Method for quantitatively evaluating storage condition of magma hydrothermal deposit based on thermal chronology

Abstract

The invention provides a method for quantitatively evaluating the storage condition of magma hydrothermal deposit based on thermochronology, which comprises the following steps: determining a sample sampling point based on field geological survey, collecting typical samples, picking out zircon and apatite monominerals, and preparing a U-Pb isotope test target and a fission track test optical slide; carrying out fission track dating on the zircon and the apatite to measure the U-Pb isotope age and (U-Th) / He isotope age of the zircon and the apatite; thermal history evolution process simulation is carried out by integrating test data, and finally, the deposit preservation condition is quantitatively calculated and evaluated according to simulation. According to the method, a scientific quantitative full-chain research scheme is provided for augmentation, denudation and thermal history evolution research of the magma hydrothermal deposit, and the technical problem of quantitative evaluation of the storage condition of the magma hydrothermal deposit is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of geological and mineral exploration, and particularly relates to a method for quantitatively evaluating the preservation of magmatic-hydrothermal deposits based on thermochronology. Background Art

[0002] Most deposits have experienced a long evolution process after their formation. Among them, the composition and occurrence of some deposits have been significantly changed, and some deposits have been subjected to strong denudation and disappearance, and only a very small number of deposits are fortunate to be preserved and outcropped on or near the surface. Therefore, the study of ore deposit geology should take into account both the ore-forming process (ore deposit genesis) and the transformation-preservation process (ore deposit change) of the deposit. The degree of deposit denudation is the most important research content of the post-ore formation change. When evaluating a deposit (point), especially during the exploration of concealed or blind ore deposits, it is often necessary to judge the occurrence of mineralization in the deep part, whether the discovered mineralization phenomenon is just outcropping on the surface or the root of the residual ore body after denudation. Although some previous studies have been carried out on the denudation degree of magmatic-hydrothermal deposits, such as through the comparative study of the zoning of primary halo abnormal elements, alteration mineral assemblages, typomorphic characteristics of minerals, and trace element contents between different ore bodies, the relative denudation degree of the ore body can be roughly understood. However, most of the research results still remain at the qualitative-semi-quantitative level.

[0003] In the past 20 years, with the rapid development of thermochronology, especially the continuous in-depth study of the multi-diffusion domain theory of potassium feldspar 40 Ar- 39 Ar, the establishment of the fission track annealing experiment and the annealing model, and the increasing maturity and wide application of the apatite, zircon (U-Th) / He dating techniques, people can better understand the uplift and denudation processes of orogenic belts. At the same time, some scholars have also tried to apply low-temperature thermochronology to the research of the thermal history evolution and denudation rate of ore deposits, and achieved good results. Thermochronology is based on the theory of mineral closure temperature. The closure temperature (Tc) is the critical temperature at which the products of radioactive decay (such as the daughter isotopes produced by decay or 238 fission tracks produced by spontaneous fission of U) cannot be preserved in mineral crystals. When the mineral cools below the closure temperature, the decay products will be retained in the mineral lattice, and the age obtained at this time is the closure (or apparent) age of the mineral ( Figure 2 ). Summary of the Invention

[0004] Technical Problem: To provide a method for quantitatively evaluating the denudation degree of magmatic-hydrothermal deposits based on thermochronology to solve the technical problem of scientifically quantitatively calculating the preservation of magmatic-hydrothermal deposits.

[0005] Technical solution: A method for quantitatively evaluating the preservation of magmatic-hydrothermal deposits based on thermochronology, comprising the following steps: Step 1, field geological survey; Step 2, determine sample sampling points according to the survey results; Step 3, collect typical samples; Step 4, respectively select zircon and apatite single minerals from the samples collected in Step 3, perform pre-treatment of (U-Th) / He isotope test samples, and respectively prepare U-Pb isotope test targets and fission track test glass slides; Step 5, respectively carry out fission track dating on the zircon and apatite obtained in Step 4; Step 6, obtain the U-Pb isotope ages of the zircon and apatite obtained in Step 4; Step 7, obtain the (U-Th) / He isotope ages of the zircon and apatite obtained in Step 4; Step 8, simulate the thermal history evolution process with the test data of Steps 5, 6 and 7; Step 9, quantitatively calculate and evaluate the preservation of the deposit according to the simulation.

[0006] Specifically, the field geological survey in Step 1 includes: Step 1.1, collect existing exploration reports, geological map literature materials of the target mining area; Step 1.2, select characteristic surface outcrops, pit outcrops, and drill cores for survey and observation, and briefly record geological occurrences, alteration and mineralization characteristics.

[0007] Specifically, in Step 2, determining the sample sampling points according to the survey results is to select typical exploration line profiles with weak alteration-mineralization based on geological information such as the occurrence of known ore bodies in the mining area and the layout of exploration projects, select surface outcrops, pit outcrops or drill cores with an elevation difference greater than 1.5 km within these profiles, and determine the sampling points according to the rule of one sample every 300 - 500 m.

[0008] Specifically, the method for collecting typical samples in Step 3 includes: mainly collecting fresh ore-forming rock masses as typical samples, calibrating the sampling position and elevation with GPS, with the weight of a single sample being greater than 2 kg, and the sample quantity being sufficient to ensure that 100 - 500 zircon and apatite single minerals can be separated.

[0009] Specifically, the method for selecting zircon and apatite single minerals and making targets in Step 4 includes: Step 4.1, crush the collected samples to 60 - 80 mesh, first perform rough selection by traditional methods, then separate zircon and apatite through heavy liquid separation and magnetic separation, and finally manually select zircon and apatite particles with a purity above 99% under a binocular microscope.

[0010] Step 4.2: Zircon and apatite single minerals are selected under a binocular stereomicroscope. Crystals with better crystal forms are chosen, and particles containing inclusions are excluded as much as possible. Parameters such as the length and width of the crystals are measured under the microscope using a scale. After the measurement, crystal photos are taken under the binocular stereomicroscope, and then they are respectively placed into niobium capsules (for zircon) and platinum capsules (for apatite).

[0011] Step 4.3: Use 400-mesh sandpaper to round the edges of a glass slide with a size of 25 mm×18 mm to prevent damage to apatite by glass debris during later polishing. The collection of zircon and apatite should be carried out on a clean and flat Teflon plate. Two drops of alcohol are dropped on the Teflon plate, and then the apatite in the sample is sprinkled into the alcohol. A pin is used to disperse the apatite particles into a nearly circular area, ensuring that there is no overlap between the apatite particles as much as possible. The required resin and bridging pieces are prepared. The resin is prepared by mixing epoxy resin and hardener in a ratio of 4:1, and vacuum treatment is carried out simultaneously to prevent the influence of bubbles on the results later. The bridging pieces are hard paper sheets with a thickness of 0.3 - 0.5 mm. After the alcohol has evaporated, 1 - 2 drops of resin are dropped on the apatite particles, and the glass slide is tilted and slowly lowered from one side of the bridging piece. Finally, the glass slide is placed steadily on the hard paper sheet and left for about 8 hours for peeling. The prepared apatite target is placed on a polishing machine for polishing. During the polishing process, as much of the inner surface of the apatite particles as possible should be polished, and the polished surface should be highly smooth. The polishing standard is to polish at least 50% of the mineral particles or the inner surfaces of 70 - 100 particles.

[0012] Step 4.4: Zircon and apatite are fixed with the resin in Step 4.3 respectively, and then their surfaces are ground and polished to prepare U-Pb isotope test targets for standby.

[0013] Specifically, the method for obtaining zircon and apatite for fission track dating in Step 5 includes: Step 5.1: Zircon is etched in an 8g NaOH + 11.5g KOH solution at a constant temperature of 210°C for about 33 hours, and apatite is etched in 6% HNO3 at a constant temperature of 25°C for 30 seconds to reveal the spontaneous fission tracks of the minerals.

[0014] Step 5.2: Use a polarized light microscope to select zircon and apatite grains with a polished surface parallel to the prismatic crystal plane and a uniform track distribution. Using a high-sensitivity and fast camera, high-resolution digital images of zircon and apatite crystals are taken with a 100-fold objective lens under the action of transmitted light and reflected light, and ensure that the pixel size of the image has been accurately calibrated.

[0015] Step 5.3: Perform track counting using a mapping protocol that conforms to the standard. Use a high-sensitivity laser ablation inductively coupled plasma mass spectrometry system to measure uranium in selected grains by the LA-ICP-MS method. Perform laser ablation on the selected grains and calibration standards (NIST-612 glass and zircon and apatite targets) for 45 s each. The laser beam spot is 30 μm and the energy is approximately 2.5 J / cm 2 , with a repetition frequency of 5 Hz.

[0016] Step 5.4: Obtain the fission track ages of zircon and apatite according to the radioactive decay formula (Formula 1). Measure the etch pit diameter (Dpar) of the particles with counted tracks, and use a radar chart to plot the distribution of the ages of each grain and the Dpar value.

[0017] (Formula 1), where λD and λf are respectively 238 the total decay constant of 235 U and the spontaneous fission decay constant, σ is 235 the thermal neutron-induced fission cross section of 238 U, I is 238 the natural isotope abundance ratio of 235 U and 2 U, ρs and ρi are respectively 206 the spontaneous fission track density of 238 U and 206 the induced fission track density of 238 U, and Φ is the thermal neutron flux.

[0018] Specifically, the method for obtaining the U-Pb isotope ages of zircon and apatite in Step 6 includes: analysis using an Agilent 7900 quadrupole ICP-MS, with a laser ablation device being an ESI NWR 193 He excimer laser. The laser ablation energy density is 5.0 J / cm 206 , the pulse is 5 Hz, and the ablation diameter is 30 μm. During the laser ablation process, helium is used as the carrier gas and argon is used as the compensation gas to adjust the sensitivity, and the two are mixed through a T-joint before entering the ICP. Each time-resolved analysis data includes the background signal for approximately 30 s before and after the analysis and the sample signal for 40 - 50 s. During the test, NIST610 and 91500 zircon ( 238U-Pb age of 31.5 Ma) was used as the monitoring standard, and two sets of NIST610 standards, 91500 zircon standards and two sets of Plešovice zircon or Durango apatite standards were inserted every 5 test points. The U-Pb isotope test data were processed by Iolite software, and then IsoplotR was used to plot and calculate the U-Pb ages of zircon and apatite.

[0019] Specifically, the method of obtaining the (U-Th) / He isotopic age of zircon and apatite in step 7 includes: Step 7.1: Helium extraction and analysis of samples were performed on an Alphachron II helium isotope mass spectrometer. Helium was extracted by heating with a 970 nm diode laser at a temperature of approximately 1300°C for 10 min. 4 He and 3 He mixture, measured by quadrupole mass spectrometry 4 He / 3 He value, 3 He is measured by 4 He standard gas calibration, the final calculation can be obtained 4 He content, 4 The measurement accuracy of He is better than 1%. The gas purification system consists of a mechanical pump, a molecular pump, an ion pump and a zirconium-aluminum pump. The system vacuum is better than 1×10 -9 torr, perform the gas extraction and analysis process at least twice under the same process.

[0020] Step 7.2: Sample 4 After the He content analysis, it was chemically dissolved and its U and Th contents were determined by inductively coupled plasma mass spectrometry (ICP-MS). 235 U- 230 Th- 145 After Nd diluent, the sample was stored in an oven at 80°C for 2 h to allow the apatite to fully dissolve. The chemical dissolution of zircon required multiple digestions in a high-pressure sealed sample dissolver. During the first digestion, 200 μL of 235 U- 230 Th- 145HF of the Nd diluent, then cap the vial, stack it in a 125 mL polytetrafluoroethylene bottle, place it in a high-pressure sealed sample dissolver, digest at 220 °C for 72 h. After the digested solution is evaporated to dryness on a hot plate at 90 °C, add 200 μL of a 7:1 HNO3:HF mixed solution, then cap it and digest on a hot plate at 90 °C for 4 h; after the mineral is dissolved, dilute it with 1 - 3 mL of double-deionized water, and finally use an Agilent 7900 inductively coupled plasma mass spectrometer to measure the U and Th contents of the sample, combined with the 4 He content measured in step 7.1, and calculate the (U-Th) / He age of the sample according to the radioactive decay formula (Formula 2).

[0021] 4 He = 8 238 U[exp(λ 238 t)-1]+7( 238 U / 137.88)[exp(λ 235 t)-1] +6 232 Th[exp(λ 232 t)-1] (Formula 2), wherein 4 He, 238 U, 235 U, 232 Th are the contents (number of atoms) of each element in the mineral at time t, t is the time accumulated by the daughter isotope 4 He produced by radioactive decay, that is, the age value to be obtained, λ 238 , λ 235 , λ 232 are respectively 238 U, 235 U, 232 the decay constants of Th.

[0022] Specifically, the thermal history evolution process simulation method in step 8 includes: Step 8.1: Evaluate the thermochronological data, and the data needs to satisfy ① the (U-Th) / He age is independent of the particle size of zircon and apatite, ② the 4 He content of zircon and apatite samples is positively correlated with the U and Th contents; Step 8.2: According to the zircon and apatite fission track ages, track lengths and the measured Dpar data of some samples, and comprehensive constraint conditions, determine the initial conditions for thermal history simulation: ① the simulated temperature ranges from the closure temperature of zircon U-Pb (average 800 °C) to the current surface temperature (average 15 °C); ② the time ranges from the zircon U-Pb isotope age to the present; ③ 100 temperature-time curves of the sample are simulated.

[0023] Step 8.3: Using the HeFTy software (Ketcham, 2005) and 4DTherm software (Fu et al., 2010), and relying on the crystal diffusion kinetics formulas (Formula 3) of Flowers et al. (2009) and Guenthner et al. (2013), simulate the thermal history evolution process of the obtained fission track test data of zircon and apatite, as well as the U-Pb and (U-Th) / He isotope test data of zircon and apatite.

[0024] D / a 2 =D0 / a 2 exp(-Ea / RT) (Formula 3), where D is the diffusion rate, D0 is the diffusion rate at infinitely high temperature, Ea is the activation energy, R is the gas constant, T is the Kelvin temperature, and a is the radius of the diffusion region.

[0025] Specifically, the method for quantitatively calculating and evaluating the preservation situation of the ore deposit in Step 9 includes: Step 9.1: Based on the least squares method, respectively construct linear relationships between the (U-Th) / He ages of zircon and apatite and the elevation of the sampling points ( Figure 2 ): y 锆石 =ax + b (1) y 磷灰石 =cx + d (2) Among them, y 锆石 is the (U-Th) / He age of zircon (unit: Ma), y 磷灰石 is the (U-Th) / He age of apatite (unit: Ma), x is the elevation of the sample (unit: m), a and c are the erosion rates (unit: m / Ma), and b and d respectively represent the depths of the isothermal surfaces of the closure temperatures of zircon and apatite (unit: m); Step 9.2: According to the calculation results of Step 9.1, obtain the average cooling rates of the ore-forming rock bodies of the magmatic hydrothermal deposit at different time periods, as well as the erosion depths of different sampling points. Combining with the geothermal gradient, further invert the cooling and exhumation history of the magmatic hydrothermal deposit; Step 9.3: According to the parameters obtained in Step 8, Step 9.1, and Step 9.2, calculate the erosion situations of the ore-forming rock bodies and ore bodies, and combine with the geological characteristics of the actual ore deposit to quantitatively evaluate the preservation situation of the ore bodies.

[0026] Principle of the invention: Based on the mineral closure temperature theory and the basic principle of radioactive decay, this invention uses the age-elevation method to quantitatively determine the erosion rate of the sample, combines high-medium-low temperature thermochronological isotope test data and conducts numerical simulations, realizing the whole-process quantitative inversion of the cooling and exhumation history of the magmatic hydrothermal deposit.

[0027] Beneficial effects: The present invention adopts the sampling strategy of age-elevation method, determines the sampling points in combination with the actual geological conditions of the mining area, and jointly uses zircon and apatite U-Pb dating, (U-Th) / He dating and fission track dating methods to simulate the thermal evolution process based on comprehensive test data, which can better solve the problem of quantitatively evaluating the preservation of magmatic hydrothermal deposits. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The accompanying drawings are used to provide an understanding of the present invention and constitute a part of the specification. Together with the following specific embodiments, they are used to explain the present invention, but do not constitute a limitation of the present invention. In the accompanying drawings: Figure 1 A flow chart of a method for quantitatively evaluating the preservation of magmatic hydrothermal deposits based on thermochronology provided by the present invention; Figure 2 A schematic diagram of the sampling strategy for the age-elevation method and how to quantitatively determine the sample erosion rate; DETAILED DESCRIPTION

[0029] The evaluation method of this embodiment is based on the thermochronology to quantitatively evaluate / calculate the degree of denudation of magmatic hydrothermal deposits, and then obtain the preservation status of the deposits, which specifically includes the following steps: Step 1: Field geological survey; Step 1.1. Through literature search, understand the geotectonic background of the relevant mineral deposits and collect the literature data on the mineral deposits published by previous researchers. In addition, actively communicate with the company where the mine is located, check the existing exploration reports and various geological maps and other relevant materials, and follow up the exploration projects and mining progress that the mine has arranged; Step 1.2: After collecting the data, conduct a preliminary analysis of the data, and determine the goals of field observations after understanding the basic geological overview. Select a suitable exploration route on the surface or in the pit so that the route can observe as many geological outcrops as possible, and the route layout needs to control the surface of the mining area to the maximum extent. At the same time, select the drill holes that cross and cut the main ore body from the completed drilling projects to form a "cross" shaped exploration profile, so as to fully reveal the geological information deep in the mining area. While observing, keep records, including outcrop location, lithology, alteration type, mineralization degree, etc., to provide necessary geological information for indoor analysis; Step 1.3: After completing the data collation and field observation records, it is necessary to roughly grasp the basic situation in the mining area. First, it is necessary to find out the main intrusive rocks developed in the mining area and their interpenetration relationship, and also to understand the main alteration types and their approximate distribution range. At the same time, it is also necessary to have a certain understanding of the output location, occurrence, and scale of the ore body. Based on the above foundation, the subsequent testing work will be more purposeful and targeted.

[0030] Step 2: Determine the sampling points according to the survey results; Based on the occurrence of known ore bodies in the mining area, geological information such as the layout of exploration engineering, etc., select typical exploration line profiles with weak alteration-mineralization, and select surface outcrops, pit outcrops or drill cores with an elevation difference greater than 1.5 km within these profiles. According to the rule of one sample every 300 - 500 m, determine the sampling points.

[0031] Step 3: Collect typical samples; The typical samples collected are mainly fresh ore-forming rock masses. The sampling positions and elevations are calibrated using GPS. The mass of a single sample is greater than 2 kg, and the sample quantity is sufficient to ensure that 100 - 500 zircon and apatite single minerals can be separated.

[0032] Step 4: Select zircon and apatite single minerals from the samples collected in Step 3 above, conduct pre-treatment for (U-Th) / He isotope test samples, and separately produce U-Pb isotope test targets and fission track test glass slides; Step 4.1: Crush the collected samples to 60 - 80 mesh. First, conduct rough selection by traditional methods, then separate zircon and apatite through heavy liquid separation and magnetic separation, and then manually select zircon and apatite particles with a purity above 99% under a binocular microscope.

[0033] Step 4.2: Select zircon and apatite single minerals under a binocular stereomicroscope, select crystals with good crystal forms, and try to exclude particles containing inclusions as much as possible; measure the crystal size under the microscope using a scale, and take a microscopic photo after measurement and put it into a niobium capsule (for zircon) and a platinum capsule (for apatite).

[0034] Step 4.3: Use 400-mesh sandpaper to remove the corners of glass slides with a specification of 25 mm × 18 mm to prevent the destruction of apatite by glass debris during later polishing; the collection of zircon and apatite should be carried out on a flat and clean Teflon plate. Drop 2 drops of alcohol on the Teflon plate, then sprinkle the apatite in the sample into the alcohol, and use a pin to disperse the apatite particles into a nearly circular area, as much as possible to ensure that there is no overlap between apatite particles; prepare the required resin and bridging pieces. The resin is prepared by mixing epoxy resin and hardener in a ratio of 4:1, and at the same time, conduct vacuum treatment to prevent the influence of later bubbles on the results. The bridging pieces are hard paper pieces with a thickness of 0.3 - 0.5 mm. After the alcohol has evaporated, drop 1 - 2 drops of resin on the apatite particles, and slowly lower the glass slide from one side of the bridging piece, and finally the glass slide is placed steadily on the hard paper piece and left for about 8 h to be peeled off; place the prepared apatite target on a polishing machine for polishing. During the polishing process, try to polish out as much of the inner surface of the apatite particles as possible, and the polished surface should be highly smooth. The polishing standard is to polish out at least 50% of the mineral particles or the inner surfaces of 70 - 100 particles.

[0035] Step 4.4: Fix zircon and apatite respectively with the gum in Step 4.3, then grind and polish the surface to prepare U-Pb isotope test targets for standby.

[0036] Step 5: Conduct fission track dating on the zircon and apatite obtained in Step 4 above respectively; Step 5.1: Etch zircon in an 8g NaOH + 11.5g KOH solution at a constant temperature of 210°C for about 33h, and etch apatite in 6% HNO3 at a constant temperature of 25°C for 30s to reveal the spontaneous fission tracks of the minerals.

[0037] Step 5.2: Use a polarized light microscope to select zircon and apatite grains with a polished surface parallel to the prismatic crystal face and a uniform track distribution. Using a high-sensitivity and fast camera, take high-resolution digital images of zircon and apatite crystals under transmitted light and reflected light with a 100-fold objective lens, and ensure that the pixel size of the images has been accurately calibrated.

[0038] Step 5.3: Use the coincidence mapping protocol for track counting, and use a high-sensitivity laser ablation inductively coupled plasma mass spectrometry system to measure uranium in the selected grains by the LA-ICP-MS method. Conduct laser ablation treatment on the selected grains and calibration standards (NIST-612 glass and zircon and apatite targets) for 45s respectively, with a laser beam spot of 30μm and an energy of about 2.5 J / cm 2 , and the repetition frequency is 5 Hz.

[0039] Step 5.4: Obtain the fission track ages of zircon and apatite according to the radioactive decay formula, measure the pit diameter (Dpar) of the particles with counted tracks, and use a radar chart to plot the distribution of the ages of each grain and the Dpar value.

[0040] Specifically, the method for obtaining the U-Pb isotope ages of zircon and apatite in Step 6 includes: analysis using an Agilent 7900 quadrupole ICP-MS, and the laser ablation device is an ESI NWR 193 He excimer laser. The laser ablation energy density is 5.0 J / cm 2 , the pulse is 5Hz, and the ablation diameter is 30μm. During the laser ablation process, helium is used as the carrier gas and argon is used as the compensation gas to adjust the sensitivity, and the two are mixed through a T-joint before entering the ICP. Each time-resolved analysis data includes the background signal for about 30 seconds before and after the analysis and the sample signal for 40 - 50 seconds. During the test, NIST610 and 91500 zircon ( 206 Pb / 238 U age is 1063 Ma) is used as the age external standard, and at the same time, the Plešovice zircon standard sample ( 206Pb / 238 U age of 339 Ma) and Durango apatite reference sample ( 206 Pb / 238 U age of 31.5 Ma) were used as monitoring reference samples. Two sets of NIST610 reference samples, 91500 zircon reference samples, and two sets of Plešovice zircon or Durango apatite reference samples were inserted every five sample points to be measured. The U-Pb isotope test data were processed using Iolite software, and then plotted and calculated using IsoplotR to obtain the U-Pb ages of zircon and apatite.

[0041] Specifically, the method for obtaining the (U-Th) / He isotope ages of zircon and apatite in step 7 includes: Step 7.1: The helium extraction and analysis of the sample were carried out on an Alphachron II helium isotope mass spectrometer. A 970 nm diode laser was used to heat and extract He gas, with a heating temperature of approximately 1300 °C and a heating duration of 10 mins. The 4 He released from the sample was mixed with 3 He, and the 4 He / 3 He value was measured using a quadrupole mass spectrometer. The measurement of 3 He was calibrated using 4 He standard gas, and finally the 4 He content in the sample could be calculated. The measurement accuracy of 4 He was better than 1%. The gas purification system consisted of a mechanical pump, a molecular pump, an ion pump, and a zirconium-aluminum pump. The system vacuum was better than 1×10 -9 torr, and the gas sampling and analysis processes were carried out at least twice under the same procedure.

[0042] Step 7.2: After the 4 He content of the sample was analyzed, it was chemically dissolved and its U and Th contents were determined using an inductively coupled plasma mass spectrometer (ICP-MS). The apatite sample in the platinum capsule was added with 235 U- 230 Th- 145 Nd diluent and stored in an oven at 80 °C for 2 h to fully dissolve the apatite; the chemical dissolution of zircon required multiple digestions in a high-pressure sealed sample dissolver. During the first digestion, 200 μL of 235 U- 230 Th- 145HF of the Nd diluent, then cap the vial, stack it in a 125 mL polytetrafluoroethylene bottle, place it in a high-pressure sealed sample dissolution vessel, digest at 220 °C for 72 h. After the digested solution is evaporated to dryness on a hot plate at 90 °C, add 200 μL of a 7:1 HNO3:HF mixture, then cap it and digest on a hot plate at 90 °C for 4 h; after the minerals are dissolved, dilute with 1 - 3 mL of double-deionized water, and finally use an Agilent 7900 inductively coupled plasma mass spectrometer to measure the U and Th contents of the sample, combined with the 4 He content measured in step 7.1, and calculate the (U-Th) / He age of the sample according to the radioactive decay formula.

[0043] Specifically, the method for simulating the thermal history evolution process in step 8 includes: Step 8.1: Evaluate the thermochronological data, and the data need to meet ① the (U-Th) / He age is independent of the particle sizes of zircon and apatite, and ② the 4 He content of zircon and apatite samples has a positive correlation with the U and Th contents; Step 8.2: According to the fission track ages, track lengths of zircon and apatite, and the measured Dpar data of some samples, and comprehensive constraint conditions, determine the initial conditions for the thermal history simulation: ① the simulated temperature ranges from the closure temperature of zircon U-Pb (average 800 °C) to the current surface temperature (average 15 °C); ② the time ranges from the zircon U-Pb isotope age to the present; ③ 100 temperature-time curves of the sample are simulated.

[0044] Step 8.3: Use the HeFTy software (Ketcham, 2005) and 4DTherm software (Fu et al., 2010), and with the help of the crystal diffusion kinetic models of Flowers et al. (2009) and Guenthner et al. (2013), simulate the thermal history evolution process of the obtained zircon and apatite fission track test data, zircon and apatite U-Pb, (U-Th) / He isotope test data.

[0045] Specifically, the method for quantitatively calculating and evaluating the deposit preservation situation in step 9 includes: Step 9.1: Based on the least squares method, respectively construct linear relationships between the (U-Th) / He ages of zircon and apatite and the elevation of the sampling point ( Figure 2 ): y 锆石 =ax + b (1) y 磷灰石 =cx + d (2) Among them, y 锆石 is the (U-Th) / He age of zircon (unit: Ma), y 磷灰石is the (U-Th) / He age of apatite (unit: Ma), x is the sample elevation (unit: m), a and c are the erosion rates (unit: m / Ma), and b and d represent the depths of the zircon and apatite closure temperature isotherms (unit: m); Step 9.2: Obtain the average cooling rates of the ore-forming rock bodies of the magmatic hydrothermal deposit at different time periods and the erosion depths of different sampling points according to the calculation results of Step 9.1. Combine the geothermal gradient to further invert the cooling and exhumation history of the magmatic hydrothermal deposit; Step 9.3: Calculate the erosion conditions of the ore-forming rock bodies and ore bodies according to the parameters obtained in Step 8, Step 9.1, and Step 9.2. Combine the geological characteristics of the actual deposit to quantitatively evaluate the preservation conditions of the ore bodies.

Claims

1. A method for quantitatively evaluating the preservation of magmatic hydrothermal deposits based on thermochronology, characterized in that It includes the following steps: Step 1, field geological survey; Step 2, determine sample sampling points according to the survey results; Step 3, collect typical samples at the sampling points; Step 4, separately select zircon and apatite single minerals from the typical samples, conduct pre-treatment of (U-Th) / He isotope test samples, and separately make U-Pb isotope test targets and fission track test glass slides; Step 5, conduct fission track dating on the zircon and apatite obtained in the above Step 4 respectively; Step 6, obtain the U-Pb isotope ages of the zircon and apatite obtained in the above Step 4; Step 7, obtain the (U-Th) / He isotope ages of the zircon and apatite obtained in the above Step 4; Step 8, conduct numerical quantitative simulation of the thermal history evolution process based on the test data of Steps 5, 6 and 7; Step 9, quantitatively calculate and evaluate the preservation situation of the ore deposit according to the simulation results.

2. The method for quantitatively evaluating the preservation of magmatic-hydrothermal deposits based on thermochronology according to claim 1, wherein: In the above Step 2, based on the known geological information within the mining area range, select a typical exploration line profile with weak alteration-mineralization, and select surface outcrops, pit outcrops or drill holes with an elevation difference greater than 1.5 km within the profile, and determine sampling points at intervals of 300 - 500 m; the geological information includes the occurrence of the ore body and the layout of exploration projects.

3. The method for quantitatively evaluating the preservation status of magmatic-hydrothermal deposits based on thermochronology according to claim 1, wherein: The typical samples collected in the above Step 3 are fresh ore-forming rock masses, and the sampling positions and elevations are calibrated using GPS, and the weight of a single sample is greater than 2 kg.

4. The method for quantitatively evaluating the preservation of magmatic-hydrothermal deposits based on thermochronology according to claim 1, characterized in that: The method for selecting zircon and apatite single minerals and making targets in the above Step 4 includes: Step 4.1, crush the collected samples and sieve them, and then separate zircon and apatite single minerals through rough selection, heavy liquid separation and magnetic separation; Step 4.2, select zircon and apatite single minerals under a binocular stereomicroscope, exclude particles containing inclusions and retain crystals with intact crystal forms, measure parameters such as the length, width and length / width ratio of zircon and apatite crystals respectively through a scale under the microscope, take microscopic photos after measurement, put zircon into niobium capsules respectively, and put apatite into platinum capsules; Step 4.3, conduct corner-rounding treatment on the glass slides, drop 2 drops of alcohol on a Teflon plate, then sprinkle the apatite in the samples into the alcohol, disperse the apatite particles into a nearly circular area to ensure that there is no overlapping phenomenon between apatite particles; after the alcohol volatilizes, drop resin on the apatite particles, place the glass slide steadily on a bridging slide, and peel it off after placing it for 8 h; polish the prepared apatite target to expose at least 50% of the mineral particles or the inner surfaces of 70 - 100 particles; Step 4.4, fix zircon and apatite with medium resin respectively, and then grind and polish the surfaces to make U-Pb isotope test targets for standby.

5. The method for quantitatively evaluating the preservation of magmatic-hydrothermal deposits based on thermochronology according to claim 1, wherein: The method for obtaining fission track dating of zircon and apatite in Step 5 includes: Step 5.1, etch zircon in an 8g NaOH + 11.5g KOH solution at a constant temperature of 210°C for 33 h, and etch apatite in 6% HNO3 at a constant temperature of 25°C for 30 s to reveal the spontaneous fission tracks of the minerals; Step 5.2: Use a polarized light microscope to select zircon and apatite grains whose polished surfaces are parallel to the prismatic crystal faces and have a uniform track distribution; use a high-sensitivity and fast camera to take high-resolution digital images of zircon and apatite crystals under transmitted light and reflected light with a 100× objective lens, and ensure that the pixel size of the images has been accurately calibrated; Step 5.3, perform track counting using a mapping protocol that conforms to the standard, and use a high-sensitivity laser ablation inductively coupled plasma mass spectrometry system to measure uranium in selected grains by the LA-ICP-MS method; perform laser ablation treatment for 45 s on the selected grain zircon, apatite target, and calibration standard NIST-612 glass. The laser beam spot size is 30 μm, the energy is 2.5 J / cm 2 , and the repetition frequency is 5 Hz; Step 5.4: According to the obtained fission track ages of zircon and apatite, measure the etch pit diameters of the grains for counting tracks, and use a radar chart to plot the age distribution and Dpar values of each grain.

6. The method for quantitatively evaluating the preservation status of magmatic-hydrothermal deposits based on thermochronology according to claim 1, wherein: The method for obtaining the U-Pb isotope ages of zircon and apatite in step 6 includes: analysis using an Agilent 7900 quadrupole ICP-MS, and a laser ablation device being an ESI NWR 193 He excimer laser; the laser ablation energy density is 5.0 J / cm 2 , the pulse is 5 Hz, and the ablation diameter is 30 μm; during the laser ablation process, helium is used as the carrier gas and argon is used as the compensation gas to adjust the sensitivity, and the two are mixed through a T-joint before entering the ICP; each time-resolved analysis data includes the background signal for 30 seconds before and after the analysis and the sample signal for 40 - 50 seconds; during the test, NIST 610 and 91500 zircon are used as external age standards, and Plešovice zircon standard sample and Durango apatite standard sample are used as monitoring standard samples respectively. Two sets of NIST 610 standard samples, 91500 zircon standard samples and two sets of Plešovice zircon or Durango apatite standard samples are inserted every 5 points to be measured; the U-Pb isotope data is processed using Iolite software, and the U-Pb ages of zircon and apatite are obtained by plotting and calculating with IsoplotR.

7. The method for quantitatively evaluating the preservation of magmatic-hydrothermal deposits based on thermochronology according to claim 1, characterized in that: The method for obtaining the (U-Th) / He isotope ages of zircon and apatite in Step 7 includes: Step 7.1: The helium extraction and analysis of the sample are carried out on an Alphachron II helium isotope mass spectrometer. A 970 nm diode laser is used to heat and extract He gas, with a heating temperature of 1300 °C and a heating duration of 10 mins. The 4 He released in the sample is mixed with 3 He, and the 4 He / 3 He value is measured using a quadrupole mass spectrometer. The measurement of 3 He is calibrated with 4 He standard gas, and finally the 4 He content in the sample is calculated. Step 7.2, Sample 4 After the He content analysis, it is chemically dissolved and the U and Th contents of the sample are analyzed by inductively coupled plasma mass spectrometry; the apatite sample in the platinum capsule is added with 235 U- 230 Th- 145 Nd diluent, and then stored in an oven at 80 °C for 2 h to fully dissolve the apatite; the chemical dissolution of zircon requires multiple digestions in a high-pressure sealed sample dissolution vessel. For the first digestion, 200 μL of 235 U- 230 Th- 145 Nd-containing HF is added to the sample, then the vial is capped and stacked in a 125 mL polytetrafluoroethylene bottle, placed in a high-pressure sealed sample dissolution vessel, and digested at 220 °C for 72 h. After the digested solution is evaporated to dryness on a hot plate at 90 °C, 200 μL of a 7:1 HNO3:HF mixture is added, then capped and digested on a hot plate at 90 °C for 4 h; after the mineral is dissolved, it is diluted with 1 - 3 mL of double-deionized water, and finally the U and Th contents of the sample are determined using an Agilent 7900 inductively coupled plasma mass spectrometer. Combining with the 4 He content measured in Step 7.1, the (U-Th) / He age of the sample is calculated according to the radioactive decay formula.

8. The method for quantitatively evaluating the preservation of magmatic hydrothermal deposits based on thermochronology according to claim 1, wherein: The method for simulating the thermal history evolution process in Step 8 includes: Step 8.

1. Evaluate the thermochronological data, and the data should meet the following requirements: ① the (U-Th) / He age is independent of the grain sizes of zircon and apatite; ② the 4 He content of zircon and apatite samples shows a positive correlation with the U and Th contents; Step 8.2: According to the fission track ages, track lengths of zircon and apatite, and the measured Dpar data of some samples, and considering comprehensive constraint conditions, determine the initial conditions for thermal history simulation: ① The simulated temperature ranges from the closure temperature of zircon U-Pb (average 800 °C) to the current surface temperature (average 15 °C); ② The time ranges from the zircon U-Pb isotope age to the present; ③ Obtain the temperature-time curve of the sample through simulation; the constraint conditions include the U-Pb ages of zircon and apatite, the (U-Th) / He ages of zircon and apatite, the fission track ages of zircon and apatite, and the sampling elevation; Step 8.3: Use HeFTy software and 4DTherm software, and with the help of the crystal diffusion kinetics model, simulate the thermal history evolution process of the obtained zircon and apatite fission track test data, and the U-Pb and (U-Th) / He isotope test data of zircon and apatite.

9. The method for quantitatively evaluating the preservation of magmatic hydrothermal deposits based on thermochronology according to claim 1, wherein: The method for quantitatively calculating and evaluating the preservation situation of the ore deposit in Step 9 includes: Step 9.1: Based on the least squares method, respectively construct linear relationships (Figure 2) between the (U-Th) / He ages of zircon and apatite and the sampling point elevation: y 锆石 = ax + b (1) y 磷灰石 = cx + d (2) Among them, y 锆石 is the (U-Th) / He age of zircon (unit: Ma), y 磷灰石 is the (U-Th) / He age of apatite (unit: Ma), x is the sample elevation (unit: m), a and c are the denudation rates (unit: m / Ma), and b and d respectively represent the depths of the isothermal surfaces of the closure temperatures of zircon and apatite (unit: m); Step 9.2: According to the calculation results in Step 9.1, obtain the average cooling rates of the ore-forming rock mass of the magmatic hydrothermal deposit at different time periods, and the erosion depths of different sampling points. Combining with the geothermal gradient, further invert the cooling and exposure history of the magmatic hydrothermal deposit; Step 9.3: According to the parameters obtained in Step 8, Step 9.1, and Step 9.2, calculate the erosion situation of the ore-forming rock mass and the ore body, and combine with the geological characteristics of the actual ore deposit to quantitatively evaluate the preservation situation of the ore body.

Images