fluid inclusions

The application of fluid inclusions in the mineralizationlijinThe department of geochemistry,Yangtze universityAbstract. Fluid inclusion analysis is an important tool in modern studies of mineral deposits, as reflected by the statistics indicating that about a quarter of the papers published in Economic Geology contain fluid inclusion studies. Fluid inclusions play an important role in the classification of mineral deposits and in the study of the composition, temperature and pressure of mineralizing fluids. Among the principal mechanisms of ore precipitation, flu-id phase separation and fluid mixing derive their key evidence mainly from studies of fluid inclusions. Data on mineralizing fluid composition obtained from fluid inclusion analysis are key to understanding how metals were transported in hydrothermal fluids. Recent progresses in metal transport in vapor have been mainly contributed by fluid inclusion studies. Data on fluid temperature and pressure from fluid inclusion studies provide important constraints on hydrodynamic models of mineralization. Most metal ore deposits are formed in the geological fluid.The formation and characteristics of hydrothermal ore deposits are closely related including temperature, pressure and composition. Although these information can be gained through the study of macro geological characteristics of ore deposit and the geochemical characteristics of the mineral , but the composition of ore-forming fluid, temperature, and pressure from fluid inclusion is the most direct evidence.Fluid inclusion is the only remain in ancient ore-forming fluid. So, the study of fluid inclusion becomes one of the important ways of genesis research naturally. For Economic Geology sampling survey, the proportion of fluid inclusion research papers, from 5% in 1975 to 15% in 1985, 27% in 1995, then remained at about 25%, about 1 in 4 papers of deposits essay involves the study of fluid inclusions. Although the fluid inclusion research has expanded to petroleum geology, magma, and the earth's interior processes, etc.its mian application in the field of ore deposit research . The application of fluid inclusions in ore deposit has a lot of monographs. but these works focus on basic principles , methods of the study and the characteristics of different deposit types.This paper mainly discusses the application of fluid inclusions in the study of ore deposits.Keywords: geochemistry, fluid inclusions, hydrothermal deposits, mineralizing fluids, ore precipitation, metal transportation.1.Fluid inclusion is one of the basis of the classification of the ore deposit According to the geological characteristics and genesis, ore deposit can be divided into different types, But at present very few scholars classify the ore deposits completely according to the geological characteristics, such as shear zone gold deposits, stratabound lead-zinc deposit or causes such as high temperaturehydrothermal, the warm liquid, syngenetic and epigenetic[1.2]. Usually the geological characteristics of the ore deposit formation and formation process environment together will be classified as a certain type, whose name can be geological features, such as jet formation process Pb-Zn- SEDEX , or typical areas such as Mississippi V alley-type Pb-Zn MVT[3.4.5], implicit in each deposit type in both the geological characteristics also causes. Different types of ore deposits formed in different geological environment, the inclusion features are different, so fluid inclusions can be used as the basis for identifying ore deposit types. For example , For the most part orogenic type gold deposits containing CO2--H2O with low salinity inclusions; Porphyry copper deposits in salt crystal with high salinity inclusions and the gas phase characteristics of inclusions; Hypabyssal are low salinity inclusions with low temperature hydrothermal deposit of aqueous inclusions; MVT deposits inclusions generally has high salinity but excluding crystal with more H2O – NaCl-CaCl2 system. Different types of inclusions characteristic Deposits are discussed in the relevant papers. It should be noted that, although the inclusions deposit types can be used as the main basis for the division, but not the sole basis. For example, the CO2-rich inclusions is most common in orogenic gold deposits, but in the pegmatite deposits associated with tungsten-tin deposits in other types of deposits are very common. Conversely, it is more effective to exclude certain deposit type with the inclusion features. For example, epithermal deposits should not turn out to be high-density CO2-rich inclusions.This paper, using gold deposits as example, attempts to setup a scientific linkage between ore geology and fluid inclusions[6]，considering that in previous published works, observations and measurements of the fluid inclusions commonly were not well interpreted. In some cases, geological data did not agree with the results obtained from fluid inclusion studies. In this paper, we first review previous classifications of gold deposits，and then，subdivide gold deposits into five classes，based on the dominant ore-formingprocesses:1intrusion-related hypothermal systems，such as porphyry-systems, breccia-pipes, IOCG and skarns; 2orogenic- or metamorphic hydrothermal type; 3epithermal-type, i. c. reworking hydrothermal deposits hosted in continental-faciesvolcanic-subvolcanicrocks;4)fine-graindisseminatedtype(Carlin type and/or Carlin-style ，i. e. reworking hydrothermal deposits hosted sediments; and 5hydrothermal metalliferous sediments related to submarine venting, such as VMS and SEDEX styles. In this work we select diagnostic geological and fluid-inclusion characteristics of these five classes of ore-systems，and clarify their key differences that can be used as genetic markers. Ore-fluids are classified into three end-members，namely reworking, metamorphic and magmatic fluids. Many ore-systems are known to form as a result of multiple fluids during multi-stage events; and their late-stage of mineralization always being caused by fluids with a high-proportion of reworking of the original ore systems or by renewed fluid flow. Therefore，the features of late-stage fluids，alteration and mineralization cannot be used to identify the origin and genetic type of an ore-system. Instead，we suggest that only the early-stage signatures can be employed to determine the origin and type of an ore-system. Reworking fluids are characterized by low-temperature(<3000C)，low-salinity and low-content of CO2，andsourced from meteoric and/or sea water; metamorphic fluids by moderate-temperature, low-salinity and high-content of CO2;and magmatic fluids by high-temperature，high-salinity and high-content of CO2. Magmatic hydrothermal ore-systems contain multi-daughter-crystal-bearing and high-salinity, C02-rich fluid inclusions; metamorphic ore-systems contain low-salinity, C02-rich fluid inclusions; and the reworking hydrothermal ore-systems contain neither daughter-crystal-bearing nor C02-rich/bearing fluid inclusions，but are populated by aqueous water-solution fluid inclusions. Finally, we discuss the tectonic settings of the ore-systems of the various classes. For examples, the orogenic-type formed during processes of crustal compression, orogenesis，metamorphism and uplift; submarine metalliferous sediments developed in the setting of rift basins; Paleozoic or earlier epithermal-type ore-systems can be preserved in accretionary orogens. It is suggested that the ore-systems and their fluid inclusions can be used as an ideal probe to trace geodynamic evolution of continents.2.The restriction of fluid inclusions to metallogenic conditionsStudy of ore deposit forming condition, the most important thing is toacquare the ore-forming hydrothermal composition, temperature and pressure[3.5]. In this regard, fluid inclusions has an irreplaceable role.In Economic Geology journals, the vast majority of papers related to inclusions belonging to the study of hydrothermal mineralization composition, temperature and pressure. In all geochemical methods of the chemical composition of the ore-forming , fluid inclusions is most direct and effective way of the chemical analysis.The main component of the ore fluids can be measured by laser Raman measurement methods together with microthermometry of fluid inclusions, the minor components and trace components can be measured with different methods of analysis.Isotope thermometer and temperature of fluid inclusions are the main determination of mineralization temperature. Isotope geological thermometer has the advantage of calculation of the pressure, the disadvantages are that it is difficult to separate evaluation mineral of isotope equilibrium and the single mineral separation more time-consuming .The method of inclusion thermometry is direct and simple, but the measured temperature is the lowest homogenizationity temperature . In most cases such as aqueous inclusions, homogenization temperature and trapping temperature are very close, and homogenization temperature that is usually considered ore-forming temperature, there is no need for the pressure correction.homogenization temperature is whether effective related to the basic principles of fluid inclusions microthermometry.The effectiveness of the fluid inclusions analysis depends on three basic assumptions, namely Roedder three principles, inclusions capturing a single, homogeneous fluid phase, the volume of inclusions unchanged , the composition of inclusions from unchanged . If any one of these three conditions is not met, the data of inclusions is invalid. it is difficult to determine whether the volume of inclusions and composition have changed ,because we cannot know the size and composition accurately.it is more difficult to determine pressure on the fluid than mineralization temperature. Although theoretically uniform as long as the measured temperature and compositionof inclusions, you can calculate the density of the fluid, and then calculates the isochore. with isochore, then know mineralization temperature by isotope thermometer, you can calculated pressure. However, due to the constant volume aqueous inclusions line slope of a large space in the temperature and pressure, small temperature difference can cause great pressure error. The isometrc slopes of oil inclusions , CO2 and CH4 are small, which we can use to estimate the pressure.3 .Fluid inclusions are key evidence of mineralization processThe study of fluid inclusions in some ore-forming process is the key evidence[6.7.8]. As is well known, the main mechanism of mineral deposits, the understanding of fluid mixing and fluid phase separation mainly come from fluid inclusions , which is one of the important contributions for modern metallogenic theory.Many of the main mineralization and magmatic intrusion related deposits occur in the contact zone . The reason is that ore-forming fluid pressure drops into the contact zone from rock mass caused by fluid phase separation. A large number of high salinity water solution containing crystal inclusions with homogenization into gas phase inclusions symbiosis has become a porphyry copper ore belt . Many gold deposits in orogenic type are thought to be caused by the pressure in the shear zone and its cyclical fluctuation H2O- CO2 fluid is caused by the phase separation, whose main basis is also from the fluid inclusions. Many of deposits found in hydrothermal of sedimentary basins such as the Mississippi valley Pb-Zn deposit, the main mineral deposits require H2S from sulfate by hydrocarbons in the basin fluid reduction of the product or the result of ore-forming fluid through the ancient reservoirs.Although this evidence can come from stable isotopes and formation of organic geochemical studies, the most convincing evidence is from fluid inclusions.Through fluid inclusions studies we can find deposits have a lot of oil inclusions, and the sphalerite has numerous small oil inclusions along with the distribution of growth rings.4 .Fluid inclusions revealling the methods of mineral transportationIn ore-forming hydrothermal metal, its main form is complex dissolved and Cl- is one of the most important complex anion[9.10].This realization comes from the experiments of high temperature and pressure, but the study of fluid inclusions salinity data is also a very important factor. Fluids are aubiquitous transport tmedium for heat and matter in most geological process. The presence of fluids in rocks may affect the chemical and physical properties, mineral reaction velocity and heatbudget of geological systems, Direct sample of geological fluids could be preserved only in fluid inclusions thatwere trapped during the growth of their host minera.l Fluid inclusions can provide us unique information for the presence and composition ofancient fluidswhich can notbe obtained by othergeochemicalmethods (e.g. the componentand evolution ofdiagenetic fluid; process of fluid-rock interaction; migration of trace elements in fluid). The study offluid inclusions thushasbecome one of the/hottest0fields in earth sciences, and plays an importantrole in studiescorrelated with geologicalprocesses.Startingwith a general introduction to the fluid inclusion properties and a summarization about the classification ofprimary-and secondary-fluid inclusions, thispaper reveiws themostrecentde-velopment in analyticalmethods in the fluid inclusion research field. We also review the currentapplication of fluid inclusions to various fields in earth sciences (e.g.metamorphic fluid, ore-deposi,t petroleum geology and biomarker).5. The restriction of fluid inclusions to metallogenic fluid dynamicsThe formation of hydrothermal ore deposits involves many complex chemical process.Here is an example to illustrate the restriction of fluid inclusions to metallogenic fluid dynamic s[7.8.10].The Lanping Basin, Y unnan Province, South-west China, contains the giant Jinding Zn-Pb depositand the newly discovered Baiyangping Cu-Co-Ag super-large deposit. The Jinding deposit, with reserves of ~200 Mt ore, grading 6108% Zn and 1129% Pb (i.e. metal resources of ~15 Mt) is the largest Zn-Pb deposit in China. The ore is hosted by Cretaceous and Tertiary terrigenous rocks. The Jinding is also the youngest sedimentary rock-hosted and the only continental super-large Zn-Pb deposit in the world. Differing from other major types of sediment-hosted Zn-Pb deposits in the world, including SST, MVT and SEDEX, the Jinding deposit represents a new type of sedimentary rock-hosted Zn-Pb mineralization. Most previous studies assumed that the mineralizing fluids were derived from within the basin and that the fluid flow was largely driven by topographic relief under a hydrostatic regime. However, the observations of hydraulic fractures and fluid inclusion data in this study indicate that the mineralizing fluid system was strongly over-pressured. The study of fluid inclusions in sphalerites and associated gangue minerals (quartz, celestite, calcite and gypsum) shows that homogenization temperatures cluster around 110-150e, with salinities of 1.6-18.0 wt% NaCl equivalent.The fluid temperature increased with the decrease in salinities during the main ore stages, and there is also a systematic westward decrease in temperature and increase in salinity in the Jinding ore district. Fluid pressures as high as (513-1364)×105Pa are indicated by CO2-rich fluid inclusions. The results of basin fluid dynamicmodeling indicate that the overpressures could not have been produced by normal sediment compaction, and the overpressure related to the thrusting may be inadequate to explain the high fluid pressures indicated by fluid inclusions. The injection of mantle-derived fluids is thought to be a viable mechanism for the build-up of high overpressures. The mixing of two types of fluids in a structural lithologic trap may have been the key dynamic process for the large-scale mineralization in the Lanping Basin: one was a mantle-derived fluid enriched in metals and CO2with higher temperatures and lower salinities, and the other was H2S-rich saline formation water with lower temperatures and higher salinities. The special hydrodynamic regime and potential contribution of mantle-derived fluids to the mineralizing system distinguish Jinding from other known sedimentary basin-asso-ciated Pb-Zn deposits.6.ConclusionsFluid inclusions as the sample of mineralization of ancient fluid ,it playsirreplaceable role in the study of ore deposit compared with other methods. Simple petrography observation can provide important basis for the division of ore deposit types. The fluid inclusions can provide the information of composition, temperature, pressure and important parameter; For some ore-forming process, such as fluid phase separation and the fluid mixing, fluid inclusions can provide the most direct evidence; Fluid inclusions play an important role in the study of mineral transportation; In recent years, the acknowledgement of metal moving in the gas phase mainly come from fluid inclusions; Fluid inclusions can provide the temperature , pressure data and important constraints for the ore-forming fluid dynamics model. Therefore, the fluid inclusions have important contribution to the theory of modern mineralization. References[1] Bodnar R J. 2003a. Introduction to fluid inclusions. In: Samson I, An-derson Aand Marshall D, eds. Fluid inclusions: Analysis and inter-pretation[M].Mineralogical Association of Canada, Short CourseSeries, 32: 1-8.[2] Bodnar R J. 2003b. Reequilibration of fluid inclusions. In: Samson I,Anderson Aand Marshall D, eds. Fluid inclusions:A nalysis and interpretation [ M].Mineralogical Association of Canada, ShortCourse Series, 32:213-231.[3]Chi G X and Lu H Z. 1991. Characteristics of fluid phase separationfields indepth-temperature coordinates with emphasis on their sig-nificance on localization of hydrothermal deposits [ J ]. ActaMineralogica Sinica, 11: 355-362 [4]Chi G, Guha J and Lu H Z. 1993. Separation mechanism in the forma-tion ofproximal and distal tin-polymetallic deposits, Xinlu ore field,southern China: Evidence from fluid inclusion data [J]. Econ.Geol., 88: 916-933.[5]Chi G, Savard M M and H†roux Y. 1995. Constraints from fluid inclusion data onthe origin of the Jubilee carbonate-hosted Zn-Pb deposit, Cape Breton, Nova Scotia[J]. The Canadian Mineralogist,33: 709-721.[6]Chi G and Savard M M. 1998a. Basinal fluid flow models related to Zn-Pbmineralization in the southern margin of the Maritimes Basin,eastern Canada[J].Econ. Geol., 93: 896-910.[7]Ulrich T, Gunther D and Heinrich C A. 1999. Gold concentrations of magmaticbrines and the metal budget of porphyry copper deposits [J]. Nature, 399: 676-679.[8] Wilkinson J J. 2001. Fluid inclusions in hydrothermal ore deposits[J].Lithos, 55:229-272.[9]Chi G, Dube B, Williamson K and Williams-Jones A E. 2006. Forma-tion of theCampbell Red Lake gold deposit by H2O-Poor, CO2-dominated fluids[J].Mineralium Deposita, 40: 726-741.[10] Chi G, Xue C, Lai J and Qing H. 2007. Sand injection and liquefactionstructuresin the Jinding Zn-Pb deposit, Y unnan, China: Indicatorsof an overpressured fluid system and implications for mineralization[J]. Econ. Geol., 102: 739-743.。