Characteristics and sources of ore-forming fluids of South Narimalahei copper polymetallic deposit in East Kunlun, Qinghai

SUN Qu, WANG Li, ZHANG Yongsheng, FAN Xingzhu, ZHANG Guofeng,SHENG Jianhua, CHEN Xiaohang and LIU Xiang

1. College of Earth Sciences, Jilin University, Changchun 130061, China;2. National Gemological Training Center, Beijing 102628, China;3. Bureau of Geologic Exploration and Mineral Development of Jilin Province, Changchun 130061, China;4. Yantai Municipal Bureau of Natural Resources and Planning, Yantai 264003, Shandong, China;5. Geological Survey of Hebei Province, Shijiazhuang 050000, China

Abstract: The South Narimalahei area is located on the north side of the Middle Kunlun fault in the eastern section of the East Kunlun composite orogenic belt. The ore body is veined and controlled by structures and se-condary fissures, which occurs in the structural alteration fracture zone in the Late Triassic granodiorite. In this deposit, copper mineralization is closely related to silicification and sericification. The formation process of the deposit includes hydrothermal mineralization and supergene oxidation. In this paper, the fluid inclusion minera-logy, microscopic temperature measurement and stable isotope studies have been carried out for ore of the main mineralization stage. The results show that the primary gas-liquid two-phase inclusions and a small amount of single-liquid inclusions are mainly developed in the quartz in the main mineralization stage. The results of microscopic temperature measurement show that the ore-forming fluid which has low temperature (151.7℃--205.8℃), low salinity(2.06wt%--4.94wt%NaCl), low density (0.86--0.92 g/cm3) and shallow formation (1.5--3.0 km) is a hydrothermal solution of NaCl-H2O system. Hydrogen and oxygen isotope results show that the ore-forming fluids mainly come from atmospheric precipitation, with a small amount of magmatic fluids participating. It is preliminarily determined that the South Narimalahei copper polymetallic deposit is a low-temperature hydrothermal vein deposit.

Keywords: copper polymetallic deposit; fluid inclusions; South Narimalahei; East Kunlun

0 Introduction

The East Kunlun orogenic belt is divided into three EW-trending tectonic units of the Northern East Kunlun Caledonian back-arc basin belt, the Middle East Kunlun basement uplift granitic belt, and the Southern East Kunlun composite accretion belt by the North Kunlun fault and the Middle Kunlun fault (Sunetal., 2009) (Fig.1). The abundant mineral resources are hosted in the East Kunlun orogenic belt. Most of the industrial ore-bearing rock bodies with the prospecting potential discovered in the orogenic belt are mostly distributed in the Middle Kunlun belt, where favorable geologic conditions for mineralization exist. The South Narimalahei copper polymetallic deposit is located in the Qimantage-Dulan Fe-Cu-Pb-Zn-W-Sn-Bi-Au-Mo metallogenic subzone on the north side of the Middle Kunlun fault in the eastern section of the East Kunlun orogenic belt (Zhangetal., 2019), where Hariza, Narimalahei and Jiadanggen copper-silver polymetallic deposits have been disco-vered (Li, 2019; Zhangetal., 2019). Since 2016, the Third Geological Survey Institute of Qinghai Pro-vince has discovered a geochemical anomaly in the 1∶25000 geochemical survey in Lumuqixi area of Dulan County, and discovered the South Narimalahei copper polymetallic deposit after further exploration. More-over, 13 mineralization belts and 15 ore bodies were subsequently delineated (Zhangetal., 2019). No in-depth study has been undertaken on the geological characteristics and genesis of the Narimalahei copper polymetallic deposit but preliminary study was performed by Liu and Li (2009), and it was suggested that the deposit is Late Triassic continental volcanic rocks related medium and low temperature hydrothermal deposits controlled by X-shaped wrench faults. The copper Polymetallic deposit located in the southern part of Narimalahei has not been studied. This paper studies the geological and geochemical characteristics of the South Narimalahei copper polymetallic deposit, focusing on intrinsic interaction between mineralization types, rock masses, wall rock alteration, the source and properties of ore-forming fluid and physical conditions of mineralization. The research compiles abundant basic geological data of the South Narimalahei area, analyses the characteristics and source of ore-forming fluids and the physical and chemical conditions of mineralization and provides theoretical basis for determining the genetic type of the ore deposit.

1 Geological characteristics of mining area

The South Narimalahei copper polymetallic deposit is located on the north side of the Middle Kunlun fault in the eastern section of the East Kunlun compo-site orogenic belt (Fig.1). The outcropped strata in the mining area consist of the Upper Triassic Elashan Formation (Fig.2), Neogene Youshashan Formation and Quaternary strata. The Upper Triassic Elashan Formation is mainly distributed in the western part of the mining area, and it is a set of continental volcanic rocks, which are distributed in a belt. The lithology is dominated by the gray-green crystalline tuff lava. The Neogene Youshashan Formation is mainly distributed in the southern part of the mining area, and consists mainly of gray, light yellow conglomerate intercalated with sandstone, mudstone intercalated with sandy limestone, etc. The Quaternary strata are mainly distributed in the southwest part of the mining area.

Fig.1 Geotectonic map of South Narimalahei area (after Li, 2018)

The structure of the mining area is dominated by the NW-trending faults, associated with small-scale NW-trending and NE-trending secondary fault belts. These secondary fault belts provide the space for the enrichment of minerals. The faults are distributed in the middle part of the mining area, and the outcrop is 200--1 500 m long and 5--15 m wide. The rock is re-latively fractured, and the original rock is mainly Late Triassic granodiorite.

1-Quaternary; 2-Neogene; 3-Early Jurassic orthoclase granite; 4-Late Triassic Orashan Formation; 5-Late Triassic monzonitic granite; 6-Late Triassic granodiorite; 7-fractured alteration zone; 8-silver mineralization zone; 9-lead mineralization zone; 10-copper mineralization zone; 11-molybdenum mineralization zone; 12-fault; 13-sampling location.Fig.2 Simplified geological map of South Narimalahei area

The mining area experienced strong magmatic activity. The magmatic rocks are widely distributed and are dominated by the Indosinian granodiorite and monzonitic granite. The Yanshanian Early Jurassic syenite granite is sporadically outcropped.

2 Geological characteristics of deposit

2.1 Ore body characteristics

Currently, 13 mineralized belts and 15 ore bodies have been delineated in the South Narimalahei mining area, including 1 silver ore body, 11 copper ore bodies, 2 lead ore bodies and 1 molybdenum ore body. The ore bodies are dominated by the vein shape, with the features as follows.

The copper ore body is mainly NE and NW-trending, and it is mostly located in the middle mining area. It mainly occurs in the Late Triassic granodiorite alteration mineralized belt (Fig.2). The ore body is 200--1 500 m long and apparently 2.64--7.40 m thick, and has the average grade of 0.19%--3.3%.

The silver ore body is mainly NW-trending, and it is mostly located in the northwestern part of the mining area. It mainly occurs in the Late Triassic monzonitic granite. The ore body is 100 m long and apparently 1.82 m thick, and has the average grade of 10.8×10-6.

The lead ore body is mainly NNW trending, and it is mostly located in the northwestern part of the mining area. It mainly occurs in the Late Triassic monzonitic granite. The ore body is 200 m long and apparently 2.7 m thick, and has the average grade of 0.34%.

The molybdenum ore body is mainly NNE-trending and is located in the northeast of the mining area. It is distributed in the Late Triassic granodiorite. The ore body is about 100 m long and apparently 1.61 m thick, with an average grade of 0.1%.

2.2 Ore characteristics

There are two types of mineralization in the South Narimalahei copper polymetallic, including the quartz vein type and the altered rock type (Fig. 3a, b). Metallic minerals mainly comprise pyrite (Fig. 3c), chalcopyrite (Fig. 3d), and a small amount of galena (Fig. 3e), sphalerite, molybdenite, limonite, malachite and azurite (Fig. 3f), etc. Gangue minerals mainly include quartz, sericite, potash feldspar, chlorite, epidote, kaolinite and calcite. The ore texture is dominated by the euhedral-semi-automatic granular, allotriomorphic granular, and metasomatic texture. The ore is mainly with network vein, agglomerate and disseminated structure (Fig. 3c).

2.3 Wall rock alteration and mineralization stages

Wall rock alteration includes potash feldsparization (Fig.4a), silicification (Fig.4b), chloritization, epidotization (Fig.4c), sericification, kaolini-zation, and carbonation, of which silicification and sericitizationare closely related to mineralization. The study shows that the ore-bearing wall rocks and their mineralization in the Narimalahei black copper polymetallic mining area are in a spatially regular distribution, which is characterized by Cu and Mo mineralization in granodiorite, where the quartz veins filled in the joints and fissures are often accompanied by pyritization, malachite mineralization and ferritization. The Ag and Pb mineralization is mainly distributed in monzonitic granite.

Fig.3 Ore types (a-b) and mineral composition (c-f) of South Narimalahei copper polymetallic deposit

a. Silicification andpotash feldsparization in granodiorite；b. silicification and chloritization in quartz vein ore；c. potash feldsparization, chloritization and epidotization in disseminated ores.Fig.4 Photographs showing features of wall rock alteration

According to field investigation and studies in the lab, the mineralization is divided into hydrothermal mineralization and supergene oxidation stages, and the former is further divided into quartz-pyrite stage (1), quartz-polymetallic sulfide stage (2) and quartz-carbonate stage (3). Stage (2) is the main mineralization stage for the pyrite, chalcopyrite, molybdenite, galena and sphalerite. The limonite, azurite and malachite are mainly formed in the supergene oxidation stage.

3 Sample collection and testing methods

The research on fluid inclusions is a key method of identifying the ore-forming fluids properties, and analyzing the source and evolution of ore-forming fluids, and the mechanism of mineral precipitation. It plays a significant role in the theoretical research on the genesis of hydrothermal deposits (Anetal., 2017). The samples for this study were collected from the mineralized quartz veins in the main mineralization stage of the copper mineralization belt in the central part of the mining area. The sampling locations are illustrated in Fig.2. Three samples, namely 18NR-5, 18NR-6 and 18NR-8 were collected respectively, and ground into the 0.2--0.3 mm thick inclusion thermo-meter slice, which were soaked in acetone for 3--4 hours and washed and dried for the research on inclusion.

Fluid inclusion petrography analysis, microscopic thermometry and laser Raman spectroscopy analysis were performed with Linkman THMS600 cooling and heating stage and Renishaw System-1000 laser Raman spectrometer in the Geological Fluid Laboratory of the Collegeof Earth Sciences in Jilin University according to the testing method proposed by Wangetal.(2008, 2014). The hydrogen-oxygen isotope analysis was conducted with Finnigan MAT 253 stable isotope ratio mass spectrometer in the Analysis and Test Research Center of Beijing Research Institute of Uranium Geology according to the test method proposed by Liuetal. (2019).

4 Research on fluid inclusions

4.1 Petrographic characteristics of fluid inclusions

The systematic petrographic observation of fluid inclusions developed in quartz in the main mineralization stage of the South Narimalahei copper polymetallic mining area was performed in this study (Fig. 5a, b). Based on the phase behavior characteristics at the room temperature, the results show that the gas-liquid and a small amount of single-liquid inclusions occur in the ore. According to the criterion of identifying the primary inclusion proposed by Andrawesetal.(1984), the primary gas-liquid fluid inclusions developed in the ore in the main mineralization stage are composed of vesicle and liquid phases at the room temperature, with gas-liquid ratio of 10%--20%, and the inclusions are in the size of 4--12 μm, and occur in the quartz particles sporadically or as group (Fig. 5c, d). At room temperature, the primary single-liquid inclusions that have the size of 4--6 μm and mostly with round and elliptic shape are composed of liquid phases and distributed sporadically or clustered with gas-liquid inclusions in quartz (Fig. 5d).

4.2 Microscopic thermometry and laser Raman spectroscopy analysis

4.2.1 Microscopic thermometry of fluid inclusions

The microscopic thermometry analysis of the primary gas-liquid inclusions in quartz in the main mi-neralization stage was performed with Linkman THMS600 cooling and heating stage. The salinity of the ore-forming fluid was acquired by the Mac Flincor calculation program based on the freezing point temperature (Brown & Hagemann, 1995). The results show that the inclusions all remain as liquid phase during the freezing-heating process, and the freezing point temperature is -1.2℃ to -2.8℃. The corresponding ore-forming fluid salinity varies from 2.06% to 4.94%, with the peak at 2.4%--4.2% (Fig. 6a). The homogenization temperature varies from 151.7℃ to 205.8℃, with a peak of 154℃ to 198℃ (Fig. 6b).

(a) Massive ore in stage 2 mineralization；(b) reticulated ore in stage 2 mineralization；(c) gas-liquid two-phase inclusions in ore quartz in stage 2 mineralization；(d) gas-liquid two-phase inclusions and single-liquid inclusions in ore quartz in stage 2 mineralization.Fig.5 Ore photos of main mineralization stage (a-b) and micrographs of fluid inclusions in quartz (c-d)

Fig.6 Histogram of homogenization salinity (a), temperature (b) and density (c) of fluid inclusions in main mineralization stage

The mineralization density calculated based on the homogenization temperature and salinity is 0.86--0.92 g/cm3(Fig. 6c).

4.2.2 Laser Raman spectroscopy analysis of fluid inclusions

In order to investigate the compositions and the source of the ore-forming fluid in the South Narimalahei copper polymetallic mining area, the single inclusion compositions laser Raman spectroscopy analysis of the primary gas-liquid inclusions in the quartz in the main mineralization stage was performed in Reni-shaw System-1000 laser Raman spectrometer. The results show that the compositions of gas-liquid inclusions (LH2O-VH2O) in the quartz is generally featured with rich gaseous and liquid water (Fig.7a, b), and the ore-forming fluid is the NaCl-H2O system.

Fig.7 Micrographs and corresponding laser Raman spectra of gas-liquid two-phase inclusions in quartz in main mineralization stage

5 Discussion

5.1 Hydrogen and oxygen isotopic compositions and their sources in ore-forming fluids

In order to analyze the source of ore-forming fluids, the hydrogen and oxygen isotope of the fluid inclusions in the quartz in the main mineralization stage was analyzed. The results show that the oxygen isotope δ18OV-SMOWof fluid inclusions in the main minera-lization stage varies from 4×10-3to 6.4×10-3, and the hydrogen isotope δDV-SMOWvaries from -87.8×10-3to -82.2×10-3(Table 1).

Table 1 Hydrogen-oxygen isotope analysis results of fluid inclusions from quartz in South Narimalahei copper polymetallic deposit

According to the peak homogenization temperature of the main metallogenic stage at 176℃, the water δ18OH2Ovalue of inclusions in the main mineralization stage was calculated as -9.4×10-3to -7.0×10-3with the formula 1000lnαquartz-water=3.38×106×T-2-3.40 (Claytonetal., 1972). The diagram of the relationship between δ18OH2O(×10-3) and δDV-SMOW(×10-3) (Fig.8) shows that the hydrogen-oxygen isotope value of the ore-forming fluid is far away from the protomagmatic water range and close to the atmospheric precipitation, indicating that the ore-forming fluid is mainly from atmospheric precipitation, and a small amount of magmatic fluids.

Fig.8 δ18OH2O-δDV-SMOW relationship diagram of ore-forming fluid (Taylor, 1974)

5.2 Ore-forming fluid properties and temperature and pressure conditions for capture

The gas-liquid fluid inclusions occur mainly in the quartz in the main metallogenic stage of the South Narimalahei copper polymetallic deposit, and the gas-liquid is relatively small and concentrated, with the content of 10%--20%. The low-temperature phase transition behavior of CO2and CH4was not observed during the frozen thermometry of the gas-liquid inclusions (He&Zhang, 1993), indicating that the ore-forming fluid is the NaCl-H2O hydrothermal fluid system. The inclusions formed at the same age are featured with similar biologic facies and facies ratio, the same homogenization type of the inclusions during the freezing-heating process (Potteretal., 1978). Also, they both homogeneous to liquid phase and have more concentrated homogenization temperature, indicating the ore-forming fluid is a homogeneous fluid when the reaction fluid inclusions are captured. The thermometry of the inclusions show the homogenization temperature varies from 151.7℃ to 205.8℃ with the peak at 154℃--198℃, the salinity of 2.06wt%--4.94wt%NaCl, and the density of 0.86--0.92 g/cm3, indicating that the ore-forming fluids are low temperature, low salinity, low density NaCl-H2O hydrothermal fluid. This is consistent with that ore-forming fluids are mainly derived from atmospheric precipitation based on the study results of hydrogen-oxygen isotope of fluid inclusions.

There is no mature method available for the calculation of metallogenic pressure. Because the microscopic thermometry of fluid inclusions causes relatively large error of the metallogenic pressure, so the calculated metallogenic depth is an approximate value. Considering the deposit geological characteristics and the studied inclusion system, the metallogenic pressure is estimated with theP(pressure)-T(temperature)-D(density) diagram of the ore fluid NaCl-H2O system compiled by Roedder and Bodnar (1980). The result shows that the metallogenic pressure is 15--30 MPa (Fig.9).

The metallogenic depth is a key part of the study of mineral deposits. Sibsonetal. (1988) established the nonlinear relationship between fluid pressure and depth in the fault zone. Sunetal. (2000) fitted the relationship between pressure and depth in sections. If the fluid pressure is lower than 40 MPa, the metallogenic depth is calculated as 1.5--3.0 km with the hydrostatic pressure gradient (10 MPa/km), reflecting the shallow mineralization.

Fig.9 P-T-D diagram of ore fluidNaCl-H2O system

5.3 Discussion on genetic types of ore deposits

The South Narimalahei copper polymetallic deposit was discovered during the pre-investigation stage in 2017--2019, and there are the research gaps in this area due to its late discovery and extremely low degree of geological research. Sunetal. (2009) believed that the mineralization is closely related to the geodynamic background and corresponds to its physical and chemical conditions such as specific strata, magma, structure, and temperature and pressure. The type of ore deposit directly affects the prospecting direction in the ore belt. The characteristics of ore deposits types are variable and different methods should be adopted for exploration of different types of ore deposits. The exploration of the vein-type hydrothermal deposits should focus on theore-controlling structures.

As mentioned above, the copper mineralization in the South Narimalahei copper polymetallic deposit occurs in the altered and fractured granodiorite. The mineralization is closely related to silicification and sericification, and the copper ore body is veined and controlled by structure and fissures. Fluids in the main ore-forming stage are NaCl-H2O system hydrothermal fluids featured with low salinity (2.06wt%--4.94wt%NaCl), low density (0.86--0.92 g/cm3), ore-forming homogenization temperature of 151.7℃--205.8℃, and estimated mineralization depth of 1.5--3.0 km. According to hydrogen and oxygen isotope analysis, the ore-forming fluids are mainly from atmos-pheric precipitation and a small amount of magmatic fluid. Therefore, the South Narimalahei copper polymetallic deposit is a low-temperature hydrothermal vein type copper polymetallic deposit.

The Late Indosinian is a key stage for transition of tectonic system in the East Kunlun area from compressional orogeny to post-orogenic extensional system, which controlled the mantle source, crust-mantle mixed source magmatic activity and the formation of a large number of hydrothermal deposits within the East Kunlun area (Sunetal., 2009). In the orogenic process, a series of NW-trending major fault structures and their derived secondary structures were formed. In the period for transition to the extensional system, the deep magma source provided thermal power for the mineralization, and the tectonic alteration fractured belt provided the fluid infiltration channels and ore-containing space for late atmospheric precipitation mineralization. It provides an important foundation for enrichment of useful constituent such as Cu. Fluid mixing and temperature reduction are possibly the main mechanism of the precipitation and minera-lization of useful constituent such as Cu.

6 Conclusions

(1) Gas-liquid two-phase inclusions are mainly developed in the ore during the main metallogenic stage of the South Narimalahei Copper Polymetallic Deposit. The uniform temperature ranges from 151.7℃ to 205.8℃, the salinity is 2.06wt%--4.94wt%NaCl, and the density is 0.86--0.92 g/cm3, indicating that the ore-forming fluid is a low-temperature, low-salinity and low-density NaCl-H2O system hydrothermal fluid.

(2) Hydrogen and oxygen isotope studies of inclusions show that the ore-forming fluids mainly come from atmospheric precipitation and a small amount of magmatic fluids. The estimated ore-forming pressure is 15--30 MPa, and the ore-forming depth is 1.5--3.0 km.

(3) It is preliminarily determined that the gene-tic type of the deposit is a low-temperature hydrothermal vein type copper polymetallic deposit.