Skarn deposit exploration method based on calcite wavelength and chemical composition variation

Abstract

The invention discloses a skarn deposit exploration method based on calcite wavelength and chemical composition variation, comprising the following steps: obtaining surrounding rock stratum short-wave infrared spectrum reflection data by using short-wave infrared spectrum technology, obtaining characteristic absorption peak position and absorption depth data of calcite near 2350 nm, and using the data to explore the regularity that the distance from the data to the metallogenic hydrothermal center is positively correlated; identifying hydrothermal calcite and obtaining Fe, Sr content and Ce anomaly (Ce / Ce N ) of the hydrothermal calcite through in-situ micro area testing technology, using the data to explore the regularity that the distance from the data to the metallogenic hydrothermal center is negatively correlated. The two sets of indexes are comprehensively used to obtain the indication result of the concealed ore body. Compared with the traditional technology, the invention only needs to identify one kind of mineral, is simple, economical and easy to operate, and the double-index combination can effectively make up for the deficiency of single method in indicating ore.

Description

Technical Field

[0001] This invention relates to the field of mineral exploration techniques and methods, and in particular to a method for prospecting skarn deposits based on variations in calcite wavelength and chemical composition. Background Technology

[0002] In magmatic hydrothermal deposits, detectable geochemical anomalies can extend outwards over a considerable area, penetrating into rocks subtly altered around the hydrothermal system, where structural or chemical anomalies are preserved in certain minerals. During deposit exploration, these anomalies often provide crucial clues indicating the location of ore bodies; therefore, understanding the physicochemical characteristics and spatial distribution of penetrating minerals within these alteration zones is essential. Taking porphyry deposits as an example, identifying subtle geochemical anomalies recorded by hydrothermal alteration minerals such as epidote, chlorite, or alunite in the surrounding rocks can indicate the concealed hydrothermal mineralization center of the porphyry mineralization system within a certain area. In the past decade or so, advancements in technologies such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and hyperspectral analysis have enabled the rapid and economical identification of mineral hyperspectral and geochemical anomalies, leading to new mineral exploration techniques—mineral exploration methods—which have been successfully applied to the exploration of porphyry and other magmatic hydrothermal deposits.

[0003] Skarn deposits occurring in carbonate rocks also exhibit widespread hidden alteration. Previous research on mineral exploration methods for skarn deposits hosted in carbonate host rocks has made some progress. For example, studies have found that the shifts in the short-wave infrared characteristic absorption peaks of mica-type minerals (Al-OH) and chlorite (Fe-OH) can effectively indicate concealed ore bodies. However, the overall research level remains weak, and more importantly, the ore-indicating range of these minerals is limited. The geochemical and imaging properties of carbonate minerals such as calcite, as exploration indicators for Carlin-type gold deposits and MVT-type deposits with carbonate host rocks, have begun to be used abroad to guide mineral exploration, but their application in skarn deposits is still insufficient. Summary of the Invention

[0004] To address the limitation of existing mineral exploration methods for skarn deposits in terms of their limited mineral indication range, this invention provides a skarn deposit exploration method based on variations in calcite wavelength and chemical composition. This method is easy to operate, requires only the identification of one mineral, and obtains its hyperspectral and geochemical composition information through relatively economical experimental methods. This provides evidence for the existence of concealed underground ore bodies over a wider range, thus guiding mineral exploration.

[0005] To achieve the above-mentioned technical objectives, this invention provides a method for exploring skarn deposits based on calcite wavelength variations, comprising: Short-wave infrared spectral reflectance data of calcite in the carbonate strata of the ore deposit were obtained, characteristic curves were plotted based on the spectral reflectance data, and characteristic absorption peaks were obtained by analyzing the characteristic curves. The parameters of the characteristic absorption peaks are calculated to obtain parameters such as the wavelength position and absorption depth of the characteristic absorption peaks. Regression analysis is performed on the characteristic absorption peak parameter data and sample spatial location data to obtain the direction and distance data of the concealed ore body, which guides the exploration.

[0006] Optionally, the process of acquiring shortwave infrared spectral reflectance data includes: Short-wave infrared spectral data of calcite in the carbonate strata surrounding the ore deposit were obtained. The short-wave infrared spectral data were analyzed and compared with the standard spectral characteristics of calcite. Data that fit the comparison were considered valid, and the short-wave infrared spectral reflectance data were recorded.

[0007] Optionally, the process of analyzing the characteristic curve includes: Based on the reflectance corresponding to different wavelength bands, spectral characteristic curves are plotted, and absorption peaks near the wavelength of 2350 nm are extracted from the characteristic curves. The absorption peaks are then identified to obtain the characteristic absorption peaks.

[0008] Optionally, the process of analyzing characteristic absorption peak parameter data includes: The parameters of the characteristic absorption peaks are calculated to obtain the main characteristic absorption peak parameter data, including peak position, absorption depth, peak height, etc. By calculating the characteristic absorption peak parameter data, the spatial variation of the characteristic absorption peak position and the spatial variation of the absorption depth are obtained. By judging the spatial variation of the peak position and the spatial variation of the absorption depth, the direction and distance data of the concealed ore body are obtained.

[0009] To achieve the above-mentioned technical objectives, the present invention also provides a method for exploring skarn deposits based on changes in the chemical composition of calcite, comprising: Geochemical composition data of hydrothermal calcite were obtained, and trace elements, including rare earth elements, were analyzed to obtain the spatial variation law of trace element content. By judging the spatial variation law of trace element content, the direction and distance data of concealed ore bodies were obtained.

[0010] Optionally, the process of obtaining geochemical composition data for hydrothermal calcite includes: Hydrothermal calcite was identified based on its mineral symbiotic relationship and cold cathodic luminescence properties, and then in-situ analysis techniques were used to analyze the chemical composition of the identified hydrothermal calcite.

[0011] Optionally, the process of analyzing geochemical composition data of hydrothermal calcite includes: By analyzing the spatial variation patterns of characteristic trace element content, and judging the spatial variation patterns of element content, the direction and distance data of concealed ore bodies can be obtained.

[0012] The present invention has the following technical effects: This invention uses shortwave infrared spectroscopy to identify calcite absorption peak data and in-situ analysis to obtain geochemical composition data of calcite. Based on the relevant data of calcite, it comprehensively judges whether there are hidden mineral bodies at depth. The application technology of this method is mature and feasible. Compared with single data, this invention uses a combination of methods to more reliably indicate mineral exploration and improve the success rate of mineral exploration. Attached Figure Description

[0013] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1 This is a schematic diagram of the method flow provided in an embodiment of the present invention.

[0015] Figure 2 This is a schematic diagram illustrating the changes in the position and depth of the characteristic absorption peaks provided in an embodiment of the present invention.

[0016] Figure 3 This is a schematic diagram of a mineral exploration model provided in an embodiment of the present invention. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] To address the key technical problem of the limited mineral exploration and ore indication range in existing skarn deposits, this invention provides the following solution.

[0019] like Figure 1 As shown, this invention provides a method for exploring skarn deposits based on the wavelength variation of calcite. The specific technical process is as follows.

[0020] Sampling: Short-wave infrared spectroscopy and hydrothermal calcite composition analysis in skarn deposits generally utilize three-dimensional core sampling from existing boreholes. The borehole spacing depends on objective conditions, typically ranging from 200 to 400 meters. For each borehole, approximately 10-20 carbonate mineral core samples are collected from near the favorable mineralization strata of the skarn ore body, with a sample borehole spacing of approximately 10 meters. Sampling is intensified in borehole sections with significant lithological variations. The collected samples primarily consist of limestone, dolomite, and marble from the surrounding strata, while also collecting samples containing hydrothermal calcite veins. Additionally, samples from corresponding strata in the region are collected as background reference values.

[0021] Spectroscopic Measurement: Samples were cleaned and air-dried. Concentration areas of carbonate minerals were selected on the smooth surfaces of each sample, with 3-7 detection points per sample. Data acquisition was conducted indoors using a conventional short-wave infrared spectrometer (e.g., ASD brand). The built-in simulated sunlight source of the acquisition unit was used to illuminate the measurement points simultaneously. During the test, the instrument was first calibrated using a white reference disk, and then calibrated again every 30 minutes using the same method. To reduce random errors, each point was measured three times. The locations of the measurement points were marked and numbered on the sample with a pencil. Different minerals were identified based on the spectral results, and experimental records were made, such as calcite and dolomite. Data was saved in .asd format for later analysis.

[0022] Spectral matching: Short-wave infrared spectral data were identified, interpreted, and processed using TSG analysis software developed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia. The main method utilizes the absorption wavelengths and depths of different minerals, resulting in different spectral morphologies, to match the mineral spectra with those in a standard optical disc library. This process completes the mineral screening and interpretation, and enables automatic identification of mineral types and relative contents. The standard short-wave infrared spectral curve of calcite measured in this invention is shown below. Figure 2 As shown, weak absorption peaks appear at approximately wavelengths of 1875 nm, 2000 nm, and 2160 nm, while a typical characteristic absorption peak, related to carbonate groups, exists near a wavelength of 2350 nm. The TSG software retains the characteristic curves for minerals with a single content of calcite and extracts spectral parameters; other data are considered invalid and discarded.

[0023] Calcite Interpretation: Calcite interpretation primarily focuses on the characteristic absorption peak around 2350 nm. Variations in the content of different cations within calcite can shift its central wavelength. The spectral data (.asd format) is opened using TSG software, and the absorption peak positions within 30 nm above and below 2350 nm are extracted; these are the characteristic absorption peak positions of calcite, typically labeled Pos2350. The absorption depths within this 30 nm range are also extracted; these are the characteristic absorption depths of calcite, typically labeled Dep2350. By determining the migration of the characteristic absorption peak wavelengths and the changes in absorption depth, and considering the spatial distribution of the sample, the spatial distribution of these two indicators is further clarified. Specifically, the shift in absorption peak position indicates changes in the chemical substances within the mineral; changes in absorption depth indicate changes in the relative content of the mineral.

[0024] Analysis: If the characteristic absorption peak position (Pos2350) of calcite is mostly concentrated between 2330-2336 nm, then there may be an ore body near the sample. If the characteristic absorption depth (Dep2350) of calcite is mostly concentrated below 0.2, then there may be an ore body near the sample. Different samples were taken from boreholes at different locations in the ore deposit, and the distance from the ore body (if any) and the hydrothermal center of the ore-forming fluid varied within the boreholes. Therefore, the position and depth of the spectral characteristic absorption peak near 2350 nm will vary.

[0025] like Figure 3 As shown, by combining geological surveys and data, the criteria for determining the presence of calcite spectral characteristics in deep ore bodies are as follows: When the characteristic absorption peak of calcite is between 2330-2336 nm and the characteristic absorption depth is less than 0.2, it indicates that the sample is within 400 m of the hydrothermal center of the rock mass, meaning the tracing range for the hydrothermal center is 400 m. The calcite Dep2350 index shows a linear positive correlation with the distance to the hydrothermal center of the rock mass. R 2 =0.44), and the specific regression equation is: y=0.0002x+0.18, where: y refers to Dep2350, and x refers to the distance from the ore-forming hydrothermal center. This feature can be used to indicate the location of the ore-forming hydrothermal center.

[0026] The intrusive rocks in the study area are mainly intermediate-acidic intrusive rocks, such as granodiorite porphyry. The study area possesses favorable host rock conditions for skarn formation and exhibits excellent host rock alteration. The three-dimensional spatial distribution of the calcite shortwave infrared characteristic absorption peaks and depths in the host rocks shows a regular variation: the absorption peak near 2350 nm is mainly concentrated between 2330-2336 nm, and the absorption depth is mainly below 0.2 nm, exhibiting a progressive variation. If these criteria are met, it indicates the presence of ore bodies at this depth in the study area, with the decreasing absorption depth pointing towards the location of the ore-forming hydrothermal center.

[0027] During the discrimination process, as the relative mass of metal atoms in calcite decreases, the spectral vibration mode shifts to the higher frequency region, manifested as a shift in the characteristic absorption peak position towards shorter wavelengths. Therefore, different proportions of Mg, Ca, Fe, and Mn elements in calcite and dolomite will affect the position of the spectral characteristic peaks. Among them, the Mg content in calcite has a strong correlation with the spectral band position and is the main factor affecting the peak position change in chemical composition. By observing the changes in the characteristic absorption peak position, the changes in the content of Mg and other metal elements in calcite can be reflected, thereby indicating the activity of ore-forming fluids.

[0028] Because organic matter content reduces the absorption peak depth of calcite, while clay impurities and particle size enhance the absorption depth, and the carbon content in limestone also significantly affects infrared spectral characteristics, carbonaceous limestone should be excluded from the study. The short-wave infrared spectral absorption depth of marble calcite is significantly higher than that of microcrystalline limestone, possibly related to the difference in calcite particle size. To minimize interference from this factor, marble samples should be excluded when extracting calcite SWIR exploration markers, and only microcrystalline limestone samples should be retained.

[0029] The technical basis of the above-mentioned technical solution of the present invention is as follows: the characteristic absorption peak position and absorption depth of calcite in the surrounding rock strata show a decreasing value with increasing closeness to mineralization, and the absorption depth shows a progressively decreasing trend. This may be due to the gradual weakening of water-rock reaction from the near end to the far end, resulting in a gradual decrease in the extracted portion of Fe and Mn elements in the surrounding rock and an increase in the content retained in the original rock. The core factor is that fluid alteration is stronger in the near end. Geologically, this is manifested by the significantly stronger alteration of kaolinite, sericitization, etc. in the near end of the ore body compared to the far end. This generates trace impurities in the limestone surrounding rock. Although short-wave infrared spectroscopy did not identify clay minerals or other minerals, these trace clay and other impurities still affected the characteristic absorption depth of the whole rock, forming a spatially regular variation.

[0030] like Figure 1 As shown, the present invention also provides a method for exploring skarn deposits based on changes in the chemical composition of calcite, and the specific technical process is as follows.

[0031] Indoor Preparation: After the aforementioned short-wave infrared spectroscopy tests, the target sample was prepared into a 30 μm probe slide and placed under a microscope equipped with a cold cathodoluminescence (CCFL) system. Thin-section observation was first performed to determine the location of different types of calcite. Subsequently, the CFL system was activated, and the vacuum process was initiated. The vacuum pressure was reduced to 0.3 Pa to achieve a stable operating state. Observation and photography under CFL were then conducted. All of the above procedures were performed in a dark indoor environment to avoid interference from other light sources. To investigate the relationship between the color intensity of carbonate minerals and mineralization, the parameters for all sample observation and photography were uniformly set as follows: accelerating voltage: 15.5 kV, beam current: 200 μA. The depth control was fixed in the image generation software, and the HDR imaging (16-bit) range was uniformly set to 0-20,000 to avoid automatic brightness adjustments for each CFL image by the image generation software.

[0032] Electron probe microanalysis: Major component analysis of carbonate minerals was performed using an electron probe microanalysis instrument (e.g., JEOL JXA-8230 tungsten filament electron probe). Experimental conditions were: current 5 nA, voltage 15 kV, beam diameter 5 μm. The oxides tested were CaO, BaO, TiO2, SrO, SiO2, ZnO, CoO, FeO, MnO, Cr2O3, V2O3, Na2O, MaO, Al2O3, K2O, and CO2. In-situ major component content data for calcite were obtained.

[0033] Hydrothermal calcite identification: Based on comprehensive analysis of mineral coexistence, cold cathodoluminescence observation, and electron probe microanalysis, hydrothermal calcite is identified. Specific identification characteristics include: mineralized hydrothermal calcite exhibits high-order white interference color, obvious polysynthetic twinning and rhombic cleavage, visible sulfides developed in calcite veins, and under cold cathodoluminescence, calcite grains generally show obvious bright red and dark color rings. The CaO content of hydrothermal calcite generally ranges from 53.0% to 56.5%.

[0034] LA-ICP-MS Analysis: In-situ trace element analysis of mineralized hydrothermal calcite was performed using a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS), such as the RESOlution 155 193 nm excimer laser ablation system, paired with a Thermo Element XR high-resolution plasma mass spectrometer. The laser beam diameter was 30 μm, the energy density was 4 J / cm², and the ablation frequency was 6 Hz. The analysis time for each sample point was 80 s, including approximately 20 s for background signal, approximately 40 s for laser ablation, and 20 s for blank signal. For every 8 sample points analyzed, a set of external standards (NIST 610 glass) and monitoring standards (NIST 612 and GSE-1G glass) were tested. Data processing was performed using Iolite software, with the Ca content of the electron probe microanalysis sample used as an internal standard. Trace element content data of the hydrothermal calcite were obtained.

[0035] like Figure 3 As shown, by combining geological surveys and data, the criteria for determining the chemical composition of calcite in deep ore bodies are as follows: The Fe content in hydrothermal calcite is concentrated at >8000×10 -6 If the sample is located within approximately 400 m of the hydrothermal center, then the Fe content in the calcite is weakly negatively correlated with the distance from the sample to the hydrothermal center. R 2 =0.34), the regression equation is y=-1.30x+2496, where: y refers to the Fe content, and x refers to the distance from the hydrothermal center of the ore-forming fluid. The Sr content in the hydrothermal calcite is concentrated >100×10 -6 If the sample is located within approximately 1000 m of the hydrothermal center, then the Sr content is negatively correlated with the distance from the sample to the hydrothermal center. R 2 =0.67), the regression equation is y=-0.08x+237, where: y refers to the Sr content, and x refers to the distance from the hydrothermal center of the ore-forming body. Rare earth elements are generally enriched in calcite, and calcite Ce anomaly (Ce / Ce*). N It exhibits a tendency to increase closer to the hydrothermal center.

[0036] The technical basis of the above-mentioned technical solution of the present invention is that the Fe content in calcite is weakly negatively correlated with the distance from the sample to the center of the hydrothermal mineralization. The technical basis is that the core controlling factor for the Fe content in hydrothermal calcite is the redox condition; under low oxygen fugacity, Fe... 2+ Due to its stable existence, Ca is prone to isomorphic substitution. 2+Entering the calcite lattice, the variation in Fe content reflects the increase in oxygen fugacity of the ore-forming fluid from near to far of the deposit. The technical basis for the higher Sr content with increasing proximity to the hydrothermal center is: Ca... 2+ With Sr 2+ Due to their similar ionic radii, ore-forming fluids readily undergo isomorphous substitution, and since magmatic hydrothermal fluids are generally rich in Sr, the Sr content in hydrothermal calcite (CaCO3) can be directly increased by the ore-forming fluids. As the ore-forming fluids migrate from porphyry to more distant sites, water-rock reactions continuously consume the Sr in the fluids, resulting in a regular variation in Sr content within hydrothermal calcite. Calcite Ce anomaly (Ce / Ce) * N The technological basis for the tendency of Ce to increase closer to the hydrothermal center is that Ce is the only rare earth element that can undergo a change in valence state in nature (Ce). 3+ and Ce 4+ In an oxidizing environment, dissolved Ce 3+ It is oxidized to insoluble Ce 4+ This results in a Ce depletion relative to other rare earth elements in the fluid, forming a negative Ce anomaly (Ce / Ce). * N <0.95); while in a reduction environment, Ce 3+ Stable presence, Ce abnormality close to 1 or positive abnormality (Ce / Ce) * N >1.05). Therefore, the oxygen fugacity of the ore-forming fluid may increase continuously as it migrates to more distant locations, which coincides with the change in redox conditions reflected by the regular changes in calcite Fe content, leading to the regular spatial variation of the Ce anomaly in hydrothermal calcite.

[0037] This invention utilizes hyperspectral technology to rapidly and cost-effectively identify characteristic absorption peaks in calcite within surrounding rock formations, serving as one indicator of the presence of deep ore bodies. Furthermore, based on in-situ chemical composition analysis, it rapidly and cost-effectively analyzes the contents of trace elements Fe and Sr, and the Ce anomaly (Ce / Ce*) in hydrothermal calcite. N This invention serves as an additional set of indicators for determining the presence of deep ore bodies. Since mineralization is influenced by numerous factors, the combination of these two indicators effectively compensates for the limitations of single-method mineralization. The invention comprehensively assesses the presence of ore and indicates the approximate location of the hydrothermal center. The primary prospecting target is skarn deposits within hydrothermal ore deposits. Compared to traditional techniques, this invention is simpler, easier to operate, requires only the identification of one mineral, reduces experimental costs, and provides evidence of the existence of concealed underground ore bodies, guiding prospecting.

[0038] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for exploring skarn deposits based on calcite wavelength variation, characterized in that, include: Shortwave infrared spectral reflectance data of calcite in the carbonate strata surrounding the ore deposit were obtained, and characteristic curves were plotted based on the spectral reflectance data. The characteristic curves are analyzed to obtain the characteristic absorption peaks; The parameters of the characteristic absorption peaks were calculated to obtain parameters such as the wavelength position and absorption depth of the characteristic absorption peaks near 2350 nm. Regression analysis was performed on the characteristic absorption peak parameter data and sample spatial location data to obtain the direction and distance data of the concealed ore body, which can guide the exploration.

2. The skarn deposit exploration method based on calcite wavelength variation according to claim 1, characterized in that: The process of acquiring shortwave infrared spectral reflectance data includes: Short-wave infrared spectral data of calcite in the carbonate strata surrounding the ore deposit were obtained. The short-wave infrared spectral data were analyzed and compared with the standard spectral characteristics of calcite. Data that fit the comparison were considered valid, and the short-wave infrared spectral reflectance data were recorded.

3. The skarn deposit exploration method based on calcite wavelength variation according to claim 1, characterized in that: The process of analyzing characteristic curves includes: Based on the reflectance corresponding to different wavelength bands, spectral characteristic curves are plotted, and absorption peaks near the wavelength of 2350 nm are extracted from the characteristic curves. The absorption peaks are then identified to obtain the characteristic absorption peaks.

4. The skarn deposit exploration method based on calcite wavelength variation according to claim 1, characterized in that: The process of analyzing characteristic absorption peak parameter data includes: The parameters of the characteristic absorption peaks were calculated to obtain the parameter data of the characteristic absorption peaks near the wavelength of 2350 nm, including the peak position and absorption depth. By calculating the characteristic absorption peak parameter data, the spatial variation of the characteristic absorption peak position and the spatial variation of the absorption depth were obtained. The characteristic absorption peak positions are concentrated between 2330-2336 nm and the characteristic absorption depth is concentrated less than 0.2, indicating the existence of concealed ore bodies. In addition, the direction and distance data of the concealed ore-forming hydrothermal center were obtained by the progressively decreasing characteristic absorption depth.

5. A method for exploring skarn deposits based on variations in the chemical composition of calcite, characterized in that, include: Geochemical composition data of hydrothermal calcite were obtained, and trace elements, including rare earth elements, were analyzed to obtain the spatial variation law of trace element content. By judging the spatial variation law of trace element content, the direction and distance data of concealed ore bodies were obtained.

6. The skarn deposit exploration method based on calcite chemical composition variations according to claim 5, characterized in that: The process of obtaining geochemical composition data for hydrothermal calcite includes: Based on the mineral symbiotic relationship of calcite and the cold cathodic luminescence properties, hydrothermal calcite (bright red ring) was identified, and then in-situ analysis techniques were used to analyze the chemical composition of the identified hydrothermal calcite.

7. The skarn deposit exploration method based on calcite chemical composition variations according to claim 5, characterized in that: The process of analyzing the geochemical composition data of hydrothermal calcite includes: Analysis of trace element contents such as Fe and Sr and Ce anomalies (Ce / Ce*) N Spatial variation patterns: By judging the spatial variation patterns of element content, the direction and distance data of the concealed ore body can be obtained.

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