Ten kilometers ascent of porphyry Cu (Au, Mo)-forming fluids in the Sanjiang region, China

Domain: earth_science
Published: 2025-03-08
Editorial outcome: minor_revision

Abstract
Porphyry-type Cu (Mo, Au) deposits are among the economically most important ore resources. The depth and mode of ore-forming fluid exsolution from their source magmas has remained poorly constrained. Here, we studied 36 magmatic rocks from six mineralized systems in the Sanjiang region of southwestern China. Melt inclusions trapped in the phenocrysts of mineralizing, felsic magmas are strongly depleted in Cl, S, and metals. Petrographic evidence, apatite volatile contents, and mass balance calculations suggest that this depletion was caused by the exsolution of aqueous fluids, which extracted 63‒97% of the Cl, S, and metals originally present in the magmas. Three independent geobarometers reveal that all the major phenocrysts in the felsic magmas crystallized within the pressure range of 0.3‒0.5 GPa. These results show that the ore-forming fluids exsolved from magmas that crystallized at mid- to upper crustal depths of ~10‒20 km, rather than from magmas crystallizing entirely within the upper crust or from magmas directly ascending from the lower crust. The fluids thus had to transport their metal and S endowments over a vertical distance of ~10 km to the site of ore precipitation, likely first via bubbles suspended in ascending porphyry magmas, and then through an interconnected fluid network.

Introduction
Porphyry-type Cu (Mo, Au) deposits provide ~75% of the world’s Cu, and considerable amounts of Mo and Au (ref. 1). Ore formation at crustal depths of a few kilometers is the product of multiple processes, which involve (1) partial melting of hydrated upper mantle, (2) crustal differentiation of mafic arc magmas, (3) exsolution of metal- and S-bearing, saline aqueous fluids from intermediate to felsic magmas, (4) emplacement of porphyry stocks or dikes that subsequently serve as fluid pathways, and (5) hydrothermal evolution and ore precipitation2–4.

Based on the high Sr/Y signature of Cu-mineralized porphyries, high-pressure differentiation of mafic arc magmas in the lower crust is recognized as a critical prerequisite for the formation of intermediate to felsic, fertile magmas5,6. However, the site and mode of ore-forming fluid exsolution from the fertile magmas are still a matter of debate (e.g., see discussions in refs. 4,7). In the traditional model, magmas accumulate in the upper crust to form large magma reservoirs, which, upon further fractionation, contribute porphyry magmas and fluids for the overlying porphyry Cu systems1–3,8–10. The large magma reservoirs have been postulated to be located at depths of 5‒15 km (ref. 1), but only very few quantitative data exist, and these rather suggest depths of only 4–10 km (refs. 8,11–14; Supplementary Table 1). On the other hand, some recent models propose that the ore-forming fluids start to exsolve already at lower- to mid-crustal depths in response to the ascent of fertile magma from the lower crust and that no large, upper crustal magma reservoirs exist at the time of mineralization5,6. Irrespective of these different models, the mechanism of fluid transfer from the source magma to the site of ore formation is not well understood4,15.

Melt inclusions, which are tiny droplets of silicate melt trapped in growing minerals, provide a unique approach to quantify magmatic volatile and metal contents before and after fluid exsolution, and, together with mineral compositions, preserve information about magmatic pressure and temperature. However, melt inclusion studies combined with thermobarometry on mineralized magmatic systems are scarce, mostly due to the difficulty of finding suitable natural samples.

Here we report on results from the Sanjiang porphyry-skarn Cu (Au, Mo) metallogenic belt in southwestern China (Fig. 1a). The study area was chosen for the following reasons: (1) the variably evolved magmatic rocks in certain magmatic systems are clearly cogenetic, and they contain well-preserved melt inclusions16,17; (2) evolutionary systematics of H2O, S, and Cl concentrations in the residual melt during magma differentiation at high pressure have been explored experimentally17; (3) melt inclusions coexist with CO2- or H2O-rich fluid inclusions in various phenocrysts; and (4) the presence of hornblende and quartz phenocrysts in mineralized felsic porphyries allows the application of two well-calibrated, independent barometers.

Results
Volatile and metal concentrations in variably evolved melts
We analyzed Cl, S, and metal concentrations in a total number of 207 melt inclusions that were hosted in olivine and clinopyroxene phenocrysts of mafic rocks (trachybasaltic andesite and trachybasalt), in clinopyroxene phenocrysts of intermediate rocks (syenite porphyry and trachyte), and in quartz, plagioclase, and K-feldspar phenocrysts of felsic rocks (rhyolite tuff, monzogranite porphyry, and quartz-monzonite porphyry) (Fig. 1; Supplementary Data 1 and 2). Most of the melt inclusions were crystallized, and thus only unexposed ones were analyzed as a whole using laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS; Methods).

The Cl and S concentrations in melt inclusions analyzed from the mafic to intermediate rocks match well with the Cl and S trends observed in residual melts of the magma differentiation experiments conducted at 1.0 GPa (ref. 17), and the W, Pb, Mo, and Ag concentrations in these melt inclusions are close to theoretical trends predicted for the case of 100% incompatibility in the crystallizing minerals (Fig. 2). The S, Cu, and Au concentrations are variably depleted compared to the theoretical trends. This is likely because S, Cu, and Au are strongly compatible in magmatic sulfides that were present in intermediate melts in both the experimental and natural samples17,18. The Cl concentrations are slightly lower than the theoretical trends, which maybe explained by the exsolution of early, CO2-rich fluids that could contain small amounts of Cl (ref. 19; the small amount of Cl that is incorporated into magmatic biotite has already been subtracted from the theoretical Cl trend shown in Fig. 2). This interpretation is supported by the presence of CO2-rich fluid inclusions in clinopyroxene and K-feldspar phenocrysts of the intermediate rocks (Supplementary Fig. 1), and also by the exsolution of CO2-rich fluids in magma differentiation experiments performed at 1.0 GPa (ref. 17). The presence of lower crustal, garnet-amphibolite and eclogite xenoliths in the intermediate rocks16 and clinopyroxene–liquid barometric results (Supplementary Data 3; Methods) suggest that the intermediate magmas formed through magma differentiation at pressures of ~0.5–1.3 GPa.

Since the concentrations of Cl and most metals (except for Cu and Au) in the trachydacitic melt inclusions (67‒70 wt% SiO2) analyzed from the intermediate rocks are still close to the theoretical trends of 100% incompatibility (Fig. 2), we conclude that the onset of aqueous fluid exsolution was not yet reached. By contrast, the Cl, S, and metal concentrations in the rhyolitic melt inclusions (75‒78 wt% SiO2) analyzed from the felsic rocks are much lower than those in the trachydacitic melt inclusions, despite higher degree of fractionation of the former melts (Fig. 2). We interpret these trends to reflect the exsolution of Cl-, S-, and metal-rich aqueous fluids, in agreement with the presence of H2O-dominated fluid inclusions coexisting with rhyolitic melt inclusions along growth zones in plagioclase phenocrysts (Fig. 3).

The F, Cl, and OH contents of clinopyroxene-hosted apatite inclusions in the intermediate rocks and of plagioclase- and potassic feldspar-hosted apatite inclusions in the felsic rocks are consistent with the interpretation that the trachydacitic melts were not saturated in aqueous fluids, whereas the rhyolitic melts were saturated (Fig. 4; see Methods for a discussion about the validity of using apatite volatile contents to constrain the timing of aqueous fluid saturation). Compositions of zircon-hosted apatite inclusions from mineralized felsic porphyries20 also suggest that these magmas were saturated in aqueous fluids (Fig. 4).

Depth and mode of fluid exsolution
The solubility of H2O in rhyolitic melts within the pressure range of 0.05‒0.50 GPa can be approximated by the equation21:

CH2Osolubility(wt%) = −12.87 × P2 + 22.76 × P + 1.989, (1)

in which P is given in GPa. The high H2O contents of glassy melt inclusions in quartz phenocrysts of the Jianchuan rhyolite tuff samples18 (Fig. 2; up to 7.0‒9.3 wt% H2O) suggest that they were trapped at pressures of at least 0.26‒0.42 GPa. These are minimum values because the activity of H2O in silicate melts was suppressed by the presence of CO2 (ref. 22), which was detected by laser Raman spectroscopy in the vapor bubble of the melt inclusions. For the same rhyolite tuff samples, slightly higher pressures (i.e., 0.3–0.5 GPa) are obtained via Al-in-hornblende barometry23 (using the composition of hornblende inclusions in quartz and K-feldspar phenocrysts; Methods; Supplementary Data 4) and Ti-in-quartz barometry24,25 (using the Ti contents of quartz phenocrysts in conjunction with reconstructed temperatures and TiO2 activities; Methods; Supplementary Data 5) (Fig. 5). For all porphyry intrusions, similar pressures of phenocryst crystallization were obtained (typically at 0.3–0.5 GPa), independent of whether they are mineralized or not16,26 (Fig. 5). Notably, the good agreement between the results obtained from the three barometers within uncertainties reinforces their validity. This comparison relies only on hornblende inclusions hosted within other phenocrysts because the composition of unshielded hornblende phenocrysts is often variably reset (see Methods). Overall, the results summarized in Fig. 5 clearly demonstrate that all major phenocrysts in the felsic magmas (i.e., quartz, hornblende, plagioclase, and K-feldspar) crystallized within the pressure range of ~0.3‒0.5 GPa, some potentially even up to 0.6 GPa. The evidence from fluid inclusion petrography (Fig. 2) and apatite volatile contents (Fig. 3) explicitly suggest that the felsic magmas were saturated in aqueous fluids at such high pressures.

The mineralization potential of the aqueous fluids was evaluated by mass balance calculations (Supplementary Data 6), based on (1) the volatile and metal concentrations in the trachydacitic vs. the rhyolitic melt inclusions (Fig. 2) and (2) the increase in magma crystallinity between these two melt compositions. To simplify the calculations, we assumed that minimal amounts of these elements were incorporated into the crystallizing minerals (i.e., plagioclase, K-feldspar, quartz, and minor amounts of biotite, amphibole, clinopyroxene, apatite, titanite, and magnetite). This is justified by the good match between most element concentrations measured in the trachydacitic melt inclusions and the theoretical trends for 100% incompatibility (Fig. 2), except for S, Cu, and Au. The latter elements could be affected by the presence of sulfides (S also by anhydrite), but the abundance of sulfides and anhydrite in the natural intermediate to felsic magmas is extremely low (typically, only 1‒2 sulfide inclusions per 150-μm-thick section; anhydrite has been observed only sporadically). Since the mass of fluids exsolved from the felsic magma significantly outweighs the mass of potentially precipitated sulfides (~5‒10 wt% fluid vs. <<0.2 wt% pyrrhotite; Supplementary Data 6), and because sulfides that are not fully enclosed within other minerals decompose during fluid exsolution27,28, we argue that the effect of sulfides on the composition of the exsolved fluids was likely insignificant. The magma crystallinity was varied in the range of 40‒60 wt% based on the typical modal abundance of phenocrysts in the felsic porphyries (Supplementary Data 1) and the fact that their whole-rock SiO2 content matches that of the trachydacitic melt inclusions (Fig. 2).

The results shown in Fig. 6 suggest that (1) the composition of the computed fluid compares well with the composition of high-temperature fluid inclusions analyzed from the Beiya, Taohua, and Yulong deposits in the Sanjiang region and the Bingham Canyon porphyry Cu-Au(-Mo) deposit in Utah (USA; associated with potassic magmas, too) and that (2) the computed fluid extracted 63‒97% of the Cl, S, and metal inventories that were present in the trachydacitic melts. Since all this extraction occurred at depths of ~10‒20 km as monitored by Al-in-hornblende barometry, Ti-in-quartz barometry, and the H2O content of glassy melt inclusions (Fig. 5), most of the Cl, S, and metals in the felsic magmas were already partitioned into aqueous fluids before some of these magmas finally ascended to shallow depth to form the porphyry stocks (Fig. 7a). Hence, the amounts of fluids that exsolved from the shallow porphyry stocks themselves seem to have played an insignificant role in the formation of the ore deposits.

The same conclusion is reached if one compares the total amount of metals present in the ore deposits with the metal content of the ore-forming magmas. The volume of the mineralized porphyry stocks at Yulong and Beiya in the Sanjiang region is likely less than 10 km3, based on a typical diameter of ≤1 km (Fig. 1d, e) and a vertical extension of ~10 km (Fig. 7a). In contrast, to provide the 6.5 Mt Cu and 0.4 Mt Mo contained in the Yulong deposit29 and the 300 t Au contained at Beiya30, at least 24 ± 13 km3, 17 ± 4 km3, and 54 ± 10 km3 of magma are required, respectively, if one assumes a magma density of 2.7 g/cm3, the metal concentrations present in the trachydacitic melt inclusions (99 ± 52 μg/g Cu; 8.8 ± 2.0 μg/g Mo; 2.0 ± 0.4 ng/g Au), and the geologically unrealistic case of 100% metal extraction efficiency from the magma and 100% metal precipitation efficiency at the ore deposit level. The actually required magma volumes should be far higher, also because the metal contents of the deposits represent only the mineable reserves, which do not include mineralized rock masses below economic cutoff ore grades (0.2 wt% Cu, 0.03 wt% Mo, and 1 g/t Au), nor the ~1/3 eroded Cu-Mo orebody at Yulong29. Hence, most of the ore-forming fluid must have originated from deeper levels, i.e., from the magma reservoirs located at 10‒20 km depth (Fig. 7a).

Distance and mechanism of fluid transport
The following lines of evidence suggest that the final emplacement of felsic porphyries and the formation of orebodies occurred at much shallower depths than the exsolution of ore-forming fluids (Figs. 5 and 7a): (1) Some felsic porphyry stocks occur within only ~2 km distance to similarly-aged volcanic tuffs, with no major faults in between (Fig. 1c); (2) Quartz vein textures and fluid inclusion types31–33 suggest that the emplacement depths of the orebodies at Yulong, Beiya, and Machangqing are of the order of 3‒4 km (refs. 34,35); (3) Small hornblende crystals in the matrix, the rim of some hornblende phenocrysts, and hornblende phenocrysts that recrystallized in-situ give Al-in-hornblende pressures of only ~0.1‒0.2 GPa, in contrast to the typically ~0.3‒0.5 GPa recorded in hornblende inclusions within other phenocrysts and in non-recrystallized core zones of hornblende phenocrysts. Consequently, the porphyry magmas and the ore-forming fluids must have migrated up over a vertical distance of at least 10 km on average from the underlying magma reservoirs to the site of ore precipitation (Fig. 7).

Initial fluid transfer was likely in the form of bubbles suspended in rising porphyry magmas (e.g., ref. 36), where depressurization promoted additional fluid exsolution. Otherwise, it is difficult to explain the fact that the phenocryst content of some felsic porphyries is higher than the rheological lock-up crystal content in magmas37 (~50 vol.%), such as at Beiya where the entire mineralized porphyry stock including its preserved apex contains ~60 vol.% phenocrysts38,39 (Supplementary Data 1). The presence of abundant fluid bubbles likely aided the transport of the very crystal-rich magmas40.

However, the large volume of the fluids required to produce the Beiya and Yulong deposits (at least 8 ± 3 to 37 ± 22 km3, calculated based on the Cu, Mo, and Au contents of the orebodies, the composition of the computed fluid, a fluid density of 0.5 g/cm3, and 100% metal precipitation efficiency) relative to the volume of the mineralized porphyry stocks (≤10 km3) renders it very unlikely that all the fluids were transferred together with the porphyry magmas to the site of mineralization (see also refs. 4,41), but rather suggests that most of the fluids ascended after the emplacement of the magmas as stocks. The low degrees of veining and alteration in deeper parts of the mineralized porphyry stocks29,30 imply that most of the fluids percolated through an interconnected fluid network at near-solidus conditions (Fig. 7b, c). The overall high crystal fractions in the porphyry magmas likely facilitated the development of the fluid network41,42 (Fig. 7b). Continuous input of fluids from the magma reservoir could ensure the fluid network to have remained open, thereby efficiently directing the fluids toward the top of the porphyry stock.

The proposed fluid transport mechanisms may be generally applicable to porphyry-type Cu deposits worldwide, considering common reports of (1) very high phenocryst contents of syn-ore porphyry intrusions40,43–45 (based on the least-altered samples), which are above the rheological lock-up value of ~50 vol.%37, (2) large tonnages of ore metals and S versus small volumes of mineralized porphyry stocks or dikes1,46,47, (3) the general occurrence of orebodies at the top of porphyry stocks1,47, and (4) the little-veined and little-altered nature of roots of mineralized porphyry stocks11,12.

In summary, this study provides support for the classical model of porphyry Cu formation because our data clearly demonstrate that mid to upper crustal magma reservoirs existed beneath the porphyry Cu (Au, Mo) deposits in the Sanjiang region and that most of the ore-forming fluids originated from these reservoirs (Fig. 7a). However, in contrast to the few depth constraints available so far, which range up to 10 km, the magma reservoirs in the Sanjiang region were situated at depths of 10‒20 km (average 15 km), whereas the orebodies themselves were situated at depths of ca. 5 km. This mean that the fluids had to ascend over a distance of ~10 km from the source region to the site of ore precipitation. Initial fluid transfer was likely in the form of bubbles suspended in the rising porphyry magma, but most of the ore-forming fluids seem to have subsequently percolated through an interconnected fluid network (Fig. 7b, c). Whereas the ore metal tonnages of the investigated systems vary by more than three orders of magnitude, the metal content (e.g., Cu, Au content) of analyzed high-temperature fluid inclusions are similar everywhere (Fig. 8). Hence, the extent of mineralization was not controlled by the metal content of the fluid, but rather by the mass of fluid exsolved from the underlying magma reservoirs, and, therefore, probably by the size of these reservoirs. Furthermore, the formation of deposits with vastly different Cu/Au ratios despite the presence of similar Cu/Au ratios in the input fluids suggests that selective metal precipitation mechanisms at the hydrothermal stage played an important role (e.g., ref. 48). A major erosion level control on the extent of mineralization in the Sanjiang porphyries can be discarded because many weakly to non-mineralized porphyry stocks occur very close to strongly mineralized ones (Fig. 1), and because the original orebodies at Beiya (Au-only) and Yulong (Cu-Mo) are largely preserved29,30.

Methods
Geologic background of the studied samples
The collision of India with the Eurasian continent triggered widespread Eocene-Oligocene potassic magmatism in the Sanjiang region along the eastern margin of the Tibetan Plateau26 (Fig. 1). Previous studies suggest that the entire range of mafic to felsic potassic magmas can be explained by differentiation of mantle-derived, potassic melts16,17. Many of the felsic, potassic porphyries (>65 wt% bulk-rock SiO2) are genetically associated with porphyry- or skarn-type Cu (Au, Mo) deposit or prospect.

We collected variably evolved potassic rocks in the Sanjiang region for petrography and in-situ geochemical microanalysis, including mafic samples from basaltic trachyandesite to trachybasalt lavas at Wozhong (n=8), intermediate samples from syenite porphyry intrusions at Jianchuan (n=4) and trachyte lavas at Madeng (n=2), and felsic samples from rhyolite tuffs at Jianchuan (n=2), and many monzogranite to quartz-monzonite porphyry intrusions at various locations (n=20) (Fig. 1; Supplementary Data 1). At the giant Yulong porphyry (-skarn) Cu-Mo deposit, the giant Beiya skarn (-porphyry) Au-Cu deposit, the small Machangqing and Xiaolongtan porphyry (-skarn) Cu (-Mo-Au) deposits, and the Taohua and Xiejiaochong porphyry Cu-Au-Mo prospects, most samples were taken within or close to sites of mineralization. In-situ zircon U-Pb dates of variably evolved potassic rocks within a specific area are indistinguishable within the analytical uncertainty of ~1‒2 million years (refs. 16,29 and references therein; Fig. 1c–e). Based on Nd-Sr-Pb isotopic and major to trace element compositions, the mafic to felsic rocks collected in and around the Jianchuan and Beiya areas appear to have evolved from a common, mantle-derived, potassic parental melt16.

LA-ICP-MS analysis of melt inclusions and fluid inclusions
As previously described in ref. 18, we utilized two LA-ICP-MS systems to analyze melt inclusions: (1) a GeoLas-Pro 193 nm ArF Excimer laser coupled with an Elan DRC-e quadrupole ICP-MS hosted at the Bavarian Geoinstitute (BGI), University of Bayreuth, Germany; (2) the same type of laser coupled with an Agilent 7900 quadrupole ICP-MS hosted at the Institute of Geological Sciences, University of Bern, Switzerland. The latter system has a much stronger detection capacity and was thus particularly used to analyze Au and Ag concentrations in relatively large melt inclusions, and Cl concentrations in rhyolitic melt inclusions. Unexposed, individual melt inclusions were drilled out as a whole, together with part of the adjacent host mineral49. The laser systems were operated at an ablation frequency of 6–10 Hz with an energy density of 3–10 J/cm2 on the sample surface. The laser beam size was adjusted in the range of 16‒90 μm, depending on the inclusion size, such that it ultimately covered the entire inclusion volume but included as little host mineral as possible. The ICP-MS systems were tuned to a ThO rate of 0.10 ± 0.05% and a rate of doubly-charged 42Ca ions of 0.25 ± 0.05% based on measurements of NIST SRM 610 and 612 glasses. GSE-1G, GSD-1G, NIST SRM 610, or Armenia rhyolite glass was used as external standard for most elements, whereas either an afghanite or scapolite standard was used for quantifying S and Cl. The NIST SRM 612 glass was used for quantifying Au concentrations. Isotopes analyzed at BGI include 23Na, 25Mg, 27Al, 30Si, 31P, 32S, 35Cl, 39K, 43Ca, 49Ti, 51V, 53Cr, 55Mn, 57Fe, 60Ni, 65Cu, 66Zn, 85Rb, 88Sr, 89Y, 90Zr, 93Nb, 98Mo, 133Cs, 137Ba, 139La, 163Dy, 173Yb, 184W, 232Th, and 238U, whereas those analyzed at the Institute of Geological Sciences include 23Na, 25Mg, 27Al, 29Si, 31P, 34S, 35Cl, 39K, 43Ca, 49Ti, 51V, 53Cr, 55Mn, 57Fe, 65Cu, 75As, 85Rb, 88Sr, 90Zr, 93Nb, 98Mo, 109Ag, 137Ba, 181Ta, 197Au, and 208Pb.

Melt inclusion analyses were quantified by subtracting the contribution of host mineral to the mixed inclusion-host signals, using either a fixed Al2O3 content or a bulk-rock trend of Al2O3 vs. SiO2 or FeOt vs. SiO2 as internal standard50 (Supplementary Data 2). The sum of all major element oxides was normalized to 100 wt%. To optimize the host subtraction, the MgO, FeOt, CaO, P2O5, or Na2O content of some melt inclusions was fixed at a value based on the composition of bulk rocks or of well-quantified, similarly-evolved melt inclusions (please see the footnote of Supplementary Data 2 for details about how the replacement values were chosen). Sulfur and Cl concentrations in melt inclusions that were analyzed at BGI were quantified using an in-house afghanite standard that contains 4.4 wt% S and 4.4 wt% Cl, though sophisticated corrections were required51. Sulfur and Cl concentrations in melt inclusions analyzed at the Institute of Geological Sciences were quantified without any correction, using the scapolite standard Sca17 that contains 2500 μg/g S and 2.9 wt% Cl. The validity of the latter results was confirmed on glassy melt inclusions through comparison with EPMA data18. Titanium concentrations in quartz hosts were quantified by normalizing the SiO2 content to 100 wt%.

Seventeen high-salinity brine inclusions were analyzed using the Agilent 7900 ICP-MS at the Institute of Geological Sciences in Bern (Supplementary Data 7). The fluid inclusions were drilled out individually, together with part of the adjacent quartz. The contribution of quartz host was numerically subtracted from the mixed signal until no Si was left in the fluid inclusion. The GSD-1G glass was used as external standard for most elements, whereas the Sca17 standard was used for quantifying S and Cl, and the NIST SRM 612 glass was used for quantifying Au. Analyzed isotopes include 23Na, 27Al, 29Si, 34S, 35Cl, 39K, 43Ca, 55Mn, 57Fe, 65Cu, 79Br, 85Rb, 88Sr, 98Mo, 109Ag, 133Cs, 182W, 197Au, and 208Pb.

While the brine inclusions had salinities of ~35‒50 wt% NaCl equiv, the absolute element concentrations were normalized to a constant salinity of 10 wt% NaCl equiv to allow for direct comparison of all fluid inclusion data. For each analysis, that nominal salinity of 10 wt% NaCl equiv was corrected for the presence of other major cations (their abundance relative to Na being known from the LA-ICP-MS analysis) to estimate the “net” Na concentration52 (all concentrations in wt%):

netNaCl = NaCl equiv − 0.5 × (KCl + FeCl2 + MnCl2 + CaCl2). (2)

EPMA analysis of clinopyroxene and amphibole
A JEOL JXA-8200 EPMA hosted at BGI was used to analyze amphiboles in felsic samples (n=17) and clinopyroxene phenocrysts in mafic to felsic samples (n=11) for the application of thermobarometry (see below). Since most felsic porphyries experienced variable degrees of alteration following phenocryst crystallization, we focused on amphibole and clinopyroxene inclusions shielded by other phenocrysts to constrain the crystallization pressure of phenocrysts. For the analysis, fully-enclosed inclusions were exposed on the sample surface by means of polishing. As a comparison and also to constrain the pressure of final magma emplacement, we additionally analyzed large, euhedral amphibole phenocrysts from their core to rim, and small, anhedral amphibole crystals in the matrix. The fact that even the core zones of some amphibole phenocrysts returned much lower pressures than amphibole inclusions hosted in other phenocrysts of the same samples (Supplementary Data 4) suggests that these amphibole phenocrysts sometimes re-equilibrated completely. This means that pressure constraints based on the composition of amphibole phenocrysts from mineralized porphyries should be treated with much caution53,54.

All the samples were coated with a ~12 nm thick carbon film. The utilized conditions were 15 kV and 10 nA, with a beam defocused to 5‒10 μm. Counting times of Na and K on peak and background before and after the peak were 10 and 5 s, respectively; for Fe, Mn, Al, Si, Ca, Ti, Cr, and Mn they were 20 and 10 s, respectively; and for Cl and F they were 60 and 30 s, respectively. The elements were standardized on albite (Na, Si), orthoclase (K), metallic Fe and Cr, enstatite (Mg), spinel (Al), wollastonite (Ca), MnTiO3 (Ti, Mn), fluorite (F), and vanadinite (Cl).

EPMA analysis of apatite
The same EPMA hosted at BGI was used for the analysis of apatite from the Jianchuan syenite porphyry and rhyolite tuff (Fig. 1c) to constrain the fluid saturation history during magma evolution. Previous studies have shown that the F-Cl-OH content of apatite can easily get modified by means of diffusion on geologically very short timescales55–57 (e.g., in days to years, depending on the grain size and temperature;), such that even in fresh lava rocks the F-Cl-OH content of apatite microphenocrysts can have diffusively re-equilibrated during cooling and thus have become different from that of small apatite inclusions fully enclosed in robust minerals58. Therefore, in mineralized porphyry stocks that experienced relatively slow cooling, phenocrysts re-equilibrated at the site of magma emplacement45,59. Rapid re-equilibration is further confirmed by our test on apatites from the very fresh Jianchuan trachyte porphyry (Supplementary Fig. 2). Since our aim is to understand the timing of fluid exsolution, we thus focused solely on apatite inclusions fully enclosed in fresh, robust phenocrysts such as zircon, clinopyroxene, plagioclase, and K-feldspar (e.g., refs. 20,45,59–61), rather than on apatite inclusions enclosed in non-robust phenocrysts such as biotite and amphibole, which have well-developed cleavages (e.g., refs. 62,63), or apatite microphenocrysts (e.g., refs. 54,64,65).

It has been demonstrated that apatite F and Cl X-ray count rates vary with exposure to the electron beam due to F and Cl migration66–68. The use of a relatively low beam energy or a relatively large beam size can reduce this effect (e.g., 10 kV accelerating voltage, 4 nA beam current, and a beam diameter defocused to 10 μm; ref. 67). To further minimize halogen migration issues, the analyzed surface of apatite crystals should be optimally parallel to their c-axis66,67. However, a beam diameter of 10 μm is often too large for the analysis of apatite inclusions, and the chance to get apatite inclusions with the c-axis exactly parallel to the sample surface is rather small. As a compromise, we thus used the analytical conditions of 15 kV, 10 nA, and 5 μm. To assess and correct the effect of halogen migration, we analyzed six gem-quality apatites from Durango, Ipira, Pakistan, Tirol, Switzerland, and Arusaha and three synthetic apatites H25, H26, and H27 as secondary standards. The F and Cl contents of the Durango apatite and the synthetic apatites have previously been determined by wet chemistry67,68. The Durango and Ipira crystals were analyzed in five different orientations (Supplementary Fig. 3). In the case of F the measured concentrations vary substantially as a function of the angle between the sample surface and the crystallographic c-axis, whereas in the case of Cl the measured concentrations remained almost constant (Supplementary Fig. 3a, b). The fitted equation in the plot in Supplementary Fig. 3c was thus employed to correct for F concentrations in the measured apatite unknowns, with the orientation of apatite inclusions estimated using polarized transmitted light microscopy. Using this approach, the uncertainties of the measured F and Cl concentrations are estimated at 5–10% and 10–15%, respectively (Supplementary Fig. 3; Supplementary Data 8).

All the apatite unknowns and secondary standards were coated with a ~12 nm thick carbon film. Fluorine and Cl were analyzed first to minimize the effect of diffusive loss during the analysis. Counting times of F, Cl, S, and Mg on peak and background before and after the peak were 60 and 30 s, respectively; P, Ca, Na, K, Si, Al, Fe, and Ti were 20 and 10 s. The elements were standardized on fluorite (F), tugtupite (Cl), apatite (Ca, P), albite (Na, Si), orthoclase (K), spinel (Al), enstatite (Mg), barite (S), MnTiO3 (Mn, Ti), and hematite (Fe).

Thermobarometry
Thermobarometry based on the composition of clinopyroxene crystals with or without coexisting silicate liquid has been widely used in magmatic systems, but different calibrations return vastly different results69. To identify the most accurate and precise expressions for application to the Sanjiang potassic magmas, we tested the various calibrations on 39 hydrous experiments conducted at 0.1–1.5 GPa and 850–1300 °C on starting materials that are compositionally similar to the Sanjiang potassic magmas17,18,70. These experimental data are best captured by the following two equations (Supplementary Fig. 4):

P(GPa) = 145.8 + 0.0197 × T(K) − 24.1 × ln T(K) + 0.0453 × H2Oliq(wt%) + 5.55 × XcpxAl(VI) + 0.805 × XcpxFe + 27.7 × XcpxK + 1.8 × XcpxJd + 4.41 × XcpxDiHd + 0.22 × ln XcpxJd − 1.77 × (XcpxAl)2 + 9.73 × (XcpxFe(M2))2 + 3.07 × (XcpxMg(M2))2 − 2.76 × (XcpxDiHd)2, (3)

and melt inclusion compositions:

ln XcpxJd = 4.60 − 0.437 × ln (XcpxJd × XliqCa × XliqFe+Mg) − 0.654 × ln (XliqMg) − 0.326 × ln XliqNa − 0.0632 × P(GPa) − 0.92 × ln XliqSi + 0.274 × ln XcpxJd, (4)

where X denotes mole fraction. Equation (3) is modified after equation (32b) of ref. 71, whereas Eq. (4) is modified after model (B) of ref. 72. Wieser et al.73 suggested that the clinopyroxene (± liquid)-based thermobarometers generally exhibit relatively large uncertainties (±0.3 GPa and ±40 °C). To find the best-matching liquid compositions for the natural clinopyroxenes from Sanjiang, we numerically related the liquid SiO2 to all the other major elements based on fitting equations in bulk-rock Harker diagrams and then varied the content of liquid SiO2 until an equilibrium Fe–Mg exchange coefficient between clinopyroxene and the liquid was found (i.e., KD(Fe–Mg) = 0.27 ± 0.03, refs. 71,72; Supplementary Data 3). The trachybasaltic andesitic melts and trachydacitic to rhyolitic melts were assumed to have contained 4 wt% H2O and 8 wt% H2O, respectively. Varying the melt H2O content by 1 wt% results in a small change of the calculated pressures (i.e., ≤0.05 GPa). Mole fractions of clinopyroxene and liquid components in Eqs. (3) and (4) were calculated using the template provided in ref. 74.

The amphibole crystals in the felsic rocks at Sanjiang are part of the low-variance mineral assemblage amphibole + plagioclase + alkali feldspar + biotite + quartz + titanite + magnetite + apatite (Supplementary Data 1) and thus can be used to calculate pressures using the formulation of ref. 23 (Supplementary Data 4). Their Altot value (in atoms per formula unit) was calculated using the template provided in ref. 75, and the corresponding pressure was calculated according to the following equation23:

P(GPa) = 0.05 + 0.0331 × Altot + 0.0995 × (Altot)2. (5)

Mutch et al.23 state that the relative pressure uncertainty is about ±16% and that the minimum absolute uncertainty is not better than 0.06 GPa. Since this barometer was calibrated also on a large dataset from natural granitic samples with independent pressure constraints, the uncertainties of our pressure estimates are unlikely to be significantly higher than the recommended values.

The crystallization pressure of quartz phenocrysts in the Sanjiang felsic rocks was calculated using the composition of quartz-hosted melt inclusions and Ti concentrations in the quartz host determined by LA-ICP-MS (Supplementary Data 5), based on the Ti-in-quartz thermobarometer of ref. 24 and the approach described in ref. 25. The solubility of Ti in quartz is described by the equation24:

log Ti (μg g−1) = −2794.3 × (1/T(K)) − 660.53 × [10 × P(GPa) / T(K)]0.35 + 5.6459. (6)

To determine the pressure, the temperature and the activity of TiO2 need to be independently constrained. Since the crystallization of the quartz phenocrysts occurred at zircon-saturated conditions, the temperature can be estimated based on zircon saturation thermometry76 applied to melt inclusions:

ln DZr/liq = −3.8 − 0.85 × (XliqNa + XliqK + 2 × XliqCa)−1 + 12900 × (1/T(K)). (7)

For the calculation of DZr/liq, the zircon is assumed to have contained a typical ZrO2 content of 62.5 wt%. The TiO2 activity in magmas is calculated based on the following solubility model77 applied to the XliqNa, XliqK, XliqCa, XliqMg, XliqFe, XliqAl, XliqSi:

log Ti(μg g−1) = 7.95 − 5305 × (1/T(K)) + 0.124 × (1/T(K)) × (XcpxDiHd × XliqNa × XliqAl) × (XliqMg × XliqFe) × (XliqNa + XliqK + 2 × (XliqCa + XliqMg + XliqFe)). (8)

The uncertainty of calculated pressures using this approach is likely better than 0.1 GPa, based on comparison with independent pressure constraints in natural magma systems25,78,79.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of Chang et al., on June 6th, 2024\nChang et al. report melt inclusion data from the Sanjiang region to constrain the depth at which fluids escaped from felsic\nmagmas. Their main findings are:\n1) Fluids were saturated at depths of 10-20 km, rather than 5-10 km.\n2) The shallow porphyry stock is too small to provide enough fluids for the formation of large deposits.\n3) The formation of deposits is controlled by the volume of fluid and mineralization efficiency rather than by high metal and\nsulfur content.\nHowever, their major conclusion is largely based on pressure estimates with considerable uncertainties and false\nassumptions, making these estimates unreliable. The remaining conclusions, which I agree with, recycle well-known\nknowledge and are not even incremental.\nAs such, the manuscript does not contain sufficient new information to warrant publication.\nBelow, I outline some major problems in the manuscript:\nFor pressure estimates, differentiating whether the depth is 5-10 km (0.15-0.25 GPa) or 10-20 km (0.25-0.52 GPa) requires a\nbarometer with a precision of 0.2 GPa, or 0.1 GPa as stated in the title (5 km vs 10 km). Unfortunately, no known barometers\nprovide such precision. For example, the extended data figure 5 clearly shows a scatter of 0.5-1 GPa on the y-axis. It should\nbe noted that this is the best-case scenario, and when applied to natural samples, which likely will not behave exactly the\nsame as experimental samples, the uncertainties will be much larger. To summarize, the barometry they use does not\nsupport their argument.\nI am unsure why these authors believe that 5-10 km is the site of the fluid source. A classical review paper on this topic is\n\"Porphyry Copper Systems\" by Sillitoe (2010, Economic Geology), which has been cited 3770 times in the past 14 years. In\nhis Figure 4, the source pluton is clearly situated at a depth of 5-15 km, and the source pluton itself extends vertically for 5-15\nkm. The authors have cited this classical paper, so it is unclear why they ignored this information.\nThey further claim that the shallow porphyry stock is too small to provide enough fluids for deposit formation, and they\nconducted a very simplified mass balance calculation. This is textbook knowledge, and there are many mass balance\nstudies on this topic from the early 2000s. It would be surprising if these authors are unaware of this. Again, Figure 4 from\nSillitoe shows that it is the source pluton at depth, rather than the shallow ones, that provides the fluids.\nTheir final finding that the volume of fluids, and hence the size of the source pluton, controls deposit formation is correct.\nHowever, this is nothing new as discussed above.", "reviewer_2": "I found this manuscript very interesting and very well executed. In recent years, several researchers have emphasized the\nimportance of deep crustal magma differentiation in generating magmatic hydrothermal ore fertility and some proposed fluid\nsaturation already at lower crustal depths. However, the proposition of mid crustal magma reservoirs directly donating the\nore-forming fluids to porphyry systems is still provocative. This manuscript provides direct evidence for this in the form of a\ndetailed silicate melt inclusion / geothermobarometry study in the Sanjiang region. I found the dataset of high quality and the\narguments based on it logical. As the findings are very important, novel and provocative, I think the manuscript is ideally\nsuited for Nature Communications. I only have few points to raise that might help to further strengthen the manuscript during\nthe revision stage:\n1) The authors discuss it the methods section, but further reinforcement of the argument that the studied magmas are coeval\nwould be beneficiary. The locations are fairly spread out, and it is unclear which mafic-intermediate magmas are coeval with\nwhich felsic ones. E.g. existing ages could be referred to more specifically and be listed in the supplementary tables.\nGeochemical links are convincingly presented in the cited 2023 Geology paper.\n2) The authors mention that magmatic sulfides are scarce. I wonder, was any magmatic anhydrite found/specifically looked\nfor?\n3) The evidence appears convincing that the magma emplaced in the porphyry fingers has mostly crystallized at 10-20 km\ndepth and that the silicate melt lost at that stage most of its budget of ore-forming constituents to an already exsolving fluid\nphase, but the mechanism of the fluid transfer still puzzles me a little bit. As the authors also pointed out, the limited\nvolume of magma transferred into the shallow crust could have only carried a limited mass of fluid phase. Thus, if I understand\ncorrectly, the basic idea is that once the fingers are emplaced, they serve as a channelway to transport a large volume of\nfluid from the large magma reservoir at 10-20 km depth. However, due to their small volume, the fingers would cool rather\nquickly and solidify as the top 10 km of the ambient crust is not very hot. How do the fluid percolation channels remain open\nfor extended periods? Or, is the advective heat transfer by the abundant fluid sufficient to keep the magma hot for a bit more\nextended time period? Even then, each finger could provide probably one pulse of fluid to the ore-forming system. Are the\nobservations in the mineralized zones consistent with this? Is the total amount of heat transferred this way sufficient to\nestablish a typical porphyry ore forming hydrothermal system? I am not trying to suggest that the answer to these questions\nis no, but I think it would be interesting to add a few sentences of discussion of these points. The transfer of fluids through an\nalready cooled and solidified finger seems unlikely as this would lead to spatially spread-out ore precipitation under a less\nsteep temperature gradient (leading to sub-economic mineralization). Also, the authors pointed out the lack of hydrothermal\nalteration in the deeper section of the porphyry stocks.\nSpecific comments:\nLine 166: It is not the solubility, just the activity of water that is reduced by CO2\nLine 216-228: Adding a bit more detailed explanation of these calculations in the methods section at the end would be\nuseful.\nLine 299-308: If similar high-Al, high P amphibole is common in porphyry stocks worldwide, this would be interesting to\nmention here.\nLine 394-397: This sentence needs clarification. Or it could be elaborated more in the methods or at least in the footnote of\nthe supplementary data tables how the replacement values were chosen.\nLine 478: The beam size on the EMPA should not affect signal intensities. The same number of electrons are spread out\nover a larger area.", "reviewer_3": "Overall this paper reads well and this is a good contribution of natural data to the existing literature and compliments existing\nexperimental work and modelling on this topic. Sufficient evidence is provided in support of the authors hypothesis and main\narguments. The calculations presented in the main text regarding the mass of metals and S in the fluid need to be detailed\neither in the manuscript or in the supplementary material – this evidence is essentially the crux of the paper and need to be\nreproducible to those who read it. To improve this manuscript, the author should consider restructuring some of the text and\nalso addressing literature that discusses an almost parallel set of processes but argue a different emphasis on the\nimportance of deep versus shallow systems.\nBelow I detail 3 themes that could be improved throughout the manuscript, followed by a line-by-line review of the text.\n1. Transfer of melt and fluids from deeper to shallower crustal levels\nOne of the main findings of the manuscript is that >90% of S and Cu and high proportions of other metals are partitioned into\nfluids exsolving at depths of up to 0.5 GPa. I think the author needs to explain more clearly why this is so significant relative\nto other models that are favoured in the literature. For example, because 90% of total system Cu has partitioned to the fluid\nphase at depths of 0.5 GPa, the significant increase in the mass of fluid exsolved during decompression bears minimal\ninfluence to the overall ‘fertility’ or ‘ore-forming potential’ of the fluid since most of its metal inventory was determined at\ndeeper levels in the crust.\nThis challenges prevailing models, which like you are in support of a ‘single-stage’ model for the formation of giant\nporphyries, but claim that the transfer of water-rich magmas from the lower crust is an essential mechanism for exsolving\nhigh masses of metal-rich fluids that determine the formation and size of the deposit (Chelle-Michou, 2017); the importance\nof deep fractionation is attributed to the accumulation of h2o and metals whilst remaining predominantly volatile\nundersaturated (Chiaradia and Caricchi, 2017; Richards 2015; Rezeau and Jagoutz, 2020).\nI do not dispute that deep fractionation of magmas plays a highly important role however I was surprised at how high the\nproportion of Cu and S partitioning in to these fluids were and hence how subordinate degassing on ascent was to adding to\nthe mass of metals conveyed by these fluids. To consolidate your conclusions I think some reference to the aforementioned\nmodels in addition to detailing the method of your calculations would be recommended.\nI think to strengthen the argument, the author should state more clearly the decoupling in concentration versus mass (Hogg\net al., 2023). A simple calculation comparing the mass of fluid required to generate the drop in concentration observed in the\nintmed to evolved MIs in Fig.1 at 0.5 GPa and compare this to the mass of fluid exsolved at 0.1-0.2 GPa (from your Al\nbarometry) would clearly emphasise to the reader that although a higher masses of FLUID exsolves at shallow levels, the\nhigher fluid-melt partition coefficient at depth, supersedes the relatively small mass of fluids at Shanjiang, resulting in 90% of\nthe metal inventory being partitioned into a smaller fluid reservoir at 0.5 GPa.\n2. Importance of sulfide saturation for the fate of S and Cu\nThe author mentions that intermediate samples are saturated in MSS which cause depletions in S and Cu and Au relative to the\nincompatible models (Fig.1).\nThe Cu MSS-melt partition coefficient ranges reaches up to 1000-10000 (Kiseeva and Wood 2013, Li and Audetat, 2015) and is\nhighest in evolved melts. Whereas Cu fluid-melt partition coefficients generally to do not exceed ~300-400 (Zajacz et\nal., 2008; Tattitch and Blundy, 2017) in evolved melts. Hence, formation of sulfides prior to degassing will cause S and Cu\nand Au to drop in the melt, as the author states.\nAlthough you conclude that fluids are causing most of the depletion in volatiles and metals listed in Fig.1 some\nacknowledgement of how 90% of Cu and S are still being delivered to fluids despite the presence of MSS is recommended.\nWithout modelling I appreciate that the above conclusions are rather speculative and not the primary focus of the paper.\nHowever I do feel it is important to comment on the state of the magma – if it is sulfide saturated, then address that e.g. mass\nof fluids degassing outweighs the effect that high Kd of Cu and Au and low mass of formation of Cu-Au-rich sulfides; or that\nperhaps degassing may have transitioned the melt into a sulfide undersaturated state (Nicholson et al., 2024).\nThe author states that only a small amount of MSS is precipitated – but could the author go a step further and perhaps\ncompare the mass of fluid versus sulfide needed to partition a given mass of Cu – keeping in line with the theme of mass\nbeing important? My suggestion would be to start with something like DCuMSS-melt / DCufluid-melt to give how much more\n[Cu] is sequestered in sulfides versus fluids. I do appreciate the limited word count of the manuscript, so please consider this\nas a suggestion.\n3. Stocks\nSpace permitting, some description of relation of stocks to magmatic system. This could be a figure in the supplement and\ndescription in the caption. You refer frequently to the fluid mass capabilities of the stock versus the magma. I think a visual\naid would be nice to show that stocks can be fed transiently by fluxing magmas – hence further emphasises your point about\nthe importance of deeper magmatic processes.\nLine-by-line review\nLine 38 – There are two prevailing models for formation of porphyries:\na. Single stage – favoured for big porphyries as discussed here, where metals are predominantly conveyed in fluids\nexsolved typically from CA magmas despite early forming sulfides (Richards, 2015; Rezeau and jagoutz, 2020).\nb. Multistage – more typical for small deposits, involves the storage of sulfides and subsequent remobilisation by later\ninfiltrating magmas (Chiaradia, 2014 and refs therein) and/or fluids (Heinrich and Connelly, 2022).\nI recommend a comment on the two prevailing models of ore formation before this sentence – with your focus being on the\nsingle stage where metals and S are mainly carried by fluids. Then this sentence highlighting importance of fluids but\nuncertainty on whether high pressure fractionation is important etc, this will bring the manuscripts main exploratory\nquestions to the forefront.\nLine 48 – mention somewhere the models that highlight the importance of deep and shallow reservoirs e.g. Rezeau and\nJagoutz (2020); Chiaradia and Caricchi (2017). I think these are valuable citations since you mention both fractionation in\ndeep and the transfer to shallow levels of the crust.\nLine 69 – what do you mean by entire? Do you mean to say homogenous? Glassy?\nLine 77: missing ‘with’ after well?\nLine 76 – line 80: reword slightly – there are only experimental data for Cl and S so MIs and experimental data fit well for\nthese elements; and MI data for S and Cl and metals correspond well to models of incompatible behaviour during magma\ndifferentiation. Also were the incompatible models based on LLDs at 1 GPa too?\nLine 80: reorganise sentence slightly – important exceptions are S, Cu and Au, which, in contrast to Ag, Mo, Pb, and W, are\nrelatively depleted compared to incompatible trends. Experimental and natural data show evidence of monosulfide solid\nsolution MSS, in which S Cu and Au are strongly compatible (Kiseeva and Wood 2013; Li and Audetat 2015).\nLine 87 – replace ‘amount’ with ‘proportion ‘\nLine 88 – theoretical trends presented in Fig.1;\nLine 115 – 118: sentence structure, I suggest something like “ since Cl and metal (*state metals specifically here because\nneed to emphasise that its not Cu and Au*) concentrations in trachydacitic MIs (67‒70 wt% SiO2) in the intermediate rocks\ndeviate negligibly from incompatible behaviour, we conclude the onset of fluid exsolution was not yet reached”.\nLine 120: delete generally substantially\nLine 121: address S and Cu and Au – you have mentioned their concentrations trace magmas to be sulfide saturated in\nearly stages (mafic to intmed compositions) but then conclude depletion is due to degassing. I do not disagree but just a line\nclarifying that either correlation in depletion across all metals suggests sulfide saturation is not dominant and then perhaps\nelaborate briefly on a suggestion listed in aforementioned review.\nLine 146: I think it would be nice to show some fluidity with Fig.1 and use the same colors for samples form mafic, intmed\nand felsic samples. For example, H2O-saturated apatite inclusions in the rhyolites will be red instead of yellow?\nLine 150: quantify the differences in Cl and F partition coefficients.\nLine 174: sentence requires citations.\nLine 175 – 178: sentence could be reworded. Do you need to state hornblende-based pressures? Could not just say this\ncomparison relies on pressures obtained from hornblende inclusions in phenocrysts to avoid resetting …\nLine 179 – 183: combine into one sentence.\nLine 199 – 202: “Below we show that such deep fluid exsolution was achieved by substantial phenocryst growth in water-\nrich felsic magmas, and that the volumes of shallowly-emplaced felsic porphyry stocks would have been far too small to\nprovide all the ore-forming fluids.” I would move sentence. First give the evidence.\nNeed to describe what stocks are and their relevance in the magmatic system and porphyries before this.\nLine 205 – 207: move this sentence to earlier when you introduce the model and replace no significant with minimal or none,\n“We assume that no significant amounts of these elements were incorporated into the crystallizing minerals.”\nLine 207 – 209: repetition of point made earlier, I don’t think you need this or remove it from earlier: Except for S, Cu, and Au,\nthe good match between theoretically predicted (for 100% incompatibility) and actually observed element concentrations at\n67‒70 wt% SiO2 in the melt (Fig. 1) suggests that this assumption is reasonable.\nLine 209 – 213: Author states Cu and Au and S are less reliable in models but these are essentially what the paper is about\nso I think more attention needs to be paid as to why sulfide saturation is not affecting the composition of fluids (i.e\nundersaturated) or that it is but because the mass of fluids exsolving (despite lower DCu fluid-melt) is more significant than\nthe mass of Cu partitioning into sulfides (despite high DCu MSS-melt) that it doesn’t matter.\nLine 221: measured ‘in’ not ‘on’\nLine 220 – 228: How did you compute these fluid compositions? Could you add this calculation to the supplement? I think to\nprove to the reader that the composition of deep fluids are a good match to natural data, there needs to be a comparison to\ncomputed fluids at e.g. 0.1 – 0.2 GPa as measured by some of the amphiboles in Fig. 4.\n“The mass balance calculations also suggest that the aqueous fluids extracted 97 ± 4% of the S, 92 ± 7% of the Cl, 91 ± 14%\nof the Cu, 88 ± 10% of the Mo, 87 ± 14% of the Ag, 84 ± 12% of the W, 72 ± 19% of the Pb, and 63 ± 26% of the Au that was\npresent in the trachydacitic melts (67‒70 wt% SiO2).”\nThis sentence is really important for the validity of your research and I think it requires some evidence based in a figure\n(subplot of Fig. 5) showing the mass equivalent of metals in the fluids relative to natural data. a further suggestion might be\nto then compare the mass of metals in fluids exsolved at this shallower level (0.1 – 0.2 GPa) to show it would be inadequate\nperhaps?\nFig.5 should highlight that yes compositions agree (a) but so do the masses (b) – making the reader aware of the decoupling\nin concentration and mass of fluids and how this is a really important thing to consider in what is required from fluids to\nacquire ore-forming potential (Hogg et al., 2023).\nIs this mass the cumulative mass of fluid exsolved from onset of volatile saturation to records in felsic samples?\nSome comment on existing models that describe similar mechanisms but de-emphasises the importance of deeper\nfractionation for generating high mass fluids but instead for accumulating water and metals that are subsequently degassing\nat shallower levels – in contrast to your conclusions (Chiaradia and Caricchi 2017; Rezeau and Jagoutz, 2020; Chelle-\nMichou, 2017 etc).\nLine 238: used a density of 2.4 g/cm3 but in Fig.3 a density of 2.7 g/cm3 is mentioned. How come?\nLine 239: what are the average concentrations that you use? Perhaps detail in a figure caption?\nLine 244: need to be more specific – conclusion is that the highest MASS of fluids, of suspected ore-forming potential, are\nexsolved at depth of 10-20 km.\nLine 273 – 275: reference to some studies that have also suggested this and demonstrated that its feasible.\nLine 276 – 285: restructure paragraph. You establish from mass balance that fluids exsolved at high pressures host 90%\nmass of Cu and S, which do not equate to the highest mass fluids. Then explain how these fluids were transferred to\nshallower levels – refer to previous studies here. Then use evidence of abundant fluid bubbles (are these in your samples?)\nto suggest initial mechanism of ascent of fluids.\nLine 292: need citations and comment on similar studies e.g. not a new concept, it is widely appreciated that the mass of S\nand Cu in porphyries is ‘anomalous’– Chiaradia and Caricchi (2017), show that periodic fluxing of hydrous magmas over\n~2Ma facilitates generation of Cu-S rich fluids.\nLine 311 - 330: this paragraph needs to pull out main points more clearly. It is important to discriminate the composition of\ncomputed fluids, existing natural data and then this study, more clearly. Which FI data in Fig. 5 are mineralised and non-\nmineralised?\nFig 5 highlights that mineralised and non-mineralised have the same concentrations – this would be a nice opportunity\nexplicitly say that concentration decouples from ore-forming potential – instead mass is important (in line with your paper).\nBingham and Sanjiang have the same range in concentrations and the same volume stock but Cu and Au mass higher at\nBingham – leads on from first conclusion, proving that mass of fluids is really important. This would be another reason to\nhave a subplot of mass here too.\nYou mention metal ratios but don’t say which? Are they in fluids or melts? And they are not quantified or shown in figures?\nI suggest remove or elaborate.\nGeneral comment – importance of the mass of the magma reservoir at the onset of volatile saturation is not addressed– I\nthink this is important to consider and briefly note in the introduction.\nLine 331: Need a final sentence that synthesises findings and main themes of paper.", "reviewer_author_response": "We sincerely acknowledge the three Reviewers (R1 ‒ an anonymous reviewer, R2 ‒ Zoltan\nZajacz, and R3 ‒ Olivia R. Hogg) for their comments, which were very helpful to improve the\nmanuscript. Below we show the reviewer comments in black, and our responses in blue.\nReviewer #1 (Remarks to the Author):\nReview of Chang et al., on June 6th, 2024\nChang et al. report melt inclusion data from the Sanjiang region to constrain the depth at\nwhich fluids escaped from felsic magmas. Their main findings are:\n1) Fluids were saturated at depths of 10-20 km, rather than 5-10 km.\n2) The shallow porphyry stock is too small to provide enough fluids for the formation of large\ndeposits.\n3) The formation of deposits is controlled by the volume of fluid and mineralization efficiency\nrather than by high metal and sulfur content.\nTo more clearly point out the main findings of this study, a summary section was added on\nlines 243‒267 (line numbers referring to the revised manuscript; the same below).\nHowever, their major conclusion is largely based on pressure estimates with considerable\nuncertainties and false assumptions, making these estimates unreliable.\nWe clearly disagree with this comment. It is unclear to us how this argument was reached.\nTests of the relevant barometers (i.e., Al-in-hornblende; Ti-in-quartz; H O content of melt\ninclusions) on natural systems in this and previous studies clearly suggest that their\nuncertainties are not significantly higher than ±0.1 GPa (please see below for more details).\nSuch uncertainties should be precise enough to reach our conclusions with regard to (1) the\ndepth of the fertile magma crystallization, (2) the depth of exsolution of the ore-forming\nfluids, and (3) the vertical distance of fluid transfer to the site of ore precipitation.\nThe remaining conclusions, which I agree with, recycle well-known knowledge and are not\neven incremental.\nWe disagree also with this comment. One of our remaining conclusions is that the size of\nporphyry Cu (Au, Mo) deposits was mainly controlled by the mass of ore-forming fluids,\nrather than by the metal concentrations in the fluids or the magmas. There are certainly\ndifferent views regarding this aspect, for example that large ore deposits formed from\nunusually metal-rich magmas (Core et al., 2006; Stern et al., 2007; Lamont et al., 2024).\nImportantly, our conclusion is well supported by the new fluid- and melt inclusion data from\nmineralized systems in the Sanjiang region. Such datasets rarely exist in the literature.\nAs such, the manuscript does not contain sufficient new information to warrant publication.\nWe demonstrate that the porphyry Cu (Au, Cu) deposits in the Sanjiang region formed by\nfluids that exsolved from mid- to upper crustal magma reservoirs. This topic is currently\nhighly debated (Sillitoe and Perelló, 2024; Heinrich, 2024; and references therein), and our\nstudy provides the first quantitative data to solve this issue. The two 2024 papers were\nadded on line 39 to further emphasize the importance of the present study.\nBelow, I outline some major problems in the manuscript:\nFor pressure estimates, differentiating whether the depth is 5-10 km (0.15-0.25 GPa) or\n10-20 km (0.25-0.52 GPa) requires a barometer with a precision of 0.2 GPa, or 0.1 GPa as\nstated in the title (5 km vs 10 km). Unfortunately, no known barometers provide such\nprecision. For example, the extended data figure 5 clearly shows a scatter of 0.5-1 GPa on the\ny-axis. It should be noted that this is the best-case scenario, and when applied to natural\nsamples, which likely will not behave exactly the same as experimental samples, the\nuncertainties will be much larger. To summarize, the barometry they use does not support\ntheir argument.\nIt is not true that \"no known barometers provide such precision\" (i.e., an overall precision of\n0.1 to 0.2 GPa). As we pointed out on lines 466‒467, the Al-in-hornblende barometer of\nMutch et al. (2016) should have an overall uncertainty of 0.06‒0.1 GPa when it is applied to\nquartz-saturated felsic magmas. Since this Al-in-hornblende barometer was calibrated also\non a large dataset from natural granitic samples with independent pressure constraints, the\nuncertainties of our pressure estimates should be not higher than the cited values. Also, as\nwas written on lines 487‒489, the Ti-in-quartz barometer of Huang and Audétat (2012) was\nrigorously tested on natural magmatic systems by comparing with independent pressure\nconstraints, which gave an overall uncertainty of better than ±0.1 GPa (Audétat, 2013;\nKularatne and Audétat, 2014; Audétat et al., 2023). Hence, our pressure estimates based on\nthese two well-calibrated barometers should have uncertainties smaller than ±0.1‒0.2 GPa.\nThis argument is also supported by the study of our own samples. For the Jianchuan\nrhyolite tuff sample, the three independent barometers (Al-in-hbl, Ti-in-quartz, and H O\ncontent of glassy melt inclusions) yielded consistent pressures within an uncertainty of ±0.1\nGPa. Also for the remaining porphyry samples, consistent pressures of 0.3‒0.5 GPa for the\ncrystallization of phenocrysts were obtained based on the two independent barometers\n(Al-in-hbl and Ti-in-quartz). As was stressed on lines 135‒137, 204‒208 and 358‒361, only\nhornblende that co-crystallized with other matrix minerals of the porphyry samples yielded\nsignificantly lower pressures at ~0.1‒0.2 GPa. This is simply because the phenocrysts\ncrystallized in deep magma reservoirs, whereas the fine-grained matrix crystallized after the\nfinal emplacement of the porphyry magmas (see Fig. 7). The overall large pressure range of\nall the analyses thus should not be taken as evidence for the argument that the barometric\nresults are of poor precision – in fact it is the careful petrography that allows to delineate the\nsequential crystallization evolution starting from deeper magma crystallization to shallow\nsolidification after magma emplacement.\nTherefore, our conclusions based on the pressure estimates are robust. We revised or\nadded the text on lines 118‒137 and 467‒470 to make the above points clearer.\nI am unsure why these authors believe that 5-10 km is the site of the fluid source. A classical\nreview paper on this topic is \"Porphyry Copper Systems\" by Sillitoe (2010, Economic\nGeology), which has been cited 3770 times in the past 14 years. In his Figure 4, the source\npluton is clearly situated at a depth of 5-15 km, and the source pluton itself extends\nvertically for 5-15 km. The authors have cited this classical paper, so it is unclear why they\nignored this information.\nThe widely cited source pluton depth of 5‒15 km given by Sillitoe (2010) and the two\nreferences therein (Cloos, 2001; Richards, 2005) seems to be an empirical estimate rather\nthan based on hard quantitative data. We made a summary of the few existing hard data,\nwhich indicate paleodepths of about 4‒10 km (see Supplementary Table 1, which is also\nprovided below). These values were reconstructed based on detailed geological mapping or\ndrill core logging of tilted or differentially uplifted porphyry systems, where the source pluton\nunderlying porphyry-type Cu deposits can be mapped directly. It is unclear where the 15 km\nlimit in the Sillitoe (2010) paper originates from. On the other hand, since the reconstruction\nrequires some interpretations (e.g., how to identify the true source pluton), it leaves room\nfor alternative models. Some recent studies argue that the ore-forming fluids could start to\nexsolve already at lower- to mid-crustal depths in response to the ascent of porphyry\nmagmas from the lower crust, without the need to construct mid- to upper crustal magma\nreservoirs (e.g., Loucks, 2021). Even Richard Sillitoe, in their recent publication (Sillitoe and\nPerelló, 2024), pointed out that these two models are equally possible. Our barometric +\nmelt inclusion study provides the first direct evidence that porphyry Cu (Au, Mo)-forming\nfluids exsolved from mid- to upper crustal magma reservoirs situated at depths of up to ~20\nkm. We revised the text on lines 35‒48 to better emphasize this controversy.\nThey further claim that the shallow porphyry stock is too small to provide enough fluids for\ndeposit formation, and they conducted a very simplified mass balance calculation. This is\ntextbook knowledge, and there are many mass balance studies on this topic from the early\n2000s. It would be surprising if these authors are unaware of this. Again, Figure 4 from\nSillitoe shows that it is the source pluton at depth, rather than the shallow ones, that\nprovides the fluids.\nSure, we know about this, but the result depends very much on the size of the porphyry\nstocks and the size of the deposits, so this mass balance calculation has to be done for each\ndeposit individually.\nTheir final finding that the volume of fluids, and hence the size of the source pluton, controls\ndeposit formation is correct. However, this is nothing new as discussed above.\nAs replied above, there are many different views about what are the key factors in controlling\nporphyry copper deposit formation. Here, we provide strong evidence based on fluid- and\nmelt inclusion compositions that the size of the source pluton was the key in the Sanjiang\nregion.\nReviewer #2 (Remarks to the Author):\nI found this manuscript very interesting and very well executed. In recent years, several\nresearchers have emphasized the importance of deep crustal magma differentiation in\ngenerating magmatic hydrothermal ore fertility and some proposed fluid saturation already\nat lower crustal depths. However, the proposition of mid crustal magma reservoirs directly\ndonating the ore-forming fluids to porphyry systems is still provocative. This manuscript\nprovides direct evidence for this in the form of a detailed silicate melt inclusion /\ngeothermobarometry study in the Sanjiang region. I found the dataset of high quality and\nthe arguments based on it logical. As the findings are very important, novel and provocative,\nI think the manuscript is ideally suited for Nature Communications. I only have few points to\nraise that might help to further strengthen the manuscript during the revision stage:\nMany thanks for the nice words! We addressed the comments below.\n1) The authors discuss it the methods section, but further reinforcement of the argument\nthat the studied magmas are coeval would be beneficiary. The locations are fairly spread out,\nand it is unclear which mafic-intermediate magmas are coeval with which felsic ones. E.g.\nexisting ages could be referred to more specifically and be listed in the supplementary tables.\nGeochemical links are convincingly presented in the cited 2023 Geology paper.\nDone. Existing ages for the studied magmas were added in the Supplementary Data 1.\n2) The authors mention that magmatic sulfides are scarce. I wonder, was any magmatic\nanhydrite found/specifically looked for?\nWe did specifically search for magmatic anhydrite in the studied samples, but it was only\nfound in a syn-mineralization felsic porphyry at Yulong (also reported in Chang et al., 2018)\nand a post-mineralization felsic porphyry dike at Beiya, and the abundances seem to be\nextremely low. We revised the text on lines 153‒156 to clarify this point.\n3) The evidence appears convincing that the magma emplaced in the porphyry fingers has\nmostly crystallized at 10-20 km depth and that the silicate melt lost at that stage most of its\nbudget of ore-forming constituents to an already exsolving fluid phase, but the mechanism\nof the fluid transfer still puzzles me a little bit. As the authors also pointed out, the limited\nvolume of magma transferred into the shallow crust could have only carried a limited mass\nof fluid phase. Thus, if I understand correctly, the basic idea is that once the fingers are\nemplaced, they serve as a channelway to transport a large volume of fluid from the large\nmagma reservoir at 10-20 km depth. However, due to their small volume, the fingers would\ncool rather quickly and solidify as the top 10 km of the ambient crust is not very hot. How do\nthe fluid percolation channels remain open for extended periods? Or, is the advective heat\ntransfer by the abundant fluid sufficient to keep the magma hot for a bit more extended time\nperiod? Even then, each finger could provide probably one pulse of fluid to the ore-forming\nsystem. Are the observations in the mineralized zones consistent with this? Is the total\namount of heat transferred this way sufficient to establish a typical porphyry ore forming\nhydrothermal system? I am not trying to suggest that the answer to these questions\nis no, but I think it would be interesting to add a few sentences of discussion of these points.\nThe transfer of fluids through an already cooled and solidified finger seems unlikely as this would\nlead to spatially spread-out ore precipitation under a less steep temperature gradient\n(leading to sub-economic mineralization). Also, the authors pointed out the lack of\nhydrothermal alteration in the deeper section of the porphyry stocks.\nYes, the mechanism of the fluid transport is indeed puzzling, but the presented evidence\n(including the simple mass balance) does not leave much room for other interpretations. We\nagree that each porphyry finger providing one pulse of fluid would cool quickly, but the fluid\ncould also percolate rather fast to the top of the porphyry finger (e.g., Cathles and Shannon,\n2007). Hence, we cannot answer whether additional heat transferred by the ascending fluid\nwould be needed to retain an interconnected fluid network, or how much would be needed.\nThe same is true for the amount of heat required to establish ore-forming hydrothermal\nsystems. These are of course very interesting questions that warrant future studies. We\nadded text on lines 231‒233 to suggest that continuous input of fluids could ensure the fluid\nnetwork to have remained open. We also added Fig. 7b, c to visualize our preferred model\nabout how the porphyry stock could serve as a fluid network to transport large volume of\nfluid from the underlying magma reservoir. This model well accounts for the observations in\nmineralized systems that are listed on lines 235‒241. Intermittent repetition of the process\nexplains the common presence of multiple pulses of porphyry fingers and associated\nmineralization in individual ore deposits (e.g., Sillitoe, 2010; Chang et al., 2017).\nSpecific comments:\nLine 166: It is not the solubility, just the activity of water that is reduced by CO2\nCorrected. Thanks for pointing out the mistake.\nLine 216-228: Adding a bit more detailed explanation of these calculations in the methods\nsection at the end would be useful.\nA table in Supplementary Data 6 was added to show how the calculations were done.\nLine 299-308: If similar high-Al, high P amphibole is common in porphyry stocks worldwide,\nthis would be interesting to mention here.\nWe expect that it is common, but almost all previous studies focused only on unshielded\namphibole phenocrysts in mineralized systems that likely experienced late recrystallization\nand alteration. More studies focusing on amphibole inclusions that are fully enclosed in\nother phenocrysts from quartz-saturated porphyries are required to allow for a solid\nsummary.\nLine 394-397: This sentence needs clarification. Or it could be elaborated more in the\nmethods or at least in the footnote of the supplementary data tables how the replacement\nvalues were chosen.\nDone. The following text was added on lines 324‒325:\" please see the footnote of\nSupplementary Data 2 for details about how the replacement values were chosen\".\nLine 478: The beam size on the EMPA should not affect signal intensities. The same number\nof electrons are spread out over a larger area.\nThank you for pointing it out. We wanted to say that a low beam energy or a large beam size\nshould be used to reduce the effect of halogen migration. The corresponding text was\nchanged on lines 397-398.\nZoltan Zajacz\nReviewer #3 (Remarks to the Author):\nThank you very much for your wealthy ideas and suggestions. We tried our best to\nincorporate most of your suggestions into the manuscript, while not adding too much text in\norder to keep the focus on the main story.\nOverall this paper reads well and this is a good contribution of natural data to the existing\nliterature and compliments existing experimental work and modelling on this topic. Sufficient\nevidence is provided in support of the authors hypothesis and main arguments. The\ncalculations presented in the main text regarding the mass of metals and S in the fluid need\nto be detailed either in the manuscript or in the supplementary material – this evidence is\nessentially the crux of the paper and need to be reproducible to those who read it. To\nimprove this manuscript, the author should consider restructuring some of the text and also\naddressing literature that discusses an almost parallel set of processes but argue a different\nemphasis on the importance of deep versus shallow systems.\nThe principle of the mass balance was elaborated by adding a calculation template in\nSupplementary Data 6. We rewrote part of the Introduction, and added the Fig. 7 and a\nsummary section to better present the main focus of this study.\nBelow I detail 3 themes that could be improved throughout the manuscript, followed by a\nline-by-line review of the text.\n1. Transfer of melt and fluids from deeper to shallower crustal levels\nOne of the main findings of the manuscript is that >90% of S and Cu and high proportions of\nother metals are partitioned into fluids exsolving at depths of up to 0.5 GPa. I think the\nauthor needs to explain more clearly why this is so significant relative to other models that\nare favoured in the literature.\nThe second paragraph of the Introduction was considerably revised to more clearly present\nour main focus and to show why it is topical and provocative. Fig. 7 and a concluding section\nwas added to summarize the main findings.\nFor example, because 90% of total system Cu has partitioned to the fluid phase at depths of\n0.5 GPa, the significant increase in the mass of fluid exsolved during decompression bears\nminimal influence to the overall ‘fertility’ or ‘ore-forming potential’ of the fluid since most of\nits metal inventory was determined at deeper levels in the crust.\nDone. The corresponding text was added on lines 174‒178.\nThis challenges prevailing models, which like you are in support of a ‘single-stage’ model for\nthe formation of giant porphyries, but claim that the transfer of water-rich magmas from the\nlower crust is an essential mechanism for exsolving high masses of metal-rich fluids that\ndetermine the formation and size of the deposit (Chelle-Michou, 2017); the importance of\ndeep fractionation is attributed to the accumulation of h2o and metals whilst remaining\npredominantly volatile undersaturated (Chiaradia and Caricchi, 2017; Richards 2015; Rezeau\nand Jagoutz, 2020).\nThere seems to be a misunderstanding. Our study actually supports prevailing models that\ninvolve (1) the formation of hydrous but ±H O-undersaturated magmas in the lower crust\n2\nand (2) the exsolution of ore-forming fluids from magmas accumulated in mid- to upper\ncrustal reservoirs (Richards, 2011; Chelle-Michou et al., 2017; Chiaradia and Caricchi, 2017).\nOur reconstructed pressures of 0.3‒0.5 GPa (crustal depths of 10‒20 km) indeed correspond\nto mid- to upper crustal levels in the Sanjiang region because the crust was up to 60 km thick\nat that time. What’s really new in the present study is that we provide direct evidence to\nsubstantiate this model. In addition, we find that the ore-forming fluids in the Sanjiang\nregion must have ascended over a surprisingly long distance of ~10 km to the site of ore\nprecipitation.\nI do not dispute that deep fractionation of magmas plays a highly important role however I\nwas surprised at how high the proportion of Cu and S partitioning in to these fluids were and\nhence how subordinate degassing on ascent was to adding to the mass of metals conveyed\nby these fluids. To consolidate your conclusions I think some reference to the\naforementioned models in addition to detailing the method of your calculations would be\nrecommended.\nTwo more references, Chelle-Michou et al. (2017) and Chiaradia and Caricchi (2017), were\nadded on line 42. The calculation table was added in the Supplementary Data 6.\nI think to strengthen the argument, the author should state more clearly the decoupling in\nconcentration versus mass (Hogg et al., 2023). A simple calculation comparing the mass of\nfluid required to generate the drop in concentration observed in the intmed to evolved MIs\nin Fig.1 at 0.5 GPa and compare this to the mass of fluid exsolved at 0.1-0.2 GPa (from your\nAl barometry) would clearly emphasise to the reader that although a higher masses of FLUID\nexsolves at shallow levels, the higher fluid-melt partition coefficient at depth, supersedes the\nrelatively small mass of fluids at Shanjiang, resulting in 90% of the metal inventory being\npartitioned into a smaller fluid reservoir at 0.5 GPa.\nFig. 8 was added to highlight that the size of ore deposits was mainly controlled by the mass\nof ore-forming fluids, rather than the metal concentrations in the fluids. Since we are able to\ndemonstrate that most of the ore-forming fluids exsolved at 0.3‒0.5 GPa, the suggested\ncomparison to the mass of fluid exsolved at 0.1‒0.2 GPa is not necessary anymore.\n2. Importance of sulfide saturation for the fate of S and Cu\nThe author mentions that intermediate samples are saturated in MSS which cause depletions\nin S and Cu and Au relative to the incompatible models (Fig.1).\nThe Cu MSS-melt partition coefficient ranges reaches up to 1000-10000 (Kiseeva and Wood\n2013, Li and Audetat, 2015) and is highest in evolved melts. Whereas Cu fluid-melt partition\ncoefficients generally to do not exceed ~300-400 (Zajacz et al., 2008; Tattitch and Blundy,\n2017) in evolved melts. Hence, formation of sulfides prior to degassing will cause S and Cu\nand Au to drop in the melt, as the author states.\nAlthough you conclude that fluids are causing most of the depletion in volatiles and metals\nlisted in Fig.1 some acknowledgement of how 90% of Cu and S are still being delivered to\nfluids despite the presence of MSS is recommended. Without modelling I appreciate that the\nabove conclusions are rather speculative and not the primary focus of the paper. However I\ndo feel it is important to comment on the state of the magma – if it is sulfide saturated, then\naddress that e.g. mass of fluids degassing outweighs the effect that high Kd of Cu and Au and\nlow mass of formation of Cu-Au-rich sulfides; or that perhaps degassing may have\ntransitioned the melt into a sulfide undersaturated state (Nicholson et al., 2024).\nThe author states that only a small amount of MSS is precipitated – but could the author go a\nstep further and perhaps compare the mass of fluid versus sulfide needed to partition a\ngiven mass of Cu – keeping in line with the theme of mass being important? My suggestion\nwould be to start with something like DCuMSS-melt / DCufluid-melt to give how much more\n[Cu] is sequestered in sulfides versus fluids. I do appreciate the limited word count of the\nmanuscript, so please consider this as a suggestion.\nThanks for the suggestion. This issue is now detailed on lines 153‒161: \"S, Cu, and Au could\nbe affected by the presence of sulfides (S also by anhydrite), but the abundance of sulfides\nand anhydrite in the natural intermediate to felsic magmas is extremely low (typically, only\n1‒2 sulfide inclusions per 150-μm-thick section; anhydrite has been observed only\nsporadically). Since the mass of fluids exsolved from the felsic magma significantly outweighs\nthe mass of potentially precipitated sulfides (~5‒10 wt% fluid vs. <<0.2 wt% pyrrhotite;\nSupplementary Data 6), and because sulfides that are not fully enclosed within other\nminerals decompose during fluid exsolution (Halter et al., 2002a; Nicholson et al., 2024),\nwe argue that the effect of sulfides on the composition of the exsolved fluids was likely\ninsignificant.\"\n3. Stocks\nSpace permitting, some description of relation of stocks to magmatic system. This could be a\nfigure in the supplement and description in the caption. You refer frequently to the fluid\nmass capabilities of the stock versus the magma. I think a visual aid would be nice to show\nthat stocks can be fed transiently by fluxing magmas – hence further emphasises your point\nabout the importance of deeper magmatic processes.\nGood suggestion! We added Fig. 7 to visualize the relationship of the deep crustal mafic\nmagma reservoir, the shallow crustal felsic magma reservoir, the porphyry stock, and the ore\ndeposit. The cartoon in Fig. 7a highlights the presence of a mid- to upper crustal magma\nreservoir that is substantially crystallized and thus already saturated in aqueous fluids, and\nthe ones in Fig. 7b, c shows the possible mechanism of efficient transfer and focusing of the\nore-forming fluids from the magma reservoir to the top of the porphyry stock.\nLine-by-line review\nLine 38 – There are two prevailing models for formation of porphyries:\na. Single stage – favoured for big porphyries as discussed here, where metals are\npredominantly conveyed in fluids exsolved typically from CA magmas despite early forming\nsulfides (Richards, 2015; Rezeau and jagoutz, 2020).\nb. Multistage – more typical for small deposits, involves the storage of sulfides and\nsubsequent remobilisation by later infiltrating magmas (Chiaradia, 2014 and refs therein)\nand/or fluids (Heinrich and Connelly, 2022).\nI recommend a comment on the two prevailing models of ore formation before this sentence\n– with your focus being on the single stage where metals and S are mainly carried by fluids.\nThen this sentence highlighting importance of fluids but uncertainty on whether high\npressure fractionation is important etc, this will bring the manuscripts main exploratory\nquestions to the forefront.\nWe acknowledge that it is still under debate whether early saturation of magmatic sulfides is\ndetrimental or whether remobilization of these sulfides is beneficial for the formation of\nporphyry Cu deposits, but these topics are beyond the scope of the present study. We are\nreluctant to comment too much on the role of sulfides.\nBased on the high Sr/Y ratio of Cu-mineralized porphyry intrusions, it seems clear that\nhigh-pressure differentiation of mafic arc magmas in the lower crust is a critical prerequisite\nfor the formation of fertile magmas. The main purpose of our study is to provide evidence\nfor where and how the fertile magmas exsolved porphyry Cu-forming fluids. The new\nevidence allows us to explicitly resolve the two prevailing models (Sillitoe and Perelló, 2024;\nHeinrich, 2024; Loucks, 2021; Sillitoe, 2010): (1) the ore-forming fluids exsolved from mid- to\nupper crustal magma reservoirs during substantial, isobaric crystallization of the fertile\nmagmas; (2) the ore-forming fluids exsolved at lower- to mid-crustal depths in response to\nthe ascent of the fertile magmas from the lower crust, with no need to construct mid- to\nupper crystal magma reservoirs. We rewrote lines 35‒48 in the Introduction to more clearly\nset the stage for this discussion, which does not have much to do with the role of magmatic\nsulfides.\nLine 48 – mention somewhere the models that highlight the importance of deep and shallow\nreservoirs e.g. Rezeau and Jagoutz (2020); Chiaradia and Caricchi (2017). I think these are\nvaluable citations since you mention both fractionation in deep and the transfer to shallow\nlevels of the crust.\nDone. We referred to two more numerical modeling papers on line 42 (Chiaradia and Carcchi,\n2017; Chelle-Michou et al., 2017) that highlight the importance of both deep and shallow\nreservoirs.\nLine 69 – what do you mean by entire? Do you mean to say homogenous? Glassy?\nWe mean that individual, unexposed melt inclusions were ablated and analyzed entirely\n(together with part of the surrounding host mineral) by LA-ICP-MS, regardless of whether\nthey are glassy or crystallized. The text on lines 69‒77 was reworded to avoid\nmisunderstanding.\nLine 77: missing ‘with’ after well?\nCorrected.\nLine 76 – line 80: reword slightly – there are only experimental data for Cl and S so MIs and\nexperimental data fit well for these elements; and MI data for S and Cl and metals\ncorrespond well to models of incompatible behaviour during magma differentiation.\nDone. See lines 78‒82.\nAlso were the incompatible models based on LLDs at 1 GPa too?\nYes – \"1 GPa\" was added on line 749 to make it clear.\nLine 80: reorganise sentence slightly – important exceptions are S, Cu and Au, which, in\ncontrast to Ag, Mo, Pb, and W, are relatively depleted compared to incompatible trends.\nExperimental and natural data show evidence of monosulfide solid solution MSS, in which S\nCu and Au are strongly compatible (Kiseeva and Wood 2013; Li and Audetat 2015).\nThe sentence was rewritten (lines 83‒86).\nLine 87 – replace ‘amount’ with ‘proportion ‘\nDone.\nLine 88 – theoretical trends presented in Fig.1;\nRevised.\nLine 115 – 118: sentence structure, I suggest something like “ since Cl and metal (*state\nmetals specifically here because need to emphasise that its not Cu and Au*) concentrations\nin trachydacitic MIs (67‒70 wt% SiO2) in the intermediate rocks deviate negligibly from\nincompatible behaviour, we conclude the onset of fluid exsolution was not yet reached”.\nThe sentence was restructured.\nLine 120: delete generally substantially\nDone.\nLine 121: address S and Cu and Au – you have mentioned their concentrations trace magmas\nto be sulfide saturated in early stages (mafic to intmed compositions) but then conclude\ndepletion is due to degassing. I do not disagree but just a line clarifying that either\ncorrelation in depletion across all metals suggests sulfide saturation is not dominant and\nthen perhaps elaborate briefly on a suggestion listed in aforementioned review.\nThis issue will be elaborated on lines 153‒161. We kept the original text here for fluidity.\nLine 146: I think it would be nice to show some fluidity with Fig.1 and use the same colors for\nsamples form mafic, intmed and felsic samples. For example, H2O-saturated apatite\ninclusions in the rhyolites will be red instead of yellow?\nDone.\nLine 150: quantify the differences in Cl and F partition coefficients.\nDone.\nLine 174: sentence requires citations.\nDone. See line 133.\nLine 175 – 178: sentence could be reworded. Do you need to state hornblende-based\npressures? Could not just say this comparison relies on pressures obtained from hornblende\ninclusions in phenocrysts to avoid resetting …\nDone.\nLine 179 – 183: combine into one sentence.\nWe like to stick to the two-sentence expression, as there is too much information to put it\ninto one sentence.\nLine 199 – 202: “Below we show that such deep fluid exsolution was achieved by substantial\nphenocryst growth in water-rich felsic magmas, and that the volumes of shallowly-emplaced\nfelsic porphyry stocks would have been far too small to provide all the ore-forming fluids.” I\nwould move sentence. First give the evidence.\nDone. This and the following two paragraphs were restructured and partly rewritten.\nNeed to describe what stocks are and their relevance in the magmatic system and porphyries\nbefore this.\nWe added the new Fig. 7 to show what is porphyry stock and its relationship to the mid- to\nupper crustal magma reservoir and the ore deposit.\nLine 205 – 207: move this sentence to earlier when you introduce the model and replace no\nsignificant with minimal or none, “We assume that no significant amounts of these elements\nwere incorporated into the crystallizing minerals.”\n\"no significant\" was replaced by \"minimal\", and the text in this part was restructured and\npartly reworded.\nLine 207 – 209: repetition of point made earlier, I don’t think you need this or remove it from\nearlier: Except for S, Cu, and Au, the good match between theoretically predicted (for 100%\nincompatibility) and actually observed element concentrations at 67‒70 wt% SiO2 in the\nmelt (Fig. 1) suggests that this assumption is reasonable.\nWe kept this sentence to emphasize that the assumption for the mass balance is valid.\nLine 209 – 213: Author states Cu and Au and S are less reliable in models but these are\nessentially what the paper is about so I think more attention needs to be paid as to why\nsulfide saturation is not affecting the composition of fluids (i.e undersaturated) or that it is\nbut because the mass of fluids exsolving (despite lower DCu fluid-melt) is more significant\nthan the mass of Cu partitioning into sulfides (despite high DCu MSS-melt) that it doesn’t\nmatter.\nThank you for this suggestion! This was detailed on lines 150‒161.\nLine 221: measured ‘in’ not ‘on’\nCorrected.\nLine 220 – 228: How did you compute these fluid compositions? Could you add this\ncalculation to the supplement?\nThe calculation table was added in the Supplementary Data 6.\nI think to prove to the reader that the composition of deep fluids are a good match to natural\ndata, there needs to be a comparison to computed fluids at e.g. 0.1 – 0.2 GPa as measured\nby some of the amphiboles in Fig. 4.\nSince our aim is to evaluate the significance of the fluids exsolved in the mid- to upper\ncrustal magma reservoir, and also for the sake of brevity, only the composition of the deep\nfluids is presented in Fig. 6a to compare with the natural fluid inclusion data.\n“The mass balance calculations also suggest that the aqueous fluids extracted 97 ± 4% of the\nS, 92 ± 7% of the Cl, 91 ± 14% of the Cu, 88 ± 10% of the Mo, 87 ± 14% of the Ag, 84 ± 12% of\nthe W, 72 ± 19% of the Pb, and 63 ± 26% of the Au that was present in the trachydacitic\nmelts (67‒70 wt% SiO2).”\nThis sentence is really important for the validity of your research and I think it requires some\nevidence based in a figure (subplot of Fig. 5) showing the mass equivalent of metals in the\nfluids relative to natural data.\nA subplot of the original Fig. 5 (now Fig. 6b) was added to visually show that the computed\ndeep fluids extracted most of the Cl, S, and metals originally present in the felsic magmas.\na further suggestion might be to then compare the mass of metals in fluids exsolved at this\nshallower level (0.1 – 0.2 GPa) to show it would be inadequate perhaps?\nDone. The corresponding text was added on lines 174‒178.\nFig.5 should highlight that yes compositions agree (a) but so do the masses (b) – making the\nreader aware of the decoupling in concentration and mass of fluids and how this is a really\nimportant thing to consider in what is required from fluids to acquire ore-forming potential\n(Hogg et al., 2023).\nAs mentioned above, Fig. 6b was added to stress the importance of the fluids exsolved at\n0.3–0.5 GPa in extracting S, Cl, and metals from fertile magmas. In addition, Fig. 8 was added\nto highlight that the size of ore deposits was mainly controlled by the mass of ore-forming\nfluids, rather than the metal concentrations in the fluids.\nIs this mass the cumulative mass of fluid exsolved from onset of volatile saturation to records\nin felsic samples?\nYes.\nSome comment on existing models that describe similar mechanisms but de-emphasises the\nimportance of deeper fractionation for generating high mass fluids but instead for\naccumulating water and metals that are subsequently degassing at shallower levels – in\ncontrast to your conclusions (Chiaradia and Caricchi 2017; Rezeau and Jagoutz, 2020;\nChelle-Michou, 2017 etc).\nAs elaborated above, our conclusion is not significantly different from the proposition of\nthese papers. However, in contrast to these papers we provide hard evidence for the\npresence of mid- to upper crustal magma reservoirs underlying porphyry Cu (Mo, Au)\ndeposits in the Sanjiang region, and that the fertile magmas experienced considerable\ncrystallization in the magma reservoir and thus exsolved the majority of the ore-forming\nfluids. We rewrote part of the introduction, added Fig. 7 and a summary section to more\nclearly present the main focus and findings.\nLine 238: used a density of 2.4 g/cm3 but in Fig.3 a density of 2.7 g/cm3 is mentioned. How\ncome?\nChanged to the same density of 2.7 g/cm3.\nLine 239: what are the average concentrations that you use? Perhaps detail in a figure\ncaption?\nThe concentrations were added on line 187.\nLine 244: need to be more specific – conclusion is that the highest MASS of fluids, of\nsuspected ore-forming potential, are exsolved at depth of 10-20 km.\nDone. Please see lines 193‒195.\nLine 273 – 275: reference to some studies that have also suggested this and demonstrated\nthat its feasible.\nThe feasibility is demonstrated in the coming text. We are not aware of previous studies that\nhave drawn conclusions specifically with regard to this issue. This actually represents one of\nthe main new findings of the present study.\nLine 276 – 285: restructure paragraph. You establish from mass balance that fluids exsolved\nat high pressures host 90% mass of Cu and S, which do not equate to the highest mass fluids.\nThen explain how these fluids were transferred to shallower levels – refer to previous studies\nhere. Then use evidence of abundant fluid bubbles (are these in your samples?) to suggest\ninitial mechanism of ascent of fluids.\nThe paragraph was revised, with referring to the paper of Tosdal and Dilles (2020). In the\noriginal text we already referred to the paper of Pacey et al. (2019) in which it is suggested\nthat abundant fluid bubbles were required for the transport of crystal-crowded porphyry\nmagmas.\nLine 292: need citations and comment on similar studies e.g. not a new concept, it is widely\nappreciated that the mass of S and Cu in porphyries is ‘anomalous’– Chiaradia and Caricchi\n(2017), show that periodic fluxing of hydrous magmas over ~2Ma facilitates generation of\nCu-S rich fluids.\nTwo references, Parmigiani et al. (2017) and Heinrich (2024), were cited to support the\nconclusion that most of the fluids could not have ascended simultaneously with the porphyry\nmagma.\nLine 311 - 330: this paragraph needs to pull out main points more clearly. It is important to\ndiscriminate the composition of computed fluids, existing natural data and then this study,\nmore clearly. Which FI data in Fig. 5 are mineralised and non-mineralised?\nThis paragraph was shortened and moved to the conclusion section. We added Fig. 8 to\nvisually support our conclusion that the size of ore deposits was mainly controlled by the\nmass of ore-forming fluids, rather than by the metal content of the fluids or the magmas.\nFig 5 highlights that mineralised and non-mineralised have the same concentrations – this\nwould be a nice opportunity explicitly say that concentration decouples from ore-forming\npotential – instead mass is important (in line with your paper).\nBingham and Sanjiang have the same range in concentrations and the same volume stock\nbut Cu and Au mass higher at Bingham – leads on from first conclusion, proving that mass of\nfluids is really important. This would be another reason to have a subplot of mass here too.\nDone. We added Fig. 8 to make this point more clear.\nYou mention metal ratios but don’t say which? Are they in fluids or melts? And they are not\nquantified or shown in figures? I suggest remove or elaborate.\nWe mean metal ratios in the orebodies. The corresponding text was reworded on line 261.\nGeneral comment – importance of the mass of the magma reservoir at the onset of volatile\nsaturation is not addressed– I think this is important to consider and briefly note in the\nintroduction.\nThis point was already addressed on lines 183‒185.\nLine 331: Need a final sentence that synthesises findings and main themes of paper.\nDone. A concluding section was added on lines 243‒267.\nReview by Olivia R Hogg."}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_2": "I am overall satisfied with the authors' responses to my comments and the changes implemented in the manuscript. I\nmaintain my opinion that this is an important contribution, and I recommend publication without further modifications.", "reviewer_3": "The revised version of this manuscript incorporates several of the review suggestions and has justified plausibly areas\nwhere my suggestions were not wholly relevant. I have no further comments and believe this paper now represents a strong\ncontribution to Nature Communications.\nThe introduction is much clearer and effectively outlines prior research, key research questions, and consequently what this\npaper will address. The additional supplement provides evidence supporting the calculations and has strengthened the\nmanuscript, and the new figures are excellent visual aids that enhance the overall clarity of the author's findings.\nThis paper is an important contribution to the field, and I look forward to seeing it published."}, "author_response": ""}]

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