Molten Salts: Fluid Inclusion Record and Role in Forming Mineral Deposits

The authors’ database (which includes data from more than 17500 publications on fluid and melt inclusions in minerals) was used to generalize information on the principal physicochemical parameters of natural mineral-forming fluids (temperature, pressure, density, salinity of aqueous solutions, and the gas composition of the fluids). For 21 minerals, data are reported on the frequency of occurrence of the homogenization temperatures of fluid inclusions in various temperature ranges, which make it possible to reveal temperature ranges most favorable for the crystallization of these minerals. Data on 5260 determinations were used to evaluate the frequency of occurrence of certain temperature and pressure ranges of natural fluids within the temperature intervals of 20–1200°C and 1–12000 bar. Within these intervals, frequencies of occurrence were evaluated for water-dominated and water-poor or water-free fluid inclusions in minerals. The former are predominant at temperatures below 600°C and pressures below 4000 bar, whereas the latter dominate at temperatures of 600–1200°C and pressures of 4000-12000 bar. Illustrative examples are presented for visually discernible magmatic water that exists as an individual high-density phase in melt inclusions in minerals from various rocks sampled worldwide (in the Caucasus, Italy, Slovakia, United States, Uzbekistan, New Zealand, Chile, and others). Attention is drawn to the fact that extensive data testify to fairly high (>1000–1500 bar) pressures during hydrothermal mineral-forming processes. These pressures are much higher not only than the hydrostatic but also the lithostatic pressures of the overlying rocks. Data on more than 18000 determinations are used to evaluate the frequency of occurrence of certain temperature and salinity ranges of mineral-forming fluids within the intervals of 20–1000°C and 0–80 wt % equiv. NaCl and certain temperature and density ranges of these fluids at 20–1000°C and 0.01–1.90 g/cm3. Information is presented on the gas analysis methods most commonly applied to natural fluids in studying fluid inclusions in minerals in 1965–2007. The average composition of the gaseous phase of natural inclusions is calculated based on more than 3000 Raman spectroscopic analyses (the most frequently used method for analyzing individual inclusions).

A near-infrared (NIR) spectroscopic method is proposed to achieve the simultaneous determination of salinity and internal pressure of fluid inclusions in natural minerals. A combination band between the anti-symmetric stretching and bending vibrations of molecular water at approximately 5180 cm-1 was observed for standard salt solutions and natural minerals containing fluid inclusions with known salinities. A curve-fitting procedure was used to analyze the change in the band shape of the combination. Justification of the calibration was confirmed by observation of fluid inclusions in natural minerals whose salinities had already been determined using microthermometry. The detection limit of the present method is 1 NaCl-eq wt. %. The minimum size of fluid inclusions that produced well-resolved spectra was approximately 30 microm. This method was applied to assess micro fluid inclusions in a natural diamond with cubic growth habit (cuboid). The salinity and residual pressure of those fluid inclusions were estimated respectively as 4.4 wt. % NaCl-eq and 0.6-0.8 GPa. The present method is complementary to Raman microscopy and microthermometry for the determination of salinity in fluid inclusions of geological samples.

The Xiaoxinancha Au-rich copper deposit is one of important Au-Cu deposits along the continental margin in Eastern China. The deposit consists of two sections: the Beishan mine (North), composed of altered rocks with veinlet-dissemination sulfides and melnicovite-dominated sulfide-quartz veins, and the Nanshan mine (South), composed of pyrrhotite-dominated sulfide-quartz veins and pure sulfide veins. The isotope compositions of noble gases extracted from fluid inclusions in ore minerals, i.e. ratios of 3He/4He, 20Ne/22Ne and 40Ar/36Ar are in the ranges of 4.45–0.08 Ra, 10.2–8.8 and 306–430, respectively. Fluid inclusions in minerals from the Nanshan mine have higher 3He/4He and 20Ne/22Ne ratios whereas those from the Beishan mine have lower 3He/4He ratios. The analysis of origin, and evolution of the ore fluids and its relations with the ore-forming stages and the ages of mineralization suggests that the initial hydrothermal fluids probably come from the melts generated by partial melting of oceanic crust with the participation of fluids from the mantle (mantle-plume type)/aesthenosphere. This also corresponds to the continental margin settings during the subduction of Izanagi ocaneic plate towards the palaeo-Asian continent (123–102 Ma). The veinlet-dissemination ore bodies of the Beishan mine were formed through replacement and crystallization of the mixed fluids generated by mixing of the ascending high-temperature boiling fluid with young crustal fluid whereas the melnicovite-dominated sulfide-quartz veins were formed subsequently by filling of the high-temperature ore fluid in fissures. Pyrrhotite-dominated sulfide-quartz veins in the Nanshan mine were formed by filling-deposition-crystallization of the moderate-temperature ore fluids and the pure sulfide veins were formed later by filling-deposition-crystallization of ore substance-rich fluids after boiling of the moderate-temperature ore fluids. The metallogenic dynamic processes can be summarized as: (1) formation of fluid- and ore substance-bearing Adakitic magma by degassing, dewatering and partial melting during subduction of the Izanagi plate; (2) separation and formation of ore fluids from the Adakitic magma; and (3) success-sive ascending of the ore fluids and final formation of the Au-rich Cu deposit of veinlet-dissemination and vein types by secondary boiling.

Information from a database, which was compiled and continuously updated by the authors of this paper and now includes information from 19500 publication on fluid and melt inclusions in minerals, is used to summarize results on the physicochemical formation parameters of hydrothermal Au, Ag, Pb, and Zn deposits. The database provides information on fluid inclusions in minerals from 970 Pb-Zn, 220 Au-Ag-Pb-Zn, and 825 Au-Ag deposits in various settings worldwide. Histograms for the homogenization temperatures of fluid inclusion are presented for the most typical minerals of the deposits. In sphalerite, most homogenization temperatures (1327 measurements) of fluid inclusions lie within the range of 50–200°C with a maximum at 100–200°C for this mineral from Pb-Zn deposits and within the range of 100–350°C (802 measurements) with a maximum at 200–300°C for this mineral from Au deposits. Data are presented on fluid pressures at Au (1495 measurements) and Pb-Zn (180 measurements) deposits. The pressure during the preore, ore-forming, and postore stages at these deposits ranged from 4–10 to 6000 bar. The reason for the high pressures during preore stages at the deposits is the relations of the fluids to acid magmatic and metamorphic processes. More than 70% of the fluid pressures values measured at Pb-Zn deposits lie within the range of 1–1500 bar. Au-Ag deposits are characterized by higher fluid pressures of 500–2000 bar (61% of the measurements). The overall ranges of the salinity and temperature of the mineral-forming fluid at Au-Ag (6778 measurements) and Pb-Zn (3395 measurements) deposits are 0.1–80 wt % equiv. NaCl and 20–800°C. Most measurements (∼64%) for Au-Ag deposits yield fluid salinity <10 wt % equiv. NaCl and temperatures of 200–400°C (63%). Fluids at Pb-Zn deposits are typically more saline (10–25 wt % equiv. NaCl, 51% measurements) and lower temperature (100–300°C, 74% measurements). Several measurements of the fluid density fall within the range of 0.8–1.2 g/cm3. The average composition of volatile components of the fluids was evaluated by various techniques. The average composition of volatile components of fluid inclusions in minerals is calculated for hydrothermal W, Au, Ag, Sn, and Pb-Zn deposits, metamorphic rocks, and all geological objects. The Au, Ag, Pb, and Zn concentrations in magmatic melts and mineral-forming fluids is evaluated based on analyses of individual inclusions.

A study of the composition of fluid inclusions in ore minerals of the Davenda Mo–porphyry deposit and the Aleksandrovskoe sulfide–quartz–gold ore deposit, as well as of fluid inclusions in minerals of igneous rocks, showed that ore-forming fluids inherit the salt and gas composition of magmatic fluids generated during crystallization of ore-bearing rocks of the Amudzhikan–Sretensky Igneous Complex, which formed simultaneously with the Au and Mo mineralization. The gold-bearing sulfide–quartz veins of the Aleksandrovskoe deposit formed with the participation of two types of hydrothermal fluids, differing in the composition of salts and the gas phase: homogeneous Ca–Na chloride fluids with CO2 and heterophasic Na–K–Fe-chloride fluids, which indicates two sources of ore-forming fluids during the formation of Au-mineralization. Na–K–Fe-chloride fluids in terms of salt and gas composition were similar to the ore-forming fluids of the Mo-mineralization of the Davenda deposit. Ore-forming Ca–Na-chloride with CO2 The fluids of the Aleksandrovskoe field are comparable in salt and gas composition with the magmatogenic fluids of quartz diorite porphyries and diorite porphyrites. Ore forming Na–K–Fe carbonate-chloride fluids of the Davenda and Aleksandrovskoe deposits show great similarity in composition to magmatic fluids of granite porphyry and emphasize the genetic identity of Mo mineralization in both deposits. The data obtained confirm the widespread opinion that a genetic relationship exists between gold mineralization and dikes of intermediate and mafic composition, and molybdenum–porphyry mineralization with granite–porphyry of the Amudzhikan–Sretensky Igneous Complex. The real agents of this genetic link are metalliferous magmatogenic fluids, the salt and gas composition of which inherit ore-forming fluids. The formation depth of productive ore mineral assemblages in veins of the Aleksandrovskoe and Davenda deposits is estimated at 7.9–7 and 8–6.3 km, respectively, which is not typical of porphyry deposits, the formation of which is characterized by shallower depths.

How and how much do hydrothermal fluids trans� port metals from the area of their involvement in the oreforming process to the zone of their precipitation? What should the composition of fluids, which may transport metals in the proportions necessary for the formation of mineral deposits, be? These problems have been studied by more than one generation of researchers in the field of ore deposits. An important role in the solution of this problem was played by the experimental and thermodynamic studies of mineral solubility in aqueous fluids at high temperatures. In spite of significant achievements in these areas, we cannot assume that the obtained results reflect the chemical composition of the natural oreforming fluid adequately, since modeling of metal behavior in it is limited by our knowledge of the thermodynamic con� stants of minerals and components of fluid. Significant achievements in improving our knowledge on the chemistry of mineralforming fluids were gained dur� ing the study of modern hydrothermal systems and fluid inclusions in minerals of hydrothermal deposits (1). A huge volume of data was obtained after the dis� covery of modern hydrothermal sources precipitating sulfide ores on the floor of the World Ocean (21° N) on the East Pacific Rise in 1978. During the past 35 years, more than 300 hydrothermal sites have been discov� ered and studied by submersibles. The major data on the chemistry of fluid and concentrations of metals in it were mostly based on the results of the analyses of its samples collected on the hydrothermal fields by sub� mersibles. However, study of fluid inclusions in miner� als from modern sulfide edifices has shown that the conditions of deposition, the evolution of salinity, and the temperature of fluids discharged on the sea floor were much more variable than followed from direct measurements (2-4). Development of new highpre� cision apparatus and analytical methods has provided completely different possibilities for the study of a wide spectrum of chemical elements in fluids trapped by fluid inclusions during mineral crystallization. Study of fluid inclusions in minerals from a number of ore deposits and analysis of the concentrations of met� als in fluids precipitating ores have been performed (5-7). Using these methods, it is very interesting to analyze the fluid captured by minerals during the for� mation of modern sulfide ores. For this purpose, we carried out investigations of fluid inclusions in miner� als from the Semenov modern ore cluster.

Melt inclusions and aqueous fluid inclusions in quartz phenocrysts from host felsic volcanics, as well as fluid inclusions in minerals of ores and wall rocks were studied at the Cu-Zn massive sulfide deposits in the Verkhneural’sk ore district, the South Urals. The high-temperature (850–1210°C) magmatic melts of volcanic rocks are normal in alkalinity and correspond to rhyolites of the tholeiitic series. The groups of predominant K-Na-type (K2O/Na2O = 0.3–1.0), less abundant Na-type (K2O/Na2O = 0.15–0.3), and K-type (K2O/Na2O = 1.9–9.3) rhyolites are distinguished. The average concentrations (wt %) of volatile components in the melts are as follows: 2.9 H2O (up to 6.5), 0.13 Cl (up to 0.28), and 0.09 F (up to 0.42). When quartz was crystallizing, the melt was heterogeneous, contained magnetite crystals and sulfide globules (pyrrhotite, pentlandite, chalcopyrite, bornite). High-density aqueous fluid inclusions, which were identified for the first time in quartz phenocrysts from felsic volcanics of the South Urals, provide evidence for real participation of magmatic water in hydrothermal ore formation. The fluids were homogenized at 124–245°C in the liquid phase; the salinity of the aqueous solution is 1.2–6.2 wt % NaCl equiv. The calculated fluid pressure is very high: 7.0–8.7 kbar at 850°C and 5.1–6.8 kbar at 700°C. The LA-ICP-MS analysis of melt and aqueous fluid inclusions in quartz phenocrysts shows a high saturation of primary magmatic fluid and melt with metals. This indicates ore potential of island-arc volcanic complexes spatially associated with massive sulfide deposits. The systematic study of fluid inclusions in minerals of ores and wall rocks at five massive sulfide deposits of the Verkhneural’sk district furnished evidence that ore-forming fluids had temperature of 375–115°C, pressure up to 1.0–0.5 kbar, chloride composition, and salinity of 0.8–11.2 (occasionally up to 22.8) wt % NaCl equiv. The H and O isotopic compositions of sericite from host metasomatic rocks suggest a substantial contribution of seawater to the composition of mineral-forming fluids. The role of magmatic water increases in the central zones of the feeding conduit and with depth. The dual nature of fluids with the prevalence of their magmatic source is supported by S, C, O, and Sr isotopic compositions. The TC parameters of the formation of massive sulfide deposits are consistent with the data on fluid inclusions from contemporary sulfide mounds on the oceanic bottom.

Ore geology, fluid inclusions and four-stage hydrothermal mineralization of the Shangfanggou giant Mo–Fe deposit in Eastern Qinling, central China

Petrogenetic analysis of fluid and melt inclusions in minerals from mantle xenoliths from the Bele pipe basanites ( North Minusa depression)

Fluid evolution of the Qiman Tagh W-Sn ore belt, East Kunlun Orogen, NW China

Fluid inclusions in minerals provide a source of geologically significant fluids. A new computerized mass spectrometric technique for analyzing volatiles in individual inclusions has been developed. The inclusions are opened by decrepitation in vacuum, and the computer recognizes the abrupt rise in pressure and controls the mass spectrometer. This scans continuously from 1 to 65 amu every 25 ms, which is within the time constraints of a bursting inclusion. The peak height for each mass in each spectrum is measured and stored along with the background data. Data are reduced after analysis. Each mass number is assigned its own attenuation factor by the computer in a preliminary analysis. This permits the determination of trace components down to 1 part in 10,000. With this system it is possible to analyze up to 225 inclusions in 1 h using a 10-mg sample of quartz, calcite, plagioclase, pyroxene, galena, or other mineral.

In the thesis, we studied fluid inclusions in minerals and mineralization epochs of Luziyuan Pb-Zn deposits in eastern of Zhenkang Yunnan province. 240 available data have been obtined by observing characteristics of fluid inclusions in sphalerite, quartz, fluorite, calcite under microscope and selecting 284 primary inclusions and secondary inclusions thermometry. Various mineralization epochs homogenization temperature, freezing temperatures, salinity frequency histogram showed a bimodal distribution mode. The fluid inclusions in sphalerite have a greatly important,it is ore-forming fluids. Its homogenization temperature distributed in 240 to 270°C (medium temperature) and 187 to 220°C (low temperature) within the range of two, (W NaCl) salinity is 5.0% to 10.6%. According to the homogenization temperature and salinity, it can be deduced Luziyuan mineralization fluids density of 0.82 to 0.96 (gcm-3). Draw the deposits are mainly two hydrothermal mineralization, a hydrothermal in medium temperature, another period of low and medium temperature. The two ore-forming fluids are medium salinity and medium-density.

Application of densimetry using micro-Raman spectroscopy for CO2 fluid inclusions: a probe for elastic strengths of mantle minerals

Fluid inclusions in minerals hold the potential to provide important data on the chemistry of the ambient fluids during mineral precipitation. Especially interesting to astrobiologists are inclusions in low-temperature minerals that may have been precipitated in the presence of microorganisms. We demonstrate that it is possible to obtain data from inclusions in chemosynthetic carbonates that precipitated by the oxidation of organic carbon around methane-bearing seepages. Chemosynthetic carbonates have been identified as a target rock for astrobiological exploration. Other surficial rock types identified as targets for astrobiological exploration include hydrothermal deposits, speleothems, stromatolites, tufas, and evaporites, each of which can contain fluid inclusions. Fracture systems below impact craters would also contain precipitates of minerals with fluid inclusions. As fluid inclusions are sealed microchambers, they preserve fluids in regions where water is now absent, such as regions of the martian surface. Although most inclusions are < 5 microns, the possibility to obtain data from the fluids, including biosignatures and physical remains of life, underscores the advantages of technological advances in the study of fluid inclusions. The crushing of bulk samples could release inclusion waters for analysis, which could be undertaken in situ on Mars.

In order to extract fluid inclusions in minerals, a new type ball-mill made of Pyrex glass designed by Kita (1981) was used. Samples used in this study were hydrothermal vein quartz, fluorite and arsenopyrite. Under suitable conditions, gases in primary inclusions could be extracted with little contribution of secondary inclusions. The conditions found adequate for quartz and fluorite samples from the Takatori mine are as follows; 1) initial grain size of the sample is 6 ∼ 10 mesh, and the amount of sample is about 4g, 2) preheating temperature of sample is set at the filling temperature of primary inclusions, and the preheating period is more than 12h in vacuum, 3) the duration period of crushing using an alumina ball for 30 ∼ 60min is necessary, and 4) the duration of gas-recovery procedure for more than 3h with the heating of the mill at the same temperature of preheating is necessary for the recovery of gasses adsorbed on the powdered sample. For quartz and fluorite samples, isotopic compositions of extracted gases including H2O, CO2 and CH4 were reproduced well. The CO2/H2O ratio obtained for fluid inclusions in quarts from the Takatori tungsten mine, Japan, varied considerably according to the difference in the condition of ball-milling. For arsenopyrite sample the isotopic results were scattered widely because SO2 was evolved during crushing even at room temperature, which may be due to the reaction between arsenopyrite and H2O. When this method is applied to extracting fluid inclusions in minerals, the experimental conditions such as 1) and 2) should be changed depending on the nature of samples, especially the filling temperature of primary inclusions is important to set the preheating temperature. Disadvantages inherent in the ball-milling method are, 1) incomplete extraction of fluid inclusions and 2) the adsorption of gases onto the powder surface. The latter can be overcome by heating the powdered sample during the recovery process of gases, which was proven by the simulation test. As far as the isotopic compositions of H2O, CO2 and CH4 extracted from fluid inclusions are concerned, disadvantages given above have no effect on the result.