A method for determining the deep extension of polymetallic ore bodies in magmatic hydrothermal deposits using fluid inclusions

Abstract

The present invention discloses a method for determining the deep extension of polymetallic ore bodies in magmatic hydrothermal deposits using fluid inclusions. This method systematically conducts geochemical research on fluid inclusions based on large-scale structural-alteration petrographic mapping to identify the physicochemical conditions, such as temperature and salinity, of the ore-forming fluids, as well as the fluid evolution characteristics, and estimate the mineralization depth. Based on the spatial distribution characteristics of geological-temperature and geological-salinity anomaly maps, a prospecting model for hydrothermal polymetallic deposits is constructed, deep-extending indicators of vertical changes in fluid inclusion temperature and salinity are extracted, and the deep-extending patterns of the polymetallic ore bodies are determined. This method solves the technical difficulty of locating deep ore bodies in magmatic hydrothermal deposits, helps infer the spatial positioning patterns of deep ore bodies (groups), and provides an important geological basis for deep resource exploration and shortening the prospecting cycle.

Description

Technical Field

[0001] The present invention relates to a method for determining the deep extension of a polymetallic ore body in a magma hydrothermal deposit by utilizing fluid inclusions, and belongs to the field of deep-earth mineral resource exploration. Background Art

[0002] The formation of hydrothermal deposits is inseparable from the participation of ore-forming fluids. Fluid inclusions, as direct records of magmatic and hydrothermal fluid activity trapped in mineral lattices, have long been recognized and utilized by geologists. Since the study of fluid inclusions was introduced into the field of Chinese geology in the 1960s, the continuous advancement and enrichment of research methods have led to a continuous increase in the depth and level of research in ore deposits and other fields. The study of fluid inclusions can provide accurate information on the physical and chemical properties of ore-forming fluids, such as temperature, salinity, composition, pressure, density, pH, and Eh. Therefore, it is one of the most important means and methods for studying the properties of ore-forming fluids, their temporal and spatial evolution characteristics, the depth of rock formation and mineralization, and the mechanism of mineralization in hydrothermal mineralization systems.

[0003] As the mining of shallow surface ore bodies is exhausted, deep prospecting breakthroughs are urgently needed. To this end, predecessors have systematically summarized and discussed the maximum mineralization depth and maximum deep extension of hydrothermal deposits using fluid inclusions, and believe that there is still a large prospecting space in the deep. Based on the current status of previous research, the following major problems still exist: (1) Most fluid inclusion studies tend to explore the origin, properties, evolution and other characteristics of ore-forming fluids from a qualitative perspective, which helps to understand the changes in fluid properties and the mineralization process in different periods. Due to the differences in the use conditions of multiple parameters, there are uncertainties and multiple solutions when exploring the characteristics of ore-forming fluids, which also makes it difficult to reconstruct the migration direction, distance, positioning mechanism, etc. of fluids during the mineralization period. (2) There is a lack of in-depth research on the quantitative evaluation of ore-forming fluid indicators from shallow to deep, and there is little mention of the rapid establishment of fluid quantitative evaluation indicators. (3) In previous fluid inclusion studies, there are relatively few studies on the spatial changes of temperature and salinity at different elevations, and they have not been effectively combined with the geological facts of the ore deposits. As a result, it is still difficult to effectively construct favorable ore-forming structural locations, and the guiding significance for deep ore body exploration is limited. Therefore, it is necessary to propose a method to quantitatively evaluate the depth and extension of ore bodies using fluid inclusion geochemical data. Summary of the Invention

[0004] In response to the existing technical difficulties, the present invention proposes a method for determining the deep extension of polymetallic ore bodies in magmatic hydrothermal deposits by using the vertical variation index of the temperature and pressure conditions of fluid inclusions. The method is carried out in the following steps:

[0005] Step 1: Conduct a field geological survey of a specific magmatic hydrothermal deposit and use large-scale structural-alteration petrographic mapping to collect mineralized-altered samples of different elevations and types from the hydrothermal deposit, and record their spatial coordinates, expressed as X and Y. Identify the paragenetic associations of the collected mineralized-altered minerals and develop a generation sequence of the mineralized-altered minerals. Then, select translucent-transparent minerals from the hydrothermal mineralization stage from the mineralized-altered samples and process them into double-sided polished fluid inclusion slices with a thickness of 0.1-0.2 mm.

[0006] Step 2: Fluid inclusion analysis and testing

[0007] The morphology, size, type, and distribution characteristics of fluid inclusions in fluid inclusion slices are observed using an optical microscope to identify primary inclusions with regular morphology and stable gas-liquid ratios captured during mineral growth. A hot and cold stage device for fluid inclusion microthermometry is used to obtain the homogenization temperature, freezing point temperature, and melting temperature of daughter minerals of primary inclusions in hydrothermal minerals at different mineralization stages. The salinity of the primary inclusions is then calculated based on the freezing point temperature and melting temperature of daughter minerals.

[0008] In order to obtain test data that can represent the properties of the fluid at a certain stage, it is necessary to eliminate the secondary inclusions in the minerals and select the primary inclusions for testing;

[0009] The division of the mineral formation sequence table is based on the comprehensive judgment of the macroscopic and microscopic relationships between minerals and veins of different periods, such as interpenetration, inclusion, contact, and replacement, to identify the symbiotic combination relationship of minerals; the mineral formation sequence of different magmatic hydrothermal deposits varies due to the differences in hydrothermal minerals;

[0010] The homogenization temperature, freezing point temperature, and melting temperature of daughter minerals were determined according to the method described in Lu Huanzhang, Fan Hongrui, Ni Pei, et al. Fluid Inclusions[M]. Beijing: Science Press, 2004: 132-208.

[0011] Step 3: Test data processing

[0012] The coordinates of each sampling point and the salinity of primary inclusions of hydrothermal minerals at different mineralization stages are used to construct a primary inclusion salinity database, which is expressed as X, Y and salinity. The coordinates of each sampling point and the homogenization temperature of primary inclusions of hydrothermal minerals at different mineralization stages are used to construct a primary inclusion homogenization temperature database, which is expressed as X, Y and homogenization temperature.

[0013] Step 4: Construction of fluid inclusion extension index and identification of ore body extension law

[0014] A. Import the primary inclusion salinity database and the primary inclusion homogenization temperature database into the data analysis software, and draw the salinity and temperature contour maps of primary inclusions in different middle sections and profiles respectively;

[0015] B. Superimpose the salinity and temperature contour maps of primary fluid inclusions in different mid-sections and profiles with the actual geological map of the deposit at the same scale to obtain the geology-temperature and geology-salinity anomaly contour maps. Take the uniform temperature and salinity at the time of the latest hydrothermal mineral formation as the lower limit of the anomaly, circle the high-value areas of temperature and salinity anomalies on the geology-temperature and geology-salinity anomaly contour maps, and analyze the distribution characteristics of the anomalies in the high-value areas, including the shape, range, number, and size of the anomalies. Summarize the changing patterns of the high-value anomaly areas, especially the changing patterns of the anomaly areas in the profile contour maps.

[0016] C. Extract qualitative and quantitative indicators to judge the depth of ore bodies

[0017] Based on the primary inclusions identified in step 2, the gas-liquid ratio and the relative content of daughter mineral inclusions are used as qualitative indicators for evaluating the deep extension of the ore body;

[0018] Based on the geological-temperature and geological-salinity anomaly contour maps obtained in step B, a temperature contour map cross-section and a salinity contour map cross-section are drawn from the surrounding rock to the ore body and from the shallow to the deep, thereby obtaining a contour line curve from the low-value anomaly area to the high-value anomaly area. Based on the curve curve, the variation range of the width, length and vertical difference of the primary inclusion temperature and salinity high-value anomaly area on the plane and on the section is obtained, and the range is used as a quantitative indicator for evaluating the depth extension of the ore body;

[0019] D. Based on the qualitative and quantitative indicators obtained in step C, when the gas-liquid ratio of primary inclusions and the number of daughter mineral inclusions in the same geological body gradually increase from the surrounding rock to the ore body and from the shallow to the deep, and at the same time, the length, width and vertical difference of the high-value abnormal areas of temperature and salinity of the primary inclusions show an increasing or sudden change trend, it is inferred that the ore body extends to the deep;

[0020] Combined with the geological facts of existing shallow middle-section deposits, the relationship between quantitative indicators and the spatial position of known shallow ore bodies is compared. The approximate migration direction of the fluid is determined based on the offset direction of the anomaly center of the high-temperature-high-salinity anomaly area in different middle sections and different profiles on the geological-temperature and geological-salinity anomaly contour maps. The deep extension distance of the polymetallic ore body is comprehensively analyzed based on the periodic variation range of length, width and vertical difference.

[0021] Advantages and technical effects of the method of the present invention:

[0022] (1) Based on the study of fluid inclusions, the uncertainty and multi-solutions in exploring the characteristics of mineralizing fluids caused by differences in the use of multiple parameters are overcome, providing a reliable solution for reconstructing the evolution of fluids during the mineralization period.

[0023] (2) This method can quickly establish quantitative evaluation indicators for fluid geochemistry from shallow to deep.

[0024] (3) This method is simple and practical, which can effectively reduce exploration costs, identify the deep extension law of the ore body, and thus shorten the deep prospecting and exploration cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a diagram for the mineralization stage division of a magmatic hydrothermal deposit in southern Hunan;

[0026] Figure 2 Micrographs of different types of fluid inclusions in a magmatic hydrothermal deposit in southern Hunan: a-liquid-rich two-phase isolated inclusion in fluorite; b-liquid-rich gas-liquid two-phase inclusion group in fluorite; c-daughter crystal-containing three-phase inclusion in fluorite; d, e-pure liquid inclusions; f, g-boiling inclusion group in fluorite, liquid-rich, gas-rich, and daughter crystal-containing three-phase inclusions; h, i-CO2-rich inclusions in fluorite; j-daughter crystal-containing three-phase inclusion in fluorite; k-gas-liquid two-phase inclusion in calcite; l-gas-liquid two-phase inclusion in garnet; L is liquid phase; V is gas phase; S is solid phase.

[0027] Figure 3 The homogenization temperature and salinity histograms of fluid inclusions in a magmatic hydrothermal deposit in southern Hunan Province are shown in the figure below: a. Homogenization temperature histogram of fluid inclusions; b. Salinity histogram of fluid inclusions. 1 - early skarn stage; 2 - late skarn stage; 3 - early sulfide stage; 4 - late sulfide stage.

[0028] Figure 4 Distribution maps of homogenized temperature and salinity anomalies in different planes and sections of a magmatic hydrothermal deposit in southern Hunan: a. Homogenized temperature contour map of fluid inclusions in altered minerals in the middle section at -136 m; b. Salinity contour map of fluid inclusions in altered minerals in the middle section at -136 m; c. Homogenized temperature contour map of fluid inclusions in altered minerals in the middle section at -256 m; d. Salinity contour map of fluid inclusions in altered minerals in the middle section at -256 m; e. Homogenized temperature contour map of fluid inclusions in altered minerals in sections at -136 m, -176 m, and the southern part of the middle section at -256 m; f. Salinity contour map of fluid inclusions in altered minerals in sections at -136 m, -176 m, and the southern part of the middle section at -256 m.

[0029] Figure 5 The prospecting model and deep prospecting direction of a magmatic hydrothermal deposit in southern Hunan; a. Metallogenic model of the Huangshaping deposit; b. Schematic diagram of the vertical "expansion-contraction-expansion" of the rock mass and alteration, and the tangential vertical fluid inclusion temperature and salinity deep extension indicators; c. Box plot of rock mass → surrounding rock fluid inclusion temperature and salinity, and the lateral evolution characteristics of inclusion types; d. Evolution pattern of the gas-liquid ratio of fluid inclusions and the relative content of daughter mineral inclusions in different mineralization stages. DETAILED DESCRIPTION

[0030] The present invention will be further described in detail below by way of examples, but the scope of protection of the present invention is not limited to the contents described above. The methods in the examples are conventional methods unless otherwise specified. The determination of homogenization temperature, freezing point temperature, melting temperature of daughter minerals and salinity conversion refer to the methods in Lu Huanzhang, Fan Hongrui, Ni Pei, et al. Fluid Inclusions [M]. Beijing: Science Press, 2004: 132-208.

[0031] Example 1: The method of the present invention was implemented in a magmatic hydrothermal copper-lead-zinc polymetallic deposit in southern Hunan, and good prospecting progress was achieved. The specific content is as follows:

[0032] This deposit is a typical deposit in southern Hunan, boasting a long mining history and excellent geological conditions for mineralization. The area boasts a rich variety of mineral resources, primarily copper, tin, tungsten, molybdenum, lead, zinc, and silver. The ore minerals are primarily sphalerite, galena, pyrrhotite, molybdenite, scheelite, pyrite, and chalcopyrite, while the gangue minerals are primarily quartz, calcite, fluorite, diopside, garnet, and actinolite. The ore grade is high, the mineralization conditions are favorable, and the area possesses excellent prospecting potential. The main mineralization types in the mining area include skarn, hydrothermal filling and replacement, and porphyry (unmineralized). The tungsten-molybdenum polymetallic ore bodies are typical skarn deposits. The mining area is characterized by intense magmatic and hydrothermal activity, and the contact zone between the rock mass and the surrounding carbonate rock, as well as intense alteration and recrystallization, is characterized by strong skarnization, sericitization, silicification, and epidote formation on the SE side of the quartz porphyry and in the contact zone between the granite porphyry and the surrounding rock. Numerous tungsten-molybdenum ore bodies develop along the skarnization zone, and the skarns are closely distributed around the rock mass contours. Intense recrystallization and hydrothermal calcitization occur within the surrounding rock mass, with alteration gradually weakening from the fault to the sides.

[0033] The specific implementation steps are as follows:

[0034] 1. Rock and mineral sample collection, sample handling and processing

[0035] On the basis of detailed field investigation of the geological characteristics of the deposit, the required research samples were collected according to different mineralization stages and different elevations (92m, -96m, -136m, -176m, -256m and the middle section of -296m), and their spatial coordinates were recorded, expressed as X and Y. They mainly include garnet skarn, sphalerite ore, quartz, fluorite and calcite;

[0036] The mineral symbiosis of the deposit was identified by analyzing macroscopic and microscopic mineralization across different periods, as well as interpenetration, inclusions, contacts, and replacements between veins. The deposit was divided into two periods and four stages: the skarn stage and the quartz-sulfide stage. The skarn stage includes the magnetite (scheelite, cassiterite) stage and the scheelite (molybdenite) stage. The magnetite (scheelite, cassiterite) stage, representing the early skarn stage, is characterized by euhedral and subhedral garnet minerals. Fluorite closely associated with epidote, hornblende, and actinolite, or replacing early garnet and filling garnet cavities, was selected as the primary mineral type for fluid inclusion analysis in the scheelite (molybdenite) stage, representing the late skarn stage. Fluorite closely associated with epidote, hornblende, and actinolite, or replacing early garnet and filling garnet cavities, was selected as the primary mineral type for fluid inclusion analysis in this stage. The sulfide stage includes the pyrite (galena, sphalerite) stage and the galena and sphalerite stages. The pyrite and molybdenite (galena and sphalerite) stage is the early sulfide stage, which is mainly characterized by molybdenite and symbiotic pyrite, which replace the early skarn minerals. Therefore, fluorite that replaces epidote, actinolite, and garnet minerals in the late skarn stage, and fluorite and calcite that are closely symbiotic with pyrite are selected as the main mineral types for fluid inclusion research in this stage. The galena and sphalerite stage is the late sulfide stage, which is characterized by sphalerite and galena replacement, and pyrite ( Figure 1 ), so fluorite and calcite, which are closely associated with lead-zinc ore, were selected as the main minerals for fluid inclusion research in this stage.

[0037] The required fluid inclusion samples were collected from skarns, quartz veins and calcite veins in the middle section of 92, the middle section of -96, the middle section of -136m, the middle section of -176m, the middle section of -256 and the middle section of -296 in the mining area. Systematic sample collection was carried out in the southern section of -136m, the southern section of -176m and the southern section of -256. The samples were processed into double-sided polished fluid inclusion slices with a thickness of 0.1-0.2mm. The samples were systematically identified in the laboratory, and representative samples were selected for detailed fluid inclusion petrographic research and microthermometry analysis.

[0038] 2. Fluid inclusion analysis and testing

[0039] Based on the microscopic identification device, the primary inclusions of the deposit are divided into four types: Type I gas inclusions: This type of inclusion is rare and is only found in the boiling inclusion group in fluorite. It is a single phase or the gas phase composition is greater than 95% at room temperature. The edges of this type of inclusion are dark and the center is slightly bright. Type II aqueous solution inclusions: It is distributed in all stages of mineralization and is the most common type of inclusion in various minerals. The individual variation range at room temperature is large and the shape is diverse. It is often elliptical, negative crystal, and irregular. According to the gas-liquid ratio of the inclusions and the homogeneous phase state during the heating process, it can be divided into three subtypes. Among them, Type IIa, liquid-rich gas-liquid two-phase inclusions are widely distributed in various types of minerals. The gas phase component is water vapor and the gas-liquid ratio is less than 50%. In fluorite, this type of inclusion is often distributed in groups or in the form of beads. It rarely appears in isolation. The individual inclusions are large (15-20μm on average), and the largest can reach 40-60μm. In calcite and garnet, this type of inclusion is relatively large. Small (average 5-10 μm) and less in number than in fluorite. In garnet, inclusions are generally distributed in its ring zone, but rarely distributed in groups. Most of them are isolated with an average gas-liquid ratio of 10-20%. In calcite, most of them are elliptical along their joints, and the long axis is much larger than the short axis. The gas-liquid ratio is generally 20%; Type IIb, gas-rich gas-liquid two-phase inclusions: can be seen in fluorite, relatively less than Type IIa, with a gas-liquid ratio of >60%, the gas phase component is water vapor, and it eventually becomes a gas phase during the heating process. In the boiling inclusion group, the gas-liquid ratio of this type of inclusion is between 70-90%, and the size of a single inclusion is 5-15 μm; Type IIc pure liquid inclusions are a single liquid phase at room temperature and are commonly found in fluorite ( Figure 2 d, e), the inclusions have clear boundaries and are often distributed in isolation. The size of the inclusions ranges from 4 to 8 μm. This type of inclusion is often seen in conjunction with type IIa and IIb inclusions. No phase change was observed in this type of inclusion during the temperature measurement. Type III CO2-rich three-phase inclusions have typical "double eyelid" features at room temperature. This type of inclusion is relatively rare and is only found in fluorite. Type IV daughter mineral-containing multiphase inclusions are developed in large quantities in fluorite. The daughter minerals are mostly bright green. During the heating process, this type of inclusion shows the characteristics of bubbles disappearing first and daughter minerals becoming uniform later. They are mostly in conjunction with type IIa and IIb. Individual sizes vary, and the largest can reach about 15 μm. During the temperature measurement process, it was also found that the daughter minerals did not disappear even after being heated to fracture ( Figure 2 ).

[0040] The uniform temperature, freezing point temperature and melting temperature data of primary inclusions at different stages were obtained by micro-temperature measuring device ( Figure 3) The salinity of fluid inclusions was calculated based on the freezing point of primary inclusions and the melting temperature of daughter minerals. The primary inclusion temperatures for different stages of the deposit were obtained as follows: Magnetite (scheelite, cassiterite) stage: Garnet is a typical representative mineral of early skarn minerals. Inclusions are mostly isolated, with occasional groups of three or two. Type IIa inclusions are predominantly small. Due to the instrument's maximum test temperature of 600°C, some inclusions in the garnet are not homogenized after heating to 600°C, indicating temperatures >600°C. Due to the dark field of view and numerous impurities in the garnet during temperature measurement at this stage, observation is difficult, resulting in fewer test results. Its homogenization temperature ranges from 503.7°C to >600°C, with an average of >568.8°C and a major peak at 547.2°C to >600°C. Salinity ranges from 16.53% to 23.11%, indicating high-temperature, high-salinity fluid properties. The test also revealed the presence of boiling inclusions in the garnet. These inclusions are classified as Type I, IIa, IIb, III, and IV, with Type IIb being the predominant. Their homogeneous phase is gaseous. These inclusions have a relatively low homogenization temperature compared to other types of inclusions in the garnet, averaging 530.8°C, and salinity ranging from 4.3% to 20.7%. In the scheelite and molybdenite phases, the homogenization temperature ranges from 196.8°C to 404.6°C, averaging 347.2°C. The freezing-point melting temperature of the inclusions ranges from 13.72% to 20.97%, with an average of 17.43%. In the quartz-sulfide period, pyrite and molybdenite (chalcopyrite) stage, type IIa, IIb and IV inclusions are mainly developed. Type IIa inclusions are commonly developed in fluorite, which are distributed in groups and are generally large, 10-20μm. The homogenization temperature of type IIa inclusions is between 118.6 and 331.8℃, with peaks at 213.8-270.6℃ and an average of 230.7℃. The salinity is between 1.4% and 22.65%, with peaks concentrated at 4.1%-14.7% and an average of 8.4%. Type IV is also common in this stage. Bubbles often reach homogenization before daughter minerals. The bubble disappearance temperature is between 154.6 and 212.4℃. When the temperature rises to 283.1℃, the daughter minerals begin to disappear until they reach complete homogenization at 361.2℃, indicating that their salinity ranges from 36.63% to 43.34% and an average of 38.9%. Sphalerite and galena stage: The main tested minerals in this stage are calcite and fluorite, which are closely related to Pb-Zn mineralization. The inclusions in calcite in this stage are small (≈7-10μm), but the inclusions in fluorite are generally large (≈15-30μm, and the largest can reach 60μm). Type IIa inclusions are mainly developed. The homogenization temperature range is between 94.3 and 207.6℃, with an average of 152.5℃. The salinity ranges from 0.53% to 14.25%, with an average of 6.69%.

[0041] 3. Data Processing

[0042] The coordinates of each sampling point and the salinity of primary inclusions of hydrothermal minerals at different mineralization stages were used to construct a primary inclusion salinity database, expressed as X, Y, and salinity. The coordinates of each sampling point and the homogenization temperature of primary inclusions of hydrothermal minerals at different mineralization stages were used to construct a primary inclusion homogenization temperature database, expressed as X, Y, and homogenization temperature. The primary inclusion salinity database and the primary inclusion homogenization temperature database were imported into MAPGIS 6.7 software to draw the salinity and temperature contour maps of primary inclusions at the -136m and -256m middle and southern joint sections, respectively.

[0043] 4. Construction of Fluid Inclusion Deep Extension Index and Identification of Orebody Deep Extension Laws

[0044] 4.1 Construction of Fluid Inclusion Deep Extension Index

[0045] In order to extract the mineralization condition information indicating the extension of polymetallic ore bodies in the horizontal and vertical directions, based on the study of typical characteristics of the deposit, the salinity and temperature contour maps of the primary inclusions in the 136m, -256m middle and southern joint sections were superimposed with the actual geological map to obtain the geological-temperature ( Figure 4 a, c, e) and geological-salinity anomaly contour maps ( Figure 4 b, d, f), taking the homogenization temperature of 152.5℃ and salinity of 6.7wt% when the latest hydrothermal minerals were formed as the lower limit of the anomaly, the high-value areas of temperature and salinity anomalies were circled on the geological-temperature and geological-salinity anomaly contour maps, and using the automatic section generation module of Mapgis 6.3 software to draw the cross-section from the rock mass to the surrounding rock, from the shallow to the deep, thus obtaining the contour line curve from the low-value anomaly area to the high-value anomaly area ( Figure 4 The right curve of each small figure is as follows:

[0046] (1) Qualitative indicators: namely, the relative content of daughter mineral inclusions and the gas-liquid ratio. In the dry skarn stage, the main inclusions are gas-rich gas-liquid two-phase inclusions and daughter crystal three-phase inclusions; in the wet skarn stage, the main inclusions are liquid-rich gas-liquid two-phase inclusions; in the metal sulfide stage, the main inclusions are liquid-rich fluid inclusions and calcite daughter crystal three-phase inclusions; in the calcite stage, the main inclusions are liquid-rich gas-liquid two-phase inclusions. The fluid inclusions in different stages correspond to their spatial output. From the rock body to the surrounding rocks on both sides, as the fluid temperature and salinity gradually decrease, the pure gas phase and gas-rich gas-liquid two-phase inclusions gradually decrease. In the metal sulfide period, there are basically no pure gas phase inclusions, only a small amount of gas-rich phase inclusions. The three-phase high-salinity brine inclusions containing daughter crystals also gradually decrease, indicating that the fluid gradually transforms from magmatic hydrothermal fluid to low-temperature, low-salinity hydrothermal fluid. In terms of inclusion size, the inclusions in fluorite in the metal sulfide period are relatively large. On the one hand, the mineral characteristics of fluorite may contribute to the formation of fluid inclusions. On the other hand, it can also be explained that the decompression boiling of the fluid causes the rapid cooling of the ore-forming fluid, resulting in rapid crystallization of minerals. Through the observation of fluid inclusions from dry skarn to calcite stages, it is found that the gas-liquid ratio of primary inclusions in the four stages generally shows a decreasing trend, ranging from 60% to 90%, 30% to 55%, ~20%, and 10% to 20%, respectively; the relative content of daughter mineral inclusions decreases significantly from the rock body to the periphery, and the content of the four stages is between ~14%, 11-9%, ~5%, and 0%, respectively.

[0047] (2) Quantitative indicators: namely, the width, length and vertical difference of the fluid inclusion homogenization temperature and salinity anomaly zone, based on the geological-temperature and geological-salinity diagrams ( Figure 4 ), the vertical change indexes of different planes and sections are obtained as follows: 1) The cross section of the temperature contour map of the middle section of -136 shows ( Figure 4 a) The temperature decreases rapidly from high temperature to low temperature from east to west. Figure 4 a); The isotherm diagram in the middle section of -256 also has similar characteristics ( Figure 4 c), and indicates that the high temperature section is the location of skarn-type ore bodies, and its west side may be the favorable section of hydrothermal vein-type lead-zinc ore bodies, and the temperature change diagram of the NW-SE inclined section ( Figure 4 a, c) show obvious fluctuations, which is closely related to the spatial concave-convex distribution characteristics of the rock mass. The part from convex to concave is the main output part of the polymetallic ore body. The uniform temperature gradient zone of hydrothermal minerals is mainly distributed along the NW direction, indicating that the ore-forming fluid has the characteristic of migrating from SE to NW ( Figure 4a, c). 2) Both in plan and cross-section, the temperature and salinity contours show a gradually decreasing trend from the altered porphyry body toward the Shidengzi Formation limestone on either side (or outward), with the average temperature changing from 550°C to 90°C and the average salinity changing from 19wt% to 4wt%. This is consistent with the geological fact that high-temperature skarn minerals near the rock body gradually transition to medium- and high-temperature skarn minerals near the surrounding rock, and ultimately evolve into medium- and low-temperature hydrothermal metasomatic alteration minerals within the surrounding rock. 3) Oblique sections of the temperature contours at the -136m and -256m sections show that the two high-temperature centers are approximately 400m apart, suggesting the possibility of a third high-temperature anomaly center and a third concealed rock body on the SE side of the mining area, approximately 400m away. 4) The vertical joint profile in the southern part of the mining area shows that the contour gradient zones in the high-temperature and high-salinity areas at -136m and -256m are large, while the gradient zone at -176m is significantly smaller. Existing prospecting projects have shown that the vertical changes in the rock mass show the characteristics of "expansion-contraction-expansion" with a period of 120 to 140m. This may be the result of the opening and closing of the structure, that is, the pulsation of the structure. The change pattern of the contour gradient zones in the high-temperature and high-salinity areas is consistent with the actual exploration results. In addition, the homogenization temperature and salinity of the fluid inclusions in each stage in the vertical direction basically do not change, indicating that the fluid properties are relatively stable from deep to shallow. 5) The main ore bodies are distributed at the transition point from the inner concave (peak) to the outer convex (trough) of the ore-forming rock mass. According to the -136 and -256 m sections, the width of the high-temperature anomaly is between 140 and 200 m. The actual width of the ore body is less than or equal to half the wavelength of the anomaly width, which is 40 to 60 m. Combining the planar distribution characteristics of the two plane anomalies, the length of the main ore body is 400–600 m (1 / 2λ≈200–300 m) ( Figure 5 ad); From the vertical anomaly distribution diagram, it can be seen that the vertical difference of the main ore body is 60m (1 / 2λ≈120m)

[0048] Under the premise that other factors at depth remain unchanged, this regular change will also exist. Therefore, the change of inclusion temperature and salinity can be used as one of the indicators to judge the depth and extension of the ore body.

[0049] 5.2 Determination of ore body extension law

[0050] Mineralization is closely related to alteration processes such as skarnization, quartzization, and calcite formation, and these alterations are closely distributed around the rock mass. Based on the previously discussed estimates of mineralization depth and the established fluid inclusion extension index, the stable vertical fluid properties at each stage indicate a stable deep mineralization environment and a consistent source of ore-forming fluids. The distribution pattern of high-temperature areas in the temperature and salinity contour maps of the central and southern sections indicates a SE-shift of the center of mineralization and alteration, indicating a SE-extending distribution pattern for rock masses tightly encased in altered rocks, which is an important prospecting direction at depth. The vertical expansion-contraction-expansion pattern of altered rocks is caused by the morphological changes of the rock mass within the vertical space. Therefore, it is believed that the deep location where the rock mass changes from convex to concave is the main production site of polymetallic ore bodies. Vertically, from the surrounding rock to the ore body, from the middle section of 56m to the middle section of -296m, the gas-liquid ratio and the number of daughter mineral inclusions of the primary inclusions of the skarn-type polymetallic ore body on the granite porphyry side gradually increase with depth. Laterally, the hydrothermal alteration closely surrounding the rock body, the temperature and salinity of the fluid inclusions show a characteristic of gradually decreasing from the center of the granite porphyry body to the surrounding rock on the W side. Therefore, it is believed that the ore body continues to extend to the depth, and its length and width increase with increasing depth. The W side of the deep rock body may be a favorable prospecting location for lead-zinc ore bodies ( Figure 5 Based on quantitative and qualitative indicators, it is speculated that the polymetallic tungsten and molybdenum ore bodies may extend for more than 1.5 km, with a period of about 400 m. Thick and large ore bodies are distributed in the depressions of the rock mass.

Claims

1. A method for determining the deep extension of polymetallic ore bodies in magmatic hydrothermal deposits using fluid inclusions, characterized in that: Proceed as follows: Step 1: Conduct a field geological survey of a specific magmatic hydrothermal deposit and use large-scale structural-alteration petrographic mapping to collect mineralized-alteration samples of different elevations and types, and record their spatial coordinates. Identify the paragenetic associations of the collected mineralized-altered minerals, and stratify the generation sequence of mineralized-altered minerals. Then, select translucent-transparent minerals from the hydrothermal mineralization stage from the mineralized-altered samples and process them into double-sided polished fluid inclusion slices. Step 2: Fluid inclusion analysis and testing The morphology, size, type, and distribution characteristics of fluid inclusions in fluid inclusion slices are observed using an optical microscope to identify primary inclusions with regular morphology and stable gas-liquid ratios captured during mineral growth. A hot and cold stage device for fluid inclusion microthermometry is used to obtain the homogenization temperature, freezing point temperature, and melting temperature of daughter minerals of primary inclusions in hydrothermal minerals at different mineralization stages. The salinity of the primary inclusions is then calculated based on the freezing point temperature and melting temperature of daughter minerals. Step 3: Test data processing The coordinates of each sampling point and the salinity of primary inclusions of hydrothermal minerals at different mineralization stages are constructed into a primary inclusion salinity database; the coordinates of each sampling point and the homogenization temperature of primary inclusions of hydrothermal minerals at different mineralization stages are constructed into a primary inclusion homogenization temperature database; Step 4: Construction of fluid inclusion extension index and identification of ore body extension law A. Import the primary inclusion salinity database and the primary inclusion homogenization temperature database into the data analysis software, and draw the salinity and temperature contour maps of primary inclusions in different middle sections and profiles respectively; B. Superimpose the salinity and temperature contour maps of primary fluid inclusions in different mid-sections and profiles with the actual geological map of the deposit at the same scale to obtain the geology-temperature and geology-salinity anomaly contour maps. Take the uniform temperature and salinity at the time of the latest hydrothermal mineral formation as the lower limit of the anomaly, circle the high-value areas of temperature and salinity anomalies on the geology-temperature and geology-salinity anomaly contour maps, and analyze the distribution characteristics of the anomalies in the high-value areas, including the shape, range, number, and size of the anomalies. Summarize the changing patterns of the high-value anomaly areas and the changing patterns of the anomaly areas in the profile contour maps. C. Extract qualitative and quantitative indicators to judge the depth of ore bodies Based on the primary inclusions identified in step 2, the gas-liquid ratio and the relative content of daughter mineral inclusions are used as qualitative indicators for evaluating the deep extension of the ore body; Based on the geological-temperature and geological-salinity anomaly contour maps obtained in step B, a temperature contour map cross-section and a salinity contour map cross-section are drawn from the surrounding rock to the ore body and from the shallow to the deep, thereby obtaining a contour line curve from the low-value anomaly area to the high-value anomaly area. Based on the curve curve, the variation range of the width, length and vertical difference of the primary inclusion temperature and salinity high-value anomaly area on the plane and on the section is obtained, and the range is used as a quantitative indicator for evaluating the depth extension of the ore body; D. Based on the qualitative and quantitative indicators obtained in step C, when the gas-liquid ratio of primary inclusions and the number of daughter mineral inclusions in the same geological body gradually increase from the surrounding rock to the ore body and from the shallow to the deep, and at the same time, the length, width and vertical difference of the high-value abnormal areas of temperature and salinity of the primary inclusions show an increasing or sudden change trend, it is inferred that the ore body extends to the deep; Combined with the geological facts of existing shallow middle-section deposits, the relationship between quantitative indicators and the spatial position of known shallow ore bodies is compared. The approximate migration direction of the fluid is determined based on the offset direction of the anomaly center of the high-temperature-high-salinity anomaly area in different middle sections and different profiles on the geological-temperature and geological-salinity anomaly contour maps. The deep extension distance of the polymetallic ore body is comprehensively analyzed based on the periodic variation range of length, width and vertical difference.

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