Pyrite is one of the most common gold-bearing sulfides in a series of hydrothermal gold deposits, such as orogenic-type, Carlin-type, and epithermal-type gold deposits [1
]. The typomorphic characteristics of pyrite, e.g., crystal habit, thermoelectricity, and chemical composition are generally controlled by geochemical conditions during ore-forming processes. Thus, typomorphic characteristics of pyrite can not only be used in studying the genesis of gold-ore deposits [2
] but also for gold-ore exploration [6
], especially for deep ore predictions [8
The Shuangwang gold deposit is a large orogenic gold deposit (>80 t Au) in the Western Qinling Orogen (WQO). It occurs in the Devonian strata and is characterized by breccia-type mineralization [10
]. Orebodies KT8 and KT9 are the two major orebodies that account for >75% of the total gold reserve [11
] of the Shuangwang gold deposit. Most of the shallow reserve has been extracted after more than 35 years of operation. Thus, it is urgent to find more gold at depth for the development of the Shuangwang gold mine. The present study focuses on the typomorphic characteristics of gold-bearing pyrite and its application to deep ore targeting in the Shuangwang gold deposit.
2. Geologic Setting
The WQO, sandwiched between the North China Block (NCB) to the north and South China Block (SCB) to the south (Figure 1
), is part of the Central Orogenic Belt of China [13
]. Due to the N–S directional orogenic processes, regional tectonic lines extend in the E–W direction, represented by the Shang-Dan and Mian-Lue sutures and many folds and faults of different levels. The WQO, as a small part of the broader Paleotethys Ocean, records 300 m.y. of evolution of the Shang-Dan and Mian-Lue Oceans [14
]. The strata of the WQO is dominated by a strongly deformed thick flysch sequence [14
], which is dominated by Devonian clastic rocks and carbonate rocks with a small amount of Carboniferous and Permian metasedimentary rocks. These rocks were deposited in the Fengtai basin in the Early Palaeozoic and metamorphosed into medium-grade (mainly greenschist) facies in the subsequent orogenic process [17
]. Many granitic intrusions are distributed along the sutures and regional faults, forming an approximately EW-trending magmatic belt. Most of these granitoids were emplaced in the Late Triassic [18
], representing the products of final collision.
There are many gold deposits in the WQO. These gold deposits are the products of Paleozoic orogenic processes between the NCB and the SCB, and have been divided into different genetic types: orogenic, Carlin, and Carlin-like [22
]. The Carlin (e.g., Jinlongshan, Dashui) and Carlin-like (e.g., Zhaishang, Liba, Yangshan) gold deposits mainly occur in the middle and southern belts of the WQO. They are hosted by weakly metamorphosed Triassic clastic and carbonate rocks [22
]. The Carlin-like type differs from the Carlin gold deposit in that they may genetically be related to the synchronous magmatism [24
]. The orogenic gold deposits (e.g., Shuangwang, Maanqiao, and Baguamiao), occurring between the two regional sutures, are closely related to WNW-trending shear zones developed in the Paleozoic metasedimentary rocks [21
]. These orogenic gold deposits, which were thought to be the result of the early subduction of the Mian-Lue oceanic crust [24
], show similar geological geochemical features. Among them, the Shuangwang deposit is characterized by its breccia-type gold ores, and is thus distinct from the other gold deposits in this area.
3. Ore-Deposit Geology
The Shuangwang gold deposit occurred in an NW-extending hydrothermal breccia belt in the Devonian strata of the Fengtai basin, which was described as a forearc basin by previous work [25
]. The strata exposed in the Shuangwang gold deposit mainly consist of weakly metamorphosed Devonian clastic rocks and carbonate rocks (Figure 2
). The total thickness of the strata is more than 5000 m. The Devonian strata, from old to young, were divided into the Wangjialeng Formation (Lower Devonian), the Gudaoling Formation (Middle Devonian), and the Xinghongpu and Jiuliping Formations (Upper Devonian) [11
]. The Wangjialeng Formation is dominated by crystalline limestone and interlaid metamorphosed siltstone and sandy slate. The Gudaoling Formation includes metamorphosed siltstone, crystalline limestone, and biolimestone interlayered with sandstone. The Xinghongpu Formation, which is the ore-hosting stratum, is composed of metamorphosed sandstone and slate (Figure 3
a,b). The Jiuliping Formation mainly consists of metasiltstone and slate.
Magmatic activity was characterized by Indosinian granitic intrusion in the Shuangwang gold district. The largest intrusion is called the Xiba pluton with an area of about 50 km2
]. The Xiba pluton extends in an NWW direction, and is about 1–3 km south to the gold breccia belt. This pluton intruded into the Devonian stratum along the axis of the Xiba fold. The pluton is mainly composed of quartz monzodiorite (Figure 3
c) and minor amounts of granodiorite with zircon U–Pb ages of ca. 218–215 Ma [26
]. Furthermore, some small Yanshanian granitic porphyries developed in the western part of the Shuangwang gold-bearing breccia belt.
The Shuangwang gold-bearing breccia belt is hosted in the marine sedimentary strata of the Xinghongpu formation. The gold-bearing breccia belt, with a length about 11.5 km and width about several meters to several hundred meters, extends in an NW direction (290–310°) between the Wangjialeng village and Wangjiazhuang village. The gold-bearing breccia belt is discontinuous on the surface and is named Breccia I–V. Some smaller breccias, such as the Miaogounao and Xiaomiaogou breccias, occur 1 km further to the north. Along the major breccia belt, a series of orebodies occur, which are named KT8, KT9, KT7, KT5, KT6, and KT2. Among them, orebodies KT8 and KT9 are by far the most important ones, as they account for more than 75% of the reserve of the Shuangwang gold deposit.
Orebody KT8 is the largest orebody (between prospecting lines 22 and 46), and dips to the NNE at about 75°. KT8 is about 650 m long with an average thickness of 30 m, extending more than 400 m at depth (Figure 4
). The gold grade of orebody KT8 is higher than that of KT9, averaging 3.08 g/t. Orebody KT9 is located between prospecting lines 3 and 20. The occurrence of the KT9 is similar to that of the KT8, and it also dips to NNE at a steep angle. It extends for more than 600 m with thickness of 18 m. The average gold grade of orebody KT9 is 1.98 g/t. Gold mineralization is better in the upper part than in the lower part of orebodies KT8 and KT9.
Gold ores are characterized by breccias in the Shuangwang gold deposit. Previous studies showed that gold-bearing breccias are the result of hydraulic fracturing induced by overpressured ore-forming fluid [10
]. Breccia ores contain country rock fragments dominated by albitized slate and siltstone (Figure 3
d–g). These fragments vary in diameter, from several millimeters to several meters, and have angular, platy, or irregular shapes, showing no obvious grinding and sorting. Some fragments can be pieced together, indicating very small movement of these fragments (Figure 3
f). The size of the fragments is negatively correlated to the gold grade, i.e., the larger the fragments, the lower the gold grade [11
]. The mineral composition of the cements includes ankerite, quartz, albite, calcite, and some pyrite [12
]. Pyrite is the chief gold-hosting mineral, and gold occurs in native form mainly in interstices between pyrite crystals or in microfractures within pyrite (Figure 3
Four ore-forming stages have been summarized by previous research [10
]. Stage I, replacement stage before brecciation with an ankerite–quartz–albite assemblage; Stage II is represented by an assemblage of quartz, albite, pyrite, and ankerite; Stage III has a pyrite–calcite–quartz assemblage; Stage IV exhibits an assemblage of fluorite, dickite, and gypsum with no gold deposition. Stage II is the major gold deposition stage in the Shuangwang gold deposit.
4. Sampling and Analytical Methods
Fifty-eight ore samples from the major ore-forming-stage ores were collected from most accessible drifts at levels 1100, 1150, 1200, 1250, and 1330 of orebodies KT8 and KT9 (1100 is the lowest operating level).
Samples were firstly crushed into 40–60 mesh grains. After panning and filtration, pyrite crystals with >99% purity were handpicked under microscope. The morphologic study of pyrite crystals was carried out under a binocular microscope at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing, China) (CUGB).
Thermoelectricity of the pyrite samples was tested by a BHTE-06 thermoelectric coefficient measuring instrument at the Genetic Mineralogy Laboratory of CUGB. The temperature of the cold end and the hot end was set to 20 and 80 °C, respectively. Fifty pyrite grains were randomly selected from each sample for measurements.
Trace elements of pyrites were tested by high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) Element I in the analytical laboratory center of the Beijing Research Institute of Uranium Geology (BRIUG) following the national standard of DZ/T0223-2001 [28
]. About 50 mg of powered pyrite was dissolved in high-pressure Teflon bombs using an HF + HNO3
mixture. The signal collection of HR-ICP-MS lasted 25 s, and the scanned mass ranges were set at 6–235 amu to cover target elements. The experimental processes were conducted under a temperature of 20 °C and relative humidity of 30%. The uncertainties of the ICP-MS analyses were estimated to be better than ±5%. According to previous research [29
], indicator elements such as Ba, Sb (supraore halo elements), Pb, Zn, Cu (near-ore halo elements), and Co, Mo, Bi (subore halo elements) were chosen to analyze their special distribution for deep ore prediction in the Shuangwang gold deposit.
7. Deep Ore Prediction
According to the comprehensive analysis of the contour maps of morphology index, P-type frequency, and indicator elements, some target areas that show at least two kinds indicating information about the morphology, thermoelectricity, and chemical composition of pyrite were delineated for future gold exploration at depth (Figure 11
Target ①: The area between prospecting lines 0 and 1 showed weak anomaly of morphology index. Supraore element Sb and near-ore elements Zn and Cu had clear anomalies. There were no P-type frequency anomalies in this area.
Target ②: Anomalies of P-type frequency and an intense Zn anomaly between prospecting lines 14 and 18 indicated possible mineralization at depth, but this target showed no morphology index anomalies.
Target ③: The area between prospecting lines 30 and 34 where anomalies of morphology index and P-type frequency showed clear deep extension. The high contents of supraore halo elements Ba and Sb, together with subore halo element anomalies, pointed to the possibility of downward extension of the orebody, or of a concealed orebody.
Target ④: The area between prospecting lines 44 and 46 showed a deep extension morphology index anomaly, together with weak anomalies of Ba, Bi, and Co. There were no P-type frequency anomalies in this area.
The four target areas above are substantiated with deep extension of proven orebodies (Figure 11
). There occurred chemical anomalies in all the four targets, whereas morphology anomalies were absent in Target ②, and thermoelectricity anomalies were missing in Targets ① and ④. Among these four targets, the third is the most prospective area with strong information from morphology index, P-type frequency, and halo elements. More attention should be paid to these four target areas in future gold exploration at depth, and especially the third target.
8. Concluding Remarks
Pyrite typomorphic features of different gold deposits have been studied in detail since the 1970s, especially in China and the former Soviet Union, not only for the genesis of gold deposits but also for gold exploration. The present pyrite typomorphic research delineated four prospective targets at the depth of orebodies KT8 and KT9 of the Shuangwang gold deposit. These targets are in agreement with the deep extension of known gold mineralization, indicating that pyrite typomorphic study is an effective way to deep gold predict orogenic gold deposits or other gold deposits of hydrothermal origin.
Indications of the crystal habit, thermoelectricity, and chemical composition of pyrite for deep gold prediction are different. The role of chemical composition is direct and obvious. Information for deep ore prediction from the crystal habit and thermoelectricity of pyrite is indirect and sometimes ambiguous, because these two parameters are affected by many factors. Considering the easy and cheap data-acquired method for crystal habit and thermoelectricity, a comprehensive study on the pyrite typomorphism of morphology, thermoelectricity, and chemical composition is suggested in order to acquire as much information as possible.