Fluid Inclusions and C–H–O–S–Pb Isotopes of the Huoluotai Porphyry Cu (Mo) Deposit in the Northern Great Xing’an Range, NE China: Implications for Ore Genesis

: The Huoluotai Cu (Mo) deposit is a recently discovered porphyry Cu deposit in the northern Great Xing’an Range, NE China. Fluid inclusion (FI) micro-thermometry results and the C–H–O–S–Pb isotope compositions of the Huoluotai Cu (Mo) deposit are presented in this study. The ore-forming process consists of the sulfide-barren quartz stage (I), the quartz + chalcopyrite ± pyrite ± molybdenite stage (II), the quartz + polymetallic sulﬁde stage (III), and the quartz + calcite ± pyrite ± ﬂuorite stage (IV). Cu mineralization occurred mainly in stage II. Four types of FIs were recognized: liquid-rich two-phase FIs (L-type), vapor-rich two-phase FIs (V-type), daughter-mineral-bearing three-phase FIs (S-type), and CO 2 -bearing FIs (C-type). In stage I, the ore-forming ﬂuids belong to an H 2 O − NaCl − CO 2 system. In stages II, III, and IV, the ore-forming ﬂuids belong to an H 2 O − NaCl system. The results of the FI micro-thermometry and H–O isotope analysis show that the ore-forming ﬂuids originated from a magmatic origin in stage I and mixed with meteoric water from stages II to IV. The S–Pb isotope results suggest that the source of the ore-forming materials has the characteristics of a crust–mantle-mixing origin. Fluid boiling occurred in stages I and II. The FI micro-thermometric data further show that Cu was mainly deposited below 400 ◦ C in stage II, suggesting that ﬂuid boiling occurring below 400 ◦ C may be the primary factor for Cu precipitation in the Huoluotai Cu (Mo) deposit.

The Huoluotai Cu (Mo) deposit, which is located in the NGXR, contains estimated industrial metal resources of >4900 t Cu and >300 t Mo and is currently being explored [8]. Previous studies have focused on the geological characteristics [9], geochronology [9,10], and geochemistry of the igneous rocks in this deposit [11]. According to these studies, the Huoluotai Cu (Mo) deposit is considered to be a porphyry deposit. The magmatic activity of the Huoluotai Cu (Mo) deposit mainly occurred in the Late Jurassic-Early Cretaceous (ca. 150-140 Ma), and Cu (Mo) mineralization was closely related to the Late Jurassic granodiorite porphyry (ca. 149 Ma; [11]). Nevertheless, the lack of comprehensive research on the ore-forming fluids limits our understanding of the ore-forming conditions of the Huoluotai Cu (Mo) deposit. Three important issues have not been resolved: (1) the origin and detailed evolution of the ore-forming fluids are unknown; (2) the source of the oreforming materials remains uncertain; and (3) the possible mineralization processes have not been determined.
To solve these problems, the present study presents micro-thermometry and laser Raman spectroscopy data of the fluid inclusions (FIs), and C-H-O-S-Pb isotope compositions. The results will provide new clues for further prospecting directions in the Huoluotai Cu (Mo) deposit, as well as the late Mesozoic porphyry Cu deposits in the NGXR.
Three types of alteration were identified in the Huoluotai Cu (Mo) deposit: potassic, chlorite-epidote, and phyllic alterations from early to late ( Figure 3A-D). From center to edge, two alteration zones can be recognized: the potassic alteration zone and the phyllic alteration zone ( Figure 2B). Hydrothermal alteration surrounds the granodiorite porphyry and is correlated strongly with the granodiorite porphyry ( Figure 2B). Potassic alteration is characterized by secondary K-feldspar and biotite ( Figure 3A,B). The ore minerals developed in the potassic alteration zone are mainly magnetite, chalcopyrite, hematite, and small amounts of molybdenite ( Figure 3E-H). Magnetite mainly developed as disseminations and/or magnetite ± quartz ± chalcopyrite veinlets. Cu (Mo) orebodies mainly developed in the potassic alteration zone. Chalcopyrite mainly developed as disseminations, masses, or quartz + chalcopyrite veins in the potassic alteration zone ( Figure 3G,H). Potassic alteration was locally overprinted by epidote and chlorite alterations. Phyllic alteration can be distinguished by secondary sericite and quartz ( Figure 3C,D). The ore minerals present in the phyllic alteration zone are mainly pyrite, chalcopyrite, and molybdenite ( Figure 3I). In the phyllic alteration zone, chalcopyrite occurs mainly as disseminations, while molybdenite occurs mainly as disseminations and/or quartz + molybdenite + pyrite ± chalcopyrite veins ( Figure 3I). Based on the mineral assemblages and crosscutting relationships between veins in the Huoluotai Cu (Mo) deposit (Figure 3), four mineralization stages were identified: the sulfidebarren quartz stage (stage I), the quartz + chalcopyrite ± pyrite ± molybdenite stage (stage II), the quartz + polymetallic sulfide stage (stage III), and the quartz + calcite ± pyrite ± fluorite stage (stage IV) ( Figure 4). Stage I is characterized by sulfide-barren quartz veins, which are rare and generally discontinuous ( Figure 3E). There is no obvious cutting relationship between the veins in stage I and those in other mineralization stages. Stage I veins are mainly concentrated in the potassic alteration zone. Copper is mainly found in stage II. Stage II veins are generally continuous and irregular ( Figure 3G) and mainly contain quartz, chalcopyrite, pyrite, and molybdenite. Stage II veins are distributed mainly in the potassic alteration zone. Stage III veins are generally continuous and straight ( Figure 3I

Fluid Inclusion Measurements
In total, 31 quartz samples were used to perform FI measurements at the Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Land and Resources, Changchun, China. Secondary FIs were not analyzed [25]. All types of FIs were selected for laser Raman spectra analyses using an RM-2000 Laser Raman microprobe. Microthermometric analyses were performed using a Linkam THMS-600 heating-freezing stage (Linkam Scientific Instruments Ltd., Epsom, UK).

H-O Isotope Analyses
Six quartz samples were selected for H-O isotope analysis at the Center of Analytical Laboratory in the Beijing Research Institute of Uranium Geology (BRIUG), China National Nuclear Corporation (Beijing, China). The H-O isotope compositions were analyzed by a Finnigan MAT253-EM mass spectrometer. The O and H isotope compositions were analyzed using the conventional BrF 5 method and the Zn reduction method, respectively [26,27]. The H-O isotope values were normalized with the Vienna Standard Mean Ocean Water (V-SMOW) standards. The analytical precision was better than 0.2‰ for δ 18 O and 2‰ for δD.

C-O Isotope Analyses
Four representative calcite samples were selected for C-O isotope analysis at BRIUG. The C-O isotope analyses of calcite were performed using the 100% phosphoric acid method [28] with a MAT-251EM mass spectrometer. δ 13 C used the Pee Dee Belemnite (PDB) standard, and δ 18 O used the SMOW standard. The analytical precision was better than 0.1‰ for δ 13 C and 0.2‰ for δ 18 O.

S-Pb Isotope Analyses
The S isotope compositions of seven representative sulfide samples were analyzed at BRIUG. The values of δ 34 S for the sulfide samples were determined on SO 2 produced by the reaction between the sulfide and cuprous oxide. δ 34 S used the Vienna Cañon Diablo Troilite (V-CDT) standard. The Pb isotope compositions of four representative sulfide samples were also analyzed at BRIUG. Sulfide samples were dissolved in a mixed solution of HF + HNO 3 at 150 • C for seven days. The Pb isotope compositions were measured using a MAT-261 thermal ionization mass spectrometer.

Microscopy Investigations of Fluid Inclusions
According to the phases, filling degree, and combination relationship of FIs at a normal temperature (25 • C), four different FI types were identified in the quartz samples from the Huoluotai Cu (Mo) deposit, as described below.
Liquid-rich (L-type) FIs are most common in four mineralization stages, which consist of two phases with V H 2 Figure 5A,C-E,G-I). The vapor phase accounts for 10-20 vol.% of the FI volume. The FIs range from 3 to 20 µm in size and are round or irregular in shape. Commonly, L-type FIs homogenize into liquid during heating. Vapor-rich (V-type) FIs were identified in stages I, II, and III but absent in stage IV. These FIs consist of two phases with V H 2 O /(V H 2 O + L H 2 O ) > 50 vol.% at~25 • C and have 60-90 vol.% vapor bubbles (Figure 5C,F,G). These FIs are normally round or oval in shape and range in size from 5 to 20 µm. Commonly, V-type FIs homogenize into vapor during heating.
Daughter-mineral-bearing three-phase (S-type) FIs were detected in stages I and II. S-type FIs consist of an aqueous liquid, a vapor bubble, and one or more solid daughter minerals at~25 • C ( Figure 5B,D,E). These FIs are normally oval or irregular in shape and range from 5 to 20 µm in size. Additionally, these FIs may contain one, two, or three daughter minerals, which can be halite or ore minerals ( Figure 5B,D,E). The halites are mainly transparent, pale white, and cubic in shape ( Figure 5B,D). The ore minerals are opaque ( Figure 5B,D,E). Most halites dissolved after the vapor-bubble disappeared during heating. CO 2 -bearing (C-type) FIs only occur in stage I. These FIs are normally round, oval, or irregular in shape and range from 8 to 15 µm in size. Most C-type FIs consist of three phases (L H 2 O + L CO 2 + V CO 2 ), and occasionally two phases (L H 2 O + L CO 2 ) at~25 • C. The latter changes into three phases with the appearance of CO 2 bubbles at~10 Figure 5A), and can be homogenized into a liquid CO 2 phase.

Micro-Thermometric Results
FI micro-thermometric data in this study are listed in Table 1 and shown in Figure 6. The salinities of L-and V-type FIs were estimated according to the HOKIEFLINCS_H 2 O-NaCl program [29]. The salinities of S-type FIs were calculated using the equations from [30]. We calculated the salinities of C-type FIs based on the melting temperatures of CO 2 clathrate in the H 2 O-CO 2 -NaCl system [31].    Figure 6B). The V-type FIs homogenized into the vapor phase at 427-471 • C (n = 12; Figure 6A). These V-type FIs yielded final ice-melting temperatures (T m-ice ) of −7.9 to −5.8 • C, corresponding to salinities of 8.9-11.6 wt.% NaCl equivalent (n = 12; Figure 6B). During heating, all S-type FIs commonly homogenized into the aqueous liquid phase, with halite daughter minerals dissolving at temperatures (T h-s ) varying from 424 to 485 • C (n = 20; Figure 6A), with calculated salinities of 50.1-57.8 wt.% NaCl equivalent (peaking at 52-56 wt.% NaCl equivalent, n = 20) ( Figure 6B). C-type FIs were completely homogenized into liquid H 2 O at temperatures (T h-total ) of 429-470 • C (n = 8) ( Figure 6A). These FIs yielded initial melting temperatures (T m-CO2 ) of solid CO 2 from −57.4 to −56.8 • C and clathrate melting temperatures (T m-cla ) from 3.5 to 5.5 • C, corresponding to salinities of 8.3-11.3 wt.% NaCl equivalent (n = 8) ( Figure 6B). The CO 2 phase homogenized into the liquid CO 2 phase at temperatures (T h-CO 2 ) of 28.2-30.5 • C.

Laser Raman Spectroscopy
The gas phase composition of some representative FIs in the four mineralization stages were selected for laser Raman spectroscopy analysis. Representative laser Raman spectra are shown in Figure 7. C-type FIs in stage I were universally found to be rich in CO 2 ( Figure 7A). Vapor phases of the V-type FIs in stages I, II, and III only contained H 2 O ( Figure 7B). L-type FIs in the four mineralization stages only contained H 2 O ( Figure 7C,D). These analytical results suggest that the ore-forming fluids of stage I represent an H 2 O-CO 2 -NaCl system that transforms into an H 2 O-NaCl system from stages II to IV.

H-O Isotopes
The H-O isotope results are listed in Table 2 and illustrated in Figure 8. Here, the δ 18 O V-SMOW values of the six quartz samples from the four mineralization stages range from 7.5 to 9.6‰. The δ 18 O H 2 O values were calculated by the quartz-water equilibrium function [32], with the peak homogenization temperatures of FIs in the same mineralization stage. The calculated δ 18 O H 2 O values for stages I, II, III, and IV are 6.5‰, 3.0‰ to 3.5‰, 0.2‰ to 1.9‰, and −2.5‰, respectively (Figure 8). The δD values for stages I, II, III, and IV are −108.1‰, −126.2‰ to −122.5‰, −132.9‰ to −130.9‰, and −139.1‰, respectively (Figure 8).

C-O Isotopes
The C-O isotope results for the calcites in stage IV are listed in Table 3 and illustrated in Figure 9. Here, the δ 13 C PDB values of the four calcite samples in stage IV range from −3.1‰ to −0.2‰. The δ 18 O SMOW values range from 6.6‰ to 8.1‰. 6 Figure 9. Plot of δ 13 C PDB vs. δ 18 O SMOW for the calcite from the Huoluotai Cu (Mo) deposit. The three main carbon sources include marine carbonate [35], sedimentary organic matter carbon [36], and magma-mantle carbonate [37].

S-Pb Isotopes
The S isotope results of the seven sulfide samples from the Huoluotai Cu (Mo) deposit are presented in Table 4 and plotted in Figure 10. The δ 34 S V-CDT values of the seven sulfides range from 0.2‰ to 3.7‰ (average = 1.9‰) (Table 4; Figure 10).  The Pb isotope compositions of the four sulfide samples from the Huoluotai Cu (Mo) deposit are listed in Table 5 and illustrated in Figure 11. Here, the Pb isotope compositions of sulfides have a limited variation in 206

Source of the Ore-forming Materials
The δ 34 S V-CDT values (0.2‰ to 3.7‰) of the seven sulfides from the Huoluotai Cu (Mo) deposit overlap those of typical porphyry deposits in the NGXR, such as the Badaguan (-2.4‰ to 3.5‰; [45]), the Chalukou (-1.9‰ to 3.6‰; [43]), the Xiaokele (-1.2‰ to 2.4‰; [5]), and the Fukeshan (-2.3‰ to 3.4‰; [4]) deposits ( Figure 10). These values are also consistent with the δ 34 S V-CDT values of typical porphyry deposits elsewhere in the world (−5‰ to 5‰; [39]). The δ 34 S V-CDT values of sulfide minerals for all these deposits fall into the magmatic sulfur range and are similar to those of mantle-derived sulfur (Figure 10; [39]). The Pb isotope compositions of sulfide minerals from the Huoluotai Cu (Mo) deposit are similar and homogeneous, indicating that they share a common Pb reservoir. In the 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb diagram ( Figure 11A), Pb isotope data for the sulfide samples are plotted between the mantle and orogene evolution curves, near the mid-ocean ridge basalt (MORB) region. In the 208 Pb/ 204 Pb vs. 206 Pb/ 204 Pb diagram ( Figure 11B), Pb isotope data are all plotted between the orogene and upper crust evolution curves (near the mantle evolution curve), showing the characteristics of a crust-mantle-mixing origin.

Estimation of Trapping Pressure
Trapping pressure can be estimated only when the actual trapping temperature is known or if fluid boiling occurred in the hydrothermal system at the time of entrapment [48,49]. During heating, L-type FIs homogenized into the aqueous liquid phase, while the V-type FIs homogenized into the vapor phase. In the Huoluotai Cu (Mo) deposit, the coexistence of L-, V-, and S-type FI assemblages ( Figure 5A-F) within the same quartz crystal in stages I and II suggests that fluid boiling occurred in those stages. The trapping pressures in stage I were estimated to range from~300 to~500 bar ( Figure 12A), assuming a simple NaCl-H 2 O system using the isobar equations given in [50]. However, the addition of CO 2 to an NaCl-H 2 O system could raise the solvus and increase the entrapment pressure ( Figure 12B; [51][52][53][54]). Therefore, in this study, the estimated trapping pressures in stage I represent only the minimum values. The trapping pressures in stage II were estimated to be between~100 and~300 bar and were mostly concentrated around~200 bar ( Figure 12A) based on the isobar equations given in [50]. There are clear decreasing trends of pressure from stages I to II ( Figure 12A). Fluid boiling did not occur in stages III and IV, so the estimated trapping pressures represent only the minimum values [54]. The minimum trapping pressures in stages III and IV were estimated to range from~30 to~150 bar and <50 bar, respectively ( Figure 12A).

Origin of the Ore-forming Fluids
The δ 18 O H 2 O values for stage I (6.5‰) were close to those of magmatic water [55], indicating that the ore-forming fluids from stage I were derived from a magmatic source. The δD values for stage I were significantly lower than those for typical magmatic water (Figure 8; [56]). Previous studies have shown that the relatively depleted isotopic values of fluids could have been caused by water-rock interactions or magma degassing [57,58]. The depleted δ 18 O H 2 O and δD isotopic characteristics were extensively recorded by fluids from an early stage in porphyry deposits of the NGXR [59], such as the Xiaokele Cu (Mo) (-1.2‰ to 2.4‰; [5]), the Fukeshan Cu (Mo) (-2.3‰ to 3.4‰; [4]), and the Chalukou Mo (-1.9‰ to 3.6‰; [43]) deposits, which were interpreted to be predominantly of magmatic origin. The δD and δ 18 O H 2 O values for stages II, III, and IV were relatively lower than those for stage I and plotted in the region between the meteoric water line and the magmatic water field (close to the magmatic water field) (Figure 8), suggesting the involvement of some meteoric water but still dominated by magmatic water. In the δ 13 C PDB vs. δ 18 O SMOW diagram (Figure 9), the C-O isotope compositions of calcite in stage IV fell into the field of granite and magmatic carbonate, indicating that a certain amount of magmatic water was still involved in the mineralization process during the late mineralization stage.

Fluid Evolution and Mineralization Process
The occurrence of L-, V-, S-, and C-type FIs in stage I quartz indicates that the initial fluids belong to an H 2 O-CO 2 -NaCl system. These fluids are characterized by high homogenization temperatures and high salinities ( Figure 6A,B). High amounts of hematite, magnetite ( Figure 3E,F), and hematite-bearing S-type FIs ( Figure 5B), but few sulfides, were found in stage I, suggesting that the initial fluids were highly oxidized [60,61]. This result is supported by the detection of oxidized gases (e.g., CO 2 ) in FIs via laser Raman spectroscopy analysis ( Figure 7A). Sulfur in the magma was mostly present in the form of sulfate due to high oxygen fugacity and CO 2 activity [41,62], but sulfide mineralization would be inevitably constrained. These kinds of initial hydrothermal fluids were observed to be particularly prone to extensive potassic alterations and quartz + K-feldspar ± magnetite ± hematite ± pyrite veins in stage I. Cu (Mo) mineralization occurred mainly in stage II. L-, V-, and S-type FIs were found in stage II quartz crystals ( Figure 5D-F), which suggests that the ore-forming fluids of stage II belong to an H 2 O-NaCl system. Hematite, magnetite, and C-type FIs were not found in stage II, suggesting that the CO 2 content and oxygen fugacity of the fluids were distinctly decreased compared to those in stage I. Such degassing of oxidized gas is considered to be a major process responsible for sulfate reduction [61]. This reduction in oxygen fugacity could be due to the escape of CO 2 caused by fluid boiling [63][64][65] and/or magnetite precipitation [66][67][68]. These results suggest that the fluid system only contained minor CO 2 in stage I, indicating that it is likely not a CO 2 -rich system. Therefore, the immiscibility of CO 2 and water is not an important process in the Huoluotai Cu (Mo) deposit. The precipitation of magnetite was found to reduce S 6+ to S 2− (i.e., 12[FeO] + H 2 SO 4 = 4Fe 3 O 4 (Magnetite) + H 2 S; [62,68]), so the acid in stage II ore-forming fluids became stronger with an increase in S 2− and H + activity [69]. More significantly, the homogenization temperatures of stage II fluids were obviously lower than those of the stage I fluids ( Figure 6). H-O isotope data in this study indicate that mixing of hydrothermal fluids and meteoric water occurred in stage II. The addition of cooler and lower-salinity meteoric water inevitably led to a decrease in temperature and salinity of the primary hotter and high-salinity magmatic water, causing the precipitation of sulfides. Furthermore, the micro-thermometric data obtained in this study showed that Cu was mainly deposited below 400 • C in stage II, which is consistent with the temperature at which hydrothermal fluids reached Cu saturation in some micro-thermometric and thermodynamic studies of FIs [70][71][72][73]. Beyond stage I, fluid boiling also occurred in stage II. Generally, fluid boiling can cause widespread hydraulic fracturing and accelerate the influx and circulation of meteoric water, which could lead to rapid deposition of sulfides. In addition, fluid boiling can lead to the exsolution of volatile components from ore-forming fluids [51], which would also promote the enrichment of metals in the ore-forming fluids [74,75]. Thus, we believe that fluid boiling occurring below 400 • C may be the primary factor for Cu precipitation in the Huoluotai Cu (Mo) deposit.
No evidence of fluid boiling was found in stages III and IV. The ore-forming fluids in stages III and IV belong to a homogeneous H 2 O-NaCl system and became more diluted and cool, as indicated by the H−O isotope data. Magmatic water mixed with meteoric water may have initiated metal sulfide precipitation in stage III [71]. The increasing meteoric water influx may have formed the quartz + calcite ± pyrite ± fluorite veins in stage IV, reflecting waning of the hydrothermal system.

Conclusions
(1) The Huoluotai Cu (Mo) deposit is a typical porphyry Cu deposit in the NGXR. The ore-forming process of this deposit consists of the sulfide-barren quartz stage (I), the quartz + chalcopyrite ± pyrite ± molybdenite stage (II), the quartz + polymetallic sulfide stage (III), and the quartz + calcite ± pyrite ± fluorite stage (IV). Cu mineralization occurred mainly in stage II.
(2) The results of FI micro-thermometry and H-O isotope analysis showed that the ore-forming fluids originated from a magmatic origin in stage I and then mixed with meteoric water from stages II to IV. The S-Pb isotope results suggest that the source of the ore-forming materials has the characteristics of a crust-mantle-mixing origin.
(3) Fluid boiling occurring below 400 • C may be the primary factor for Cu precipitation in the Huoluotai Cu (Mo) deposit.