Since the early 1970s, the general survey, exploration and research of Shaxi porphyry copper deposit in Lujiang have been highly valued by the Ministry of Geology and Minerals, the Ministry of Metallurgy and the Nonferrous Metals Corporation. The T _ (327) Geological Team of Anhui Province has been exploring in this area for a long time, and 258,500 tons of medium grade (Cu ≥ 0.4%) and low grade (Cu = 0.2% ~ 0.4%) have been delineated. On the basis of detailed analysis of geological and drilling data of predecessors, especially Team 327 of Anhui Bureau of Geology and Mineral Resources, and through detailed field investigation and analysis, a theoretical model of structural shielding and ore-controlling and rock-controlling in the core of the anticline in this area is put forward, and a copper-rich iron cap is found in the core of the calamus mountain anticline outside the original detailed investigation area in the south of the mining area according to this model. The analysis results of three samples show that the high grade of copper is 1475 × 10. In cooperation with the special group of Institute of Geophysics, Chinese Academy of Sciences, the electromagnetic exploration was carried out in the prediction area of Changpu Mountain, and the comparative experimental study of No.9 exploration line was carried out in Shaxi known mining area. The results show that there is a highly polarized area in the area of Changpu Mountain, which is similar to the abnormal shape of Shaxi mining area and slightly stronger. The abnormal body is distributed in the northeast, with a longitudinal length greater than 1500m and a width greater than 500m, and the buried depth of the abnormal body is about 100m from the surface. The preliminary research results show that the anomaly is large in scale and is caused by sulfide mineralization. Based on the comprehensive study of geology and geochemistry in the whole region, it is considered that the anomaly is caused by a shallow-buried porphyry copper mine similar to Shaxi mining area, that is, there is a large-scale porphyry copper mine prospect prediction area in the area of Changpu Mountain.
(2) Regional geological background and geochemical characteristics of the deposit.
Shaxi-Changfushan porphyry copper deposit is located in the northern margin of the middle section of the iron-copper metallogenic belt in the middle and lower reaches of the Yangtze River. The main fault of the Tanlu fault runs through the whole area from the west side of the mining area and is close to the Luzong volcanic basin in the east. The mining area is located in the composite part of the Tanlu fault zone and the Fanshan-Tongling deep fault zone (Chang et al., 19 1). The strata in the mining area are exposed simply. In addition to Quaternary modern sediments and Cretaceous red glutenite, there are continental volcanic rocks of Upper Jurassic-Lower Cretaceous in the northwest, east and southeast of the mining area. As the surrounding rocks of ore-bearing rocks, there are middle and lower Jurassic continental lacustrine clastic rocks, upper Devonian-middle and lower Silurian continental marine clastic rocks and bay lagoon clastic rocks. Shaxi porphyry copper deposits are mostly found in porphyries, mainly synbiotic porphyry and spiderlike porphyry. Shaxi mining area is geographically divided into Qipanshan, Tongquanshan, Shizishan and Duanlongjing from north to south. The four ore blocks are actually a whole, distributed continuously in the northeast, and controlled by the NNE composite anticline in the whole area. The predicted area of Changpu Mountain is located in the south of Duanlongjing fault, and the structural line is slightly inclined to the east, which constitutes a relatively independent mineralization area with Shaxi deposit, but they are closely related in genesis (327 Geological Team,1982; Wang Kuiren et al., 1993). See Figure 9- 1 for the geological overview of the whole region.
Based on the above analysis, we purposefully selected the Changpushan area south of Longjing in Shaxi mining area for field investigation, and found the mineralization point and the copper-rich iron cap, the symbol of porphyry copper deposit, on the surface. The analysis results of three surface samples show that the copper content is between (560 ~ 1475) × 10-6 (Table 9- 1), which is much higher than the copper content in the surface mineralization point of Shaxi deposit and its associated elements such as lead, zinc, cobalt and mineralization elements S, F, Cl and As. Moreover, the mineralization point is located in the core of anticlinorium in Shaxi-Changfushan area, the main structure of the whole region (Figure 9-2), which provides good conditions for the formation and ore-hosting of deep ore bodies, so we think that the mark of this point can be used as the prospecting mark of porphyry copper deposits.
Fig. 9- 1 Geological-magmatic ore-controlling schematic diagram of Shaxi-Changfushan porphyry copper deposit in Lujiang, central Anhui.
1. Quaternary sediments; 2. Meso-Cenozoic red beds; 3. Mesozoic volcanic rocks; 4. Mesozoic clastic rocks; 5. Paleozoic clastic rocks; 6. diorite; 7. Shallow diorite porphyry; 8. Ore-bearing diorite porphyry and biotite amphibolite porphyry; 9. Shallow volcanic rocks; 10. Intrusive breccia; 1 1. Burst breccia; 12. Infer anticline axis; 13. Secondary syncline; 14. Fault; 15. Location of Shaxi Mining Area: (1) Qipanshan, (2) Fengtai Mountain, (3) Tongquan Mountain, (4) Gushan Mountain, (5) Duanlongjing Mountain, (6) Longtou Mountain; 16. Location of metallogenic prediction area in Changpu Mountain
Figure 9-2 Measured Geological Profile of I-I Line in Shaxi Mining Area, Lujiang County, Anhui Province (according to Geological Team 327 of An Wei Bureau of Geology and Mineral Resources)
A. polarizability and resistivity curves; B. Geological profile of borehole of Tongquanshan No.9 exploration line in Shaxi porphyry copper mine area, Anhui Province. 1. Middle and lower Jurassic clastic rocks; 2. Lower Jurassic clastic rocks; 3. Synchrosite diorite porphyry; 4. Middle porphyry plagioclase amphibolite porphyry; 5. Intrusive breccia; 6. oxidation zone boundary; 7. Ore bodies; 8. Infer the axis trace of composite anticline and its derived faults (NNE); 9. NE-trending fault
Table 9- 1 Elemental Analysis Results of Copper-rich Iron Cap on the Surface of Acorus calamus Mountain (wB/ 10-6)
The analysis was completed in the central laboratory of East China Institute of Metallurgical Geology, and the analysis method was atomic absorption spectrometry.
(3) Geophysical investigation and anomaly analysis
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Geophysical investigation in this area is based on geological and geochemical research. The selection of working area and the layout of survey lines cover the abnormal sections found in previous geological exploration and geochemical survey. Considering that there may be porphyry copper mineralization conditions in the contact zone between Silurian strata (S 1-2) and diorite body in the survey area, the survey line has a certain extension, which is conducive to a comprehensive understanding from normal field to abnormal area. In this work, a total of 6 profiles have been measured (one of which is the measured profile of known ore bodies in Shaxi mining area of Lujiang), and the survey line basically runs northwest-southeast, and the measured area of the prediction area is about 0.6km2.
According to the geological conditions of the study area, IP method and high-precision magnetic method are mainly selected. The instruments used in the electrical method are IPR-8 receiver and IPT- 1 transmitter imported from Canada. This method is an intermediate recursive method, which is matched with symmetric quadrupole sounding. Canadian IGS-2 magnetometer is selected as the magnetic method, and the diurnal variation correction of magnetic data is carried out. In this area, all kinds of instruments used in electrical and magnetic methods work stably, and the measured data are repeatable and the results are reliable. The selected instruments and methods well reflect the information of geological bodies in this area. According to the principle from known to unknown, and fully consider the particularity of geological conditions. First, we chose Line 9 in Shaxi Mining Area as the experimental profile (Figure 9-3). It can be seen from the measured profile that the concealed ore body corresponds well to the anomaly, and the anomaly electrical method has the characteristics of low resistance and high polarization, and the polarization rate is as high as 40‰. The magnetic method above the ore body is negative anomaly. Zhongshaxi diorite can't be detected in the northwest of the experimental section because of the reservoir barrier, and the magnetic field measured near Zhongshaxi Township is 49267.9nT. By analyzing the experimental section, it can be seen that the magnetic method above the ore body is negative anomaly, which may reflect alteration, and the high polarization by electric method is relatively wide and slow, which may reflect the large mineralization halo range of metal sulfide around the ore body.
2. Electrical measurement results
In the profile measured in the prediction area, the line 1 ~ 4 has similar characteristics (Figure 9-4). Its polarizability suddenly changed from about 20‰ of the normal Silurian system (S 1-2) in the east to over 40‰ when it entered the western anomaly area, and the highest value was about 60‰. However, after entering the Zhongshaxi diorite body, the polarization value rapidly drops below 8‰. The polarization anomaly is very obvious. And the anomaly is generally consistent with the direction of the contact zone between Silurian (S 1-2) and Zhongshaxi diorite body in the area, which is a belt near northeast and can be divided into two belts. The anomaly value in the western belt is slightly higher, which may be caused by shallow burial depth. According to the sounding results, the buried depth of the roof of the highly polarized body is less than100 m, and the resistivity changes greatly due to factors such as topography, but the abnormal zone of low resistivity near the rock mass can still be seen, which also reflects the corresponding characteristics of the high sulfide mineralization area and the known mining area in the prediction area. After a slight deflection to the southeast, the abnormal zone is sandwiched between lines 2 ~ 5. The abnormal area can reach 0.6km2 (Figure 9-5).
Figure 9-3 Experimental Contrast Profile (According to Geological Team 327 of Anhui Bureau of Geology and Mineral Resources)
1-loose sediment; 2- shale; 3- mudstone; 4— argillaceous siltstone; 5— Silty mudstone; 6- limonite silty mudstone; 7-limonite silicified silty mudstone; 8- argillaceous siltstone containing limonite; 9-Limonite silicified argillaceous siltstone; 10-siltstone; 1 1- diorite porphyry; 12- sampling position and quantity; 13- Quaternary; 14-Middle Silurian; 15- Yanshanian diorite porphyry
Fig. 9-4 Profile of Apparent Resistivity and Polarization (ρs, Ms) in the prediction area of calamus copper mine in Lujiang, Anhui Province.
It can be seen from the bathymetric curve that the bathymetric curve of the predicted area 1 line 67 has a very consistent correspondence with the bathymetric curve of the known ore body Shaxi line 9, and the polarization of the predicted area is 49‰, which is much higher than that of the known ore body (Figure 9-6). In addition, in the non-mineralized area, the deep rock polarizability does not increase much (Figure 9-7), which shows that this method is very effective in this area.
Fig. 9-5 Plan of polarization measurement results in the prediction area of calamus porphyry copper deposit in Lujiang, Anhui Province
3. Magnetic measurement results
In order to cooperate with the electrical exploration, we have also done the verification work of magnetic survey, and measured along all the electrical exploration lines. * * * There are 6 magnetic profiles. The predicted area 1 ~ 4 line has similar profile characteristics. The magnetic field is an inclined step belt from west to east and from north to south. Line 3 is a steep step belt with increasing magnetic field value to the west of 580 points, and it is smooth in the east. Except for a small local anomaly at 240 on the curve of line 4, point 520 corresponds to point 580 on line 3. There are local anomalies on the line from 740 to 880 1, and the amplitude can reach100 nt; No.460 in Line 2 has a disturbance change of nearly 20nT, and the shape of No.600 corresponds to No.580 in Line 3, while Line 5 reflects a completely different feature from the previous lines, that is, there is a certain fluctuation at a higher magnetic field level, reflecting that the shallow part under S 1-2 stratum is rock mass. Figure 9-8 is a magnetic survey plan, which is much richer than the regional aeromagnetic data. In addition to the shape of the cascade zone, secondary anomalies reflecting the shapes of dikes and small rock masses can also be seen.
Figure 9-6 IP Sounding Curve Profile of Lujiang-Changfushan Porphyry Copper Mine in Lujiang County, Anhui Province.
The above picture shows 670 points on the L line; The picture below shows No.560, No.9 exploration line in Shaxi mining area. Ms polarizability; apparent resistivity
Figure 9-7 Schematic Diagram of Quadrupole Symmetrical Sounding Area
Figure 9-8 Magnetic Survey Plan of Puchangshan Porphyry Copper Deposit Prediction Area in Lujiang, Anhui Province
Figure 9-9 Shaxi Mining Area Line 17 Section (according to the data of 327 Geological Team of Anhui Bureau of Geology and Mineral Resources)
4. Ore-controlling model of structural shielding in the core of anticline in the predicted area of Changpu Mountain.
The mineral assemblage of Shaxi deposit is simple, mainly including chalcopyrite, pyrite, pseudomagnetite, bornite, magnetite, chalcocite, celestite and arsenopyrite. The alteration features are: K-feldspar belt, sericitization belt and Panqingyan belt from inside to outside, but the zoning boundary is not obvious, and there is obvious zoning only in the main ore-bearing rocks-biotite diorite porphyry and biotite diorite porphyry. Magmatic rocks in the mining area are widely distributed, with strong magmatism and long-term continuous activity. Its composition is a set of complex rocks with different stages of calcium-alkali series homology. The upper and lower limits of magmatic activity time are relatively clear: volcanic rocks overlap with Xiangshan Group of Middle and Lower Jurassic, while rock bodies obviously invade Xiangshan Group, while there are many volcanic rocks in the red beds of Lower Cretaceous, and some intrusive rocks and gravel. It is known that the isotopic age of diorite porphyry in ore-bearing rock mass is 173 ~ 123 Ma, which is equivalent to late Jurassic-early Cretaceous, that is, it belongs to Yanshanian product.
After studying Shaxi mining area, Dong Shuwen (1984) put forward a broom structure ore-controlling model: Zhongshaxi rock mass throws three small rocks northward, one of which contains ore and the two wings do not. The model can give the distribution characteristics of ore bodies. However, according to the analysis of existing profile lines and borehole data of Shaxi deposit, the main ore bodies of Shaxi deposit are inverted U-shaped, and most of them are distributed in the core of anticline (Figure 9-9). The surrounding rock is mainly silicified clastic rock of altered Gaojiabian Formation, and the rock mass intermittently exposed along the core can be regarded as the outcrop after the core of anticline is denuded. Non-mineralized rock bodies exposed on both wings are generally shallow igneous rocks. In the core of the anticline, the ore-bearing hydrothermal solution rich in volatile matter enriches ore under the cover of the roof, but it is obviously not conducive to mineralization in the two wings. Since Indosinian, the large-scale left-lateral translation of the Tanlu fault (Xu J.W, 1993) has also played a leading role in the tectonic framework of this area, and a series of NE-NE-distributed pinnate tensile faults have been formed on the east side of the core of the compound anticline, which is a good channel for the later magmatic hydrothermal rise, while the core of the anticline and its collapsed parts are ore-bearing. The above geological structural conditions provide favorable conditions for the formation of porphyry copper deposits in the whole region.
On the basis of predecessors' work, combined with the measured stratum profile on the surface, we put forward the structural shielding and rock and ore control model at the core of anticline (Figure 9- 10). The application of this model can well explain the distribution of ore bodies, the distribution of coal seam thickness and the essential reasons for the absence of ore in the rock mass on the two wings of Shaxi deposit. According to this model, the copper-rich iron cap found in the field investigation in Changfushan area, where Silurian strata are distributed in the south of Duan Longjing, is the embodiment of the guiding ideology of prospecting. Both theory and practice strongly prove the guiding role of this theoretical model in searching for porphyry copper deposits in this area.
5. Discussion results
Based on the comprehensive geological and geophysical research results (electrical and magnetic methods), it can be seen that the anomalies delineated by induced polarization method in this area have a certain scale, and the anomalies are similar in magnitude to the adjacent Zhongshaxi ore body, and locally higher than the known mining area. Such a high polarizability value of 40 ‰ ~ 60 ‰ can only reflect the characteristics of underground sulfides in this area. The characteristics of abnormal and known mining areas are generally low resistivity and high polarization. The structural position corresponding to the anomaly is equivalent to that of the known mining area. The magnetic field intensity measured by magnetic method in this area is higher than that in the known mining area (the difference is about 100nT), which may reflect that the caprock in this area is thin. According to the above data, this area is a promising copper prospecting area. Among them, 740 ~ 650 points on the 1 line can be used as the preferred exploration and verification profile in the shallow part of the area.
Figure 9- 10 Conceptual Model of Tectonic-Magmatic Mineralization in Puchangshan Porphyry Copper Deposit Prediction Area
1. sandstone; 2. argillaceous siltstone; 3. diorite; 4. Copper-rich iron cap; 5. Speculate the existing form of ore body
To sum up, the forecast area of Changpu Mountain is the prospect area of porphyry copper deposits. Geological, physical and geochemical evidences all support the ore-controlling model of structural shielding in the core of anticline. We can see that the mineralization anomalies in the area are mainly delineated by induced polarization method, which has sufficient geological and geophysical basis and has been demonstrated by experts. In addition, the scale of mineralization anomaly is large, and its magnitude is similar to that of the adjacent Zhongshaxi ore body, and locally higher than that of the known mining area. And the anomaly is similar to the known mining area, which is a low resistance and high polarization anomaly. The structure corresponding to the anomaly is equivalent to the known mining area. Based on the analysis of the geological structure background and structural position of the whole area, we think that the predicted area of Changpu Mountain is a porphyry copper mine with great ore prospects.