Metallogenic age and metallogenic model

(1) Geological foundation

According to the analysis of basic geological data, it is considered that the main metallogenic period of gold deposits in this area should be Mesozoic Cretaceous. The basis is as follows: ① Gold deposits occur in mylonitized cataclasts and granitic cataclasts in the footwall of the main fracture surface of Yishui-Tangtou fault, and the formation age of cataclasts is consistent with the strong activity time of Yishui-Tangtou fault in Mesozoic, that is, Cretaceous. ② The formation of gold deposits is closely related to Mesozoic magmatism. Altered diorite was found in the hole ZK 10 1 in Longquan station gold deposit. The rock type is similar to that of Tongjing rock mass, which is the product of Mesozoic magmatism. The formation of gold deposits in this area should be after the formation of rock mass.

(2) Age characteristics of potassium-argon isotopes.

In the process of gold mineralization in this area, potash feldspar is an important altered mineral and an important indicator mineral to judge the mineralization intensity. In this study, K-Ar isotopic dating samples were taken for the metallogenic age of gold deposits in this area, and the potash feldspar minerals from Longquanzhan Gold Mine, Nanxiaoyao Gold Mine and Niujiaxiaohe Gold Mine were sent to the isotope room of the Institute of Mineral Resources of China Academy of Sciences, and the alteration age of gold deposits in this area was determined. Instruments used: MM5400 static vacuum mass spectrometer; Test conditions: When the sample melts at about 1500℃, add an accurate quantitative 38Ar diluent, determine the isotope ratio of (40Ar/38Ar)m and (38Ar/36Ar)m after mixed dilution, calculate the radioactive source of the sample, and then calculate its age according to the K content of the sample. See Table 5-22 for the original test data and Table 5-23 for the test results. The results show that the formation age of gold deposits in this area should be Mesozoic Cretaceous. Nanxiaoyao gold deposit is located on the near east-west fault, and its age is quite different from that of Longquanzhan gold deposit, which may imply that the near east-west fault and its deposits formed late.

Table 5-22 K-Ar dating results

Table 5-23 Metallogenic Age Results

According to the study of China University of Geology, Yinan Gold Mine in Shandong Province and Geological Team 8 of Shandong Bureau of Geology and Mineral Resources 1992, the K-Ar dating data of 20 subvolcanic rocks and single minerals in Tongjing area of yinan county are mainly between1/KLOC-0 ~126 Ma; The age of another Rb-Sr isochron is 1 13.4 Ma, which can also provide evidence for the formation time of gold deposits in this area.

(3) U-Pb age characteristics of a single zircon microdomain.

1) sampling, processing and preparation. U-Pb age samples of single zircon were collected from Yinan gold mine, Nanxiaoyao gold mine and Shibu rock mass in Yishu fault zone. The samples of Nanxiaoyao gold deposit were collected at the depth of 30 m. The ore is pyritized silicified diorite cataclastic rock. Pyrite with larger particle size 1 ~ 2 mm is concentrated in the edge of the fragment, and pyrite with smaller particle size of 0.005~0.5 mm is concentrated in the fragment. The sample of Yinan gold mine is taken from the currently mined gold mine, which is pyritized silicified diorite and Shibu rock mass is adamellite.

After the ore is crushed to 40 meshes, it is roughly washed, then finely washed, and the samples are subjected to high intensity magnetic separation, electromagnetic separation and heavy liquid separation. Single zircon of 10 g was selected from heavy minerals under binoculars. After magnetic separation and density separation, elutriation and purification, zircon samples and standard samples were fixed on the sample target with epoxy resin, and the surface of the sample target was polished and polished until fresh zircon slices were exposed. After the zircon on the target was photographed with reflected light and transmitted light under the microscope (Figure 5- 12, Figure 5- 13), the CL analysis was carried out. The State Key Laboratory of Continental Dynamics of Northwest University has determined the U-Pb age of a single zircon.

Fig. 5- 12 transmission light image of zircon NX03T-5 in Nanxiaoyao Gold Mine

2) Check the test conditions and test accuracy. On-line U-Pb dating of single zircon was carried out by using Agilent 7500 ICP-MS from the State Key Laboratory of Continental Dynamics of Northwestern University, ComPex 102 ArF excimer laser (working substance ArF, wavelength 193nm) from Lambda Physik Company of Germany and GeoLas 200M optical system from MicroLas Company. The diameter of laser beam spot is 30 μ m, and the depth of laser denudation sample is 20 ~ 40 μ m. In the experiment, he is used as the carrier gas of the denuded substance, and NIST SRM 6 10, the standard reference material of synthetic silicate glass developed by National Institute of Standards and Technology, USA, is used to optimize the instrument. The sampling method is single-point erosion, and the data acquisition is peak-jumping. After completing the sample determination at 4 ~ 5 measuring points, add the standard sample at one time. Zircon samples were measured twice before and after NIST SRM 6 10, and analyzed at 15 ~ 20. Zircon age adopts international standard zircon 9 1500 as external standard, element content adopts NIST SRM 6 10 as external standard and 29Si as internal standard. Test conditions and analysis accuracy meet relevant quality requirements.

Fig. 5- 13 transmission light image of YNG zircon in Tongjing gold mine, Yinan.

3) Analysis method. Preparation of zircon sample target: First, a single zircon mineral is stuck on double-sided tape, and then fixed with colorless and transparent epoxy resin [194]. After the epoxy resin is completely cured, it is polished until zircon exposes a plane, but it does not need gold plating. The size of the sample target can be determined according to the size of the laser ablation pool, and a plurality of zircon samples and standard zircon samples can be pasted on the same sample target to improve the space utilization ratio of the laser ablation pool [208, 209].

Zircon U-Pb dating is carried out in ICP-MS quadrupole ICP-MS Elan6 100DRC with dynamic reaction cell. The instrument can operate in standard mode and DRC mode [209]. The laser ablation system is a German GeoLas 200M optical system, which has the following advantages: a nearly perfect flat-topped beam is formed on the surface of the sample; The same energy density can be provided for different spot beams; Highly transparent substances that are extremely difficult to denude, such as timely, carbonate and fluoride, can control denudation [2 10]. The laser beam diameter can vary in the range of 4 ~ 120 micron, the single pulse energy can reach 2 10 mJ, the highest repetition frequency is 20 Hz, the average power is 4 W, and the energy density is more than 28 J/cm2 after uniform focusing by the optical system. When the sample or mineral is easy to be eroded, the energy density can be reduced by reducing the laser high voltage or adjusting the attenuator.

In the experiment, he was used as a carrier gas for bare substances. NlsTsRM6 10, a standard reference material for synthetic silicate glass developed by the National Institute of Standards and Technology, is used to optimize the instrument, so that the instrument can achieve the highest sensitivity, the lowest oxide yield, the lowest background value and stable signal. The sampling method is single-point erosion, and the data acquisition adopts a peak-jumping method with a mass peak [195].

The sensitivity and signal stability of LA-CPMS zircon dating will directly affect the error of the final isotope ratio, so the optimization of instrument parameters will directly affect the final result. Taking advantage of the advantages of deep ultraviolet laser being easily absorbed by the sample and high energy, the sensitivity of 238U is generally greater than 800 cps/μ g g-1under the condition of 40 micron laser ablation spot diameter in this study. However, in general, the content of 238U in zircon samples is greater than > 50 μg/g/g, so the signal of 238U is greater than > 40000 cps. Quadrupole mass spectrometry can get good results in accuracy and precision [192].

4) Zircon U-Pb isotopic dating results. Zircons separated from Nanxiaoyao gold mine are mostly authigenic medium-long columns, and the corresponding transmission light images show that zircon develops banded structure (Figure 5- 14, Figure 5- 15), which belongs to magmatic crystalline zircon, but ordinary zircon develops dark gray edges to form core-mantle structure. In this study, the U-Pb isotopic dating of 30 zircons was completed by laser probe mass spectrometry (single zircon LA-CPMS method). The results of dating data are shown in Table 5-24, as shown in Figures 5- 16 and 5- 17. The average value of 95% dating data is concentrated in (2435 65438+). As shown in Figure 5- 18 and Figure 5- 19, zircon measuring points are concentrated on the harmonic line, and the corresponding weighted average statistical ages of 206Pb/238U surface age are (2438 13)Ma and (2503 54)Ma, which can represent the formation age of rocks. It is the completion period of continental crust proliferation and cratonization in this area, and this tectonic-thermal event is very strong in this area, which is consistent with the evidence obtained from basic geological data in this area. The lower intersection points (1 16 20) Ma and (99.7) ma may be the reconstruction ages of late thermal events, which represent the historical records of late intense thermal events in this area, because this age is characterized by the most intense volcanic-magmatic activity in Cretaceous and the most intense activity in Yishu fault zone.

Fig. 5- 14 Cathodoluminescence Image of Zircon in Nanxiaoyao Gold Mine

Fig. 5- 15 Cathodoluminescence Image of Zircon in Tongjing Gold Mine, Yinan

5) Discussion on geological significance. In this study, the U-Pb isotopic age of zircon from Nanxiaoyao gold mine was determined. The results show that it is an intrusive rock formed in Paleoproterozoic, (2438 13)Ma basically represents the age of the rock mass, and the metamorphic deformation characteristics of the rock mass also indicate that it is an ancient rock mass. But from the analysis of regional mineralization, the formation of gold deposits is closely related to Mesozoic magmatism [2 1 1, 2 12]. When studying the metallogenic age of gold deposits in this area, the author determined the potash feldspar minerals in Longquan Station Gold Mine, Nanxiaoyao Gold Mine and Niujiaxiaohe Gold Mine by K-Ar method. Their age values are (141.92 2.06) ma, (94.291.38) ma and (95.921.40) ma, respectively. The gold deposits in this area should have been formed in the middle and late Early Cretaceous [265438]. In addition, 20 pieces of whole rock and single mineral K-Ar dating data were measured in Tongjing area of yinan county, with age value of1/kloc-0 ~126ma, and another group of Rb-Sr isochron dating data was 1 13.4 Ma [6]. However, the age of the lower intersection point in zircon U-Pb isotopic dating in this study is (1 16 20) Ma, which may represent the approximate time limit of regional mineralization, which is basically consistent with the age in the above results and similar to the research result of Jiaodong gold mine [2 13].

Fig. 5- 16 zircon surface age spectrum in sample NX03

Fig. 5-5- 17 YNG zircon surface age spectrum

Table 5-24 zircon U-Pb isotope laser probe mass spectrometry dating results of Nanxiaoyao (NX03) Tongjing (YNG 1) gold deposit

sequential

sequential

5. 1.5.2 Metallogenic stage

According to the characteristics of altered minerals and their interpenetration, the gold mineralization in this area can be roughly divided into four metallogenic stages:

A. Yingshi-pyrite stage: the main minerals are Yingshi and pyrite, which are early mineralization, but the mineralization is weak.

B. The phase of Yingshi-sericite-pyrite: the main minerals are Yingshi, sericite and pyrite. At this stage, mylonite cataclastic rocks generally experienced pyritization and sericitization alteration, which led to pyritization and sericitization alteration, but the gold mineralization was weak.

Fig. 5- 18 zircon U Pb isotopic age harmonic diagram of laser probe mass spectrometry in Nanxiaoyao gold mine (NX03)

C. gold-quartz-polymetallic sulfide stage: the mineral components generated in this stage are complex, mainly pyrite and quartz, followed by chalcopyrite, galena, sphalerite, silver-gold ore and natural gold. Among them, it is round and irregular, and most of them are plastic flow structures. Pyrite is mostly autotype-semi-autotype granular and disseminated, and some of them gather into veinlets; Gold minerals and polymetallic sulfides are mostly distributed along cracks or crystal gaps of pyrite.

Fig. 5- 19 laser probe mass spectrometry zircon U-Pb isotopic age harmonic diagram of Yinan gold mine

D Carbonization stage: it is the late stage of mineralization, and the minerals formed in this stage are mainly calcite veins and brass minerals, among which carbonate minerals are mostly produced in veinlets, and chalcopyrite is mostly authigenic crystals, which are massive and veined in calcite veins.

5. 1.5.3 metallogenic model

According to the above analysis of mineralization and the source of ore-forming materials, combined with the basic geological and mineral data, the following ideas are put forward for the gold mineralization model in this area.

1) During Archean, gold-rich intermediate-basic magma from mantle (or core) erupted strongly, forming the initial source bed of Taishan Group, which is a set of volcanic rocks mainly composed of intermediate-basic extrusive rocks and has the nature of greenstone belt.

2) Archean-Proterozoic multi-stage regional metamorphism, especially the first act metamorphism in the late Neoarchean, formed a thermal anticline (granulite facies belt) spreading in the northeast. In the process of migration from high energy band to low energy band, the ore-forming materials in Taishan Group (source bed) are continuously extracted and mineralized along the axial foliation and the low-pressure expansion space of Taishan Group, forming a metamorphic hydrothermal gold deposit. The collision orogeny and magmatic activity in Jinning period not only produced extensive ductile shear deformation, but also was the initial background of gold deposits in this area.

3) During the early Yanshanian Mesozoic, due to the strong activity of Yishu fault, a series of NE-trending fault structures were formed in the area, and a large-scale tectonic-magmatic thermal event occurred, and the whole uplift area was activated again on a large scale. The upwelling of rock juice, which is mainly composed of intermediate acid components, has widely explained and reformed Proterozoic adamellite. The Mesozoic rock mass followed the activated east-west basement structure, which led to the peak of thermal events in this area, which was enough to activate and migrate gold in source beds and derived source rocks. In the process of magma evolution, volatile components gradually increase, and gold forms soluble complexes with volatile components and alkali (elements such as K and Na) and enters the fluid phase. Under the influence of a series of physical and chemical actions such as temperature and pressure, ore-bearing hydrothermal solution always migrates from high energy band to low energy band, and precipitates and enriches in the NE-trending brittle fractures and fractures, forming gold deposits. When the ore-bearing hydrothermal solution flows through the wide structural fracture zone, the addition of Tianshui accelerates the precipitation of gold, forming a large metasomatic (altered rock type) gold deposit in the fracture zone; But when it is filled in structural fractures, it forms a relatively small and rich gold deposit (time-pulse type); Skarn-type gold deposits (contact metasomatism type) are formed in the contact part between intermediate-acid rock mass and limestone. With the passage of time, the ore-bearing hydrothermal solution showed a drift from magmatic water to atmospheric precipitation.

Figure 5-20 Metallogenic Model Map of Gold Polymetallic Mining Area in Yishu Fault Zone

4) After the intense activity in the early Yanshan period, it temporarily eased, and the tectonic activity in the late Yanshan period intensified again, and large-scale volcanic eruption and magma intrusion resurfaced. However, small-scale volcanic eruption and latent volcanic intrusion were mainly formed along the intersection of east-west and northeast faults in the area, which brought the deep gold to the shallow crust again, and the ore-forming materials migrated along the existing faults, further overlapping with the early gold deposits, forming an independent gold deposit in the same lava-volcanic activity area. At the same time, polymetallic deposits such as copper, lead and zinc were formed along the fault structure, ending the metallogenic process of endogenetic gold and polymetallic deposits in Yishu fault zone. Figure 5-20 briefly shows the metallogenic model.