C-type Inclusions Research Articles

Genesis studies of Li–Rb deposits in pegmatites from Bailongshan, western China: Evidence from chronology, fluid inclusions, and H–O isotope analysis

The Bailongshan Pegmatite deposit, located in the West Kunlun Orogenic Belt, Northwest China, is a newly discovered, super-large Li–Rb (Be–Ta–Nb) rare-metal deposit. Since complex magmatic-hydrothermal processes are responsible for the mineralization of such rare-element pegmatites, it is desirable to study the evolution and sources of ore-forming fluids to analyze the genesis of ore deposits. In this study, the 40Ar/39Ar plateau ages of muscovite and biotite were determined to be 171.36 ± 1.87 and 172.39 ± 1.66 Ma, respectively, indicating that the duration of hydrothermal mineralization was approximately 170 Ma. Based on the zonal nature of the mineral assemblage, the Bailongshan area was divided into four zones and stages (I–IV), namely the albite–quartz–lithium tourmaline (AQT, stage I), albite–quartz-bearing mica (AQM, stage II), albite–quartz–spodumene (AQS, stage Ⅲ), and spodumene–quartz (SQ, stage IV) zones. Among these, AQS and SQ were the main ore-bearing areas. In terms of the fluid inclusions found in quartz and spodumene, the different types include a gas-rich phase (V-type), a liquid-rich phase (L-type), a daughter mineral-bearing three-phase (S-type), and a carbon dioxide-bearing phase (C-type). In stage I, the homogeneous temperatures of the V- and S-type fluid inclusions varied from 365 to 415 °C, while their corresponding salinities were 8.5–12.9 and 44.8–47.2 wt% NaCl equiv., respectively. In stage II, the homogeneous temperature and salinity of the L-type inclusions were 315–365 °C and 9.9–13.3 wt% NaCl equiv., respectively, while in stages Ⅲ and IV, the homogeneous temperatures of the L- and S-type fluid inclusions were between 235 and 335 °C, while their salinities were 7.2–12.3 and 32.1–37.0 wt% NaCl equiv., respectively. Furthermore, for the C-type inclusions, the homogeneous temperature and salinity were 235–320 °C and 4.9–10.6 wt% NaCl equiv., respectively. The laser Raman results showed that the fluid in the metallogenic stage was an H2O–NaCl–CO2–CH4 system. Based on the homogeneous temperature and salinity results, the fluid capture pressure from stage III to stage IV was calculated to be 280–150 MPa, and the depth of the capture was >6 km. Moreover, the H–O isotope results suggested that the early ore-forming fluids are mainly magmatic hydrothermal fluids, whereas the later (stage IV) mineralizing fluids may be mixed with a small amount of meteoric water. The subsequent immiscibility of the fluid may be one of the factors responsible for the discharge and precipitation of minerals.

Genesis and fluid evolution of the Hatu orogenic gold deposit in the West Junggar, Western China

The West Junggar in Xinjiang, western China, represents a significant gold mineralization belt hosting over 200 gold deposits, with the Hatu gold deposit being the largest among them. In this study, ore geology and fluid inclusion assemblages of quartz samples from the Hatu gold deposit were investigated in an effort to clarify the mineralization process and its relationship with the geodynamic settings. The mineralization process is delineated into early (pyrite-albite-quartz veins), middle-a (quartz-ankerite veins), middle-b (quartz-ankerite-sulfide-native gold veins), and late (quartz-calcite veins) stages. The early stage veins suggest a compressional setting while the middle-a and middle-b stage veins suggest a tensional shear setting. The late stage veins are typically filled fissures cutting through earlier veins. Scanning electron microscope-cathodoluminescence (SEM-CL) reveals that early stage quartz (Q1) exhibits concentric CL-oscillatory growth zoning, while middle-a stage quartz (Q2a) displays unidirectional zoning and ranges from CL-bright to CL-dark, middle-b stage quartz (Q2b) is CL-bright and weakly zoned, and late-stage euhedral quartz (Q3) shows sector zoning transitioning from CL-gray to CL-dark. Quartz that formed in these stages developed three types of fluid inclusions: pure CO2 (PC-type), CO2–H2O (C-type), and H2O–NaCl (W-type). The early stage quartz contains C- and W-type fluid inclusions homogenizing at 309–345 °C, while the late stage quartz contains only W-type fluid inclusions with homogenization temperatures of 164–229 °C. Microthermometry of fluid inclusion indicated the evolutionary of the metallogenic fluid from a high-temperature, CO2-rich, and minor CH4 metamorphic fluid to a low-temperature, CO2-poor meteoric fluid. From the early to middle-a stages, fluid boiling caused the unloading of ore-forming elements whereas fluid mixing led to the precipitation of polymetallic sulfides and gold in the middle-b stage. Trapping pressures in the middle-b stage were estimated using C-type inclusions, illustrating that gold mineralization took placed at depth of 8.0 km. We propose classifying the Hatu gold deposit as an orogenic deposit, originating from the collisional orogenesis between the Yili-Kazakhstan and Siberian continents in the Late Carboniferous.

Ore Mineralogy, Fluid Inclusion Geochemistry, and Zircon U-Pb Geochronology of the Nanmingshui Gold Deposit in East Junggar, Xinjiang, Northwest China: Implications for Ore Genesis

The Nanmingshui gold deposit, located in the eastern segment of the Kalamaili gold belt (KGB), is hosted by the sub-greenschist facies rocks of the Lower Carboniferous Jiangbasitao Formation. The genesis of this deposit, however, has been debated for decades because of controversial constraints on the P-T-X conditions and origins of hydrothermal fluid and mineralization age. In this study, we present gold-bearing sulfide compositions, fluid inclusions, H-O isotopes, and the results of hydrothermal zircon U-Pb dating to provide new insights into the genesis of the gold deposit. Three gold mineralization stages are recognized: quartz–pyrite–minor native gold veins (early), quartz–tourmaline–arsenopyrite–pyrite–gold–polymetallic sulfide veins (middle), and quartz–calcite veinlets (late). Gold predominantly occurs as native gold with high fineness ranging from 941 to 944 in sulfides and quartz, and some as solid solutions (Au+) within the lattice of pyrite and arsenopyrite. Three types of primary fluid inclusions are identified in hydrothermal quartz: CO2-H2O (C-type), aqueous (W-type), and pure CO2 (PC-type) inclusions. The early-stage quartz mainly contains C-type and minor W-type inclusions, with total homogenization temperatures (Th) of 220–339 °C, salinities of 0.4–3.7 wt.% NaCl eqv., and bulk densities of 0.66–1.01 g/cm3. All three types of inclusions are observed in the middle-stage quartz, of which the C- and W-type inclusions yield Th values of 190–361 °C, with salinities of 0.4–6.0 wt.% NaCl eqv. and bulk densities of 0.69–0.99 g/cm3. The late-stage quartz contains only W-type inclusions that have lower Th values of 172–287 °C, higher salinities of 1.4–6.9 wt.% NaCl eqv., and bulk densities of 0.79–0.95 g/cm3. Trapping pressures estimated from C-type inclusions in the early and middle stages cluster at 280–340 MPa and 220–310 MPa, respectively, corresponding to metallogenic depths of 10–13 km and 8–11 km. The H-O isotopic compositions (δ18Owater = 1.8–10.9‰, δD = −99 to −62.9‰) and microthermometric data indicate that the ore-forming fluids belong to medium–high-temperature, low-salinity, medium-density, and CO2-rich-H2O-NaCl ± CH4 ± N2 systems, probably originating from metamorphic water. Fluid immiscibility is a crucial mechanism for gold precipitation. Additionally, the U-Pb dating of hydrothermal zircons, from the auriferous quartz–tourmaline vein, yield a weighted mean 206Pb/238U age of 314.6 ± 9.6 Ma. Taking all of the above, the Nanmingshui deposit can be reasonably classed as a typical mesozonal orogenic gold deposit in the KGB, which was formed in a Late Carboniferous tectonic transition from syn-collision between the Jiangjunmiao accretionary complex and Yemaquan arc to post-collision in the East Junggar Orogen. Our results serve to better understand the gold mineralization and genesis of the Late Paleozoic orogenic system in the Kalamaili area, Xinjiang.

Genesis of the Shiyaogou porphyry Mo deposit at East Qinling, China: Evidence from geochronological, fluid inclusion, geochemical whole-rock and isotope studies

The Shiyaogou deposit in Luanchuan County, China, is a large concealed porphyry Mo deposit within the East Qinling metallogenic belt (EQMB). Major and trace element analyses reveal that the host granite is defined by high silica (SiO2 = 71.56 to 75.25%) and alkali (Na2O + K2O = 6.91 to 7.88%) contents and has typical I-type characteristics. Zircons derived from the pluton yield U–Pb ages of 133.2 ± 1.1 Ma, indicating an early Cretaceous magmatic event in the EQMB. The εHf (t) values and TDM2 ages range from − 15.1 to − 11.2 and from 2.1 to 1.9 Ga, respectively, suggesting that the granite mainly originated from the lower crust melting with the involvement of mantle components. The mineralization process can be divided into three stages: (1) quartz + K-feldspar + pyrite + molybdenite; (2) quartz + pyrite + molybdenite; and (3) quartz + calcite veins. Three different types of fluid inclusions (FIs) are present in quartz veins: NaCl–H2O (W-type), CO2–H2O (C-type), and daughter mineral-bearing (S-type). Hematite is recognized as a daughter mineral in S-type inclusions of stage 1, whereas the daughter sulfide and reducing CH4 gases can be observed in the stage 2 quartz, suggesting that the ore-forming fluids were initially oxidizing and then evolved to more reducing conditions. The FIs in stages 1 mainly display Th above 428 °C with salinities up to 57.62 wt% NaCl eqv; in stage 2, the FIs display Th of 132–423 °C and salinities of 3.23–50.03 wt% NaCl eqv; in stage 3, the FIs homogenized in the range of 117–247 °C with low salinities of 2.74–8.95 wt% NaCl eqv. The coexisting W-, S- and/or C-type inclusions homogenized at similar temperatures, but contain different salinities during stages 1 and 2, suggesting that the fluids boiled during both stages. Our study reveals that the ore-forming fluids are magmatic in origin. These high-temperature magmatic fluids gradually evolved from high salinities and high CO2 contents to a mixture of magmatic and meteoric water with lower temperature, lower salinity, and lower CO2 content. The precipitation of Mo mainly resulted from the decreasing temperature and salinity. Additionally, the changes in pH and CO2 content due to boiling and the water–rock reaction could have contributed toward the precipitation of Mo.

Geochronology, fluid inclusions, and isotopic characteristics of the Xiaoshan gold deposit, Henan Province, China

The Xiaoshan gold deposit of western Henan Province is located in the southern margin of the North China craton (NCC). Ore bodies of the deposit occur as veins in the fault zones. The ore-forming process can be divided into three stages: an early quartz–pyrite stage, a middle quartz–polymetallic sulfide stage, and a late quartz–carbonate stage. The LA-ICP-MS zircon U–Pb age of the Houhe granite, which is related to Au mineralization of the Xiaoshan deposit, was 139–137 Ma. The initial 176Hf/177Hf values of the Houhe intrusion are 0.282126–0.282368, with the εHf(t), TDM1(Hf), and TDM2(Hf) values being −19.9 to −11.3, 1242–1583 Ma, and 1912–2454 Ma, respectively, indicating that the Houhe granite is formed by partial melting of the lower crust and that the source is mainly the Taihua Group. Two types of fluid inclusions (FIs) have been distinguished in various quartz veins including liquid-rich (WL-type) and H2O–CO2 (C-type) inclusions. The FIs in the quartz of the quartz–pyrite stage comprise WL- and C-type inclusions; their homogenization temperatures, salinities, and densities vary from 300 °C to 393 °C, 1.6 wt% to 11.0 wt% NaCl eqv., and 0.57 to 0.82 g/cm3, respectively. The FIs in the quartz of the quartz–polymetallic sulfide stage consist of WL- and C-type inclusions; their homogenization temperatures, salinities, and densities range from 261 °C to 298 °C, 1.1 wt% to 11.8 wt% NaCl eqv., and 0.74 to 0.89 g/cm3, respectively. Only WL-type inclusions are observed in the quartz of the quartz–carbonate stage; their homogenization temperatures, salinities, and densities vary from 193 °C to 258 °C, 2.2 wt% to 12.7 wt% NaCl eqv., and 0.87 to 0.97 g/cm3, respectively. The ore-forming fluids of the Xiaoshan gold deposit are generally characterized by medium temperature, moderate–low salinity, low density and belonging to an H2O–NaCl ± CO2 system. The δ18OH2O values calculated for ore-bearing quartz vary from 0.7‰ to 4.5‰, and the δDV-SMOW values from bulk extraction of fluid inclusion water vary from −69.5‰ to −47.8‰, suggesting that the ore-forming fluid consists dominantly of magmatic water. The δ34SV-CDT values range from 0.37‰ to 3.9‰. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of the ore minerals are in the ranges of 17.391–17.728, 15.420–15.577, and 37.420–37.923, respectively. The data from the S and Pb isotopic systems indicate that the ore-forming materials came from granitic magma. The Xiaoshan gold deposit is a mesothermal lode-gold deposit of magmatic-hydrothermal origin. Fluid phase separation and temperature decrease were the dominant mechanisms for the deposition of ore-forming materials.

Origin and evolution of ore-forming fluids of the Larong W-(Mo) deposit, eastern Tibet: Constraints from fluid inclusions, H-O isotopes, and scheelite geochemistry

The Larong porphyry W-(Mo) deposit, formed in the main collisional orogenic stage of Lhasa and Qiangtang terranes, is the first-discovered giant W-(Mo) deposit in the Leiwuqi-Zogang metallogenic belt, Sanjiang region. The W-Mo mineralization occurs as veins and stock-works mainly hosted in the granodiorite porphyry, monzogranite porphyry, biotite granite porphyry, and surrounding schist. Four types of fluid inclusions (FIs) including PC-type (pure vapor CO2), C-type (CO2-H2O), W-type (aqueous two-phase) and L-type (pure liquid H2O) FIs have been identified from Stage I quartz-scheelite ± alkaline feldspar veins, Stage II (including three sub-stages) quartz-molybdenite ± pyrite ± chalcopyrite veins, and Stage III quartz ± calcite ± pyrite veins. The total homogenization temperatures of FIs vary widely (187–437 °C) with multiple peaks, indicating multistage fluid activities. The ore-forming fluids of the main mineralization stages are characterized by CO2 enrichment, high-medium temperature (~270–440 °C), and medium–low salinity (2.4–14.4 wt% NaCl equiv.). The homogenization temperature of W-type inclusions is obviously higher than that of C-type inclusions, and the salinity is relatively lower, implying multiple immiscibility of fluids with continuous escape of CO2. H-O isotopic compositions suggest that the primary ore-forming fluids dominantly originated from magma water. Mo concentrations and δEu values in scheelite indicate a decreasing trend of redox state of ore fluids. From Stage I to Stage II to Stage III, the relatively higher-temperature, CO2-rich, and oxidized fluids gradually evolved into relatively lower-temperature, CO2-poor, and reduced fluids. Here we propose that the decrease of temperature and oxygen fugacity and increase of S2- activity and PH values of the ore fluids caused by continuous fluid-rock interaction and loss of CO2 could account for the successive mineralization of W and Mo at the Larong deposit.

Insights into the genesis of orogenic gold deposits from the Zhengchong gold field, northeastern Hunan Province, China

The Zhengchong gold deposit is located in the central Jiangnan Orogen Belt, where gold ore occurs mainly as quartz veins, altered slates, and minor breccias hosted by shear zones in the Lengjiaxi Group. Based on its vein crosscutting relationships, mineral assemblages, and paragenetic sequences, mineralization in this deposit is divided into four stages: Stage 1: quartz(Q1)–pyrite stage, Stage 2: quartz(Q2)–pyrite–arsenopyrite stage, Stage 3: quartz(Q3)–pyrite–polymetallic sulfide mineral stage, and Stage 4: quartz(Q4)–carbonate stage. Two types of fluid inclusions have been recognized in the quartz grains: CO2-bearing fluid inclusions (C-type) and aqueous fluid inclusions (W-type). Fluid inclusions in hydrothermal quartz yield homogeneous temperatures and salinities of 312–406 °C and 4.65–9.21 wt% NaClequiv (Stage 1), 252–324 °C and 2.20–8.10 wt% NaClequiv (Stage 2), 193–296 °C and 2.24–14.04 wt% NaClequiv (Stage 3), and 127–209 °C and 2.57–6.59 wt% NaClequiv (Stage 4). The co-occurrence of these inclusion types despite their variable fluid salinities can be explained by fluid boiling during Stages 2 and 3 of mineralization, possibly as a result of episodic pressure decreases linked to fault-valve activity. Laser Raman spectroscopic analysis results reveal that small quantities of CH4 and N2 are present in the Stage 2–3 fluid inclusions. Fluid inclusion data and microthermometric results show that the ore fluids were low-salinity and CO2-rich, yielding both CO2 inclusions (C-type) and aqueous inclusions (W-type). The trapping pressures estimated from the C-type inclusions are 80–180 MPa for Stages 2 and 3, yielding gold mineralization depths of ∼7–8 km. The mineralizing fluids had calculated δ18OH2O values ranging from +3.7‰ to +8.2‰ and δD values from −75.2‰ to −61.8‰, indicating derivation mainly from metamorphic fluids. The sulfide minerals yield δ34S values of −8.9 to −0.1‰ and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 17.83–18.12, 15.67–15.76, and 38.03–38.62, respectively, which differ significantly from the isotopic composition of the granodiorite (δ34S = −2.4‰ to +1.0‰; 206Pb/204Pb = 18.57–19.28; 207Pb/204Pb = 15.69–15.76; 208Pb/204Pb = 38.97–40.34). However, the S–Pb isotopic compositions of the sulfide minerals are similar to those of slates of the Lengjiaxi Group in the study area. The Zhengchong gold deposit probably originated from a metamorphic fluid system characterized by low salinity, low density, and CO2 enrichment with limited admixture of magmatic fluids. Based on similarities to other orogenic gold deposits, we infer that the Zhengchong orebody represents an orogenic gold deposit formed during the Late Ordovician Jiangnan Orogeny caused by collision of the Yangtze and Cathaysia blocks.

Fluid evolution of the Qiman Tagh W-Sn ore belt, East Kunlun Orogen, NW China

The Qiman Tagh W-Sn ore belt is located in the westernmost sector of the East Kunlun Orogen, NW China. It has been recognized as a unique W-Sn belt that formed in the early Paleozoic and related to closure of the Proto-Tethys. To understand the evolution of ore-forming fluids and its relationship with the tectonic setting of East Kunlun Orogen, we report the results obtained from fluid inclusion and H-O isotopic studies of ores and quartz veins for the Qiman Tagh W-Sn ore belt. Mineralization in Qiman Tagh includes four stages characterized by quartz-cassiterite-wolframite assemblage stage 1, quartz ± scheelite assemblage stage 2, quartz-polymetallic sulfides stage 3, and ore-barren veins stage 4. The former two stages are conducive to mineralization, while the latter two stages are less important. The fluid inclusions are distinguished between CO2-H2O (C-type) and NaCl-H2O (W-type) in composition, containing a trace of CH4, N2, C2H6, SO2, and CO32–. Cassiterite and quartz in stage 1, instead of wolframite, contain a great deal of C-type inclusions. All inclusions in minerals of stage 1 yield homogenization temperatures of 230.1–384.1 °C (peaking at 310–320 °C), with salinities lower than 14.76 wt% NaCl equiv. and bulk densities of 0.63–0.89 g/cm3. The stage 2 minerals contain both two types of inclusions, yielding homogenization temperatures of 183.4–335.9 °C (peaking at 280–290 °C), with salinities lower than 14.53 wt% NaCl equiv. and bulk densities of 0.66–0.97 g/cm3. Fluid inclusions in minerals of stages 3 and 4 are mainly W-type and homogenized at temperatures of 140.6–277.6 °C (peaking 210–220 °C), and 116.9–255.1 °C (peaking 160–170 °C), respectively. The H-O isotopic systematics shows that the fluids were dominated by magmatic water in stages 1 and 2, but by meteoric water in stages 3 and 4. Integrating all the geological and geochemical data, we conclude that the fluids forming the Qiman Tagh W-Sn ore belt evolved from granite-derived, CO2-rich and reducing, to meteoric water-dominated, CO2-poor and oxidizing. Fluid immiscibility, cooling and interaction with rocks are main mechanisms for metallic deposition.

Geology, geochronology and geochemistry of the Dabate Cu–Mo deposit, northern Chinese Tien Shan: Implications for spatial separation of copper and molybdenum mineralization

The Dabate deposit is a porphyry Cu–Mo deposit located in the Sailimu Block, northern Chinese Tien Shan. The deposit shows a clear separation of Cu and Mo mineralization, with Cu mineralization occurring along the edge of the rhyolitic porphyry, and Mo mineralization in the inner part of the rhyolitic porphyry. Sm-Nd isotopic analyses of six fluorite samples from the Cu sulfides–fluorite veins yield an isochron age of 298.1 ± 5.0 Ma, and Re-Os isotopic analyses of four molybdenite samples from the molybdenite–quartz veins yield an isochron age of 299.6 ± 3.2 Ma. These geochronological data and accompanying geologic evidence indicate that the Cu and Mo mineralization belongs to the same hydrothermal system and are closely related to the rhyolitic porphyry, which has a youngest zircon 206Pb/238U age of 297.3 ± 8.0 Ma.Rare earth element (REE) and fluid inclusion studies were carried out for fluorite in the Cu ores. The fluorite related to Cu mineralization have high ΣREE (63.4–216.9 ppm) and show generally similar REE patterns with the rhyolitic porphyry, suggesting that the fluid of Cu mineralization is closely related to the rhyolitic porphyry. The slight LREE-depletion and Ce-depletion of fluorite relative to the rhyolitic porphyry may be due to preferential partitioning of HREE into the fluid in neutral to weak alkaline conditions and to mixing with meteoric water which is characterized by Ce-depletion. Three types of fluid inclusions were recognized in the fluorite from the Cu sulfides–fluorite veins: liquid–vapor (LV-type), CO2-bearing (C-type) and solid mineral-bearing (S-type). The LV-type inclusions have homogenization (vapor to liquid) temperatures from 175 to 328 °C and salinities of 0.4∼14.9 wt% NaCl eqv. (mode at 0∼8 wt% NaCl eqv.), and is of H2O + NaCl composition. The C-type inclusions have homogenization (vapor to liquid) temperatures from 206 to 279 °C and salinities of 1.6∼8.5 wt% NaCl eqv., and is of H2O + CO2 + NaCl + CH4 composition. The S-type inclusions have partial homogenization (vapor to liquid) temperatures of 173–233 °C and salinities of 1.2–7.9 wt% NaCl eqv., and is of H2O + NaCl composition. The solid particle in S-type inclusions did not change during heating up to 550 °C, indicating that the solid is not daughter mineral and the S-type inclusions are actually the same as the LV-type inclusions. The LV-type inclusions have microthermometric attributes similar to those typically found in the late stages of porphyry deposits, whereas the C-type inclusions have similar compositions as the inclusions in the Mo ores as documented in previous studies at Dabate. The presence of both LV-type and C-type inclusions suggests that the Cu mineralization may be related to mixing between meteoric water and magmatic fluids, as is also indicated by the REE of fluorite.Comparing with data from Mo ores reported in previous studies, it is suggested that the Mo-stage fluids were more reduced and acidic and Mo mineralization may be mainly related to fluid pressure decrease and immiscibility, whereas the Cu-stage fluids were more oxidized, neutral to weakly alkaline and Cu mineralization may be mainly controlled by mixing and cooling. The vapor phase produced by fluid immiscibility in the deep area may have carried Cu, As, Ag, HF, CO2 and migrated to the outer part of the porphyry, precipitated Cu sulfides, arsenopyrite and fluorite by mixing with meteoric water from the country rocks. Whereas the liquid phase produced by fluid immiscibility at depth may have carried Mo, NaCl and migrated to the middle-west part of the rhyolitic porphyry, become immiscible and precipitated molybdenite.

Geology and fluid inclusion geochemistry of the Zijinshan high-sulfidation epithermal Cu-Au deposit, Fujian Province, SE China: Implication for deep exploration targeting

The giant Zijinshan Cu-Au deposit in the Zijinshan orefield, Fujian Province, southeastern China, is the first high-sulfidation epithermal deposit identified in mainland China. The Cu and Au orebodies occur as veins and pods in the NW-trending faults and breccias zones. Intensive and pervasive alteration is characterized by downward and outward zoning from intensively leached silicic alteration (or vuggy quartz, Q), through alunite-quartz-pyrite alteration (advanced aragillic alteration, Q-Alu) and the quartz-alunite alteration overprinting the sericite alteration (Q-Alu-Di-Srt), to sericite (or phyllic) alteration zone. Gold mineralization mainly occurs in the silicic alteration zone, while the copper mineralization is confined in the alunite-quartz alteration zone. On the basis of detailed petrographic study, an early porphyry type mineralization stage, characterized by chalcopyrite-bornite-molybdenite-pyrite-sericite assemblage is recognized at Zijinshan, which is subsequently and strongly overprinted by the sulfate alteration and high-sulfidation Cu-Au mineralization stages. A supergene stage is also identified at shallow depth of the deposit, with gold largely enriched but copper commonly leached. The ore-forming fluids related to the early sericite alteration and possibly porphyry type mineralization are high-temperature, high-salinity magmatic water, characterized by the presence of the halite-bearing inclusions. The CO2-H2O (C-type) inclusions are mostly vapor-rich and abundantly identified in the samples with advance argillic alteration (alunite alteration). They are regarded to be the buoyant vapor phase by fluid boiling of a single-phase, low- to moderate-salinity magmatic fluid at depth, where a separated saline phase and related porphyry mineralization might be expected. A group of secondary inclusions coexisting with enargite grains are recognized in the samples with alunite alteration and suggested to be trapped from the ore-forming fluids of the Cu-Au mineralization stage. The total homogenization temperatures and salinities of the secondary inclusions are below 300°C (peaking at 260–280°C) and under 10wt% NaCl eqv., respectively. Intensive fluid boiling is the major mechanism for the formation of the giant high-sulfidation Cu (covellite-, digenite-dominated) orebodies in the Zijinshan deposit. It is a deep-seated high-sulfidation epithermal deposit according to the estimated depth of 1.4–2.1km from the C-type inclusions in quartz grains from the alunite alteration zone. By fluid inclusion mapping of a nearly NW-trending cross section profile, the isotherms are extrapolated using the average total homogenization temperatures and the possible heat source is suggested. It is indicated that the heat source and possible concealed porphyry mineralization nearly coeval to the high-sulfidation Cu-Au mineralization at Zijinshan might be located at the southeastern Zijinshan deposit or the northern area between the Zijinshan and Wuziqilong deposits, where deep drilling is encouraged.

Geology and hydrothermal evolution of the Baituyingzi porphyry Mo (Cu) deposit, eastern Inner Mongolia, NE China: Implications for Mo and Cu precipitation mechanisms in CO2-rich fluids

The Baituyingzi Mo (Cu) deposit is a newly identified porphyry deposit located in the southern part of the Xilamulun Mo metallogenic belt, Northeast China. Mo (Cu) mineralization mainly occurs as quartz veinlets and stockwork veinlets in the potassic altered Baituyingzi monzogranite porphyry. A detailed investigation of fluid inclusions and OH isotopes from hydrothermal veins at Baituyingzi allow us to establish the evolving history of the fluid system, as well as understand factors controlling CuMo precipitation in CO2-rich fluids.Six vein types are recognized at Baituyingzi, which belong to five hydrothermal stages: (1) UST quartz vein, formed during magmatic-hydrothermal transitional stage; (2) A1 vein (sulfide-barren quartz vein) and A2 vein (quartz–pyrite–chalcopyrite±molybdenite vein, Cu mineralization), related to potassic alteration; (3) B vein (quartz–molybdenite±chalcopyrite±pyrite vein, Mo mineralization), formed during the potassic-sericitic transitional stage; (4) D vein (quartz–pyrite±molybdenite±chalcopyrite vein), associated with sericitic alteration; and (5) late stage L vein (quartz–carbonate vein).Four types of fluid inclusions have been observed in these veins, i.e., liquid-rich inclusions (L-type), vapor-rich inclusions (V-type), polyphase high-salinity inclusions (S-type), and CO2-rich inclusions (C-type).UST quartz veins contain abundant S- and co-existing V-type inclusions, accompanied by minor melt inclusions, indicating that the parent fluids were likely in the form of bubbly magma (~10wt% NaCl equiv.) that was derived from a deep magma chamber.A1 veins are dominated by coexisting S- and V-type inclusions that homogenized at temperatures ranging from 400° to 548.5°C and pressures between 300 and 750bar, which may represent the result of parent fluid immiscibility (vapor-brine) at depth.Apart from S- and V-type inclusions, Cu-bearing A2 veins are especially rich in isolated L-type inclusions (homogenized at 347 to 430°C) that share the same salinity range as V-type inclusions (homogenized at 375 to 470°C), suggesting that the vapor gradually contracted to a liquid phase at around 350bar. Because the system was still in a lithostatic condition (revealed by textures of A2 veins), Cu precipitation at Baituyingzi was thus largely controlled by temperature reduction (to below 400°C) near the vapor-liquid surface in relatively oxidized fluids (indicated by the precipitation of abundant hematite).C-type inclusions (homogenize at 275 to 350°C) seem to be more widely developed, and they commonly co-exist with L-type inclusions (homogenized at 340 to 380°C) of similar salinity in B veins, implying that they unmixed at temperatures below 350°C and pressures below 200bar. Considering that the system had already fluctuated to hydrostatic conditions (revealed by clear open-space filling textures), as well as considering the occurrence of minor feldspar-destructive alteration halos in B veins, Mo precipitation would likely be a result of water-CO2 immiscibility at temperatures below 350°C with a sharp pressure reduction to below 200bar and an increase in fluid acidity.D veins contain plentiful L- and minor C– and S-type inclusions that homogenized at lower temperatures (240 to 350°C) and pressures (30 to 150bar). The wide feldspar-destructive alteration halos indicate that the fluids became progressively more acidic and reacted intensely with wall-rocks.L veins only contain low temperature (168 to 280°C) and low salinity (0.4 to 10.9wt% NaCl equiv.) L-type inclusions, implying that meteoric water was introduced to the barren magmatic fluids, leading to precipitation of a large amount of remnant material.δ18O and δD isotope ratios in quartz (δ18Ofluid=3.54‰ to 7.51‰, δD=−107‰ to −87‰) also confirm a primary magmatic origin for ore-forming fluids in earlier veins, except for L veins.Therefore, fluid evolution at Baituyingzi was characterized by relatively continuous, system-scale cooling along an ascent path gradually straddling the vapor-brine, vapor-liquid, and water–CO2 surface in the vapor-dominated H2OCO2NaCl fluid system, whereas the subtle temporal change in P-T-redox conditions and acid balance of the evolutionary paths of the magmatic fluid may be responsible for Cu and Mo separation at the Baituyingzi CO2–rich systems. Additionally, water-CO2 immiscibility seems to be an additional important mechanism that complicates Mo precipitation in CO2–rich systems.

Fluid inclusion study of the Wunugetu Cu–Mo deposit, Inner Mongolia, China

Hydrothermal alteration and mineralization at the Wunugetu porphyry Cu–Mo deposit, China, include four stages, i.e., the early stage characterized by quartz, K-feldspar and minor mineralization, followed by a molybdenum mineralization stage associated with potassic alteration, copper mineralization associated with sericitization, and the last Pb–Zn mineralization stage associated with carbonation. Hydrothermal quartz contains three types of fluid inclusions, namely aqueous (W-type), daughter mineral-bearing (S-type) and CO2-rich (C-type) inclusion, with the latter two types absent in the late stage. Fluid inclusions in the early stage display homogenization temperatures above 510°C, with salinities up to 75.8 wt.% NaCl equivalent. The presence of S-type inclusions containing anhydrite and hematite daughter minerals and C-type inclusions indicates an oxidizing, CO2-bearing environment. Fluid inclusions in the Mo- and Cu-mineralization stages yield homogenization temperatures of 342–508°C and 241–336°C, and salinities of 8.6–49.4 and 6.3–35.7 wt.% NaCl equivalent, respectively. The presence of chalcopyrite instead of hematite and anhydrite daughter minerals in S-type inclusions indicates a decreasing of oxygen fugacity. In the late stage, fluid inclusions yield homogenization temperatures of 115–234°C and salinities lower than 12.4 wt.% NaCl equivalent. It is concluded that the early stage fluids were CO2 bearing, magmatic in origin, and characterized by high temperature, high salinity, and high oxygen fugacity. Phase separation occurred during the Mo- and Cu-mineralization stages, resulting in CO2 release, oxygen fugacity decrease and rapid precipitation of sulfides. The late-stage fluids were meteoric in origin and characterized by low temperature, low salinity, and CO2 poor.