The porphyry type of hydrothermal mineral deposits is of paramount economic importance because deposits of this type host much of the world’s reserves of Cu, Mo, and Re, as well as much Re and remarkable reserves of critical elements such as Ag, Pd, Te, Se, Bi, Zn, and Pb (e.g., ^[1][2][3][4][1,2,3,4]). Porphyry deposits are conventionally classified into mineralogical–geochemical types according to their dominant valuable components, e.g., [3]. The most economically important porphyry-type deposits are those of the Cu (Au), Cu–Mo (Au), Mo, and Au types.

Porphyry deposits have been studied for a long time, and their features have been much discussed in many reviews, e.g., [5]. Detailed reviews were published on the geology and geotectonic settings of porphyry deposits, zoning of the wall-rock metasomatites, and spatiotemporal relations between the ore mineralization and magmatism in porphyry systems, e.g., ^{[3][4][6][7][8][9]}[3,4,6,7,8,9].

Porphyry deposits contain large reserves of ores with low Cu, Mo, and Au concentrations and are genetically linked to the emplacements and crystallization of melts ranging from diorite to granite in composition. Magma bodies that generate porphyry mineralization are usually constrained to plate margins, e.g., ^{[1][2][3][4][7][8]}[1,2,3,4,7,8]. Porphyry deposits are usually zonal [10] and are stockworks of disseminated and stringer accumulations of sulfides and oxides hosted in large (up to 10 km³) volumes of hydrothermally altered rocks, which were produced by the large-scale circulation of hydrothermal fluids at upper crustal levels ^{[2][3][4][11][12][13][14]}[2,3,4,11,12,13,14]. Porphyry deposits were found in continental magmatic belts worldwide, showing evidence of spatiotemporal and genetic relations to hypabyssal porphyritic diorite and granodiorite intrusions, which were produced by water-rich magmas.

According to the orthomagmatic model ^{[4][10][15][16]}[4,10,15,16], porphyry copper mineralized magmas are usually emplaced into upper crustal levels (at depths of approximately 5–10 km). The gradually cooling melts approach their saturation with volatile components ^[17][18][17,18]. As soon as the melt reaches its saturation with volatiles, a phase of aqueous magmatic fluid (whose salinity is not high) is separated from the melt ^[17][18][17,18], and some elements (including sulfur, chlorine, copper, and some other metals) are, therewith, transferred from the melt into the aqueous phase to form mineralized aqueous magmatic fluid ^{[3][19][20][21][22][23][24][25][26][27][28][29][30][31]}[3,19,20,21,22,23,24,25,26,27,28,29,30,31]. Magmatic fluid ascends along fractures and cracks into the already-solid parts of the intrusions, alters host rocks, and comes, due to the pressure decrease, to the field of two-phase equilibrium, in which pressure is lower than 1300 bar (i.e., the region of ore deposition). In this region, the fluid exsolves (heterogenizes) into two phases: chloride brine and a low-density aqueous fluid ^[32][33][32,33]. The heterogenization of the fluid triggers the onset of ore deposition. The fluids then continue cooling, interacting with rocks and diluted meteoric waters, and depositing ore and gangue minerals ^{[4][12][14][16]}[4,12,14,16]. Relics of the mineralizing fluids are captured as fluid inclusions, which can provide a record of the evolution of the parameters and composition of the fluids with time [34]. Many researchers have demonstrated that parameters of mineral-hosted fluid inclusions in porphyry hydrothermal systems systematically vary in space and with time, e.g., ^{[26][31][34][35][36][37][38][39]}[26,31,34,35,36,37,38,39]. A model developed for H₂O–NaCl fluid is able to realistically describe the distributions of various types of fluid inclusions (halite-bearing brine inclusions, gas inclusions, and liquid-rich two-phase inclusions) over the volumes of porphyry copper deposits [40]. Assemblages of brine and gas fluid inclusions mark domains with high-grade ores. It is important to specify that the fluid inclusion assemblage (FIA) is usually defined as the most finely discriminated group of cogenetic fluid inclusions occupying an individual petrographic feature (e.g., crystal growth-zone or healed fracture), as is unambiguously recognizable by microscopic methods. This model is able to predict the composition and parameters of fluid inclusions captured by minerals when porphyry fluid–magma systems spatiotemporally develop, which enables one to utilize these data for various practical purposes. Although porphyry deposits were studied in much detail, it is still interesting to correlate current understandings of their genesis with the available data on mineral-hosted fluid inclusions in porphyry Cu–Mo–Au systems.

It is pertinent to mention the following reviews of fluid inclusions in the minerals of porphyry systems, e.g., ^{[5][26][34][38][41][42][43][44][45]}[5,26,34,38,41,42,43,44,45]. Some reviews were devoted to the parameters and composition of fluid inclusions in minerals from porphyry deposits, e.g., ^[31][46][47][31,46,47]. Fluid inclusions hosted in minerals at porphyry deposits are still actively studied, and extensive newly acquired data were published after the aforementioned reviews. Differences between the physicochemical parameters of fluids that produced all of the four types of porphyry deposits in the Cu–Mo–Au system still have not been adequately analyzed in the literature, likely because of the very broad variations in the homogenization temperature and salinity of the fluid inclusions.

2. Characterization of Fluids at Porphyry Deposits

When fluid inclusions are characterized, their descriptions are, conventionally, begun with descriptions of how these inclusions look like at room temperature. Fluid inclusions hosted in minerals from porphyry deposits can be grouped into the following three types, according to the phase composition of these inclusions at room temperature (Figure 12). Type 1. Fluid inclusions of high-salinity brines: these inclusions contain an aqueous solution of high salinity (>26–30 wt % equiv. NaCl), a vapor bubble, and one or more daughter crystals. Among the latter, halite, sylvite, opaque or transparent red crystals of hematite and anhydrite have been identified [34]. The triangular transparent phases are usually identified as chalcopyrite crystals ^[26][31][34][26,31,34], which provide evidence of a high Cu concentration in the fluid. Recent Raman spectroscopic studies have identified daughter phases of javorieite KFeCl₃ ^[48][124] and magnetite ^[49][125]. Type 2. Gas-rich fluid inclusions (> 70 ± 10 vol % gas). Type 3. Aqueous salt two-phase fluid inclusions, whose salinity ranges from low to intermediate, and which contain a gas bubble that occupies 30 ± 10 vol %. Some authors, [46] and others, distinguish an individual type of gas fluid inclusions of intermediate density, which contains an aqueous solution and vapor in equal proportions (L ≈ V), and which, also, sometimes host a small opaque crystal. It is thought that these fluid inclusions captured magmatic fluid at a high temperature and pressure in the region of homogeneous fluid above the vapor−liquid boundary in the system H₂O−NaCl, and sometimes contain CO₂ ^[50][51][52][51,85,87]. Such fluid could be entrapped in fluid inclusions at deeper levels than those where the ore mineralization was deposited. Due to this, such fluid inclusions cannot be numerous, and they cannot significantly modify the general situation. Herein, rwesearchers attribute these inclusions to type 2, if such inclusions are mentioned in publications. Figure 23 and Table 12 show the variations of parameters of mineralizing fluids collectively for all of the fluid inclusions, without subdividing them into types, but only the general ranges of the homogenization temperatures, salinity, and densities of the fluids. Note that the data on low-density fluid inclusions containing a gas phase can be incomplete due to purely technical reasons, namely, due to the small volumes of aqueous fluids in these inclusions, which often make it impossible to microthermometrically study these inclusions. This shall be taken into account when dealing with the data on fluid inclusions. The gas constituent of mineralizing fluid at porphyry deposits is not discussed in this papentryr because such information is still insufficient and because this information cannot characterize all types of porphyry deposits. Overall ranges of the physicochemical parameters. The principal parameters of mineralizing fluids at individual porphyry deposits of various types in the Cu−Mo−Au system are listed in Table 12. In general, the ranges of the principal physicochemical parameters of fluids at porphyry deposits are fairly broad, as follows from the data on 2414 groups of fluid inclusions (Figure 23, Table 23): the homogenization temperatures of the fluid inclusions range from 90 to 957 °C (388 °C on average). As seen in the histogram in Figure 34, most of the fluids were entrapped into fluid inclusions at temperatures of 200 to 500 °C. Note that publications usually present homogenization temperatures of fluid inclusions. However, it is now acknowledged that mineral-forming processes at porphyry deposits begin when the fluid becomes heterogeneous ^[32][33][32,33]. It is known that if fluid inclusions are entrapped on a two-phase equilibrium curve, the homogenization temperatures of these inclusions are equal to their entrapment temperatures [34]. Hence, the homogenization temperatures of early mineral-hosted fluid inclusions at porphyry deposits usually correspond to their entrapment temperatures, which allowed reusearchers to discuss the fluid temperatures here.