idea to be interesting and creative and my tendency is to encourage editors to publish\nthis sort of thing. One problem is that the Stokes Law modeling is too simplistic to be a passably\nreliable way of representing the system at hand. The discussion that follows is speculative and not\nsupported by any evidence that I consider to be strong enough to support what they are saying. The\nideas are plausible, but there really isn’t enough basis for putting them forward (see my more detailed\ncomments below). I think that I have to recommend rejection. If the authors could find stronger\nevidence and undertake more suitable modeling it would be appropriate to resubmit. I have made\nsome suggestions for editing the text on the PDF file and my specific comments are listed below.\nWe thank Dr. Tomkins for taking the time to revise our manuscript! We also appreciate his interest in\nthe topic being discussed in our paper and the insightful comments. We have made all the minor\ncorrections from the attached pdf file and responded to his comments point-by-point below.\nSpecific Comments\nTable S1: There are a lot more than those listed in this table. If you’re aiming for a comprehensive\nlist, you have a bit of work to do (e.g., do a quick search on the Au-Bi association). If you’re aiming\nfor a list of some examples, this is probably OK, but you should indicate that the list is just an\nexemplar of a bigger set of known deposits.\nAgreed. This list summarizes some examples (line 310 and Table S1).\nLine 46: Careful here. Many people have recently postulated that nano-particulate gold is important\nin ore formation, so your wording here is inaccurate. It would be better to say that few people have\ninvestigated the role of nano-sized melt droplets in ore formation, which is what you’re actually doing.\nAlso, be aware that work similar to yours has been published already, so you should be citing that\nand indicating how your work is different, which it is (you should do a comprehensive search for\nmore, such as amongst the 42 papers that cite the one I’m listing here HASTIE 2021):\nAgreed! We thank Dr. Tomkins for the recommended bibliography. We have rephrased lines 44–46.\nLine 59: If you’re looking at inclusions trapped in mineralised quartz veins in an ore deposit, you\ncannot glean insights into the early stages of metal transport. The inclusions are trapped as quartz\ncrystallises, so the thermo-chemical conditions at the time of fluid infiltration are not even preserved\nwell (the conditions have to change in order for quartz to form).\nYes and no. Fluid inclusion studies constitute a reasonable approach to thermochemical conditions of\nmineral deposition but, for various reasons (which may lead to a long dissertation), they are never\n100% beyond reasonable doubt. Serious fluid inclusionists aim to ensure the best possible way to\nperform representative analyses through scrupulous petrographic and microthermometric\ndeterminations, which we did (see Cano et al., 2023, 2024). Our methodological procedure on the\nmatter was canonical, and aimed to overcome the fact that, as we all know, fluid inclusions sample\nactual mineralizing fluids in a discontinuous and somewhat serendipitous manner. In this sense, fluid\ninclusion studies have similar limitations as the studies on the fossil record, but a discontinuous record\ndoes not invalidate any study in which all due precautions were taken, and all methodological or\ninstrumental good practices were employed.\nWe agree that if all metals were being transported in solution, then the observed polymineral\ninclusions should have precipitated in-situ as mineralized quartz was growing in response to\nphysicochemical variations in ore-bearing fluids (as Reviewer mentioned). Direct precipitation of\nfocalized Au-Ag ores grading in the wt% scale found at El Hilo would have required episodic\ninjections of large volumes of hydrothermal fluid. However, this ~5 cm-wide bonanza zone lacks any\nstructure suggestive of episodic fluid percolation (e.g., colloform-banded or crustiform; Fig. 1a),\nwhich points to a single metal-rich fluid injection. Also, the extent of hydrothermal veining at El Hilo\nis comparable to that around neighboring less mineralized veins, suggesting similar volumes of\ncirculating fluids. Alternatively, precipitating such amounts of Au-Ag from a single metal-rich fluid\ninjection would have required unrealistically high Au-Ag solubilities (lines 117–125). These\nobservations suggest that part of Au-Ag-bearing nanomaterials were probably already present within\nthe fluid at the onset of in-situ quartz + metal precipitation. In this sense, these “carried-away”\nnanomaterials would represent an “early” stage of metal transport.\nLine 61: “…mechanic[al] transport of metallic nanomelt precursors…” What is this supposed to\nmean? The precursor to a nanomelt droplet would be an aqueous complex like AuHS- for example,\nand talking about mechanical transport of that makes no sense. Perhaps just delete the word\n“precursors”? Or use language that explains what you mean.\nAgreed. We have corrected “precursors” to “droplets”.\nLine 119: Yes, but be a little careful here; it applies for nanoparticles < ~5 nm. See: Dick et al., 2002,\nSize-dependent melting of silica-encapsulated gold nanoparticles. J. Am. Chem. Soc. 124, 10, 2312–\n2317.\nWe thank Dr. Tomkins for the recommended paper. We have reformulated lines 137–139 to be more\nprecise. According to Guisbiers et al. (2016), the melting point of Au-Ag alloys is size- and shape-\ndependent and decreases as Ag contents increase—even in nanoparticles up to 10 nm in diameter.\nProvided that some of the observed nanoparticles reside in this size interval, we think that the size\ndid facilitate the formation of nano-droplets.\nLines 120-123: I have no problem with the idea that nanos can sometimes serve as nuclei for growth\nof bigger things, whether they be minerals or melt droplets. However, I don’t think their existence in\na mineralised quartz vein, amongst some larger polymetallic melt droplets, is evidence that they\nformed early. It’s just evidence that they existed at the same place and time. It is plausible that they\ncould have formed a long way away and been carried to the site of deposition, but it’s probably\nmore likely that they formed as the quartz hosting them crystallised, given that this would have been\nin response to changing T, P and/or X conditions (X being chemistry).\nAgreed. By “early”, we meant at some point between the source and the deposition site, not\nnecessarily in the causative intrusion (lines 237–239). In this way, part of the micro- or nano-sized\nPMIs must have formed “early” given the solubility limitations expected for a single hydrothermal\nfluid injection responsible for a narrow, Au-Ag over-enriched, bonanza-type ore. We kindly refer the\nReviewer to a previous response (“If you’re looking at inclusions trapped in mineralised…”).\nYou could delete the over interpretation from here and leave the overall point til later after you’ve\ndiscussed physical carriage.\nDone (lines 139–142).\nFig 3a, b: In a, move the numbers down onto the x-axis and perhaps find a better way to indicate the\nsize bins (why not use whole numbers to save space?).\nDone.\nIn b, why is the critical size in the middle of the fluid ascent rate range? Would be more\nconservative to be at the bottom – that’s also similar to the size of many of your particles, which might\nexplain why they’re there.\nAgreed. However, we have modified the original Fig. 3b to present the new model.\nLine 185: “…beyond which they would have sank and been lost.” Hydrothermal plumbing systems\nare not continuous vertical pipes through which things can be carried or sink. They are tortured\npathways, commonly with a significant horizontal component, that are activated by fluid-mediated\ndeformation. Yes, small particles can be carried when the flow rate is high enough, but no, particles\nwill not sink; they just will be too heavy to be carried along. See next comment.\nAgreed. We have rewritten this whole section (lines 232–319).\nGeneral point about the usage of Stokes Law: Using the simple version of Stokes Law for this exercise\nis a major simplification of all hydrothermal systems, which inherently have complex structurally\ncontrolled fluid flow pathways. Commonly the structures that allow fluid transit are activated by\naddition of fluid from below in a dynamic environment (increased pore fluid pressure causes the rocks\nto break, allowing fluid transmission). The simple version of Stokes Law applies to a continuous body\nof fluid, such as a vertical pipe with frictionless walls, that dense particles sink through; there is no\ntortuosity in the migration path. Those working on particle sinking through melt networks in a solid\nframework of crystals have derived modifications of Stokes Law to deal with the tortuosity problem.\nBut this still wouldn’t apply directly to your case. Perhaps somebody has already attempted to\nnumerically model carriage of dense particles in a hydrothermal fluid system?\nWe fully agree that fluid flow and PMIs transport occurs in a complex network of fractures, with\nvariations in fracture aperture and rugosity. Modeling these processes through such a system would\nrequire modeling (a) fluid flow, (b) mass transport of particles, and (c) heat flow/transfer. Some\napproaches simplify the fracture network by an equivalent representation using the cubic law for fluid\nflow and assuming that the fracture network can be represented as a continuum. Such an approach\nrequires extensive work (e.g., Liu et al., 2025) and is beyond the scope of our work.\nHowever, to show the feasibility of PMIs transport, as a first approximation, we have modelled this\nnetwork as a single tube filled with solid particles (among which are the PMIs), employing the\nformalism of fluidized beds (equation 1); these particles fill the fracture space up to some fraction (1–\nφ), while the rest of the space is occupied by the fluid (φ). We now estimate the minimum velocity\nfor particles within such a system/tube to be mobilized/fluidized by the flowing fluid. Flow rates\nwould also need to overcome the settling of particles (equation 2), which now we calculate from an\nexpression valid for a wide range of Reynolds numbers (Fig. 3b), including turbulence. We also\nestimate the minimum pressure gradient (of course, the pressure gradient is independent of pressure\nmagnitude) needed to fluidize the granular bed along a given length (Fig. 3c) of an existing fracture\nnetwork (i.e., fault systems at Natividad). We emphasize that tortuosity within the fracture network\nwould increase the pressure gradient needed to mobilize the bed (lines 190–192).\nOur new approach shows that PMIs, especially nanos, are readily fluidized (<10–5 m/s) and settle\nslowly (<10–2 m/s; Fig. 3b). These results suggest that if particles settle at some point due to fluid\nstagnation under an impermeable barrier, they could be easily fluidized as more fluid is added from\nbelow, and ascent is resumed. Therefore, hydrothermal solutions can potentially fluidize and drag\nnanoPMIs, even in tortuous pathways across the uppermost continental crust, considering ascent rates\nfor hydrothermal fluids between 100 and 10–2 m/s (Delaney et al., 1987; Okamoto and Tsuchiya, 2009;\nDissanayake et al., 2014). This velocity range is conservative, given that some hydrothermal plumes\nreach up to 6.2 m/s (Dissanayake et al., 2014).\nAlthough our model may be regarded as simplistic, previous authors have employed a similar\napproach to explain the mechanical transport of dense particles within hydrothermal fluids (e.g.,\nOliver et al., 2006; Okamoto and Tsuchiya, 2009). For example, Oliver et al. (2006) modelled the\ncarriage of rock fragments up to 10 m in size by magmatic-derived fluids over ~10 km, which resulted\nin “fluidized breccias” at the Cloncurry district, Australia. We partly based our approach on theirs, as\nthey also calculated fluidization and particle settling velocities as well as the minimum pressure\ngradient necessary to maintain fluidization conditions along distances up to 10 km. Moreover,\nOkamoto and Tsuchiya (2009) modelled settling velocities of quartz crystals up to 500 µm long during\nfluid ascent within vein-forming fractures in the Sambagawa Belt, Japan. They also used an approach\nsimilar to Oliver’s (and ours), although they simplified their particle settling model by using the\nStoke’s law (Re<1).\nCurrently, Figure 3b and the discussion surrounding it are largely irrelevant because of this over\nsimplification. The discussion on whether a given size particle can be transported is not based on\nappropriate modeling. Are the fluid ascent rates given an average of the ascent rate through a broad\nsection of crust, or are they local ascent rates applicable to a particular structure?\nAscent rates were taken from authors who measured or calculated ascent rates of hydrothermal fluids\nin black smokers (Delaney et al., 1987; Dissanayake et al., 2014) and vein-forming fractures\n(Okamoto and Tsuchiya, 2009). We took these ascent rates as average values from the PMI source(s)\nto the deposition site. Our aim was not to model the transport of PMIs across a broad section of the\ncrust, given that intermediate-sulfidation epithermal deposits are usually emplaced less than 4 km\nfrom their causative intrusion (Camprubí and Albinson, 2007; Sillitoe, 2010; Wilkinson et al., 2013),\nwhich is the uppermost section of the crust. Nevertheless, it is worth noting that some authors\n(Schettino et al., 2023) have shown that polymineral inclusions (solidified melts) can be physically\ntransported by ascending silicate melts from the mantle to the upper continental crust.\nOne possible way to deal with this problem is to acknowledge the difficulty of modeling a complex\nsystem, show that the simple Stokes Law modeling finds that sub-micron particles settle at rates 3\norders of magnitude slower than estimated fluid ascent rates (Fig 3b), and therefore suggest that\nnanodroplets are likely to be easily transported long distances in hydrothermal systems (not the\ncautious tone of the text). The other way to deal with the problem is to numerically model the complex\nsystem, which is perhaps what’s needed for a Nature Comms paper.\nAgreed.\nLines 185-189: “Recent studies8,9,15,27 have shown that shallow boiling, cooling, and mixing with\nexternal fluids (such as meteoric water or seawater) promote the coalescence of nano-sized materials\nin hydrothermal fluids. These processes are characteristic of the shallow epithermal\nenvironment28,29, suggesting that some polymetallic melts from El Hilo should have originated\noutside this environment.” Sure, coalescence is likely promoted in that environment, but this doesn’t\nrequire that the particles formed outside that environment. Given the dramatic changes in P, T and X,\nit’s more likely that they formed when the fluids experienced boiling, cooling, mixing. I agree that it’s\nplausible that some of them formed prior to reaching the site of deposition.\nWe agree with Dr. Tomkins that placing the exact site of nanomelt formation is challenging. For this\nreason, we have suggested in lines 237–239 and 309–310 that nanomelts could have formed anywhere\nbetween the magmatic source and the deposition site. Nevertheless, the magmatic input to ores is\nshown by sulfides and sulfosalts with δ34S between –3.2‰ and –0.3‰ (Cano et al., 2024), which\nVCDT\nmatches the –3‰ to 3‰ range of mantellic S (Chaussidon and Lorand, 1990). The epithermal\nenvironment may have a vertical extend of ~2 km in similar geologic settings (e.g., Camprubí and\nAlbinson, 2007), which gives plenty of space for nanomelts to form and grow during fluid transit as\na result of boiling, fluid-rock interactions, and conductive heat loss. Regardless of the site of\nformation, very high precious metal contents at El Hilo added to the abundance of Au-Ag enriched\nPMIs suggest that some nanomelts were likely physically transported from a certain source (magmatic\nor epithermal) to the final site—bonanzas are hard to explain by “regular” precipitation from solution\n(Au-Ag solubilities in the order of ppbs; e.g., Seward et al., 2014; Pearce et al., 2015; Wagner et al.,\n2016; Liu et al., 2019).\nWe cannot discard any potential source of nanomelts in the magmatic-hydrothermal system,\nespecially considering that LMCE-bearing mineral assemblages (i.e., former nanomelts) have been\nreported in porphyry deposits that represent the roots of some types of epithermal deposits (e.g.,\nElatsite in Bulgaria; González-Jiménez et al., 2021) and constitute a good proxy for the likelihood of\na magmatic source for nanomelts.\nCITED REFERENCES\nCamprubí, A., and Albinson, T., 2007, Epithermal deposits in México - Update of current knowledge\nand an empirical reclassification: Special Paper of the Geological Society of America, v. 422,\np. 377–415, doi:10.1130/2007.2422(14).\nCano, N., Camprubí, A., Proenza, J.A., and González-Partida, E., 2024, Genesis and evolution of the\nNatividad Au-Ag(-Ge) epithermal deposit in southern Mexico: Unravelling the final chapters\nof the Sierra Madre del Sur metallogeny: Ore Geology Reviews,\ndoi:https://doi.org/10.1016/j.oregeorev.2024.105979.\nChaussidon, M., and Lorand, J.P., 1990, Sulphur isotope composition of orogenic spinel lherzolite\nmassifs from Ariege (North-Eastern Pyrenees, France): An ion microprobe study:\nGeochimica et Cosmochimica Acta, v. 54, p. 2835–2846, doi:10.1016/0016-7037(90)90018-\nG.\nDelaney, J.R., Mogk, D.W., and Mottl, M.J., 1987, Quartz-cemented breccias from the Mid- Atlantic\nRidge: samples of a high-salinity hydrothermal upflow zone.: Journal of Geophysical\nResearch, v. 92, p. 9175–9192, doi:10.1029/JB092iB09p09175.\nDissanayake, A.L., Yapa, P.D., and Nakata, K., 2014, Simulation of hydrothermal vents in the Izena\nCauldron, Mid Okinawa trough, Japan and other Pacific locations: Journal of Hydro-\nEnvironment Research, v. 8, p. 343–357, doi:10.1016/j.jher.2014.05.003.\nGonzález-Jiménez, J.M., Piña, R., Kerestedjian, T.N., Gervilla, F., Borrajo, I., Pablo, J.F. de, Proenza,\nJ.A., Tornos, F., Roqué, J., and Nieto, F., 2021, Mechanisms for Pd–Au enrichment in\nporphyry-epithermal ores of the Elatsite deposit, Bulgaria: Journal of Geochemical\nExploration, v. 220, p. 106664, doi:10.1016/j.gexplo.2020.106664.\nGuisbiers, G., Mendoza-Cruz, R., Bazán-Díaz, L., Velázquez-Salazar, J.J., Mendoza-Perez, R.,\nRobledo-Torres, J.A., Rodriguez-Lopez, J.L., Montejano-Carrizales, J.M., Whetten, R.L.,\nand José-Yacamán, M., 2016, Electrum, the gold-silver alloy, from the bulk scale to the\nnanoscale: Synthesis, properties, and segregation rules: ACS Nano, v. 10, p. 188–198,\ndoi:10.1021/acsnano.5b05755.\nLiu, W., Chen, M., Yang, Y., Mei, Y., Etschmann, B., Brugger, J., and Johannessen, B., 2019, Colloidal\ngold in sulphur and citrate-bearing hydrothermal fluids: An experimental study: Ore Geology\nReviews, v. 114, p. 103142, doi:10.1016/j.oregeorev.2019.103142.\nLiu, B., Kumar, D., and Ghassemi, A., 2025, Modeling proppant transport and settlement in 3D\nfracture networks in geothermal reservoirs: Geothermics, v. 125,\ndoi:10.1016/j.geothermics.2024.103176.\nOkamoto, A., and Tsuchiya, N., 2009, Velocity of vertical fluid ascent within vein-forming fractures:\nGeology, v. 37, p. 563–566, doi:10.1130/G25680A.1.\nOliver, N.H.S., Rubenach, M.J., Fu, B., Baker, T., and Blenkinsop, T.G., 2006, Granite-related\noverpressure and volatile release in the mid crust: fluidized breccias from the Cloncurry\nDistrict, Australia: Geofluids, v. 6, p. 346–358, doi:10.1111/j.1468-8123.2006.00155.x.\nPearce, M.A., White, A.J.R., Fisher, L.A., Hough, R.M., and Cleverley, J.S., 2015, Gold deposition\ncaused by carbonation of biotite during late-stage fluid flow: Lithos, v. 239, p. 114–127,\ndoi:10.1016/j.lithos.2015.10.010.\nSchettino, E., González-Jiménez, J.M., Marchesi, C., Palozza, F., Blanco-Quintero, I.F., Gervilla, F.,\nBraga, R., Garrido, C.J., and Fiorentini, M., 2023, Mantle-to-crust metal transfer by\nnanomelts: Communications Earth and Environment, v. 4, p. 1–9, doi:10.1038/s43247-023-\n00918-y.\nSeward, T.M., Williams-Jones, A.E., and Migdisov, A.A., 2014, The chemistry of metal transport and\ndeposition by ore-forming hydrothermal fluids, in Holland, H.. and Turekian, K.K. eds.,\nTreatise on Geochemistry: Second Edition, Oxford, Elsevier Ltd., p. 29–57,\ndoi:10.1016/B978-0-08-095975-7.01102-5.\nSillitoe, R.H., 2010, Porphyry copper systems: Economic Geology, v. 105, p. 3–41,\ndoi:10.2113/gsecongeo.105.1.3.\nWagner, T., Fusswinkel, T., Wälle, M., and Heinrich, C., 2016, Microanalysis of fluid inclusions in\ncrustal hydrothermal systems using laser ablation methods: Elements, v. 12, p. 323–328.\nWilkinson, J.J., Simmons, S.F., and Stoffell, B., 2013, How metalliferous brines line Mexican\nepithermal veins with silver: Scientific Reports, v. 3, p. 1–7, doi:10.1038/srep02057.\nResponse to the comments by Reviewer 3\n(Reviewer’s comments are italicized and our responses are in blue)\nThis manuscript introduced a new mechanism of enrichment of noble metals in magmatic-\nhydrothermal ore forming systems. The authors propose that nano-micro sized melts may be present\nand suspend in hydrothermal fluids and be transported by fluids to form mineralization. Evidence\nprovided includes SEM, EDS, TEM and EPMA analyses, etc. The idea reads interesting with relevant\ndata and observations. However, I have some concerns related to whether the melts could be stable\nin the fluids at the T-P-salinity conditions as discussed, as well as if the calculation of the fluids’\ncapability to transport and maintain melts suspension is accurate enough. Please see below:\nWe thank Reviewer 3 for taking the time to revise our manuscript and for the positive feedback about\nthe novelty of our investigation. Their suggestions have considerably improved it. We have responded\nto their comments point-by-point below.\n1. It is important to consider the chemical stability of the noble metal melts in the hydrothermal fluids\nwith relatively high T and salinity. At conditions of El Hilo fluids, e.g., 100-400ºC, up to 20wt% NaCl,\nthe fluids may dissolve significant amounts of metals, in particular when considering that chloride\ncomplexes are thought to be important species for Au, Ag, and PGEs in Cl-dominated fluids and\ncontribute to their hydrothermal transportation. Hence, it is important to provide constraints on if\nthe nano-micro metal melts can be chemically stable as well in such fluids. I suggest the authors to\nprovide thermodynamic calculations of the relevant metals and if possible, the fluid composition\nmeasurement of the fluid inclusions to demonstrate what the solubility of metals is.\nWe agree with Reviewer’s comment. At fluid conditions of El Hilo, part of the Au and Ag load was\nprobably transported in solution as AgCl – and/or AuHS –. However, available solubility models for\n2 2\nhydrothermal fluids at 10 wt% NaCl equiv., pH ~5, and 350 °C (Kouzmanov and Pokrovski, 2012)—\nwhich are conditions similar to those of El Hilo fluids—yield solubility limits of ~0.1 ppm Au and 50\nppm Ag. These data agree with calculations by other authors that suggest Au solubilities on the order\nof ppbs (e.g., Seward et al., 2014; Pearce et al., 2015; Wagner et al., 2016; Liu et al., 2019).\nConsidering this, the dissolution of large amounts of Au and Ag present at El Hilo (in the wt% scale)\nwould have required large volumes of hydrothermal fluids, which are not reflected in the extent of\nquartz veining and/or alteration haloes when compared to neighboring low-grade or unmineralized\nveins (lines 117–125). Therefore, this hydrothermal pulse (El Hilo bonanza represents a single fluid\ninjection, as it lacks any laminated textures) could have not possibly dissolved all precious metals\ncontained in suspended materials (PMIs), which probably hindered their dissolution and consequently\nfavored their chemical stability with the media. Nevertheless, we think that metal-rich nanomelts\nwere not inert because their could have potentially scavenged Au and Ag from the solutions (e.g.,\nTooth et al., 2011), and hence the very high Au and Ag contents of PMIs (2 wt% Au and 31 wt% Ag).\nAs for the second part of the comment, previous authors have performed thermodynamic calculations\nand solubility models for metals in hydrothermal fluids at different temperature, salinity, and pH\nranges (e.g., Kouzmanov and Pokrovski, 2012; Seward et al., 2014; Pearce et al., 2015; Liu et al.,\n2019). Their results have shown that Au and Ag solubilities are on the order of ppbs, which would\nnot account for the formation of precious metal bonanzas (McLeish et al., 2021; Petrella et al., 2022),\nas already noted. In addition, we did not consider microchemical analyses on fluid inclusions because\nthose measurements would not provide trustworthy constraints on metal solubility given the\noccurrence of suspended metal-rich nanomaterials. Analyzing fluid inclusions (e.g., Fig. 1c) by, for\nexample, LA-ICP-MS would yield unreliable Au and Ag contents (Au-Ag in solution + Au-Ag in\nsuspension).\n2. The calculation of the capability of upwelling fluids to mechanically transport particles is based\non the assumption of the fluids of a fixed density of 840kg/m3 at 340ºC and 300 bar. This was then\napplied to calculate the settling velocity of fluids with various salinities. However, it is important to\nnote that the salinity has a significant influence on the fluid density as well. I have done some\ncalculations myself: for fluid at 340ºC, 300bar, the density of fluids for 0m, 2m, and 4m NaCl would\nbe 670, 793.7, and 879.14kg/m3 respectively. There is a big variation there, and what is the influence\nof such on the settling velocity? The authors state that the salinity has an insignificant impact on the\nsettling velocity because they ignored salinity’s impact on density. I suggest the authors to revise their\ncalculations and provide a more accurate estimate.\nWe fully agree with Reviewer’s comment and are thankful for performing their own calculations.\nAfter calculating fluidization and settling velocities of particles carried by fluid with densities of 670,\n793.7, and 840 kg/m3, we confirmed that density (and hence salinity) has no remarkable impact\non the results (see figure below and Table S3). Thus, we have performed new calculations of\nfluidization conditions and settling velocities at a fixed density (837 km/m3; average density of fluid\ninclusions from El Hilo, see Table S3). Instead, we noted that other variables, such as the porosity of\nthe bed of suspended particles (fluid reservoir with melt droplets) or the fluid regime (Reynolds\nnumber), have a more significant effect on fluidization and settling velocities than fluid density. For\nthese reasons, our new models contemplate different porosities (0.3, 0.5, and 0.7) and fluid regimes\n(Re = 1, 10, and 100).\nCalculation of fluidization (φ = 0.3, 0.5, and 0.7) and settling (Re = 1, 10, and 100) velocities of\nparticles within fluids with contrasting densities: 670 kg/m3 (0 wt% NaCl ), 794 kg/m3 (10 wt%\neq\nNaCl ), and 840 kg/m3 (19 wt% NaCl ). Density variations are indicated by the three lines for each\neq eq\nφ and Re value; for example: fluidization velocities are represented by gray (19 wt% NaCl ), orange\neq\n(10 wt% NaCl ), and blue (0 wt% NaCl ) lines at φ = 0.3. This plot shows that density has a\neq eq\nnegligible effect on fluidization velocities of particles with any given size, and this effect is even\nlower on the settling velocities (all three lines for Re = 1, 10, and 100 lie on top of each other).\nCITED REFERENCES\nKouzmanov, K., and Pokrovski, G.S., 2012, Hydrothermal Controls on Metal Distribution in\nPorphyry Cu (-Mo-Au) Systems: Society of Economic Geologists, Special publication, v. 16,\np. 573–618, doi:10.5382/sp.16.22.\nLiu, W., Chen, M., Yang, Y., Mei, Y., Etschmann, B., Brugger, J., and Johannessen, B., 2019, Colloidal\ngold in sulphur and citrate-bearing hydrothermal fluids: An experimental study: Ore Geology\nReviews, v. 114, p. 103142, doi:10.1016/j.oregeorev.2019.103142.\nMcLeish, D.F., Williams-Jones, A.E., Vasyukova, O. V., Clark, J.R., and Board, W.S., 2021, Colloidal\ntransport and flocculation are the cause of the hyperenrichment of gold in nature: Proceedings\nof the National Academy of Sciences of the United States of America, v. 118, p. 1–6,\ndoi:10.1073/pnas.2100689118.\nPearce, M.A., White, A.J.R., Fisher, L.A., Hough, R.M., and Cleverley, J.S., 2015, Gold deposition\ncaused by carbonation of biotite during late-stage fluid flow: Lithos, v. 239, p. 114–127,\ndoi:10.1016/j.lithos.2015.10.010.\nPetrella, L., Thébaud, N., Fougerouse, D., Tattitch, B., Martin, L., Turner, S., Suvorova, A., and Gain,\nS., 2022, Nanoparticle suspensions from carbon-rich fluid make high-grade gold deposits:\nNature Communications, v. 13, p. 1–9, doi:10.1038/s41467-022-31447-5.\nSeward, T.M., Williams-Jones, A.E., and Migdisov, A.A., 2014, The chemistry of metal transport and\ndeposition by ore-forming hydrothermal fluids, in Holland, H.. and Turekian, K.K. eds.,\nTreatise on Geochemistry: Second Edition, Oxford, Elsevier Ltd., p. 29–57,\ndoi:10.1016/B978-0-08-095975-7.01102-5.\nTooth, B., Ciobanu, C.L., Green, L., and Neill, B.O., 2011, Bi-melt formation and gold scavenging\nfrom hydrothermal fluids : An experimental study: Geochimica et Cosmochimica Acta, v. 75,\np. 5423–5443, doi:10.1016/j.gca.2011.07.020.\nWagner, T., Fusswinkel, T., Wälle, M., and Heinrich, C., 2016, Microanalysis of fluid inclusions in\ncrustal hydrothermal systems using laser ablation methods: Elements, v. 12, p. 323–328.."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have reviewed the revised manuscript and am satisfied with the authors' responses to my comments. I believe\nthe points raised during the initial review have been addressed thoroughly and with appropriate consideration. The revisions have\nsignificantly improved the clarity and strength of the manuscript.\nI have no further comments and consider the manuscript, in its current form, to be suitable for publication.\nSummary of other concerns from this revision, in light of Reviewer #3’s comments:\nReviewer #3 highlights an important point about the need to provide the density of the PMIs. This is critical since these\nmetals are very dense, which affects their ability to be transported upwards. The authors could explore whether silica gel (as\ndescribed in McNab et al, 2024 Coprecipitation of amorphous silica and gold nanoparticles contributes to gold\nhyperenrichment- GEOLOGY) might carry these metals, potentially mitigating the density issue. However, the gel would also\nneed continuous porosity for transport.\nIt is unclear whether metals are transported as a melt suspended in fluid or as solid nano- or microparticles (example line\n257). If transported as a melt, what processes cause solidification at the site of mineralisation? The explanation provided—\nboiling leads to deposition—is vague and there is confusion between the coalescence of melt vs solid particles. Both\nreferences cited here 16 and 35 are describing the coalescence of solid metallic nanomaterial not melts. It is likely that the\ncoalescence mechanism between a solid particle and melt differ and this point should be addressed in the discussion.\nThere are some inconsistencies in the description of the material being transported in the proposed model. For instance, in\nlines 126 and 135, the authors refer to a “metal-rich melt,” while later, in lines 246 and 263, they describe “sulfide melts” or\n“metal-rich sulfide nano melts.” This distinction is significant and needs to be clarified.\nIn L244, the manuscript states that precious metals avoid dissolution in hydrothermal fluids, which is likely dur to their low\nsolubility but it is not the case for sulfur that has a high solubility in these systems. Then how does S remain in the melt\nphase rather than transitioning to the fluid phase? The stability of a sulfide melt under these conditions requires further\nexplanation, as sulfide melts are typically associated with magmatic rather than aqueous environments.\nThe authors also reference the Bi collector model, where precious metals partition into a Bi melt phase in an aqueous\nsystem. While this is a well-documented process, the manuscript instead describes a sulfide melt, which may behave\ndifferently in such an environment. The stability of a sulfide melt under hydrothermal conditions, particularly its ability to\npersist and transport metals, needs to be addressed further in my opinion.\nAdditionally, if the melt is described as “metal-rich,” does it imply a sulfide-dominated composition or one dominated by\nprecious metals with low solubility? These two scenarios could have distinct implications for melt stability, transport\nproperties, and deposition mechanisms. Clear distinctions between these terms and their respective roles in the proposed\nmodel would help resolve potential confusion and strengthen the study’s argument.", "reviewer_2": "I've reviewed the changes the authors have made since the previous review and I can see that they've put a lot of effort into\naddressing each comment. With regards to my previous comments, my main criticism was around the too-simplistic use of\nStoke's Law as a way of exploring the carriage of micro- and nano-scale particles. The authors have completely redone the\nrelevant part of the manuscipt and it is now much more rigorous. As I read through this new material, I was wondering, if the\nparticles are so easy to transport, what would make them accumulate? And then, at the end of the section (Page 14, lines\n253-260), the authors discuss boiling at the site of accumulation and metal-upgrading. I guess this is a reasonable way to\nround out this section, and that question of nano-droplet/particle accumulation is one that the research community will\ncontinue to ponder for some time. I am happy to recommend that the article be published now.", "reviewer_3": "The revised manuscript provides some good improvements from their previous version. More evidence was added to\nsupport the mechanical transport of PMIs. This is good, but I think there are still some more careful justifications are needed\nregarding this new transport mechanism of precious metals. The manuscript should not be published before these points\ncan be well addressed.\nThese are:\n1. The author in their reply and new calculations show that porosity has a more significant influence on fluidization and\nsettling velocities. This is apparent. However, as the calculated porosity was ‘set’ to 0.3, 0.5, 0.7 in their calculation, no\nexplanation was provided how such numbers are based on. Well, to me, even 0.3 looks like a big overestimate of porosity by\nlooking at their SEM of PMIs. The electrum was even more dense and less porous compared to other parts of the PMIs.\nAs the porosity of PMIs is showing such important influence on their mechanical transport in fluids (1 to >1.5 orders of\nmagnitude by looking at their new figure, comparing PMI porosity 0.3 to 0.7), it is necessary to provide the data on the PMI\ndensity: what is the PMI porosity in your sample?\n2. The question then comes to the density calculation of the PMIs. It is clear if the PMIs are too dense then they will\nprecipitate fast, not possible at all for them to be transported by a fluid. The provided density calculations of PMIs were\nbased on mineral data (solid) and the numbers were calculated by using mixtures of such through compositional data.\nThere are two major flaws there: (1) how different will it be for a melt mixture and a solid mixture when they have the same\ncomposition? (2) the observed particle size seems to be solid rather than a polymetallic melt, which may have significantly\nsmaller size but higher density. If such, it will be more difficult to transport them in fluid.\n3. Then, since the electrum are more dense and has higher density than other parts of PMIs, the author should explain better\nhow these things with such contrasted physical properties can be hold together then a fluid is trying to flush them. It is very\nlikely the less dense part which also have higher porosity will be effectively separated with Ag-Au electrum.\n4. The temperature range for these melts was assumed to be <400ºC, yet, the melting point of mentioned phases (see their\nsupplementary table S2) are mostly much higher (eg, Ag is ~960ºC, Au is 1064ºC). The author need to explain better in the\ntext how such melt can be stable in such a low T circumstances.\n5. The impact of the proposed mechanism should be better discussed. It seems that even if the mechanism is true, it is more\nof a localized phenomenon and may not really explain large-scale precious metal mineralization, especially for Au. It may\nexplain El Hilo, but how to apply this to other cases? Please provide more insights."}, "author_response": "Response to the comments on the manuscript:\nA transient non-soluble noble metal transport in hydrothermal ore systems\nNéstor Cano, José M. González-Jiménez, Antoni Camprubí, Eric Morales-Casique, and Eduardo\nGonzález-Partida\nResponse to the comments by Reviewer 1\n(Reviewer’s comments are italicized and our responses are in blue)\nI have reviewed the revised manuscript and am satisfied with the authors' responses to my comments.\nI believe the points raised during the initial review have been addressed thoroughly and with\nappropriate consideration. The revisions have significantly improved the clarity and strength of the\nmanuscript. I have no further comments and consider the manuscript, in its current form, to be\nsuitable for publication.\nWe thank Reviewer 1 for taking the time to rereview our manuscript and for their positive feedback.\nCertainly, their comments helped us to improve the paper considerably.\nSummary of other concerns from this revision, in light of Reviewer #3’s comments:\nReviewer #3 highlights an important point about the need to provide the density of the PMIs. This is\ncritical since these metals are very dense, which affects their ability to be transported upwards. The\nauthors could explore whether silica gel (as described in McNab et al, 2024 Coprecipitation of\namorphous silica and gold nanoparticles contributes to gold hyperenrichment- GEOLOGY) might\ncarry these metals, potentially mitigating the density issue. However, the gel would also need\ncontinuous porosity for transport.\nWe agree that the density of PMIs is a critical factor influencing their ability to remain suspended in\nthe fluid during ascent and have incorporated this consideration into our model (Fig. 3b). First, we\nestimated the density of PMIs based on the areal distribution of mineral species and their ideal\ndensities (sourced from mindat.org). Using data from 100 PMIs, we calculated an average density of\n6.3 ± 1.3 g/cm³ (1σ) (see Supplementary Data S2, sheet “PMI composition and density,” and lines\n373–380).\nNext, using this average density of 6.3 g/cm³, we computed the fluidization and settling velocities of\nmicro- and nano-sized materials (lines 390–394 and Supplementary Data S3). Our results demonstrate\nthat the mineralizing fluid could indeed transport such high-density materials upwards—despite the\nsignificant density difference (0.8 g/cm³ for the fluid vs. 6.3 g/cm³ for the melt)—due to the small\nsize of the materials (generally <5 µm; Fig. 3a–b).\nWhile silica gels were likely present, as suggested by the occurrence of jigsaw quartz at El Hilo,\ntheir primary role was associated with in-situ boiling-induced deposition of PMIs rather than facilitating\nthe long-distance transport of nano- and micromaterials (Cano et al., 2023).\nIt is unclear whether metals are transported as a melt suspended in fluid or as solid nano- or\nmicroparticles (example line 257). If transported as a melt, what processes cause solidification at the\nsite of mineralisation? The explanation provided—boiling leads to deposition—is vague and there is\nconfusion between the coalescence of melt vs solid particles. Both references cited here 16 and 35 are\ndescribing the coalescence of solid metallic nanomaterial not melts. It is likely that the\ncoalescence mechanism between a solid particle and melt differ and this point should be addressed in the\ndiscussion.\nAgreed. Our SEM and TEM observations indicate that, prior to deposition, metals were transported\nas a fluid-mediated melt enriched in low-melting-point chalcophile elements (e.g., As, Sb, Se; Fig.\n4a). However, in-situ processes at the deposition site, such as boiling and conductive cooling, were\nlikely to have depressed the system's temperature, thus triggering the solidification of metal-rich\nsulfide-sulfosalt melts (referred to hereafter simply as “metal-rich melts”) after their trapping in\nquartz (lines 233–240). Solidified metal-rich melts are equivalent to polymineral inclusions or PMIs\n(lines 53–54, 145–147, 164–165, and 151–152). We have modified several parts of the text to clarify\nthis issue.\nRegarding the coalescence of melts vs. solid particles, the chemical similarity observed between\nnano- and micron-scale polymineral inclusions (lines 229–232; Figs. 1e–j and 2b–m) supports the\ncoalescence of (nano)melts rather than solids. Since references 16 and 35 address the coalescence of\nsolid nanomaterials from colloidal suspensions, we have reformulated lines 236–237 to distinguish\nour findings from these cases.\nThere are some inconsistencies in the description of the material being transported in the proposed\nmodel. For instance, in lines 126 and 135, the authors refer to a “metal-rich melt,” while later, in\nlines 246 and 263, they describe “sulfide melts” or “metal-rich sulfide nano melts.” This distinction\nis significant and needs to be clarified.\nAgreed. Considering Reviewer 1’s previous comments on this issue and the average composition of\nPMIs (61.3 wt% Ag, 2.4 wt% Au, 4.6 wt% Sb, 2.3 wt% As, 15.5 wt% S, 1.8 wt% Se, 6.0 wt% Cu,\nand 3.9 wt% Fe; see Supplementary Data S2), we have chosen to refer to these as “metal-rich sulfide-\nsulfosalt melts” (although we used “metal-rich melts” throughout the text for brevity; lines 135–136).\nThis terminology has been consistently applied.\nIn L244, the manuscript states that precious metals avoid dissolution in hydrothermal fluids, which\nis likely dur to their low solubility but it is not the case for sulfur that has a high solubility in these\nsystems. Then how does S remain in the melt phase rather than transitioning to the fluid phase? The\nstability of a sulfide melt under these conditions requires further explanation, as sulfide melts are\ntypically associated with magmatic rather than aqueous environments.\nThe authors also reference the Bi collector model, where precious metals partition into a Bi melt\nphase in an aqueous system. While this is a well-documented process, the manuscript instead\ndescribes a sulfide melt, which may behave differently in such an environment. The stability of a\nsulfide melt under hydrothermal conditions, particularly its ability to persist and transport metals,\nneeds to be addressed further in my opinion.\nAdditionally, if the melt is described as “metal-rich,” does it imply a sulfide-dominated composition\nor one dominated by precious metals with low solubility? These two scenarios could have distinct\nimplications for melt stability, transport properties, and deposition mechanisms. Clear distinctions\nbetween these terms and their respective roles in the proposed model would help resolve potential\nconfusion and strengthen the study’s argument.\nWe agree with these comments. The studied solidified metal-rich melts (i.e., PMIs) are predominantly\ncomposed of metals (primarily Ag, ~60 wt.%) and metalloids, together accounting for ~85 wt.%, with\n~15 wt.% S (Supplementary Data S2). With our data, we cannot explain how S persisted in the fluid,\nand there is no available literature on the fate of S from metal-dominated sulfide-sulfosalt melts in\nlow-temperature hydrothermal solutions. Nevertheless, it is reasonable that the precious metal-\ndominated composition of these melts—with components such as Ag and Au exhibiting very low\nsolubility in fluids—prevented the preferential dissolution of certain constituents (e.g., S, As, Sb).\nThe composition of PMIs is relatively homogeneous, and we did not observe any S-free inclusions,\naside from a few electrum inclusions (see figure below and Supplementary Data S2). Therefore, we\nclassified all PMIs as former metal-rich melts, grouping them into a single type of solidified melt. As\na result, no differentiation between melt types was necessary in our model, which otherwise would\nhave had implications for melt stability, transport properties, and deposition mechanisms as\nhighlighted by Reviewer 1. To maintain consistency, we have double-checked and ensured the\nsystematic use of the terms “metal-rich sulfide-sulfosalt melt” or, simply, “metal-rich melts”\nthroughout the text.\nWhisker and box plot showing the composition of PMIs (Figure taken from the Supplementary Data\nS2).\nAlthough most studies on melt-mediated mineralization have focused on Bi- and Te-rich ores\n(Douglas et al., 2000; Tooth et al., 2008, 2011; Jian et al., 2022, 2024), some authors have also\nrecognized melt-suggestive features in S-bearing assemblages from low temperature (<400 °C) ore\nsystems (Ciobanu et al., 2006; Cave et al., 2019; Sousa-Guimarães et al., 2019; Vikent’eva and\nBortnikov, 2023; Cano et al., 2024). Notably, some studies have proposed that S-bearing melts may\nact as potential collectors of Au and Ag from fluids (Ciobanu et al., 2006; Cano et al., 2024), which\nagrees with our observations (lines 225–227). Supplementary Data S1 highlights several examples of\nsuch observations.\nCITED REFERENCES\nCano, N., González-Jiménez, J.M., Camprubí, A., Domínguez-Carretero, D., González-Partida, E.,\nand Proenza, J.A., 2023, Nanomaterial accumulation in boiling brines enhances epithermal\nbonanzas: Scientific Reports, v. 13, p. 1–7, doi:10.1038/s41598-023-41756-4.\nCano, N., González-Jiménez, J.M., Camprubí, A., Proenza, J.A., and González-Partida, E., 2024,\nMacro-to-nanoscale investigation unlocks gold and silver enrichment by lead-bismuth\nmetallic melts in the Switchback epithermal deposit, southern Mexico: American\nMineralogist, v. In Press.\nCave, B.J., Barnes, S.J., Pitcairn, I.K., Sack, P.J., Kuikka, H., Johnson, S.C., and Duran, C.J., 2019,\nMulti-stage precipitation and redistribution of gold, and its collection by lead-bismuth and\nlead immiscible liquids in a reduced-intrusion related gold system (RIRGS); Dublin Gulch,\nwestern Canada: Ore Geology Reviews, v. 106, p. 28–55,\ndoi:10.1016/j.oregeorev.2019.01.010.\nCiobanu, C.L., Cook, N.J., Damian, F., and Damian, G., 2006, Gold scavenged by bismuth melts: An\nexample from Alpine shear-remobilizates in the Highiş Massif, Romania: Mineralogy and\nPetrology, v. 87, p. 351–384, doi:10.1007/s00710-006-0125-9.\nDouglas, N., Mavrogenes, J., Hack, A., and England, R., 2000, The liquid bismuth collector model:\nan alternative gold deposition mechanism, in Silbeck, C.G. and Hubble, T.C.T. eds.,\nUnderstanding planet Earth; on the stanting blocks of the third millenium, 15th Australian\nGeological Convention, Sydney, Geological Society of Australia, p. 135.\nJian, W., Mao, J., Cook, N.J., Chen, L., Xie, G., Xu, J., Song, S., Hao, J., Li, R., and Liu, J., 2022,\nIntracrystalline migration of polymetallic Au-rich melts in multistage hydrothermal systems:\nexample from the Xiaoqinling lode gold district, central China: Mineralium Deposita, v. 57,\np. 147–154, doi:10.1007/s00126-021-01090-z.\nJian, W., Mao, J., Lehmann, B., Cook, N.J., Li, J., Song, S., and Zhu, L., 2024, Hyper-enrichment of\ngold via quartz fracturing and growth of polymetallic melt droplets: Geology, v. 52, p. 411–\n416, doi:10.1130/G51875.1/6275576/g51875.pdf.\nSousa-Guimarães, F., Cabral, A.R., Lehmann, B., Rios, F.J., Ávila, M.A.B., de Castro, M.P., and\nQueiroga, G.N., 2019, Bismuth-melt trails trapped in cassiterite–quartz veins: Terra Nova, v.\n31, p. 358–365, doi:10.1111/ter.12391.\nTooth, B., Brugger, J., Ciobanu, C., and Liu, W., 2008, Modeling of gold scavenging by bismuth melts\ncoexisting with hydrothermal fluids: Geology, v. 36, p. 815–818, doi:10.1130/G25093A.1.\nTooth, B., Ciobanu, C.L., Green, L., and Neill, B.O., 2011, Bi-melt formation and gold scavenging\nfrom hydrothermal fluids : An experimental study: Geochimica et Cosmochimica Acta, v. 75,\np. 5423–5443, doi:10.1016/j.gca.2011.07.020.\nVikent’eva, O. V., and Bortnikov, N.S., 2023, Evidence of Mineral Melting in Ores of the Svetlinsk\nGold Deposit, South Urals, Russia: Doklady Earth Sciences, v. 513, p. 1321–1325,\ndoi:10.1134/S1028334X2360233X.\nResponse to the comments by Reviewer 2 (Dr. Andrew Tomkins)\n(Reviewer’s comments are italicized and our responses are in blue)\nI've reviewed the changes the authors have made since the previous review and I can see that they've\nput a lot of effort into addressing each comment. With regards to my previous comments, my main\ncriticism was around the too-simplistic use of Stoke's Law as a way of exploring the carriage of micro-\nand nano-scale particles. The authors have completely redone the relevant part of the manuscipt and\nit is now much more rigorous. As I read through this new material, I was wondering, if the particles\nare so easy to transport, what would make them accumulate? And then, at the end of the section (Page\n14, lines 253-260), the authors discuss boiling at the site of accumulation and metal-upgrading. I\nguess this is a reasonable way to round out this section, and that question of nano-droplet/particle\naccumulation is one that the research community will continue to ponder for some time. I am happy\nto recommend that the article be published now.\nWe thank Dr. Tomkins for taking the time to rerevise our manuscript and for his positive feedback!\nResponse to the comments by Reviewer 3\n(Reviewer’s comments are italicized and our responses are in blue)\nThe revised manuscript provides some good improvements from their previous version. More evidence\nwas added to support the mechanical transport of PMIs. This is good, but I think there are still some\nmore careful justifications are needed regarding this new transport mechanism of precious metals.\nThe manuscript should not be published before these points can be well addressed.\nWe thank Reviewer 3 for taking the time to rereview our manuscript and for acknowledging the\nimprovements from its previous version.\n1. The author in their reply and new calculations show that porosity has a more significant influence\non fluidization and settling velocities. This is apparent. However, as the calculated porosity was ‘set’\nto 0.3, 0.5, 0.7 in their calculation, no explanation was provided how such numbers are based on.\nAgreed. Porosity affects fluidization velocity but has minimal influence on settling velocity, where\nthe Reynolds number plays a more prominent role. As noted in lines 403–405, porosity values of 0.3,\n0.5, and 0.7 were initially used to represent varying fluid (φ) to melt droplet+particle (1–φ) ratios\nwithin fractures in the hydrothermal system. In the (re)revised version of the manuscript, we have\nalso performed calculations for porosities of 0.9 to represent a fluid-dominated medium (lines 213–\n216) with some suspended metal-rich sulfide-sulfosalt melts—which probably offers a better\nrepresentation of the mechanical transport mechanism being addressed. The new results for porosity\nat 0.9 agree with previous ones (at 0.3, 0.5, and 0.7), suggesting that micro- and nanomelt droplets\ncould indeed have been fluidized even under very low fluid flow rates (Fig. 3b).\nWell, to me, even 0.3 looks like a big overestimate of porosity by looking at their SEM of PMIs. The\nelectrum was even more dense and less porous compared to other parts of the PMIs. As the porosity\nof PMIs is showing such important influence on their mechanical transport in fluids (1 to >1.5 orders\nof magnitude by looking at their new figure, comparing PMI porosity 0.3 to 0.7), it is necessary to\nprovide the data on the PMI density: what is the PMI porosity in your sample?\nIt is important to note that porosities (φ) of 0.3, 0.5, 0.7, and 0.9 used in calculations are NOT for\nPMIs but for the fluid + suspended droplets/particles within the fractures, where the proportion of\nfluid is φ and that of droplet+particle is 1–φ (lines 181–182 and 403–405). Our results suggest that\nPMIs were molten before their entrapment, thus they were virtually not porous.\nIt is not possible to reliably determine the original porosity of the “particle bed” (i.e., hydrothermal\nfluid + melt droplets) being fluidized. The quartz/PMI ratio observed in SEM and TEM images (Figs.\n1b–d, S1) does not correspond to a specific porosity. Assuming that this ratio represents the original\nporosity implies that all the fluid contents in solution crystallized as quartz, which is an\noversimplification. Hydrothermal fluid crystallization is rarely 100% efficient, often leaving behind\na residual liquid phase. Sequential mineral precipitation, influenced by changes in temperature, fO ,\n2\nfS , and fluid composition, typically results in fractionation of the residual fluid. In open systems like\n2\nthe fracture-controlled Natividad deposit, such residual fluids can migrate before complete\ncrystallization occurs.\nAt El Hilo, the transporting agent for PMIs was an epithermal fluid (lines 21–24). It is therefore\nreasonable to assume that the proportion of fluid (φ) was significantly greater than that of the\nsuspended PMIs (1–φ) during transit, representing a high-porosity medium (lines 213–216). Given\nthese uncertainties and the impracticality of defining a specific porosity, we calculated fluidization\nvelocities for a range of porosities within the fracture network (0.3, 0.5, 0.7, and 0.9) to reflect various\nscenarios wherein melt droplets were (or were not) transported by fluids. This approach allowed us\nto provide a more comprehensive representation of the natural hydrothermal system. Moreover, it is\nworth remembering that fluid flow across the epithermal environment does not only occur in open\nspaces along the entire mineralized body: the “tails” of fractures may have rather low permeabilities,\nand even in the most permeable sections of the fractures, permeability is drastically reduced as\nminerals precipitate. The analyzed associations (banded and massive veins; Fig. 1a) belong to sections\nin the mineralization in which high permeabilities can be reasonably assumed. If there were\nmineralization types that denoted very limited permeability (i.e., veinlets or stringers), we might\ndiscuss our results otherwise.\nAnyway, evaluating the behavior of fluidization velocities as a function of porosity within the\nfractures helped us to emphasize that metal-rich sulfide-sulfosalt droplets could have been readily\nfluidized, even at low flow rates and very low pressure gradients (Fig. 3b–c).\n2. The question then comes to the density calculation of the PMIs. It is clear if the PMIs are too dense\nthen they will precipitate fast, not possible at all for them to be transported by a fluid.\nYes, metal-rich sulfide-sulfosalt melts were significantly denser than the fluid phase. In earlier\nversions of the manuscript, we provided density estimates for PMIs based on the areal distribution of\nmineral species and their ideal densities (sourced from mindat.org). Using data from 100 PMIs, we\ncalculated an average density of 6.3 ± 1.3 g/cm³ (1σ). This information is detailed in Supplementary\nData S2 (sheet “PMI composition and density”) and explained in lines 373–380.\nDespite the substantial density difference between the fluid (0.8 g/cm³) and the melts (6.3 g/cm³), our\ncalculations demonstrate that the mineralizing fluid could effectively transport these high-density\nmaterials upwards, mainly due to their nano and micron-scale sizes (Fig. 3a–c; lines 191–197;\nSupplementary Data S3). Larger droplets would have not been possibly transported by the fluids,\nwhich would explain the size threshold of PMIs at El Hilo around 40 µm (Fig. 3a).\nThe provided density calculations of PMIs were based on mineral data (solid) and the numbers were\ncalculated by using mixtures of such through compositional data. There are two major flaws there:\n(1) how different will it be for a melt mixture and a solid mixture when they have the same\ncomposition?\nAs detailed above, we calculated the density of PMIs based on the proportion of their constituent\nminerals (see Supplementary Data S2). As Reviewer 3 correctly noted, these densities represent solid\nPMIs, not their molten counterparts (i.e., metal-rich sulfide-sulfosalt melts). However, it is important\nto recognize that solid metallic minerals generally have higher densities than their liquid forms (lines\n392–394). This is due to the closely packed structures of solids, which maximize packing efficiency,\nwhereas liquids lack long-range atomic order, resulting in increased atomic spacing and volume—\nand thus lower density. For instance, solid Ag has a density of 10.5 g/cm³ compared to 9.3 g/cm³ for\nliquid Ag, with similar trends observed for Au, Cu, Pb, S, Se, and others.\nConsidering these differences, if the mineralizing fluid was capable of transporting materials with\ndensities of 6.3 g/cm³ (the density of solidified PMIs; lines 392–394), it would have certainly been\nable to carry less dense materials, such as molten PMIs (Fig. 3b). Therefore, our conclusion regarding\nthe suspension-like transport of metal-rich melts remains well-supported and consistent with these\nobservations.\n(2) the observed particle size seems to be solid rather than a polymetallic melt, which may have\nsignificantly smaller size but higher density. If such, it will be more difficult to transport them in fluid.\nOur SEM and TEM analyses of PMI-rich quartz reveal textures—such as blebby shapes, curvilinear\nor cuspate boundaries, and triple points (lines 112–114)—that suggest PMIs originated as metal-rich\nmelts. These melts, enriched in low-melting-point chalcophile elements (LMCE; As, Sb, Se), were\nstable within the epithermal fluid (lines 132–147). The observed textures indicate that the metal-rich\nsulfide-sulfosalt melts remained liquid during transport and even after entrapment, crystallizing only\nas the system cooled down (lines 237–240).\nAs Reviewer 3 noted, the solidified PMIs are denser and would have been more challenging to\ntransport via ascending fluids compared to their molten counterparts. Therefore, our calculations\nusing the density of solid PMIs (6.3 g/cm³) emphasize the capability of hydrothermal solutions to\ntransport high-density micro- and nanomaterials, even under low flow rates (Fig. 3b). This further\nsupports the feasibility of fluid-mediated transport of metal-rich melts in such systems (Fig. 4a).\n3. Then, since the electrum are more dense and has higher density than other parts of PMIs, the\nauthor should explain better how these things with such contrasted physical properties can be hold\ntogether then a fluid is trying to flush them. It is very likely the less dense part which also have higher\nporosity will be effectively separated with Ag-Au electrum.\nMinerals forming PMIs did not need to remain held together during transport, as they represented\n“homogeneous” multi-component melt droplets comprising Ag-Au-Cu-Pb(-Fe-Zn)-As-Sb-S-Se\n(lines 112–117 and 132–147). Sinking of denser phases, such as electrum, would have only occurred\nif electrum had fractionated from the metal-rich melt before its entrapment. However, electrum\nfractionation from the melt was unlikely, as indicated by FE-SEM and TEM images showing that\nmicro- and nano-PMIs consist of electrum, Ag sulfosalts, and base metal sulfides (Figs. 1e–j and 2b–\nm). Otherwise, electrum would be systematically missing from PMIs.\n4. The temperature range for these melts was assumed to be <400ºC, yet, the melting point of\nmentioned phases (see their supplementary Data S2) are mostly much higher (eg, Ag is ~960ºC, Au\nis 1064ºC). The author need to explain better in the text how such melt can be stable in such a low T\ncircumstances.\nAgreed, but numerous experimental, thermodynamic, and empirical studies have shown that the\nmelting points of noble metals (e.g., Ag, Au, PGEs) can be significantly lowered, even to below 400\n°C, when they are associated with low-melting-point chalcophile elements such as Bi, Te, Sb, Se, As,\nand Pb (lines 34–44; Douglas et al., 2000; Frost et al., 2002; Tomkins et al., 2006, 2007; Tooth et al.,\n2008, 2011; Cave et al., 2019; Jian et al., 2022, 2024; Cano et al., 2024).\nThe point raised by Reviewer 3 is explained in lines 132–145: “To date, no experimental investigation\nfully elucidates the intricate relationships among cotectics, eutectics, and thermal boundaries within\nthe multi-component system Ag-Au-Cu-Pb(-Fe-Zn)-As-Sb-S-Se, especially at the nano-realm.\nNevertheless, the existence of metal-rich As-Sb-S-Se melts (referred to hereafter simply as “metal-\nrich melts”, since metals account for >70 wt%; Supplementary Data S2) is consistent with fluid\ntemperatures of ~270–400 °C17 exceeding binary and ternary eutectic points of the simpler systems:\nAu-Pb (212 °C), As-S (310 °C), Au-Sb (360 °C), Ag-As-S (280 °C), Pb-Sb-S (240 °C), and Pb-As-S\n(305 °C)2,4,18. Accordingly, the stability of metal-rich melts in low-temperature (<400 °C)\nhydrothermal fluids is favored in (1) complex multicomponent systems containing low-melting-point\nchalcophile elements (LMCE), namely As, Sb, Se, Pb, Bi, etc.2,5; and (2) some metallic nanomaterials\n(e.g., Au-Ag alloy nanoparticles <10 nm in size), whose melting temperature decreases dramatically\nat the nanoscale compared with bulk counterparts19,20.”\n5. The impact of the proposed mechanism should be better discussed. It seems that even if the\nmechanism is true, it is more of a localized phenomenon and may not really explain large-scale\nprecious metal mineralization, especially for Au. It may explain El Hilo, but how to apply this to other\ncases? Please provide more insights.\nAgreed. Although the formation of bonanza deposits, such as El Hilo, is highly constrained in both\nspace and time during the evolution of mineralizing systems, the processes that lead to their formation\noffer valuable insights into metal transport mechanisms (e.g., Saunders and Burke, 2017; McLeish et\nal., 2021; Petrella et al., 2022; Cano et al., 2023; Jian et al., 2024). Our findings suggest that the El\nHilo deposit formed through the accumulation of metal-rich materials that were mechanically\ntransported by fluids from an ex-situ source located at a distance (Figs. 3b–c and 4a).\nAs discussed in the final paragraph of the discussion, several studies have documented the occurrence\nof Au-Ag-LMCE melts associated with ore-forming fluids at low to moderate temperatures (~100–\n400 °C) and salinities (<20 wt% NaCl equiv.) across various deposit types, including epithermal,\nskarn, and greisen (see Supplementary Data S1; Bettencourt et al., 2005; Kouzmanov and Pokrovski,\n2012; Bodnar et al., 2014). These deposits are produced by mixtures of magmatic metal-rich brines\nand low-salinity fluids (Camprubí and Albinson, 2007; Wilkinson et al., 2013; Bodnar et al., 2014;\nFekete et al., 2016; Rottier et al., 2018) and may undergo comparable evolutionary processes to those\ndescribed for El Hilo (Fig. 4a). Consequently, our observations provide a potential explanation for\nthe enrichment of noble metals in hydrothermal fluids, which could contribute to high-grade\nmineralization in various geological environments. We have modified lines 327–330 following\nReviewer’s recommendations.\nThis paper does not propose nanomaterial entrainment as the primary mechanism for ore formation\nat the Natividad epithermal deposit. Instead, it suggests this process as a plausible explanation for\nexceptionally high-grade bonanza zones, such as El Hilo. The conventional understanding of metal\ntransport in solution—via chloride or thiosulfide species—remains unchallenged. Rather, we view\nnanomaterial entrainment as a complementary mechanism to transport in solution, playing a key role\nin the formation of bonanzas. While the universal occurrence of nanomaterials in epithermal deposits\nis a promising area for future research, it is beyond the scope of this manuscript.\nCITED REFERENCES\nBettencourt, J.S., Leite, W.B., Goraieb, C.L., Sparrenberger, I., Bello, R.M.S., and Payolla, B.L.,\n2005, Sn-polymetallic greisen-type deposits associated with late-stage rapakivi granites,\nBrazil: Fluid inclusion and stable isotope characteristics: Lithos, v. 80, p. 363–386,\ndoi:10.1016/j.lithos.2004.03.060.\nBodnar, R.J., Lecumberri-Sanchez, P., Moncada, D., and Steele-MacInnis, M., 2014, Fluid Inclusions\nin Hydrothermal Ore Deposits (H. . Holland & K. K. Turekian, Eds.): Oxford, Elsevier Ltd.,\nv. 13, 119–142 p., doi:10.1016/B978-0-08-095975-7.01105-0.\nCamprubí, A., and Albinson, T., 2007, Epithermal deposits in México - Update of current knowledge\nand an empirical reclassification: Special Paper of the Geological Society of America, v. 422,\np. 377–415, doi:10.1130/2007.2422(14).\nCano, N., González-Jiménez, J.M., Camprubí, A., Domínguez-Carretero, D., González-Partida, E.,\nand Proenza, J.A., 2023, Nanomaterial accumulation in boiling brines enhances epithermal\nbonanzas: Scientific Reports, v. 13, p. 1–7, doi:10.1038/s41598-023-41756-4.\nCano, N., González-Jiménez, J.M., Camprubí, A., Proenza, J.A., and González-Partida, E., 2024,\nMacro-to-nanoscale investigation unlocks gold and silver enrichment by lead-bismuth\nmetallic melts in the Switchback epithermal deposit, southern Mexico: American\nMineralogist, v. 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The improved\nconsistency in describing the transported material strengthens the study and makes the interpretation more accessible. I am\noverall satisfied with the revisions and am happy to recommend the manuscript for publication.", "reviewer_3": "I have reviewed the revision and am satisfied with the efforts that the authors made to improve the manuscript. I believe they\nhave well addressed the remaining of my concerns and recommend the acceptance of this manuscript for publication."}, "author_response": ""}]

Decision letter