Dating Magmatic Hornblende and Biotite and Hydrothermal Sericite by Laser Probe Technique: Constraints on Genesis of Wangershan Gold Deposit, Eastern Shandong Province, China

INTRODUCTION

Eastern Shandong Province represents the largest gold concentration region in China, with annual production > 55-60 t gold and present reserve of more than 900 t gold, accounting for a quarter of the total gold production in the country. A majority of the gold deposits is hosted in the Jurassic-Cretaceous granitoid intrusions. Timing of gold mineralization in eastern Shandong has been a major focus of many previous studies. K-Ar, Rb-Sr and conventional U-Pb isotopic dating in the last two decades have yielded a wide range of mineralization ages, ranging from over 180 Ma to 40 Ma (Zhang et al., 1994; Li and Yang, 1993; Luo and Wu, 1987). Scatter of age data has led to fairly ambiguous interpretations with respect to the ore genesis. New SHRIMP zircon U-Pb data from major granitoids, which are the hosts of most important gold deposits in eastern Shandong, have placed indirect, but reasonable, constraints on the timing of gold emplacement at 130-120 Ma (Zhang 2002; Wang et al., 1998). However, whether gold mineralization and magmatism were genetically associated remains undetermined, largely due to lack of systematic dating on various hydrothermal and intrusive minerals that have distinct closure temperatures.

The Jiaojia-Xincheng gold camp is one of the most important active mining centers in this region, containing several large to medium gold deposits including Jiaojia, Xincheng, Wangershan, Hedong and Jiehe (Fig. 1). In this study, we present high-quality ⁴⁰Ar/³⁹Ar age data for the Au-bearing, hydrothermal sericite from the Wangershan deposit, and for magmatic hornblende and biotite collected from the Shangzhuang granites spatially adjacent to the Wangershan deposit. These data provide useful constraints on ore genesis of regional significance.

Figure  1.  Simplified geological map of the Jiaojia-Xincheng gold camp showing the occurrence of the Wangershan deposit and the sites of samples used for this study (modified from No.6 Geological Team of Shandong, 1983. unpublished). JXF. Jiaojia-Xincheng fault; WHF. Wangershan-Hedong fault.

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GEOLOGICAL SETTING AND MINERALIZATION

Eastern Shandong Province is situated in the southeastern margin of the Precambrian North China craton (NCC). There are four main lithologic sequences (SBGMR, 1991) : (1) Archean metamorphic complex consisting primarily of granulite, gneiss, amphibolite and biotite schist, with a thickness more than 5 000 m, (2) Early-Middle Proterozoic metamorphic and sedimentary rocks, (3) Mesozoic granitoid intrusions that are traditionally classified into the Linglong, the Luanjiahe, and the Guojialing types on the basis of textures and compositions, and (4) Jurassic to Cretaceous continental sedimentary rocks and Early Cretaceous volcanic sequences within the Jiaolai basin. Since Middle Jurassic, tectonic evolution of eastern Shandong has been dominated by the westward oblique subduction of the Inazagi-Pacific plate underneath the Eurasian plate (Fitches et al., 1991; Kimura et al., 1990; Xu et al., 1987). This oblique subduction has led to the formation of the more than 5 000 km long Tan-Lu strike slip fault extending from south China northeastward to the Russia Far East (Li et al., 2001; Wang and Mo, 1995), and numerous granitoid intrusions in east China. The Tan-Lu fault was characterized by transpression and transtension in Jurassic and Cretaceous, respectively. Widespread NE-NNE-trending faults in eastern Shandong were interpreted as secondary structures of the Tan-Lu fault (Xu et al., 1987).

The Wangershan deposit (E120°07′52″, N37°24'04″) contains more than 60 t gold at an average grade of 9-10 g·t^-1. Mineralization is hosted in the Moshan granodiorite, and structurally controlled by theNNE-striking Wangershan-Hedong fault (Fig. 1). The Moshan granodiorite is intruded by the Shangzhuang granite. Rock-forming minerals of the granite are primarily K-feldspar (19 %), plagioclase (48 %), quartz (22 %), biotite (9 %) and hornblende (2 %), with accessory minerals including titanite, magnetite, zircon, apatite and trace amount of ilmenite (Wang et al., 1998). Gold ores are closely associated with intense hydrothermal alteration (Fig. 1). Primary alteration minerals include sericite, muscovite, quartz and pyrite (Fig. 2a). Auriferous alteration zones are generally 1-2 km long, several to a few hundred meters wide, and continuous for 400-1 200 m down dip. Pyrite is the most abundant metallic mineral, typically occurring as disseminated aggregates and thin stringers. Minor sulfide minerals are chalcopyrite, galena and sphalerite. Native gold and electrum occur as inclusions within pyrite and other sulfides, as irregular infillings in microfractures of pyrite, or as individual grains along cleavage planes of sericite and muscovite (Fig. 2b). Stable isotopes and fluid inclusion data on Au-bearing quartz indicate that gold precipitated primarily from low to intermediate salinity (5 wt%-13 wt% NaCl equivalent), and ¹⁸O and CO₂-rich liquids at ~250-350 ℃ (Yang, 1998; Li and Yang, 1993).

Figure  2.  Photomicrographs showing the characteristic assemblages of hydrothermal alteration (a) and the intimate textural relationship between the hydrothermal sericite and gold (b).

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SAMPLING AND METHODOLOGY

Alteration sample (Wangershan07) was collected from underground workings. Samples of magmatic rock (SZ1 and SZ2) were from the Shangzhuang granite (Fig. 1). Sampling sites of these magmatic samples are 1-2 km away from the deposit and alteration zone, ensuring freshness of, and absence of hydrothermal alteration on, the magmatic assemblages. After petrographic examination, samples were crushed, washed in distilled water in an ultrasonic bath for one hour, and dried. Pristine igneous biotite and hornblende grains (1.2-2.6 mm) and hydrothermal sericite aggregates (0.2-1.0 mm, and > 99 % purity) were screened under a binocular microscope. Ten to twenty pure grains from each sample were loaded into irradiation disks along with Fish Canyon (nominal age of 28.02 Ma) standard (Renne et al., 1998). The disks were wrapped in Al-foil, vacuum-sealed in silica glass tubes, and irradiated for 14 hours at the B-1 CLICIT facility at the Radiation Center, Oregon State University, USA. Sample and flux monitor irradiation geometry followed those of Vasconcelos (1999). After a two-month cooling period, 2-3 grains of each sample were analyzed by the laser incremental-heating ⁴⁰Ar/³⁹Ar method at the UQ-AGES (the University of Queensland-Argon Geochronology in Earth Sciences) laboratory. The beam from a continuous wave argon ion laser was focused through an optical system onto each group of the grains. Argon was extracted at consecutively higher laser powers for a time period of 54-61 s at each designed laser level. The gases released during step-heating were purified and then admitted to the adjacent MAP 215 mass spectrometer for argon isotopic analysis. The program used for age calculation was written by Alan Denio.

The criteria used for the identification of plateaus, or undisturbed portions of the ⁴⁰Ar/³⁹Ar age spectra, is the existence of at least three contiguous steps with concordant ages within 2σ error and containing a significant proportion of the ³⁹Ar released (generally more than 80 % of the total ³⁹Ar released).

RESULTS

Results of the incremental heating analyses on magmatic and hydrothermal minerals are tabulated in Table 1 and illustrated as apparent age spectra in Figs. 3-5. All dates were quoted at the 95 % confidence level (2σ). Apparent ages of individual analyses from multiple grains of the same sample were also plotted as ideograms (Fig. 3d, Fig. 4c, Fig. 5d). The ideogram is an age-probability diagram, which is based on the assumption that errors in a date have a Gaussian distribution (Deino and Potts, 1990). If the most probable peaks correspond to well-defined plateau ages, the ideogram method reliably identifies the most probable crystallization/cooling age for the sample.

Table  1.  Analytical results of argon isotopes by ⁴⁰Ar/³⁹ Ar laser incremental heating technique

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Figure  3.  ⁴⁰Ar/³⁹Ar age spectra of magmatic hornblende (a-c) and the ideogram plot of all apparent ages (d). The excellent reproducibility of plateau ages among distinct grains from the same sample demonstrates the precision and reliability of the analyses.

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Figure  4.  Well-defined and internal consistent plateau ages of biotite grains (a-b) and the ideogram illustrating the most probable peak age for this mineral (c).

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Figure  5.  Three separates of hydrothermal sericite extracted from gold ore have yielded reproducible plateau ages of (120.6±0.3) Ma to (120.0 ± 0.4) Ma (2σ: a-c), as confirmed by the ideogram age of 120.2 Ma (d).

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All minerals yielded well-defined and high precision ⁴⁰Ar/³⁹ Arplateau ages.Three hornblende grainsfrom the sample SZ1 gave plateau ages between (128.1±0.8) Ma and (127.5±0.6) Ma (Fig. 3); whereas two biotite separatesof the sample SZ2 yieldedidentical age at (124.4±0.3) Ma and (124.1±0.3) Ma (Fig. 4).Hydrothermal sericite yielded reproducible ages ranging from (120.6±0.3) Ma to (120.0±0.4) Ma (Fig. 5).Ideograms illustrated inFigs.3-5 reveal that the hornblende, biotite and sericite have peak ages at 128.1, 124.0 and 120.2 Ma, respectively, consistent with their corresponding plateau ages.

Most analyses of hornblende and biotite show relatively low apparent ages with large uncertainties and lower percentage of radiogenic ⁴⁰Ar in the initial steps (Figs. 3 and 4), suggestive of partial argon loss from, and atmospheric argon incorporated into, the minerals dated.Argon loss from the host minerals is supported by the integrated ages 1-3 Ma younger than the corresponding plateau ages (Figs. 3 and 4).Spectrum of sample Wangershan07-01 is characterized by a significant older apparent age in the first step than the plateau age obtained by subsequent steps (Fig. 5a).This spectrum may reflect the occurrence of minor ³⁹Ar loss by re-coil during irradiation in the reactor and/or brake-out procedure preceding the isotopic analysis (Vasconcelos, 1999; McDougall and Harrison, 1988), and the recoiled ³⁹Ar has been completed lost to the surrounding atmosphere (Vasconcelos, 1999).

The excellent reproducibility of plateau ages among two or three grains from each sample substantiates the reliability and high quality of the present chronological study (Figs. 3-5). The shapes of the spectra of sericite indicate that excess argon and mixture of multiple phases do not occur in our samples. Characteristics of the apparent spectra also suggest that this hydrothermal mineral neither contains any older contaminant minerals (e.g., magmatic plagioclase), nor has been thermally disturbed during their history since precipitation.

DISCUSSION

The close spatial relation between gold deposits and granitoid intrusions in eastern Shandong Province has been widely recognized. However, whether or not mineralization and magmatism were genetically related has been a matter of controversy, largely due to scarcity of systematic, reliable dating on these two geologic processes. Previous isotopic dating, by K-Ar, Rb-Sr and conventional U-Pb techniques, have yielded a wide spectrum of age data for the granite ranging from > 2 000 Ma to 130 Ma (SBGMR, 1991; Xu et al., 1989; Hu et al., 1987). These results are questionable and cannot be valid estimates for magmatism because of the bulk nature of the methods used and the ubiquitous inherited zircons in these intrusions, as identified by recent work (Zhang, 2002; Wang et al., 1998).

Recent application of modern techniques of isotopic dating has significantly improved our understanding in various geologic events of this region. Wang et al. (1998) initiated SHRIMP U-Pb zircon dating on eastern Shandong granitoids, producing two age groups for their formation from 165-150 Ma (Linglong and Luanjiahe types) and from 130-126 Ma (Guojialing type), respectively. Age data of the Guojialing type, which is the host of a number of major gold deposits, together with the SHRIMP zircon age of a post-mineralization dike (120 Ma), were also used to indirectly, but reliably, constrain the timing of gold emplacement at 126-120 Ma (Zhang, 2002; Wang et al., 1998). These data have been confirmed by similar work subsequently performed by Guan et al. (1998) and Zhang (2002). Our ⁴⁰Ar/³⁹Ar dating on hornblende and biotite from the Shangzhuang granite (Guojialing type) yielded plateau ages at 128.4 and 124 Ma, respectively, consistent with previous SHRIMP U-Pb data (Guan et al., 1998). The argon closure temperatures for hornblende and biotite are ~500 ℃ and 300-350 ℃, respectively (McDougall and Harrison, 1988). These allow an estimate for the cooling rate of the intrusion at about 50 ℃/Ma.

Sericite aggregates investigated in this study yielded well-developed plateau age and ideogram age at ca. 120.3 Ma. The sericite separates selected for this study were unequivocally hydrothermal and grew during emplacement of the mineralizing fluids (Fig. 2a), and intimately intergrown with gold grains (Fig. 2b). Therefore, their ⁴⁰Ar/³⁹Ar age by laser incremental heating analysis can give direct constraints for bracketing the timing of hydrothermal alteration and related mineralization. Two types of fluid inclusions in alteration quartz have been recognized, i.e., two-phase aqueous inclusions with vapor bubbles varying from 5 % to 60 % of the inclusion volume, and three-phase H₂O-CO₂-vapor inclusions containing up to 5-15 per cent liquid CO₂ (Yang, 1998; Li and Yang, 1993). The temperatures of hydrothermal fluids responsible for the formation of gold ores are inferred to range from 143 ℃ to 334 ℃, with a concentration between 220-325 ℃ (Yang, 1998), by homogenization temperatures of primary inclusions. These values are slightly lower than or approximate to the closure temperature for Ar diffusion in sericite (300-350 ℃) at moderate cooling rates (McDougall and Harrison, 1988). We therefore interpret the ⁴⁰Ar/³⁹Ar cooling ages to date hydrothermal alteration and related gold mineralization in the Wangershan deposit.

The mineralization age of the Wangershan deposit obtained in our study is in excellent consistence with those of other major gold deposits in eastern Shandong. Yang and Zhou (2001) obtained the Rb-Sr age (121.6 Ma) for the formation of the Linglong gold field by direct dating auriferous pyrite. Using the same method as in this study, Zhang et al. (2003) analyzed the ⁴⁰Ar/³⁹Ar ages for auriferous sericite from the Cangshang gold deposit at (121.3±0.2) Ma. The Pengjiakuang gold deposit, a new type of mineralization recently identified in the northern margin of the Jiaolai basin in eastern Shandong Province, has also been dated by the ⁴⁰Ar/³⁹Ar and Rb-Sr methods at 128-117 Ma (Zhang et al., 2003). These suggest that pervasive gold mineralization in eastern Shandong occurred in the middle of Early Cretaceous.

Temporal relation between the cooling ages of magmatic hornblende and biotite indicates that the Shangzhuang granite cooled down very slowly. Slow cooling of the rocks may reflect existence of hot lithosphere underneath eastern Shandong in the Early Cretaceous (Zhai et al., 2002). The temporal gap of ca.4 million years between hydrothermal sericite and magmatic biotite, and the closely spatial association between the Wangershan deposit and the Shangzhuang granite (Fig. 1), suggest that gold mineralization could be genetically linked to the Early Cretaceous magmatism.

There are abundant intermediate to mafic dikes (mostly diorite and lamprophyres) occurring in most gold districts in this region. SHRIMP U-Pb zircon dating demonstrates that these dikes were formed predominantly at 121-119 Ma (Zhang, 2002).⁴⁰Ar/³⁹Ar dating of phlogopite extracted from representative mafic dikes also gave the same age of 120.3-119.2 Ma (Qiu et al., 2001). These indicate that gold mineralization for the Wangershan gold deposit, as well as for most deposits in eastern Shandong, was coeval to the emplacement of the widespread intermediate to mafic dike swarm. Coupling of pervasive gold mineralization with formation of the dike swarm may reflect significant transition of tectonic regime from transpression to transtension, presumably related to continental lithosphere thinning and resulting crustal extension of the Early Cretaceous in eastern China (Mao et al., 2002; Zhai et al., 2002).

CONCLUSION

Well-defined and extremely reproducible plateau ages of magmatic and hydrothermal minerals confirm that laser incremental ⁴⁰Ar/³⁹Ar dating method is a robust and promising approach in resolving various thermal events in the geological past of a region. Hornblende and biotite yielded internally consistent plateau ages at 128.1-127.5 and 124.4-124.1 Ma, respectively, while Au-related sericite bracketed the mineralization ages at 120.6-120.0 Ma. Temporal association between the magmatic and hydrothermal minerals suggests that genesis of gold mineralization could be tentatively related to the magmatism of Early Cretaceous. The mineralization age is in good agreement with timing of other major deposits constrained by previous studies, indicating that pervasive gold emplacement in eastern Shandong Province took place contemporaneously. The coincidence of formation ages between the major gold deposits and intermediate-mafic dikes indicate that they are genetically linked under the same tectonic setting, i.e., Early Cretaceous lithosphere thinning and related extension in eastern China.