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Keywords:

  • fluid inclusions;
  • Dahu Au–Mo deposit;
  • orogenic Mo deposit;
  • Xiaoqinling terrane;
  • Qinling Orogen;
  • China

Abstract

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 REGIONAL AND ORE GEOLOGY
  5. 3 SAMPLES AND METHODS
  6. 4 FLUID INCLUSION STUDIES
  7. 5 THE O–H ISOTOPE CONSTRAINTS
  8. 6 DISCUSSION
  9. 7 CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

The Xiaoqinling gold field in central China is the second largest orogenic Au province in China. In the Xiaoqinling area, the Dahu Au–Mo deposit is typical because it is one of the five early-discovered large gold deposits, but also unique for its northernmost location and Mo-association. This study shows that the deposit is a fault-controlled lode system formed by a three-stage hydrothermal process. The early-stage pyrite–quartz veins are structurally deformed and broken. The middle-stage molybdenite–pyrite–quartz stockworks (mostly coaxial) infill the cracks of the early-stage veins and minerals. The late-stage open-space filling quartz–carbonate veinlets show a comb-like texture. The early- and late-stage quartz only contains the fluid inclusions of CO2–H2O and H2O–NaCl types, respectively, suggesting an evolution from CO2-rich to CO2-poor; whereas the middle-stage quartz contains fluid inclusions of CO2–H2O, H2O–NaCl, pure CO2 and daughter mineral-bearing types, supporting a boiling fluid system, because they can occur in a microscopic domain of a single crystal, homogenize divergently at similar temperatures clustering between 293 and 410 °C, and show contrasting salinities of <13.6 wt.% and >26.3 wt.% NaCl equiv. The inclusions in early-stage quartz are homogenized at 402–503 °C, with salinities of 4.5–10.4 wt.% NaCl equiv.; and those in late-stage quartz are homogenized at temperatures of <251 °C, with salinities of 14.4–14.8 wt.% NaCl equiv. The trapping pressures of inclusions are estimated to be 138–331 MPa in the early stage and 78–237 MPa in the middle stage, implying that the fluid system alternately fluctuated from lithostatic to hydrostatic, which was controlled by a fault–valve mechanism at depths of >11 km and ~7.8 km, respectively. Hydrogen and oxygen isotope signatures indicate that the fluids originated from metamorphic devolatilization in the early stage, then mixed with meteoric water in the middle stage, and finally replaced by meteoric water in the late stage. Hence, it is concluded, that metal precipitation at the Dahu deposit was mainly caused by fluid boiling, followed by fluid mixing, along with a trans-compressing at the transitional zone from a magmatic arc to back-arc basin, which resulted from the Triassic northward subduction of the Mian-Lue oceanic slab. © 2014 The Authors. Geological Journal published by John Wiley & Sons Ltd.

1 INTRODUCTION

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 REGIONAL AND ORE GEOLOGY
  5. 3 SAMPLES AND METHODS
  6. 4 FLUID INCLUSION STUDIES
  7. 5 THE O–H ISOTOPE CONSTRAINTS
  8. 6 DISCUSSION
  9. 7 CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

Orogenic gold deposits are a distinct class that has been the main source of world gold production. Groves et al. (1998) defined orogenic gold deposits to be structurally-controlled lode systems formed by low-salinity, CO2-rich metamorphic fluids after regional peak-metamorphism during accretionary orogenies (Goldfarb et al., 2001; Kerrich et al., 2005; Chen, 2006; Groves and Beirlein, 2007; Pirajno, 2009). Chen and his coauthors (e.g. Chen and Fu, 1992; Chen et al., 2004, 2005a, b, 2007a, 2012; Pirajno, 2009, 2013; Chen, 2013; Zhou et al., 2014) developed a group of genetic models at deposit-, orefield/terrane- and province/orogen-scales for the orogenic and related types of Au deposits formed in continental collision orogeny. They summarized the diagnostic characteristics of the orogenic Au deposits differing from other types of Au deposits (Chen et al., 2007b), and proposed the potentials of orogenic-type Ag, Sb, Pb–Zn, Cu, Mo deposits, on the basis of: (1) the geochemical similarity of Ag, Pb–Zn, Mo and Cu with Au, and (2) the proved Au–Ag, Au–Pb–Ag and Au–Cu associations in the well-known orogenic gold deposits (Chen, 2006). Such a synthesis facilitated subsequent exploration and research for the orogenic-type deposits including Ag (Chen et al., 2004, 2005b; Zhang and Chen, 2005; Han et al., 2014), Pb–Zn (Qi et al., 2007; Zhang et al., 2012; Zheng et al., 2013), Cu (W.B. Li et al., 2008; Zheng et al., 2012, 2014; Zhong et al., 2012, 2013) and Mo (Li et al., 2011a, b, 2014a; Deng et al., 2014) deposits. This shows the importance of gaining insight into the hydrothermal processes forming the orogenic deposits.

Fault-controlled lode Mo deposits have been rarely reported in the previous studies (Li et al., 2007). In the eastern Qinling Orogen (Fig. 1), such a kind of Mo deposits were recently discovered at Zhifang (Deng et al., 2014), Longmendian (Li et al., 2011b, 2014a) and Zhaiwa (Deng et al., 2013a, b), as well as at depth in the Dahu gold mine (Li et al., 2011a; Ni et al., 2012).

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Figure 1. (A, B) Main structural regions in China. (C) Geological map showing the distribution of gold deposits in the Xiaoqinling gold field (modified after Chen, 2006). This figure is available in colour online at wileyonlinelibrary.com/journal/gj

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The Dahu deposit was initially prospected and mined as an orogenic Au deposit, with a proven reserve of 38 t Au at an average grade of 6.8 g/t (Mao et al., 2002). It is one of the five early-discovered large (each has reserve of 20–100 t Au) gold deposits in the Xiaoqinling gold field (Fig. 1C) which ranks the second largest gold-producing province in China (Chen et al., 1998; Mao et al., 2002) following the Jiaodong gold province (Chen et al., 2005a). Underground exploiting of the gold ores has revealed the economic significance of the Mo mineralization in the deeper part of auriferous quartz veins, and together with drilling, proved a Mo metal reserve of 100 000 t, with an average grade of 0.24% (Mao et al., 2008). This recognition of the Au–Mo association made the Dahu deposit extremely unique in the structurally-controlled lode gold deposits in both the Xiaoqinling gold field and the world, because no orogenic-type gold deposit has been reported to associate with Mo. However, whether the Dahu Au–Mo mineralization is of orogenic-type remains unclear. In this contribution, we report the results obtained from the studies in geology and fluid inclusions and isotope geochemistry, and thereby decipher the hydrothermal process and ore-forming mechanism of the Dahu deposit.

2 REGIONAL AND ORE GEOLOGY

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 REGIONAL AND ORE GEOLOGY
  5. 3 SAMPLES AND METHODS
  6. 4 FLUID INCLUSION STUDIES
  7. 5 THE O–H ISOTOPE CONSTRAINTS
  8. 6 DISCUSSION
  9. 7 CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

The E–W-trending Xiaoqinling Terrane is bound by the San-Bao Fault to the north and the Xiaohe Fault to the south (Fig. 1) (Kerrich et al., 2000; Mao et al., 2002; Chen et al., 2009; Jiang et al., 2009). The main lithostratigraphic unit in this area is the Neoarchaean to Palaeoproterozoic Taihua Supergroup, a suite of amphibolite- to granulite-facies metamorphic rocks consisting of graphite schists, marbles, quartzites, banded iron-formations, gneisses and amphibolites (Chen and Zhao, 1997). This supergroup was further divided into the Beizi, Dangzehe, and Shuidigou groups in ascending sequence, with isotope ages of 3.0–2.55 Ga, 2.5–2.3 Ga, and 2.3–2.1 Ga, respectively (Chen and Zhao, 1997). Granitic magmatism in this area is represented by the Xiaohe biotite granite of Mesoproterozoic age, and the Huashan, Wenyu, and Niangniangshan biotite granites of Jurassic–Cretaceous age (Mao et al., 2010; Fig. 1C).

The Dahu deposit is located in the northern margin of the Xiaoqinling Terrane, and hosted by migmatitic granites and biotite–plagioclase gneisses of the Taihua Supergroup (Fig. 2A). Gold and molybdenum mineralization are associated with a series of quartz veins controlled by ENE- and WNW-trending and north-dipping faults, namely F1, F8, F7, F35, F5, and F6 from north to south (Fig. 2A). Fault F5 is the main ore-controlling structure, and hosts most of the orebodies. Structural analysis has revealed that these faults evolved from Triassic–Jurassic south-directed thrusting to Cretaceous north-directed normal faulting (Zhang et al., 2009).

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Figure 2. (A) Simplified geological tectonic map of the Dahu Au–Mo deposit and (B) N–S cross-section (modified from Li et al., 2011a). This figure is available in colour online at wileyonlinelibrary.com/journal/gj

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The quartz vein-type Mo orebodies and Au orebodies can be controlled by the same fault, with the former located in the deeper part (Fig. 2). The orebodies are usually 10 to 100 cm thick, with a stable extension. The S35 quartz–molybdenite vein is hosted in the F35 ductile shear zone which is approximately 900 m long and 10 to 50 m wide on the surface. Molybdenite is the major Mo-containing mineral, and occurs in two styles, i.e. radiated aggregates disseminated in milky quartz veins (Fig. 3A and 3B) and membrane-like powder in the cracks of quartz veins. Au occurs as native Au inclusions mainly in pyrite, minor in quartz, or associated with pyrite–galena–quartz veinlets that fill fissures in cubic pyrite (Mao et al., 2002). Other metallic minerals include pyrite, chalcopyrite, galena, sphalerite, pyrrhotite, bornite, and covellite. Gangue minerals primarily include quartz, K-feldspar, plagioclase, calcite, sericite and chlorite.

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Figure 3. Ore fabrics of the Dahu Au–Mo deposit under the microscope. (A) Coarse-grained molybdenite–quartz veins; (B) fine-grained molybdenite aggregates occurring along the fissures of quartz; (C) K-feldspathization (microcline); (D) Brecciated early-stage quartz and pyrite were replaced and filled by middle-stage chalcopyrite–galena–quartz; (E) Middle-stage molybdenite–chalcopyrite–galena–quartz veins filling the fissures of early-stage pyrite; (F) Early-stage pyrite replaced by molybdenite; (G) Late-stage quartz–carbonate vein cross-cutting the altered wall rocks; (H) Calcite veins filling the cracks in K-feldspar. Abbreviation: Cc, calcite; Cpy, chalcopyrite; Gle, galena; Kfs, K-feldspar; Mo, molybdenite; Py, pyrite; Qz, quartz. This figure is available in colour online at wileyonlinelibrary.com/journal/gj

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Field investigation of ore petrography, texture and cross-cutting relationships shows that the Au–Mo mineralization is a multi-stage process that includes three stages. The early stage is represented by quartz, (cubic) pyrite, and K-feldspar. Euhedral pyrite and milky quartz are present in quartz veins and occasionally inter-grown with K-feldspar (Fig. 3C). The middle stage is represented by disseminated veinlets composed of quartz, molybdenite and other sulphides such as pyrite, chalcopyrite and galena (Fig. 3D). These molybdenite–quartz (±pyrite ± chalcopyrite ± galena) veins generally fill open-space cracks and/or fissures in pre-existing pyrite (Fig. 3E) that is occasionally replaced by anhedral molybdenite (Fig. 3F). The middle-stage quartz is poorly transparent, and thus is distinguished from the early-stage milky quartz. The late stage is characterized by comb-like quartz–carbonate veinlets that cut earlier veins (Fig. 3G and 3H). Quartz crystals usually grow from parallel walls toward the centre, with subsequent infilling of calcite crystals, indicating an opening and extensional condition (Chen et al., 2005a, 2007b). The formation of calcite is presumed to be slightly later than quartz, although they are probably deposited from a same fluid system (Chen et al., 2005a, 2007b).

3 SAMPLES AND METHODS

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 REGIONAL AND ORE GEOLOGY
  5. 3 SAMPLES AND METHODS
  6. 4 FLUID INCLUSION STUDIES
  7. 5 THE O–H ISOTOPE CONSTRAINTS
  8. 6 DISCUSSION
  9. 7 CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

The samples analysed in this study were selected from the S35 quartz vein, and doubly polished to 0.3 mm-thick sections for fluid inclusion (FI) investigations. Eighteen doubly polished sections were examined petrographically, of which 12 were selected for detailed microthermometric measurements and laser Raman spectroscopic analyses, including two sections from the early stage, eight from the middle stage, and two from the late stage.

Microthermometric measurements were conducted using a LINKAM THMSG600 heating–freezing stage in the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. The heating/freezing rate was 0.2 °C/min to 5 °C/min, but decreased to <0.2 °C/min near the phase transformation points. The heating–freezing stage was calibrated using the standard of synthetic FIs produced by America FLUID INC. Temperature errors were estimated at ±0.5 °C in the range of −120 to −70 °C and ±0.2 °C in the range of −70 °C to 500 °C.

Laser Raman spectroscopy detection was performed using a Raman micro-spectrometer (InVia Reflex, Renishaw Group, Gloucestershire, UK) to determine the composition of a single inclusion. The slit width was set to 50 µm, and the resulting resolution was ±1 cm−1. The objective was a Leitz 20× with a working distance of 15 mm. An argon ion laser with a wavelength of 514.5 nm operating at 20 mW was used to illuminate the sample for Raman signal generation. Each spectrum was collected within an accumulation time of 30 s and a wavelength of 100 to 4000 cm−1.

4 FLUID INCLUSION STUDIES

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 REGIONAL AND ORE GEOLOGY
  5. 3 SAMPLES AND METHODS
  6. 4 FLUID INCLUSION STUDIES
  7. 5 THE O–H ISOTOPE CONSTRAINTS
  8. 6 DISCUSSION
  9. 7 CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

4.1 Types of FIs

Fluid inclusions were classified based on their compositions (detected by laser Raman spectroscopy), phases at room temperature, and phase transitions observed during heating/cooling. Four types of FIs were identified.

4.1.1 P-type – pure carbonic inclusion

The P-type FIs are pure CO2(±CH4) inclusions (Fig. 4A), and occur as a single phase at room temperature though vapour CO2 bubbles can appear during cooling. They are isolated or randomly distributed within quartz as clusters (Figs. 4A and 5A), and generally show negative crystal or oval shapes, with sizes ranging from 5 to 20 µm. The Raman spectra show the peaks of CO2 (Fig. 5A).

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Figure 4. Representatives of fluid inclusions in the Dahu Au–Mo deposit. (A) Pure carbonic inclusions. (B) P-type inclusions coexisting with C-type inclusions including subtypes C1 and C2. (C) Typical three-phase subtype C2 inclusion, with vapour/(liquid + vapour) ratio of ~45%. (D) The secondary and/or pseudosecondary W-type inclusions, showing a directional distribution in the X-shape fissures of quartz. (E) Opaque mineral-bearing S-type inclusion coexisting with C-type inclusion. (F) Halite-bearing S-type inclusion. This figure is available in colour online at wileyonlinelibrary.com/journal/gj.

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image

Figure 5. Laser Raman spectra of the typical fluid inclusions. (A) The unique peaks of CO2 in P-type fluid inclusion. (B) CO2-peak of the vapour CO2 phase in C-type inclusion. (C) Peaks of liquid H2O phase in C-type inclusion. (D) High H2O-peak and low CO2-peak of the vapour phase in W-type inclusion. This figure is available in colour online at wileyonlinelibrary.com/journal/gj

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4.1.2 C-type – CO2–H2O–NaCl inclusion

The C-type inclusions are the most abundant population. They are two-phase (vapour CO2 + liquid H2O) or three-phase (vapour CO2 + liquid CO2 + liquid H2O) CO2–H2O–NaCl or carbonic–aqueous systems (Fig. 4B and 4C), with CO2 phases accounting for 10% to 90% of the total volume. They can be subdivided into C1 and C2 subtypes, with the (VCO2 + LCO2)/(VCO2 + LCO2 + LH2O) being >50% (Fig. 4B) and <50% (Fig. 4C), respectively. These inclusions show negative crystal or oval shapes, and range from 5 µm to 25 µm in size. They occur isolatedly or as clusters. In the Raman spectra, the vapour phases (VCO2) of C-type inclusions show the peaks of CO2 and H2O (Fig. 5B), but the liquid phases (LH2O) do not show the CO2 peaks (Fig. 5C).

4.1.3 W-type – H2O–NaCl inclusion

The W-type (aqueous) FIs are two phases (VH2O + LH2O) H2O–NaCl systems at room temperature. They generally show strip, oval or irregular shapes with sizes of 5–25 µm, and usually appear as secondary inclusions along the trails cross-cutting the quartz host (Fig. 4D). Some of these inclusions occur randomly and/or intergrow with C-type inclusions (Fig. 5D). Raman analysis reveals a trace of CO2 in the vapour bubble, though no individual phase of CO2 can be observed (Fig. 5D).

4.1.4 S-type – Daughter mineral-bearing inclusion

The S-type FIs contain halite and opaque daughter minerals, and show oval or round shapes with sizes ranging from 5 µm to 15 µm in diameter. They usually contain CO2 and coexist with C-type inclusions (Fig. 4E). The most transparent minerals are proven to be halite by Raman spectrographic detection. The opaque minerals, however, cannot be identified due to their motility during irradiation.

Not all types of FIs are present in each vein, and their abundances vary in minerals of different stages. In early-stage veins, C-type FIs are the predominant type with oval, negative crystal, or irregular shapes. Compared with the early-stage veins, the middle-stage veins contain all the four types of FIs (Figs. 4B, 4E, and 5D). Moreover, the (VCO2 + LCO2)/(VCO2 + LCO2 + LH2O) (abbreviated to ‘Φ’ hereafter) values of C-type vary from 0.1 to 0.9 (Fig. 4B). The W-type inclusions are mostly distributed along the trails cross-cutting the quartz host, indicating their secondary origin. In late-stage veins, only the W-type FIs can be observed.

4.2 Microthermometry

The C-type inclusions in early-stage quartz yield clathrate melting temperatures (Tmclath) between 4.1 °C and 7.7 °C, and partial homogenization temperatures (ThCO2) between 15.0 °C and 30.2 °C. However, most C-type FIs decrepitate at temperatures of 256–365 °C before total homogenization because of high inner-pressure. Total homogenization has been observed for only two inclusions in this study, yielding homogenization temperatures of 402 °C and 503 °C, respectively (Table 1). This suggests that the early-stage mineralization occurred at high temperature (probably >400 °C) and high pressure (decrepitation before homogenization).

Table 1. Microthermometric data and pressure estimation of fluid inclusions of the Dahu deposit
StageTypeTmice (°C)Tmchath (°C)ThCO2 (°C)Thtotal (°C)Thd (°C)Salinity (wt.%)Pressure (MPa)
  1. Abbreviations: Tmice, final melting temperature of ice; Tmclath, final melting temperature of clathrate; ThCO2, homogenization temperature of CO2; Thtotal, total homogenization temperature; Thd, decrepitation temperature.

EarlyC 4.1–7.715.0–30.2402, 503256–3654.5–10.4138–331
MiddleC 1.7–9.97.1–31.2293–410215–4640.2–13.678–237
 S 3.8–5.331.1–32.5 225–251>26.3% 
 W−10.8 to −10.4  227–251/14.4–14.8 

In middle-stage quartz, the C-type inclusions yield Tmclath values of 1.7–9.9 °C and ThCO2 values of 7.1–31.2 °C. The C-type FIs with Φ > 0.5 decrepitate at 215–464 °C before total vapour-direction homogenization, while those with Φ < 0.5 are homogenized to liquid at 293 °C to 410 °C (Table 1, Fig. 6). Two CO2-bearing S-type inclusions (containing both halite and opaque minerals) yield clathrate melting temperatures of 3.8 °C and 5.3 °C, and partial homogenization temperatures of 31.1 °C and 32.5 °C, respectively. The CO2 homogenization temperatures are slightly higher than the critical CO2 temperature of 30.977 °C (Lu et al., 2004). During heating, the majority of S-type FIs are decrepitated at 225–251 °C before complete dissolution of daughter mineral because of high inner-pressure. Due to the existence of halite, the salinities of S-type FIs can be estimated to be higher than 26.3 wt.% NaCl equiv. (Hagemann and Luders, 2003; Qi et al., 2007). Integrating the observations that the S-type FIs coexist with the C-type FIs, which show contrasting volumetric percentages of CO2, and are homogenized to different phases at similar temperatures, we can conclude that the fluids in the middle stage might be boiled or phase separated (Fan et al., 2003; Lu et al., 2004; Pirajno, 2009).

image

Figure 6. Histograms of homogenization and decrepitation temperatures of fluid inclusions.

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In the late stage, the W-type FIs are too small to be suitable for microthermometric measurement. However, considering that the W-type FIs in the middle-stage quartz distribute along the trails cross-cutting the quartz crystals, we use these inclusions to represent the late-stage fluids. These inclusions yield ice-melting temperatures of −10.8 to −10.4 °C, and total homogenization temperatures of 227 to 251 °C (Table 1).

Salinities of C-type and W-type inclusions were calculated using the reference data of Bodnar (1993) and Roedder (1984), respectively. The fluid salinities in the early and late stages are estimated to be 4.5–10.4 and 14.4–14.8 wt.% NaCl equiv., respectively. The salinities of FIs in the middle stage fall in two contrasting groups of 0.2–13.6 and >26.3 wt.% NaCI equiv., possibly resulting from phase separation (Table 1).

5 THE O–H ISOTOPE CONSTRAINTS

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 REGIONAL AND ORE GEOLOGY
  5. 3 SAMPLES AND METHODS
  6. 4 FLUID INCLUSION STUDIES
  7. 5 THE O–H ISOTOPE CONSTRAINTS
  8. 6 DISCUSSION
  9. 7 CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

The oxygen and hydrogen isotopes are commonly used as a powerful tool to trace the nature, source and evolution of the ore-forming fluids. We here summarize all the available hydrogen and oxygen isotope data of the Dahu Au–Mo deposit to constrain the origin and evolution of the ore-forming fluids (Table 2). The oxygen isotope ratios of water in equilibrium with quartz were calculated using the fractionation formulas of Clayton et al. (1972).

Table 2. Hydrogen and oxygen isotope of quartz from the Dahu Au–Mo deposit (‰)
Sample No.δ18Oquartzδ18OwaterδDT (°C)StageReference
  1. Note: δ18Owater is calculated using equations of 1000lnαquartz–water = 3.38 × 106 T−2 − 3.40 (Clayton et al., 1972). T (°C) values are the averages of fluid inclusion homogenization temperatures for individual samples or mineralization stages.

 10.27.1−95450EarlyLuan et al. (1991)
 10.67.5−43450EarlyZhai et al. (2002)
F7-1911.38.2−95450EarlyJian (2010)
F7-1110.67.5−90450EarlyJian (2010)
F5-505-111.28.1−94450EarlyJian (2010)
YM540-B311.07.9−90450EarlyJian (2010)
YM540-B610.57.4−92450EarlyJian (2010)
s35-412.08.9−94450EarlyJian (2010)
470-1911.28.1−55450EarlyJian (2010)
470-f7-1810.67.5−83450EarlyJian (2010)
690-510.27.1−54450EarlyJian (2010)
540-b111.88.7−73450EarlyJian (2010)
d540-3-a11.48.3−91450EarlyJian (2010)
470-f7-2011.18.0−83450EarlyJian (2010)
505-B3811.78.6−87450EarlyJian (2010)
505-B1-211.98.8−81450EarlyJian (2010)
DH-B911.23.9−89290MiddleJian (2010)
435-B311.84.5−93290MiddleJian (2010)
435-B311.92.5−62253LateJian (2010)

The calculated δ18Owater values of the early-stage fluids vary from 7.1 to 8.9‰, with an average of 8.0‰. The δD values range from −43‰ to −95‰. Figure 7 shows that the data plot in the range of magmatic and/or metamorphic fluids. As mentioned above, the FIs in the early-stage veins are predominated by the C-type, which suggests the initial fluids are metamorphic in origin (Groves et al., 1998; Chen et al., 2007b).

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Figure 7. δ18O–δD plots of the ore fluids at the Dahu Au–Mo deposit (Base map from Taylor, 1997). This figure is available in colour online at wileyonlinelibrary.com/journal/gj

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The δ18Owater ratio of the late-stage fluids is 2.5‰, which, together with the paired δD value of −62‰, plots toward the meteoric water line in Figure 7. The δ18Owater values of the middle-stage fluids, however, range from 3.9 to 4.5‰ (Table 2), which are lower than those of the early-stage fluids, but higher than those of the late-stage fluids, suggesting a hybridization of the deep-sourced metamorphic fluids with the shallow-sourced meteoric water. The δD values of the middle-stage fluids range from −93‰ to −89‰, with an average of −91‰, lower than those of the early- and late-stage fluids (Table 2). This seems to be incongruous with the input of meteoric water, but agrees well with the majority of orogenic-type sulphide-bearing lode deposits, because sulphide deposition causes a δD-decrease of the fluids (for details see Chen et al., 2005b; Chen and Zhang, 2008).

The δ18O values of quartz at the Dahu deposit range from 10.2‰ to 12.0‰ (Table 2), which are consistent with those of metamorphic rock-hosted orogenic Au deposits (generally >10‰; Goldfarb et al., 2005). The calculated δ18Owater values (2.5–8.9‰), falling in the range of lode Au deposits in the Xiaoqinling orefield (δ18Owater = −8.6‰ to 9.6‰; Luan et al., 1985; Chen and Fu, 1992; Li et al., 1996; Lu et al., 2003; Zhao et al., 2011; Zhou et al., 2014). The δD values of −43‰ to −95‰ of the fluids (Table 2) are also within the δD range of most lode Au deposits (Fig. 7; McCuaig and Kerrich, 1998; Ridley and Diamond, 2000; Chen et al., 2012).

6 DISCUSSION

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 REGIONAL AND ORE GEOLOGY
  5. 3 SAMPLES AND METHODS
  6. 4 FLUID INCLUSION STUDIES
  7. 5 THE O–H ISOTOPE CONSTRAINTS
  8. 6 DISCUSSION
  9. 7 CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

6.1 Nature and evolution of the ore-forming fluids

Fluid inclusions are considered as potential indicators of the genesis of hydrothermal mineral systems (Chen et al., 2007b; Pirajno, 2009; Li et al., 2012a, b; Deng et al., 2013a, 2014; Zhou et al., 2014). Hydrothermal quartz at the Dahu Au–Mo deposit contains various types of inclusions, which can be used to probe into the nature of fluids and ore genesis.

Based on petrographic, microthermometric and laser Raman results presented above, we envisage a fluid system of CO2–H2O–NaCl. The initial fluids are featured by high temperature, CO2-rich and low to moderate salinity, as indicated by the presence of primary C-type inclusions but absence of S-type inclusions. In the middle stage, daughter mineral-bearing S-type inclusions can be observed, and coexist with the P-type and C-type inclusions with various CO2 contents. Considering the fact that all three types of FIs can be observed in a microscopic domain of a middle-stage quartz crystal, and they divergently homogenize at similar temperatures and show contrasting salinities, we conclude that the fluids have boiled in the middle stage. The fluid boiling might cause the bimodal distribution of fluid salinities, temperature cooling of the fluids, decrease of CO2 contents in the fluids, and consequently, rapid precipitation of abundant sulphides. In correspondence, the late stage fluids display low salinity and low temperatures, as indicated by the microthermometric characteristics of the secondary W-type inclusion trails cross-cutting the middle-stage crystals, and the primary W-type inclusions in the late-stage minerals.

It can be envisaged that fluid boiling resulted in wall-rock hydraulic fracturing that provided conduits for the inflow and circulation of the meteoric water (Chen and Fu, 1992; Chen et al., 2005b; Zhou et al., 2014). Meteoric water flowed into the ore-forming system and mixed with the existing metamorphic fluids, causing drastic changes in physico-chemical conditions of the fluid system, which should become meteoric-dominated in the late stage along with inflow of meteoric water. This interpretation is supported by the above mentioned H–O isotope signatures, in particular, the gradual decrease in δ18Owater values from early to late stages (Table 2, Fig. 7).

6.2 Pressure estimation and implication for ore genesis

The minimum trapping pressures of C-type FIs are estimated using the Flincor program (Brown, 1989) and reference data of Bowers and Helgeson (1983). The trapping pressures in the early and middle stages range from 138 to 331 MPa and 78 to 237 MPa, respectively (Table 1, Fig. 7). Furthermore, in the middle stage, the maximum of estimated pressure is approximately three times that of the minimum pressure, which corresponds to the lithostatic and hydraulic pressure, respectively. This kind of lithostatic–hydraulic alternating fluid systems can be interpreted using the fault–valve model (Sibson et al., 1988; Cox et al., 2001), and has been identified at many orogenic-type mineral systems, such as the Zhifang Mo deposit (Deng et al., 2014), Shanggong Au deposit (Fan et al., 1998), Lengshuibeigou Pb–Zn–Ag denposit (Qi et al., 2007), Yindonggou Ag deposit (Zhang and Chen, 2005) and Wenyu Au deposit (Zhou et al., 2014) in the Qinling Orogen; the Sanshandao (Fan et al., 2003) and Linglong (Zhang et al., 2007) gold deposits in the Jiaodong gold province; the Tiemurte Pb–Zn deposit (Zhang et al., 2012), Wulasigou (Zheng et al., 2012) and Qiaxia (Zheng et al., 2014) Cu deposits in Chinese Altay; and the Huogeqi (Zhong et al., 2012, 2013) and Bainaimiao W.B. Li et al., (2008) Cu deposits in Inner Mongolia. Assuming the density of the Taihua Supergroup is 3 g/cm3, the ore-forming depths are ~7.8 km in the middle stage and ~11 km in the early stage, respectively. This indicates that the crust of the Xiaoqinling orefield was uplifted by at least 3 km from early to middle stages, according well with the concept of Orogeny or Mountain-building.

The densities of CO2 in C-type FIs are calculated according to their partial homogenization temperature and manner (to liquid or vapour phases) and total homogenization temperature (Touret and Bottinga, 1979). The densities of CO2 in the early and middle stages range from 0.61 to 0.89 g/cm3 and 0.33 to 0.88 g/cm3, respectively (Table 1, Fig. 8). These results suggest that the fast uplift processes typically characterized the orogeny. Therefore, the Dahu Au–Mo deposit was considered as a mineral system formed under a syn-orogenic setting. Furthermore, the Dahu deposit was a typical fault-controlled lode deposit where the orebodies were generally located inside the ductile shear zones and fault. Moreover, the ore-forming fluids were carbonic fluids with low- to moderate-salinity and CO2-rich characteristics, similar to those of typical orogenic deposits (Groves et al., 1998; Goldfarb et al., 2001; Chen et al., 2007b; Pirajno, 2009; Phillips and Powell, 2010). The temperatures of the ore-forming fluids ranged from 220 °C to 500 °C, and the estimated mineralization depth ranged from 7.8 km to 11 km, which corresponded to the mesozonal–hypozonal class of the crustal continuum model (Groves et al., 1998; Chen, 2006). In general, the Dahu Au–Mo deposit should be a mesozonal–hypozonal orogenic-type mineral system.

image

Figure 8. Representative isochores for C-type fluid inclusions of early and middle stages. The isochores are calculated using the Flincor program (Brown, 1989) and the formula of Bowers and Helgeson (1983). This figure is available in colour online at wileyonlinelibrary.com/journal/gj

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6.3 Tectonic setting of hydrothermal mineralization

The Dahu Au–Mo deposit is located in the northernmost Xiaoqinling orefield, which belongs to the Qinling Orogen. The mineralization age of the Dahu Au–Mo deposit is constrained to 216–218 Ma by the Re–Os isochron age of molybdenite (N. Li et al., 2008) and the SHRIMP U–Th–Pb age of hydrothermal monazite (Li et al., 2011a). The Triassic tectonic setting of the Qinling Orogen has been debated for decades, with the view of syn- to post-collisional tectonism being mostly favoured by many geologists (see Chen et al., 2009; Li et al., 2014b and references therein). However, in the southern Qinling Orogen and its adjacent areas, the marine strata with ages of Devonian–Triassic are widely developed, indicating that the Qinling Palaeo-ocean (Mian-Lue Ocean and Shang-Dan Ocean) still existed in the Triassic, at least locally existed. The Jurassic strata are rarely reported and usually questionably and re-assessed to be Early Cretaceous (e.g. Mao et al., 2013), suggesting that the Jurassic Era might be a peak-uplift time. Since the beginning of the Cretaceous, the intermontane molasses can be widely and locally observed all over the Qinling Orogen, indicating a post-collision extensional setting (Chen and Fu, 1992; Zhang et al., 2001, 2002). Recently, Chen et al. (2009) proposed that the Triassic tectonic setting in Qinling area was analogous to the present Mediterranean Sea, contemporaneously accommodating oceanic plate subduction and inter-continental collision, as well as a gradual transition from subduction to collision. Li et al. (2014b) compiled geological, geochemical and geochronological data of Triassic granitoids from the Qinling Orogen, and concluded that continental collision between the Yangtze and the North China cratons occurred at about 200 Ma. The Pb–Sr–Nd isotopic studies of the Dahu deposit revealed that the ore-forming fluids must be sourced from a depleted, subducted oceanic slab (Ni et al., 2012). Another study on the Huanglongpu Mo deposit in East Qinling Mo mineralization belts also indicated the presence of B-type subduction in the Qinling at approximately 220 Ma (Xu et al., 2009).

Thus, combined with previous Pb–Sr–Nd studies (Ni et al., 2012), we inferred that the ore-forming fluids of the Dahu Au–Mo deposit formed through metamorphic devolatilization. The fluids migrated upward via shear zones into hydraulic fracture zones in rocks of low tensile strength. The small volumes of the shear zones focused the fluids to form the Dahu Au–Mo deposit (Fig. 9).

image

Figure 9. Triassic tectonic–magmatic–metallogenic schematic model in the Qinling Orogen (after Ni et al., 2012). This figure is available in colour online at wileyonlinelibrary.com/journal/gj

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7 CONCLUSIONS

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 REGIONAL AND ORE GEOLOGY
  5. 3 SAMPLES AND METHODS
  6. 4 FLUID INCLUSION STUDIES
  7. 5 THE O–H ISOTOPE CONSTRAINTS
  8. 6 DISCUSSION
  9. 7 CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

The Dahu Au–Mo deposit is one of five large Au deposits in the Xiaoqinling gold field which is the second largest orogenic Au province in China following the Jiaodong gold province. It is a typical fault-controlled lode system, but also is unique for its Au–Mo association in both Xiaoqinling and the world.

The deposit was formed by a three-stage hydrothermal process. The early, middle and late stages are represented by the formation of pyrite–quartz veins that are structurally deformed and broken, the molybdenite–pyrite–quartz stockworks infilling fissures of the early-stage veins and minerals, and the open-space filling quartz–carbonate veinlets.

Four types of fluid inclusions, i.e. CO2–H2O, H2O–NaCl, pure CO2 and daughter mineral-bearing, have been observed in the Dahu mineral system, but the early- and late-stage quartz only contains the CO2–H2O and H2O–NaCl inclusions, respectively. This indicates that the fluids evolved from CO2-rich to CO2-poor, and that the fluids boiled in the middle stage, which is supported by boiling fluid inclusion assemblages.

Estimation of the trapping pressures of fluid inclusions show that the fluid system alternately fluctuated from lithostatic to hydrostatic, and was controlled by a fault–valve mechanism. The mineralization occurred at a depth of >11 km in the early stage and at ~7.8 km in the middle stage, which is deduced from the trapping pressures of 138–331 and 78–237 MPa, respectively.

Microthermometric data of fluid inclusions and H–O isotope geochemical signatures consistently show that the fluids originated from metamorphic devolatilization in the early stage, then mixed with meteoric water in the middle stage, and finally replaced by meteoric water in the late stage.

The precipitation of metals at the Dahu deposit was mainly caused by fluid boiling, followed by fluid mixing, along with a trans-compression at the transitional zone from a magmatic arc to back-arc basin, which resulted from the Triassic northward subduction of the Mian-Lue oceanic slab.

ACKNOWLEDGEMENTS

  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 REGIONAL AND ORE GEOLOGY
  5. 3 SAMPLES AND METHODS
  6. 4 FLUID INCLUSION STUDIES
  7. 5 THE O–H ISOTOPE CONSTRAINTS
  8. 6 DISCUSSION
  9. 7 CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

This work was supported by National Natural Science Foundation of China (Grant Nos. 40730421 and 41003019), the Excellent Ph.D Thesis Program of Beijing (No. 20131000104) and Science Foundation of China University of Petroleum, Beijing (No. YJRC-2013-01). The authors are also grateful to the Henan Institute of Geology Survey and the Henan Institute of Nonferrous Metal Exploration for assistance during the field trip. Comments and suggestions from two anonymous reviewers greatly improved the quality of the paper.

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  1. Top of page
  2. Abstract
  3. 1 INTRODUCTION
  4. 2 REGIONAL AND ORE GEOLOGY
  5. 3 SAMPLES AND METHODS
  6. 4 FLUID INCLUSION STUDIES
  7. 5 THE O–H ISOTOPE CONSTRAINTS
  8. 6 DISCUSSION
  9. 7 CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES
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