Depth estimation method

Abstract

To provide a method capable of easily estimating a position of a deepest part of a mineral vein in a short period.SOLUTION: A depth estimation method of a hydrothermal vein such as a hydrothermal vein-type gold deposit, includes the steps of: measuring a temperature of each of a plurality of fluid inclusions contained in quartz collected from different depths of a target vein under prescribed conditions, for example, by microthermometry to calculate a temperature and a salt concentration of hydrothermal water when the hydrothermal water is incorporated into the quartz; calculating a boiling curve representing a relation between a boiling point and a depth of the hydrothermal water having the salt concentration; and estimating a position of a deepest part of the target vein by collating the boiling curve with a relation curve between the temperature and the depth of the hydrothermal water using an ancient groundwater surface as a reference, under geological conditions including the target vein.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for estimating the depth of a metal deposit, and more particularly to a method for estimating the depth of a hydrothermal deposit containing valuable metals such as gold, silver, copper, and rare metals. [Background technology]

[0002] Precious metals such as gold (Au) and silver (Ag), as well as valuable metals such as copper, zinc, and rare metals, exist underground in the form of metal deposits, and various exploration methods are known for them. Examples include seismic exploration, which uses the refraction and reflection of artificially generated elastic waves at boundaries of different physical properties to estimate geological structure; gravity exploration, which estimates underground structure by analyzing gravity anomalies that appear, reflecting the underground density distribution; electrical exploration, which estimates underground structure by electrically measuring the unique attributes of each mineral, such as electrical resistivity; and magnetic exploration, which estimates underground structure by measuring the Earth's magnetic field on land or in the air to determine the distribution of magnetic materials underground. Generally, after narrowing down the area where metal deposits exist to a certain extent using these exploration methods, efficient exploration can be achieved by drilling and excavation.

[0003] For example, Patent Document 1 discloses a method for predicting the geological condition of natural ground, which includes the steps of acquiring multiple types of drilling data ahead of the face in an already excavated part of the ground based on the drilling speed, impact pressure, torque, feed pressure, etc. during drilling, acquiring geological information in the already excavated part of the ground, analyzing the drilling data using multivariate analysis to create a first geological prediction formula, and adjusting the range of variation between the geological information calculated using the first geological prediction formula and the geological information itself to create a second geological prediction formula.

[0004] Furthermore, Patent Document 2 discloses a black ore exploration method in which, in the wide-area exploration stage, after selecting an exploration target area within a certain range, the exploration target is narrowed down, and as an exploration guideline, it discloses a technique in which first traces of the dacite lava dome in the final stage of acidic lava eruption are captured, then alteration halos of alkali metals and alkaline earth metals are tracked (precision exploration), and further, if necessary, mineralization halos of heavy metals are tracked. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-111289 [Patent Document 2] Japanese Patent Application Publication No. 53-072701 Summary of the Invention [Problem to be solved by the invention]

[0006] Metal deposits can consist of multiple large, tabular veins, and knowing the location of their deepest parts makes it possible to efficiently plan drilling. However, it is difficult to effectively estimate the location of the deepest part of a vein using the above-mentioned exploration methods, and in reality, it is necessary to rely on an exploration method in which core samples collected by drilling are analyzed repeatedly from the surface to the depths, which requires a large amount of money and a long time to estimate the deepest part of a deposit.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide a method that can estimate the position of the deepest part of a mineral vein easily and in a short period of time. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, the depth estimation method of the present invention is characterized by comprising the steps of: measuring the temperature of each of a plurality of fluid inclusions contained in quartz collected from different depths of a target vein under specified conditions to determine the temperature and salt concentration of the hot water when it is incorporated into the quartz; calculating a boiling curve showing the relationship between the boiling point of the hot water having the salt concentration and depth; and estimating the position of the deepest part of the target vein by comparing the boiling curve with the relationship curve between the temperature and depth of the hot water using the ancient groundwater level as a reference in the geological conditions including the target vein. [Effects of the Invention]

[0009] According to the present invention, the location of the deepest part of a mineral vein can be estimated easily and in a short period of time. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a flowchart of a depth estimation method according to an embodiment of the present invention. [Figure 2] 1 is a vertical cross-sectional view of a stratum schematically illustrating the process of generation of a mineral vein that is the subject of a depth estimation method according to an embodiment of the present invention. [Figure 3] 1 is a photograph of a specific example of a fluid inclusion handled by a depth estimation method according to an embodiment of the present invention. [Figure 4] 1 is a specific example of a boiling curve calculated by a depth estimation method according to an embodiment of the present invention. [Figure 5] This is a graph showing the relationship between hot water temperature and depth used in the depth estimation method of an embodiment of the present invention, and shows that the boiling depth can be determined by comparing the boiling curve with the old groundwater table as a reference. [Figure 6] 1 is a histogram showing a specific example of the measurement results of the homogenization temperature of fluid inclusions used in the depth estimation method of an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] An embodiment of a depth estimation method for a mineral vein according to the present invention will be described below. As shown in Fig. 1, the depth estimation method according to the present invention includes the following steps: a first step S1 in which a test drilling is performed on a hydrothermal vein containing, for example, gold (Au) as a target mineral vein, and the temperature of each of a plurality of fluid inclusions contained in quartz collected from different depths is measured using a measuring device under predetermined conditions to thereby determine the temperature and salinity of the hydrothermal fluid when the hydrothermal fluid is incorporated into the quartz; a second step S2 in which a boiling curve showing the relationship between the boiling point and depth of the hydrothermal fluid having the salinity determined in the first step S1 is calculated; a third step S3 in which a paleogroundwater level of the stratum containing the target mineral vein is determined; and a fourth step S4 in which the lower limit of the mineral vein is estimated by comparing the boiling curve with the temperature-depth curve of the hydrothermal fluid under the geological conditions containing the target mineral vein, using the paleogroundwater level as a reference.

[0012] Below, each of these first step S1 to fourth step S4 will be explained in detail using an example in which the target vein is a hydrothermal vein-type gold deposit. As shown in Figure 2, hydrothermal water, which is generated mainly by underground meteoric water being heated by geological phenomena such as magma, rises to the surface due to its high vapor pressure. As it rises, it reacts with the rocks that serve as its pathway, causing changes in temperature, pressure, carbon dioxide concentration, pH, and other factors. As a result, the reactions shown in Equations 1 to 4 below occur, and when the water rises above a certain depth, it boils and the minerals contained in the hydrothermal water crystallize, including gold. These crystallized minerals fill cracks in the strata, such as faults, forming tabular veins.

[0013] [Formula 1] HCO3 - +H + =H2CO3→CO2+H2O [Formula 2] KAl3Si2O 10 (OH)2+6SiO2+2K + →3KAlSi3O8+2H + [Formula 3] 2HCO3 - +Ca 2+ →CaCO3+CO2+H2O [Formula 4] AuHS 0 +0.5H2 → Au+H2S Au(HS)2 - +H + +0.5H2→Au+2H2S

[0014] Therefore, if the depth at which the boiling begins (hereinafter also referred to as the boiling depth) is known, the location of the deepest part (lower limit) of the mineral vein can be estimated. In the depth estimation method of an embodiment of the present invention, a boiling model of salt-containing water in an isothermal open system is used to estimate the boiling depth. Specifically, the boiling point of isothermal water with a constant salt concentration filled in a vertically open container is 100°C at the top because it is under atmospheric pressure. However, as the depth from the liquid surface increases, the boiling point gradually increases due to the hydrostatic pressure of the liquid head. By comparing this boiling model with the hydrothermal fluid temperature at each depth determined by microthermometry of fluid inclusions, the boiling depth of the hydrothermal fluid can be determined. Because minerals begin to crystallize from the boiling depth of the hydrothermal fluid, the location of the deepest part of the mineral vein can be estimated by knowing the boiling depth.

[0015] (1) 1st process S1 Specifically, in step S1, drilling is performed on the target vein formed as described above to collect multiple core samples from different depths, and fluid inclusions contained in quartz are sampled from each of these core samples. Fluid inclusions are hydrothermal fluids trapped within minerals such as quartz in hydrothermal veins formed from hydrothermal fluids. The hydrothermal fluid is trapped in the quartz as saturated water (i.e., water in equilibrium with saturated steam) at temperatures of approximately 150–300°C. Upon subsequent cooling, the gas separates from the liquid and exists in the form of bubbles, as shown in Figure 3. Microthermometric analysis of these fluid inclusions allows the temperature of the hydrothermal fluid at the time of its trapping into the quartz (hereinafter also referred to as the hydrothermal fluid temperature) and the concentration of sodium chloride (NaCl), a salt contained in the hydrothermal fluid, to be determined.

[0016] Microthermometry is a technique for measuring the freezing point and homogenization temperature of fluid inclusions. It can be performed using measuring equipment such as a USGS-type gas flow heating / cooling table. The samples used for this measurement are thin sections of fluid inclusions polished on both sides. To measure the freezing point, the sample is first cooled to freeze the liquid phase, and then gradually heated to measure the temperature at which the ice disappears (i.e., the freezing point). The presence of salt causes freezing point depression, lowering the freezing point below 0°C, the freezing point of pure water. Therefore, the measured freezing point can be used to determine the salt concentration of the hydrothermal fluid. Since the salt contained in the hydrothermal fluid in the fluid inclusions is primarily sodium chloride, the salt concentration calculated above can be considered the sodium chloride concentration. Once the salt concentration (unit: wt%) is determined, the freezing point depression θ (unit: °C) can be calculated, for example, using Equation 5 below. [Formula 5] Salt concentration = 0.00 + 1.78θ - 0.0442θ 2 +0.000557θ 3

[0017] On the other hand, when measuring the homogenization temperature, the sample is gradually heated, causing the liquid phase to expand and the gas bubbles to contract, ultimately resulting in a single liquid phase. The temperature at which this single phase is reached is the homogenization temperature. Since the pressure when the hot water is taken in is thought to be approximately equal to the saturated vapor pressure, the homogenization temperature can be thought of as approximately equal to the temperature when the hot water is taken in by the quartz.

[0018] (2)Second process S2 Next, in the second step S2, a boiling curve is calculated, which shows the relationship between the boiling point and depth of the hot water having the salinity determined by the microthermometry. This boiling curve can be calculated, for example, by the method described in a paper entitled "The Effect of Salinity on the Maximum Thermal Gradient of a Hydrothermal System at Hydrostatic Pressure" by John L. Haas, JR. (Economic Geology, vol. 66, 1971, pp. 940-946). Specifically, first, using the following equations 6 to 9, the temperature of pure water having the same vapor pressure as the temperature of hot water selected at regular temperature intervals (for example, 10°C intervals) within a range of, for example, 100°C to 370°C is calculated. Here, T x is the temperature of the hot water (unit: absolute temperature K), and T0 is the temperature T x It is the temperature (unit: absolute temperature K) of pure water that has the same vapor pressure as the hot water.

[0019] [Formula 6] TIFF2025186073000002.tif15140[Formula 7] TIFF2025186073000003.tif9140[Formula 8] TIFF2025186073000004.tif8140[Formula 9] TIFF2025186073000005.tif15140

[0020] The variable x in the above formulas 8 to 9 is the molar concentration (unit: mol / 1000 g H2O) of sodium chloride in the hydrothermal fluid contained in the fluid inclusions obtained in the first step S1, and the coefficients in the above formulas 7 to 9 are as follows: If the sodium chloride concentration of the hydrothermal fluid contained in the fluid inclusions varies depending on the depth at which it is collected, the arithmetic mean of the sodium chloride concentrations of all samples may be used as the variable x, or the sodium chloride concentration obtained by arithmetic averaging for each depth and the temperature T of the hydrothermal fluid may be used as the variable x. x By creating a correlation equation with the temperature T x You can also assign a function of x to the variable x. TIFF2025186073000006.tif44149

[0021] Next, by substituting the temperature T0 of the pure water corresponding to the hot water obtained in the above formula 6 into the following formula 10, the temperature T0 corresponding to the pure water at temperature T0 is obtained. x Calculate the vapor pressure of hot water. [Formula 10] TIFF2025186073000007.tif68143

[0022] p and T calculated using the above formulas 6 and 10 x From the relationship "dp / dT x " and perform the integral calculation shown in Equation 11 below, the distance Δh from the liquid depth h0 of the hot water column at temperature T0 to the liquid depth h of temperature T can be calculated. In other words, the temperature T of the hot water when the liquid column is deeper by the distance Δh from the liquid depth h0 of temperature T0. Note that ρ is the density of the hot water (unit: g / cm 3 ) where c is the dimension conversion factor. [Formula 11] TIFF2025186073000008.tif17140

[0023] Figure 4 shows a boiling curve created by plotting the calculation results of Equation 11 above on a graph with the hot water temperature (unit: °C) on the horizontal axis and the liquid depth (unit: m) on the vertical axis, using the salt concentration (unit: wt%) as a parameter. As can be seen from the inner graph in Figure 4, which enlarges the range from 90°C to 150°C, the liquid level of 0 m, indicated by the dotted line, is under atmospheric pressure (1.013 barA), so pure water with a salt concentration of 0 wt% has a boiling point of 100°C, and it can be seen that as the salt concentration increases by 5 wt% up to 25 wt%, the boiling point increases due to molar boiling point elevation.

[0024] (3) Third step S3 Next, in the third step S3, the paleogroundwater table is determined under the geological conditions that include the target vein. This paleogroundwater table can be determined, for example, by visually inspecting the multiple core samples collected from various depths in the first step S1. Specifically, the paleogroundwater table, which is the ground surface at the time the hydrothermal vein was formed, is generally discolored a slightly reddish-purple color due to oxidation caused by exposure to the surface, unlike core samples from other strata, and can therefore be easily identified by visual inspection.

[0025] (4) 4th process S4 Finally, in step S4, as shown in Figure 5, the relationship between the hydrothermal fluid temperature and depth determined in step S1 is plotted on a graph with the vertical axis representing distance from the current surface (unit: m) and the horizontal axis representing temperature (unit: °C), and a curve representing the relationship between the hydrothermal fluid temperature and depth is drawn by interpolating and extrapolating these plotted points. Because the homogenization temperature measured by microthermometry, which is the basis for the hydrothermal fluid temperature, can vary even for vein samples from the same depth, it is preferable to use a homogenization temperature within the range of the most frequent value in the histogram, as shown in the area surrounded by a dotted line in Figure 6. In this case, the relationship curve between the hydrothermal fluid temperature and depth will have a temperature range corresponding to the range of the most frequent value, as shown by the thick hatched line in Figure 5.

[0026] The paleogroundwater level determined in step S3 (shown by the dashed-dotted line) is plotted on the graph in Figure 5, which depicts the relationship between hydrothermal fluid temperature and depth. The boiling curve calculated in step S2 (shown by the dotted line) is then superimposed on this paleogroundwater level. For reference, the boiling curve for pure water is also shown in Figure 5 as a two-dot dashed line. As mentioned above, the boiling curve was created using an isothermal open-system boiling model. Therefore, the upper end of the boiling curve is aligned with the paleogroundwater level. Since the paleogroundwater level was under atmospheric pressure when it existed, the temperature at the upper end of the boiling curve is set to 100°C. By matching the boiling curve to the hydrothermal fluid temperature-depth relationship curve in this way, the boiling depth can be calculated. Specifically, the boiling depth can be determined from the point where the boiling curve intersects with the hydrothermal fluid temperature-depth relationship curve. As mentioned above, if the relationship curve has a temperature range, the boiling depth is expressed as a temperature range with upper and lower limits, as indicated by the two asterisks in Figure 5.

[0027] While the depth estimation method of the present invention has been described above based on an embodiment, the present invention is not limited to the above embodiment and can include various modifications and alternatives without departing from the spirit of the present invention. For example, in the depth estimation method of the above embodiment of the present invention, the target mineral vein for depth estimation is a hydrothermal vein-type gold deposit, but this is not limited thereto. Precious metals such as silver (Ag) and platinum (Pt), non-ferrous metals such as copper, zinc, and lead, and rare metals such as indium, antimony, bismuth, germanium, gallium, tellurium, and selenium are also thought to crystallize from hydrothermal fluids by a mechanism similar to that of gold, and therefore the boiling depth can be determined in a similar manner.

Claims

1. A depth estimation method comprising the steps of: determining the temperature and salt concentration of the hot water when it is incorporated into the quartz by measuring the temperature under specified conditions for each of a plurality of fluid inclusions contained in quartz collected from different depths of a target vein; calculating a boiling curve showing the relationship between the boiling point of the hot water having the salt concentration and depth; and estimating the position of the deepest part of the target vein by comparing the boiling curve with the relationship curve between the temperature and depth of the hot water using the ancient groundwater level as a reference in the geological conditions including the target vein.

2. The depth estimation method of claim 1 , wherein the temperature measurements are measurements of freezing point and homogenization temperature by microthermometry.

3. The depth estimation method according to claim 1 , wherein the position of the deepest part is a boiling depth obtained as an intersection of the relationship curve and the boiling curve.

4. The depth estimation method according to claim 1 , wherein the target vein is a hydrothermal vein-type gold deposit.

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