Variscan volcanic activity in the west section of Awulale occurred with the formation and development of Carboniferous-Permian rift in Yili, which had a great influence on the formation of gold, copper, lead and zinc and was the main ore-controlling factor. Volcanic activities in the Early Carboniferous were mainly acidic fissure eruption and basic central eruption. The iron deposits related to submarine volcanic eruption-sedimentation in the early and middle Carboniferous include Chagangnuoer and Shikubutai, and skarnization was superimposed due to granite intrusion in the middle and late Variscan.
The controlling factor of mineralization is Carboniferous rift environment, and the subsidence of rift basin is accompanied by tensile faults and cracks, which leads to the eruption of submarine volcanoes and the formation of a large number of volcanic-subvolcanic rocks.
Geological factors The main ore-controlling factor of the deposit is the skarn belt formed by contact metasomatism between carbonate rocks of Carboniferous Dahalajunshan Formation and syenite porphyry in the middle Variscan period. The distribution of skarn is basically consistent with the contact zone, with a width of 200 ~ 300 m and a length of more than 2km, showing an east-west zonal distribution. Iron ore deposits occur in this skarn belt.
Structural factors According to the regional data, the north and south sides of the mining area are nearly east-west compressive and torsional faults, which are a long-term active structural weak zone affected by regional faults. The fault structure in the mining area is well developed, which is conducive to the intrusion of magmatic rocks. After magmatic rocks invaded to form skarn belt, the tectonic movement weakened and the integrity of skarn was maintained. Because of the good trap property of skarn roof, a large amount of gas and liquid are concentrated and cannot escape, which leads to gas explosion, which breaks the surrounding rock of skarn and forms the cryptoexplosive breccia zone under high internal pressure. Breccia magnetite prepared for iron ore has the following characteristics:
1) breccia is relatively simple in composition, all of which are gray green, light gray white and dark gray black skarn breccia.
2) The breccia is angular in shape, and a few breccias are slightly dissolved. In the later period, when the iron ore slurry cemented the breccia, it was mainly filled, but most of the breccia could be spliced into a whole, belonging to the nature of broken breccia. There is a certain displacement between breccia, which indicates that breccia flows with pulp.
3) Rock alteration is intense and superimposed for many times, mainly hydrothermal alteration.
Based on the above conditions and factors, the genesis of Zhanbei Iron Mine is a multi-stage complex metallogenic process, and the characteristics of the deposit can be summarized as follows:
The main ore body is medium-sized, and the shape and distribution of the ore body are controlled by the contact zone, which is like a layered ore body. The ore is dominated by massive structure, followed by disseminated and breccia, with metasomatic and granular structure. Ore minerals are mainly magnetite, sulfides are mainly pyrite and pyrrhotite, gangue minerals are mainly skarn minerals, a small amount of carbonate minerals, with a total iron grade of 20% ~ 60%, high sulfur and low phosphorus, accompanied by a very small amount of Zn. These characteristics are very similar to Daye iron mine with contact metasomatism-hydrothermal origin. It is concluded that the formation of iron deposits is contact metasomatism-hydrothermal iron deposits, that is, skarn deposits.
2. Physical field characteristics
The regional Bouguer gravity anomaly shows that the gravity isoline is distributed in the east-west direction, and the Beizhan iron mine is located in the gravity cascade zone. On the Bouguer gravity anomaly map of 1: 1 10,000, this cascade zone is the location of the Nilek-Aragou deep fault, and the gravity value of this cascade zone changes as high as 200× 10-5m/s2. In the residual gravity anomaly, it shows a stable negative gravity anomaly with SN strike, and the preparation of iron ore is one of them.
On the magnetic map of 1: 1 Wanhang, the prepared iron mine is located in the transition zone with east-west high magnetic anomaly in the south and low negative magnetic anomaly in the north; After △T polarization, the high magnetic anomaly in the south is more obvious, and the prepared iron ore is located on the high magnetic anomaly, while the vertical first derivative diagram shows the high magnetic anomaly in more detail, which is more closely related to iron ore (Figure 3-28).
Figure 3-28 Geological, Mineral and Geophysical Analysis Diagram of Typical Iron Deposit in Hejing North Station, Xinjiang (modified according to the data of Xinjiang Bureau of Geology and Mineral Resources)
Beizhan Iron Mine is located in the aeromagnetic anomaly area of 1:50000 (Figure 3-29). The anomaly in this area can be clearly seen on the aeromagnetic △T section plane, which is a peak anomaly superimposed on the northern edge of the positive anomaly zone. The abnormal curve is sharp, the gradient is steep, the intensity is high, the shape is regular, there is obvious negative value on the north side, and the maximum amplitude is as high as 910nt; . On the plane of △T isoline, it is a regular negative anomaly with the range of about1.8km×1.5km. ..
According to the physical characteristics of the mining area, magnetite is the main strong magnetic rock (ore) in the mining area, with the highest magnetic susceptibility of130015×10-5Si, the common value of 68285× 10-5SI and the highest residual magnetization of 32900×/. Pyritized magnetite and intermediate-basic rocks take the second place, and other rocks are generally low in magnetism (Table 3-9).
Table 3-9 Table of Rock Magnetic Characteristics of Prepared Iron Ore
Fig. 3-29 Aeromagnetic Anomaly and Geological Analysis Map of Hexian County1:50,000 Iron Mine (modified according to the data of Xinjiang Bureau of Geology and Mineral Resources)
Characteristics of geophysical anomalies in the mining area: 1: 2000 magnetic anomalies are nearly equiaxed, mainly positive magnetic anomalies, with high intensity (Figure 3-30), with a maximum of 39576nT, generally 20,000-30,000 nt, steep gradient, good continuity and near east-west direction. The east-west length 1000m and the north-south width are 400m m, and the δT curve changes sharply near the 0 line, forming a strong serpentine bend, and the abnormal peak basically corresponds to the surface ore body. Judging from the occurrence of abnormal negative values on the north side, the ore body tends to the north, and the drilling results are consistent with the speculation.
Typical profile analysis: the 0-line exploration line profile is located in the middle of the magnetic anomaly area of Beizhan Iron Mine. The magnetic anomaly is serrated, with sharp changes and steep gradient. The two wings of magnetic anomaly are basically symmetrical, and there is a weak negative anomaly on the north side. The abnormal peaks are located at 370 points, 405 points and 495 points respectively from south to north, and the corresponding △T extreme values are 36776nT, 39576nT and 1, 2 1 and 544 nt respectively. Forward calculation shows that the buried depth of the ore body roof is about 9m, and the buried depth downward is about 500m (Figure 3-3 1).
3. Main ore-controlling factors
The main ore-controlling factors are summarized in the following table 3- 10.
Figure 3-30 Prepare the plan of 1: 2000 magnetic isoline of iron ore area.
Figure 3-3 1 Comprehensive Survey Profile of Geophysical Method for No.0 Exploration Line of Beizhan Iron Mine
Table 3- 10 Metallogenic Elements of Marine Volcanic Iron Deposit in Hejing County, Xinjiang
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4. Chronological research
The samples in this study are collected from various volcanic rocks in the mining area, which are outcrops and good drilling holes. The lithology is fine-grained diorite, almond-shaped andesite, dacite, granite porphyry and tuff. The sample is fresh and pure, and there is no foreign matter mixed in. Each sample is about 2kg, 8 * * *, and the major elements, trace elements and rare earth elements are tested and analyzed. The samples used in the experiment are ground into 200-mesh powder with a tungsten carbide bowl, and the bowl is washed with tap water and wiped with alcohol every time the samples are changed, so as to prevent the pollution between samples. The main elements of the whole rock are determined by X-ray fluorescence (XRF) on the X-ray fluorescence spectrometer. After the sample powder is melted into a glass block, the test accuracy is better than 65438 0%. The loss on ignition (LOI) is obtained by baking in a high temperature oven (1000℃) for 90 minutes. Trace elements and rare earth elements are dissolved in an autoclave with two acids (HNO3+HF). Plasma mass spectrometer (ICPMS；; Agilent 7500a) is used to determine the content of elements. The error of elements with content higher than 10 ppm is less than 5%, and the error of elements with content lower than 10 ppm is less than 10%.
The samples tested by La-ICP-MS zircon U-Pb dating method are all taken from fresh rocks. In this experiment, four groups of samples were extracted from magnetite ore bodies (B0 1) with coordinates of 479 1674 and 5382929. The coordinates of granite porphyry in the south of the mining area (XD 10) are 479 1008 and 5382432; Roof skarn (B03) coordinates 4791714,53282948; The coordinates of the floor skarn (B04) are 479 1526, 5382850. The samples are taken from the fresh surface of the mining area. In order to meet the needs of the test, 3 ~ 5 kg samples were taken from each sample. The samples were sent to Hebei Geological and Mineral Investigation and Research Institute. After the sample is crushed, zircon is separated by conventional gravity and magnetic separation method and purified by binoculars. The selected zircon is sent to Peking University Institute of Mineralogy of Rock Deposits for dating. Zircon samples and standard samples were put on epoxy resin by transmission electron microscope and polished. Li See et al. (2009) studied the specific preparation method and zircon dating process. The zircon structure was observed by transmission light and reflection light micrographs and cathodoluminescence image analysis, and suitable positioning points were selected for dating and data analysis and interpretation. The U-Th-Pb isotopic ratio of zircon samples is corrected by standard zircon 9 1500 (White et al., 1995). The isotope ratio and age error of single point analysis are 1σ, and the data structure is processed by IsoPlot software (Ludwig, 2003).
(1)LA-ICP-MS zircon U-Pb dating
The main types of zircon in this time are magmatic crystalline zircon and baddeleyite. The crystalline form of magmatic crystalline zircon is complete, self-crystallized, mainly columnar, with magmatic crystalline zone. Baddeleyite is round; The test error is small. According to the measured 204Pb, normal lead is corrected, and the data are interpreted and analyzed to determine the stratigraphic age. Sample B0 1 selected 1 10 zircon particles and 100 detrital zircon particles were spotted with a laser probe. In 1 10 zircon particles, 17 effective tests have been completed (Table 3- 165438+). The mass fractions of U and Th in zircon are (20.75 ~1078.09) ×10-6 and (16.36 ~ 5606.34) ×10-6b0/-4, respectively. 8. The weighted average age of 206U/238Pb at B0 1-26 is 304.2 1. B0 1 contains 100 detrital zircon, and the effective test of 18 has been completed. The harmonic age of these 18 zircons is 335.5 1.2 Ma (MSWD = 0.67) (Figure 3-32B), which belongs to the late Early Carboniferous and represents magnetite bodies. XD 10 isotopic dating shows that its harmonic age is 301.36 0.93 ma (Figure 3-32C), which belongs to the early Late Carboniferous and can represent the formation age of granite porphyry. Sample B03 is taken from the surrounding rock, and the effective point is 3 1 sample. The mass fractions of U and Th in zircon are (48.1~182.22) ×10-6 and (40.56 ~ 105, respectively. The U/Th values of magmatic zircon are all greater than 0. 1 (0.4 1 ~ 0.92), and the harmonic ages of nine measuring points such as B03-4 and B03- 17 are 342.4+1.3ma (mswd =/kloc-). Sample B04 is taken from the surrounding rock of the ore body, and the effective number is 18. This sample shows three groups of ages. The first group is B04- 1, B04-1,B03- 18, and their ages are 134.59Ma, 134.85Ma and18 respectively. The second group is B04-8, B04-9, B04- 12, with the age of 245.34Ma, 244.76 Ma, 234. 10Ma respectively, and its harmonic age is 244.61.1ma (ms
Table 3- 1 1 LA-ICP-MS preparation of iron ore zircon U-Pb analysis results
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Table 3- 12 LA-ICP-MS Hf-LU Isotope Analysis Results of Prepared Iron Ore
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(2) Zircon Hf-Lu isotope
The Hf-Lu isotopes of zircon in three samples of Beizhan Iron Mine were analyzed and tested. The test results (Table 3- 12) show that the measuring points are located in the same magmatic oscillation ring. According to the test data, zircon has a high ratio of 176Lu/ 177 Hf (mostly greater than 0.002). The ratio of 176Lu/ 177Hf varies from 0.0007 1 ~ 0.036880 ppm, with an average value of 0.004374ppm, indicating that zircon has a high accumulation of radioactive Hf after formation, and the measured176lu/1. The isotopic composition of Hf in iron ore is relatively simple,176 HF/177 HF = 0.25438+0209 ~ 0.986. εHf(t) varies widely, ranging from -5 1.69 ~ 3.9. The single-stage model age TMD 1 is 528 ~ 3622Ma, and the two-stage model age TMD2 is 58 1 ~ 3396Ma. The age of the two-stage model can better reflect the average age of provenance materials in the crust. The results of Hf isotope test show that the rocks for iron ore preparation are from partial melting of Proterozoic and Archean crustal materials, and the age of the two-stage model is older than that of the one-stage model, indicating that the source materials have existed in the crust for a long time.
5. Metallogenic model
(1) regional geological background
Beizhan Iron Mine is located in Carboniferous rift zone of Yili microplate in Tarim plate. In the Early Carboniferous, a extensional rift occurred in the Ili microplate. In the extension stage, tholeiite series and calc-alkaline series bimodal volcanic rocks were deposited and closed in the late Carboniferous.
(2) Metallogenic geological environment
The strata in the mining area are the Lower Carboniferous Dahalajunshan Formation, and the lithology is mainly a set of basic volcanic lava of coastal facies, followed by the rock combination of acidic volcanic lava with a small amount of pyroclastic rocks and normal sedimentary rocks. Petrochemical types belong to intracontinental tholeiite series and calc-alkaline series. The second lithologic member is the ore-bearing bottom, and the rock is mainly gray banded limestone. Thin limestone, dolomite marble, dolomite, marble limestone locally.
Figure 3-32 Preparation of U-Pb Age Histogram and Harmonic Diagram of Zircon from Iron Ore
The structure of the mining area is simple, and the overall performance is monoclinic structure inclined to the north. The Devonian strata in the north of the mining area form Feilai peak through nappe structure. There is a volcanic mechanism with belt eruption in the east of the mining area, and molten volcanic breccia can be seen near the crater.
Magmatic rocks exposed in the mining area mainly include syenite porphyry, diorite vein and diabase vein. The syenite porphyry is directly related to mineralization.
The syenite porphyry is light yellow with crystal structure. The phenocrysts are mainly syenite, plagioclase and potash feldspar, accounting for about 30%. It is irregular granular, clustered into dots, with erosion boundary, and the size is 0.8 ~ 3.2 mm; Feldspar and potash feldspar are semi-authigenic particles with a particle size of 0.9 ~ 4 mm, showing strong argillization and kaolinization. The matrix is fine-grained and consists of potash feldspar, plagioclase and quartz. The particle size is 0. 1 ~ 0.8 mm ... In time, fine marginal facies appeared locally in adamellite porphyry.
The syenite porphyry was formed in the early Carboniferous, and diorite and diabase veins were formed in the late Permian.
Metamorphic rocks in mining areas mainly belong to skarn contact metasomatic metamorphic rocks formed by contact metasomatic metamorphism. Skarns are distributed in belts, with a length of 1 ~ 1.5 km and a width of 200 ~ 300 m. The ore body is located in the skarn belt, and the near skarn belongs to simple skarn, mainly epidote diopside skarn.
(3) Combined distribution and occurrence of ore bodies
* * * There are 6 ore bodies in the mining area, mainly Fe3 ore body. Fe3 ore bodies generally have branches and compound veins. The total length of the ore body is 630m, the controlled depth is 380m, the thickness of the ore body is 5.12 ~139.72m, and the average thickness is 61.85m. The overall strike of the ore body is 97, and the dip angle is 47 ~ 74, with steep upward and gentle downward. According to the drilling structure and magnetic anomaly, the ore body has a lateral tendency to the east.
(4) Ore types and mineral assemblages
According to the ore structure, ore types can be divided into dense massive magnetite, breccia magnetite and disseminated magnetite, and magnetite is the main metal mineral, accounting for 85% ~ 87%. Followed by pyrite, pyrrhotite, sphalerite and chalcopyrite; The gangue minerals are mainly epidote, chlorite and diopside, followed by tourmaline, serpentine, andradite, tremolite, muscovite and calcite. The iron content in the ore is 20% ~ 64%, and the magnetic iron content is 89.53%. The harmful group is mainly S, with the content of 0.3% ~ 4.24%, with an average of 3.45%; The average content of silica is 65438 04.08%. TiO 20.38%, (K2O+MgO)/(SiO2+Al2O3) = 0.54 ~ 0.91,with an average of 0.67, belonging to self-fluxing ore. The natural type of ore is single magnetite.
(5) Ore structure
Magnetite has micro-autotype and semi-autotype granular metamorphic structure; Pyrite is heteromorphic-semi-automorphic granular with a particle size of 0.06 ~ 0.8 mm; Pyrrhotite is heteromorphic-semi-automorphic, disseminated or veinlets.
The main structural types of ore are dense massive, disseminated and breccia structures. The magnetite content in dense massive magnetite accounts for 50% ~ 70%. According to the uniformity of magnetite distribution, the characteristics of disseminated magnetite can be divided into dense disseminated structure and sparse disseminated structure. Mineral breccias with breccia structure are mostly epidote or diopside skarn, with particle size of 5 ~ 20 mm and content of 65,438+00% ~ 40%. Small breccia fragments can be spliced into bighorn breccia. Cement is magnetite.
(6) Division and distribution of mineralization stages
The metallogenic stage of iron ore preparation can be divided into three stages:
1) Volcanic sedimentary period: In the early Carboniferous, with the formation of the Awulale Rift, the original iron ore layer was deposited in the volcanic rocks of the Dahalajunshan Formation deposited during the rift activity. At this stage, the source of iron mainly comes from volcanic activity, forming a low-grade primitive iron ore layer. The pocket monster magnetite was born.
2) skarnization transformation period: the iron ore body formed during volcanic activity was strongly reformed by magmatic hydrothermal activity in the later period, forming a close symbiosis with skarn. This stage can be divided into three periods:
In the early oxide-silicate stage, the emplacement of sychrosite porphyry occurred contact metasomatism metamorphism with Carboniferous strata, and the original pig iron ore bed was skarnized in the early stage, and the iron ore grade in the original ore bed in the rock mass decreased, and the iron ore grade in the external contact zone became rich, resulting in the second generation magnetite.
In the sulfide stage, pyrite, a small amount of chalcopyrite, sphalerite and other sulfides were generated in the later stage of metasomatism;
Carbonate veinlets in carbonate stage are filled in early skarn and magnetite fractures.
3) Supergene oxidation stage: due to the high altitude of the deposit, the weathering is mainly glacial gouging, and the oxidation degree of the ore is low, with occasional jarosite, malachite and limonite, and the oxidation zone is undeveloped.
(7) Division and distribution of mineralized alteration zones
The ore body of Beizhan Iron Mine is located in the skarn zone, and the alteration from the ore body to both sides is skarnization, carbonation, serpentine, silicification or marble.
The skarnization phenomenon in skarnized mining areas is widespread, mostly developed near ore bodies, and the alteration is mainly diopside, epidote, hornblende, wollastonite, garnet, actinolite, tourmaline and other skarns with different mineral compositions. The skarnization zoning is obvious, and the roof and floor of the ore body are epidote diabase skarn belts, which are distributed around the ore body. The outer side of epidote diorite skarn belt is complex skarn, which is composed of tremolite, wollastonite, hornblende, garnet, actinolite, tourmaline and other characteristic metamorphic minerals.
Carbonization is generally developed at the edge of ore bodies, showing carbonate veins and irregular veins, which is residual hydrothermal alteration in the later stage of mineralization and has no close relationship with mineralization.
Serpentine mineralization is mostly hydrothermal alteration after mineralization, and generally develops along cracks or joints. Mainly manifested as aggregate or bundle fiber serpentine, yellow-green, waxy luster, with dislocation scratches. Generally not closely related to mineralization.
Marbling develops at the edge of the contact zone, generally far away from the ore body.
(8) genetic mechanism of the deposit
Beizhan iron deposit belongs to volcanic sedimentary iron deposit related to volcanic activity. The deposit has experienced two metallogenic stages. Volcanic sedimentary iron ore was formed by early volcanic sedimentation, and the early ore body was further transformed into mineralization by later skarnization.
The formation of marine volcanic iron deposits is directly related to volcanic activity. During the early Carboniferous extension, with volcanic eruption, a large number of minerals were ejected and deposited in low-lying areas of the basin.
With the late magmatic activity, quartz monzonite invaded the vicinity of the original ore bed, and the original iron ore bed was transformed by the contact metasomatism of magmatic hydrothermal solution. The metallogenic model of this deposit is shown in Figure 3-33.
(9) Prospecting signs
The geotectonic environment of iron ore production in the Carboniferous rift zone of Awulale is the Carboniferous rift in Awulale. In this rift zone, there are Zhibo iron mine, Chagangnuoer iron mine and Shikebutai iron mine, which constitute the metallogenic series of iron deposits in the rift zone. Therefore, the Carboniferous rift zone in Awulale is a favorable area for finding iron deposits related to volcanic activities in the rift.
Magnetite and pyrrhotite are the main ore minerals for preparing high magnetic anomaly iron ore, which have strong magnetism. The high magnetic anomaly above 3000nT found by surface magnetic survey is an important prospecting indicator for finding and delineating volcanic sedimentary magnetite bodies. The high magnetic anomaly greater than 9000nT is basically consistent with the surface distribution boundary of ore bodies. The shape of magnetic anomaly can be used to judge the trend of ore bodies and predict hidden ore bodies, and the effect is good.
The place where volcanic institutions prepare for iron ore production is the depression in the volcanic activity zone, which is close to the volcanic institutions, but not inside the volcanic institutions. Looking for volcanic depressions near volcanic institutions is the prospecting direction of the same type of deposits.
The early carboniferous skarn in the contact zone of the syenite porphyry is a shallow intrusive rock mass emplaced in the early carboniferous, which forms a strong contact metasomatic skarn in contact with the ore-bearing volcanic rocks, and some areas enrich the ore bodies by transforming them.
Magnetite outcrops and transformed magnetite have strong weathering resistance, and surface outcrops are easy to identify. The transformed magnetite can be well preserved in the water system where the ore body is located, which is one of the most direct indicators for prospecting.
Figure 3-33 Metallogenic Model of Volcanic Sedimentary Magnetite in Hejing County