J. Cent. South Univ. (2012) 19: 2627-2633
DOI: 10.1007/s11771-012-1320-y
Fluid inclusion characteristics of Longmenshan copper-polymetallic deposit in Yueshan, Anhui Province, China
LIU Zhong-fa(刘忠法)1, SHAO Yong-jun(邵拥军)1, SHU Zhi-ming(疏志明)2,
PENG Nan-hai(彭南海)1, XIE You-liang(谢友良)1, ZHANG Yu(张宇)1
1. School of Geoscience and Info-Physics, Central South University, Changsha 410083, China;
2. Tongling Nonferrous Metals Group Co., Ltd., Tongling 244000, China
? Central South University Press and Springer-Verlag Berlin Heidelberg 2012
Abstract: The types, composition and physico-chemical conditions of primary fluid inclusions were researched. The results show that the primary fluid inclusions contain vapor and liquid phase type (Type I), daughter mineral-bearing type (Type II) and pure liquid phase type (Type III). The compositions of vapor are mainly H2O and CO2 with a tiny amounts of CH4 and H 2; the liquid phase mainly contains Mg2+, Ca2+, Na+, K+, Cl- and 
, and w(Na+)/w(K+)>1; the homogenization temperatures of the primary fluid inclusions can be divided into 190-250 ℃, 250-340 ℃ and 360-420 ℃, corresponding to the salinities of 4%-9%, 9%-14%, and 14%-20.43% (NaCl equivalent mass fraction), respectively. The mineralization process can be divided into three episodes: the silicatization stage, the quartz-sulfide stage, and the carbonatization stage, and all of them are associated with the ore-forming hydrothermal fluid activity. The origin of the hydrothermal fluid is from magmatic water mainly, and later it mixes with the groundwater and meteoric water, which lead to the decrease of temperature and salinity. The decrease of salinity, temperature and pressure are the main causes of the metallogenic elements unloading and enriching in the favorable position.
Key words: fluid inclusions; metallogenic physico-chemical condition; ore-forming fluid; Longmenshan copper-polymetallic deposit
1 Introduction
Longmenshan copper-polymetallic deposit is a skarn-type deposit in Yueshan ore field, which is geotectonically located in the middle of the fracture zone along Yangtze River of the lower Yangtze platform depression in the north margin of the Yangtze platform [1]. Extensive researches have been done in ore field from petrogenic age, geochemical characteristics of the intrusion, genesis of rock body [2-6], geodynamics [7-8], geological features of deposit, ore-controlling factors, metallogenic regularity, ore-forming fluid, and genesis of deposit [1, 9-13]. The results show that the intrusion formed between 131 Ma and 138 Ma, early Yanshanian, and it is alkali-rich. Impure carbonate rocks of Triassic, the contact system and the early Yanshanian dioritic rock provide good conditions for mineralization in this area. Compared with previous studies on the Yueshan ore field and the middle and lower reaches of the Yangtze River area [14-16], so far, the researches about the Longmenshan deposit are relatively scarce, especially on fluid inclusion. The study of fluid inclusion is a direct and effective method to obtain the information of ore-forming fluid [17]. So, the study of fluid inclusion has great significance to reveal the physical and chemical conditions and the origin of ore-forming fluid. The microthermometry and composition of fluid inclusion in garnet, diopside, quartz, calcite and chalcopyrite were determined in this area. Based on this, the physical and chemical conditions and the origin of ore-forming fluid and ore-forming process were discussed. This research will provide new basis for understanding the genesis of deposit and guiding the prospecting around this area, meanwhile, it makes up for the inadequate research of ore-forming fluid.
2 Regional geology
Yueshan is the typical ore field of the Yangtze River copper, iron and gold metallogenic belt, which is an important metallogenic belt in China with complex structure [1]. In this area, the distributions of the intrusion and mineral are controlled by contact zone, near SN and NE basement faulted structures [10] (Fig. 1). Longmenshan copper-polymetallic deposit is a famous deposit in Yueshan ore field, located in contact zone between the north branch of early Yanshanian Yueshan intrusion and low-middle Triassic carbonate rocks. The stratigraphy related with mineralization is dolomitic limestone intersected with thin layer of gypsum of Yueshan group and limestone of Nanlinghu group of middle and lower Triassic in this area. The ore-controlling structure is mainly contact zone system. The intrusion which is related to the deposit is mainly a set of middle- shallow formation high-K calc-alkaline dioritic rocks, with w(Na2O)>w(K2O), containing diorite, quartz diorite and K-diorite formed stock and vein. The 40Ar/39Ar dating of hornblende is 136 Ma in intrusion which is early Yanshanian intrusive rock [18]. The ore bodies tend northwest, with the dip of 20°-50°, and form mainly phacoidal, stratoid, and partly pocket with a bifurcation along the strike. The ore types are classified as dense massive chalcopyrite ore, magnetite-chalcopyrite ore, skarn type bornite-chalcopyrite ore, massive magnetite ore and chalcopyrite-magnetite ore and so on. Ore textures contain automorphic-hypidiomorphic crystal granular texture, xenomorphic-granular texture and mainly metasomatic texture, and the ore structures are distinguished as massive structure, dense impregnation structure, vein structure and banded structure and so on. Wall-rock alteration is strong, and mainly contains garnet skarnization, diopside skarnization, scapolite skarnization, marmarization and epidotization, chloritization, serpentinization, silicification, carbonation and kaolinisation.
Fig. 1 Geological map of Yueshan area of Anhui Province (modified from Ref. [8]): 1-Diorite; 2-K-diorite; 3-Geological boundary; 4-deposits; 5-Fault; 6-Basement rift; D-Devonian; D-C-Devonian-Carboniferous; T-Triassic; J-Juassic
3 Characteristics and microthermometry of fluid inclusions
3.1 Sample collection and analysis method
On the basis of detailed geological field survey, the garnet, diopside, quartz and calcite samples of the mineralization stage were collected for studying. First, the 0.2-0.3 mm double sides were prepared in order to observe the lithofacies and fluid inclusions by using microscope, and then, the representative samples were selected and could be used to measure the temperature of the fluid inclusions. The compositions of the fluid inclusions trapped in garnet, diopside, quartz, calcite and chalcopyrite were analyzed by using the group analysis of fluid inclusion.
The thermometry of fluid inclusion was determined by using Linkam THMSG-600 type heating and cooling stage in the Microthermometry Measurement Laboratory of School of Geoscience and Info-Physics, Central South University, Changsha, China. The measured temperature ranged from 195 to 600 ℃. The compositions of vapor and liquid were determined by using Varian-3400 type gas chromatograph (USA) and DX-120 type ion chromatograph (USA) in the Laboratory of Vapor and Liquid Composition Determination of School of Geoscience and Info-Physics, Central South University, Changsha, China.
3.2 Characteristics of fluid inclusions
The primary fluid inclusions were discovered in garnet, diopside, quartz and calcite through the microscope observation. Most of them are characterized with irregular, polygonal, round shape in the spatial, and a few with negative crystal type. The size of the fluid inclusion is small, the maximum is 11.9 μm, and the minimum is less than 1 μm (Fig. 2).
At room temperature, the primary fluid inclusions of Longmenshan deposit were divided into three types: vapor and liquid two phase type (Type ?), daughter mineral-bearing type (Type II) and pure liquid phase type (Type III) on the basis of the phase characteristics (ratio of vapor to liquid in the fluid inclusions). The vapor and liquid phase inclusions (type I) are composed of solution phase and bubble phase with the ratio of gas-to-liquid between 15% and 40%,. And the Type I inclusions become homogeneous liquid-phase after heating. Daughter minerals-bearing fluid inclusions (Type II) are composed of solution phase, bubble phase and daughter minerals which are mainly NaCl. Pure liquid phases (Type III) are mainly composed of liquid phase. Type I fluid inclusions account for 75%-85% of the total, and others account for 15%. Fluid inclusions with daughter minerals occur mainly in diopside and quartz.
Fig. 2 Microphotographs of fluid inclusions from Longmenshan copper-polymetallic deposit: (a) In calcite; (b) In diopside; (c) In quartz; (d) In garnet
3.3 Microthermometry of fluid inclusions
Forty samples of vapor-liquid two phase type fluid inclusions (Type I) were measured in this work (Table 1). The homogenization temperature of the fluid inclusions in Longmenshan copper-polymetallic deposit has a wide range from Table 1, Fig. 3 and Fig. 4, with the minimum temperature of 191.3 ℃, and the maximum temperature of 441.3 ℃. It could be divided into three intervals, namely middle-low temperature (190-250 ℃), middle temperature (250-340 ℃) and high temperature (360-420 ℃). This indicates that the evolution of ore-forming fluid goes through three stages that are high temperature stage, middle temperature stage and middle-low temperature stage.
Table 1 Fluid inclusion data from Longmenshan copper-polymetallic deposit
Fig. 3 Histogram showing homogenization temperatures of fluid inclusions from Longmenshan deposit
Fig. 4 Histogram showing salinities of fluid inclusions from Longmenshan copper-polymetallic deposit
According to Tm(ice), the salinity of the fluid inclusions could be calculated using the FLINCOR software, and the range is from 1.32% to 20.43% (NaCl equivalent mass fraction). The salinities could be divided into high salinity of 14%-20.43%, middle salinity of 9%-14% and low salinity of 4%-9%.
According to complete homogenization temperature, salinity and density graph, the density of fluid inclusions could be divided into high-density group (density of 0.776-1.004 g/cm3) and middle-low-density group (density of 0.607-0.948 g/cm3).
From the graph of salinity versus homogenization temperature about fluid inclusions in the Longmenshan copper-polymetallic deposit (Fig. 5), it can be seen that the fluid inclusions in sample LMS-566-1 and LMS-575-8 have high salinity, but the homogenization temperature of sample LMS-575-8 is relatively low. The homogenization temperature of ore-forming fluid with the same phase tends to fall from deep to shallow [19], therefore, the sample LMS-575-8 with relatively low homogenization temperature may be caused by the captured shallow ore-forming fluid with the same phase. The fluid inclusions of the sample LMS-566-1 and LMS-575-8 are captured during the silicate phase, which has relationship with the hydrothermal ore-forming solution activity. The homogenization temperatures of fluid inclusions in sample TNJ-4 concentrate in the middle temperature range, and its individual temperature is very low, while its salinity is higher. This shows that the fluid inclusions of sample TNJ-4 may be captured during the quartz-sulfide period, representing the activities of ore-forming fluid in the quartz-sulfide phase which has relationship to the hydrothermal ore-forming solution activity. The salinity and temperature of fluid inclusions of sample LMS-530-11 concentrate in the middle-low range, and the temperature approaches that of sample TNJ-4, but the salinity is lower. The fluid inclusions of the sample LMS-530-11 are captured in the transition period from quartz-sulfide phase to carbonate phase, and it presents the activities of ore-forming fluid in the carbonate phase which has relationship to the hydrothermal ore-forming solution activity. Both the temperature and salinity have falling trend as ore-forming process goes along. The visible fluid inclusions in terminal skarn minerals are not found, so, they are not studied.
Fig. 5 Salinity vs homogenization temperature plot of fluid inclusions from Longmenshan copper-polymetallic deposit
4 Composition of ore-forming fluid
The single mineral was used to analyze the ingredients of the fluid inclusions with purity large than 98%. The results are given in Table 2 and Table 3.
From Table 2, it can be seen that the hydrothermal ore-forming solution is a salt solution with complex compositions which are rich in Cl-, 
, Ca2+, Na+, K+ and Mg2+. The liquid compositions of the ore-forming fluid include Cl- and , without F-. The cation include Ca2+, Na+, K+, Mg2+, and the order by the levels is Ca2+>Na+>K+>Mg2+, with w(Na+)/w(K+)>1. It is consistent with the characteristic of the intrusion with w(Na2O)>w(K2O). The Na+ is much less than the summation of the Ca2+ and Mg2+ which are dominant in the cation of the ore-forming fluid, reflecting that the ore-forming fluid is rich in alkaline earth metals. Ca2+ is more than Mg2+ in ore-forming fluid, which is related to the fact that the main component of the wall rock is not MgCO3 but CaCO3, and the host strata is mainly not the dolomite but the limestone. This shows that the wall rock participates in the water-rock reaction of ore-forming fluid, and the ore-forming fluid is subjected to the assimilation of the wall rock. In addition to Cl-, the content of is also high, which shows that the ore-forming fluid is relatively rich in sulfur. like Cl- plays an important role during the process of forming the complex.
Table 2 Composition of liquid phase in fluid inclusions (mass fraction)
Table 3 Composition of gaseous phase in fluid inclusions (mass fraction)
Table 3 shows that the gas composition is also rich in CO2 and contains a small amount of CH4, in addition to H2O, but no O2. It is consistent with the result of the microscopic study of the fluid inclusions.
The w(Na+)/w(K+) and w(F-)/w(Cl-) of ore-forming fluid are the symbols that can be used to judge the source of ore-forming fluid [20]. Generally, the w(Na+?)/w(K+) ratio of magmatic hydrotherm is less than 1. After calculation, w(Na+)/w(K+) of garnet is equal to 0.60, and that of Chalcopyrite is equal to 0.92, with magmatic hydrothermal characteristic, which shows that the source of ore-forming fluid is magmatic water. The w(Na+)/w(K+) of quartz and calcite are greater than 1, indicating that the late ore-forming fluid is mixed water.
The mass ratio of H2O to CO2 can be used to reflect the intensity of mineralizing process and favorable degree of mineralization [21-22]: the smaller the ratio, the stronger the mineralizing process, and the better the mineralization. The mass ratio of H2O to CO2 in the carbonate phase is the largest, the second is in the silicate phase, the smallest is in quartz-sulfide phase in this area, and this is consistent with the characteristics of mineralizing process of skarn type deposit.
5 Discussion
5.1 Salinity of ore-forming fluid
From the physical and chemical conditions of fluid inclusions and the composition of ore-forming fluid, it can be seen that the salinity of fluid inclusions has their own characteristics in every phase of mineralization. The first phase (the silicate phase which has relationship with the hydrothermal ore-forming solution activity) corresponds to the high salinity group, and the salinity range is 14.1%-20.43% (NaCl equivalent mass fraction). The second phase (the quartz-sulfide phase which has relationship to the hydrothermal ore-forming solution activity) mainly corresponds to middle-high salinity range which is mainly 9%-14% (NaCl equivalent mass fraction). The salinity of the third phase (the carbonate phase which has relationship to the hydrothermal ore-forming solution activity) is 5.62%-9% (NaCl equivalent mass fraction). From Fig. 4, it can be known that the salinity of ore-forming fluid has a decrease trend with the temperature decreasing. Changes of salinity may be caused by the dilution when ore-forming fluid is mixed with the external fluids. The source of the late ore-forming fluid is mixed water which may be the hydrothermal solution mixed by magmatic water and groundwater (or atmospheric precipitation) or magmatic water and hot geothermal brine.
5.2 Ore-forming process
The original high-temperature and high-pressure ore-forming fluid was suffered from tectonic activities, especially fault activities, which made the pressure release abruptly, caused the pressure decrease and boiling of the fluid, and led to the phase separation of carbon dioxide in ore-forming fluid. According to the research of WILLIAMS-JONES and FERREIRA [23], in the early period, carbon dioxide and water thermal fluid were complete immiscibile; when migrating to shallow crust, low-secondary salinity water fluid and carbon dioxide became immiscible, then a large amount of gas was isolated, which led to retrograde metamorphism and the mineralization of copper.
w(Na+)/w(K+)>1 of liquid ingredients shows that Na+ is rich in ore-forming fluid, which is identified with w(Na2O)>w(K2O) in the intrusion, and reveals the inner link of ore-forming fluid and magma. The content of anionic is higher in the area. The reason may be that the S2- is oxidized in the higher oxidation state with CO2 or the early high temperature magmatic water enters the sulfate-resistant hot brine.
Ore-forming fluid in the area is rich in CO2, which can adjust pH value of ore-forming fluid and increase complexation degree of anions and cations, causing that the Cu2+ has larger solubility in the ore-forming fluid with a lower salinity along with the ore-forming process. The ore-forming process in this area can be roughly divided into three stages. The ore-forming fluid with high temperature, high salinity and high-density features occurs in the first stage. The contact metasomatism happens when the ore-forming fluid meets the wall rock during the emplacement process. With the mineralization going along, skarn minerals and metal minerals form in contact zones, and the main minerals are garnet, diopside and magnetite. Groundwater or atmospheric precipitate is mixed with magmatic water in the second stage, which reduces the temperature and salinity, causes the volatilization of CO2, brings about un-mixing phenomenon of fluid, and improves water-rock interaction; ore-forming elements are uninstalled in favorable positions, and finally enriched in mine. In addition, oxidation causes S2- concentration and decrease of pH in the process of oxidating from S2- to , and leads to the further precipitation of metallogenic material. This stage primarily generates quartz, lots of sulfide and small calcite. The temperature and salinity of fluid are reduced further with the ore-forming process going along, the groundwater or atmospheric precipitation composition increases remarkably, hydrothermal fluid system is cooled down gradually, precipitation and gathering stop, and eventually, the ore-forming process ends. This stage primarily generates calcite. Thus, it can be seen that the second stage is the main mineralization phase, and plays a good role in superposition and enrichment of early mineralization phase.
6 Conclusions
1) The primary fluid inclusions can be divided into gas and liquid phase type (Type I), daughter minerals-bearing type (Type II) and pure liquid type (Type III), and gas and liquid phase type (Type I) dominates.
2) The homogenization temperature of fluid inclusions is mainly between 180 and 420 ℃, and it can be divided into three temperature ranges: 190-250 ℃, 250-340 ℃ and 360-420 nd INATEout actyion℃. The salinities of fluid inclusions (NaCl equivalent fraction) can be divided into 14%-20.43%, 9%-14% and 4%-9%. Changes of salinity may be caused by the dilution when ore-forming fluid is mixed with the groundwater or atmospheric precipitate.
3) The gas compositions of fluid inclusions are mainly CO2 and H2O, containing a small amount of CH4 and H2, but no O2. The liquid ingredients are Mg2+, Ca2+, Na+, K+, Cl- and . The source of ore-forming fluid is magmatic water, and the later ore-forming fluid is mixed water. The combination type of ore-forming fluid is Na++Ca2++K+-Cl- ().
4) The early ore-forming fluid carrying mineral is mixed with the outside fluid, such as hot brine, groundwater and atmospheric precipitation with the ore-forming process going along. This leads to the decrease of salinity, temperature and pressure. The immiscible phenomenon comes into being in the ore-forming fluid, with the process of water-rock interaction, and finally, the metallogenic elements are unloaded and enriched in the favorable position.
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(Edited by YANG Bing)
Foundation item: Project(200911007-04) supported by the Special Funds for Scientific Research of Land and Natural Resources, China; Project (2007CB411405) supported by the National Basic Research Program of China; Project(20109901) supported by the National Crisis Office of China
Received date: 2011-07-27; Accepted date: 2011-12-01
Corresponding author: LIU Zhong-fa, PhD; Tel: +86-13574886497; E-mail: liuzhongfa_ok@126.com
