I. Geological characteristics of regional mineralization
Hanxing-type iron deposit is a typical skarn-type iron deposit, and most of the ore bodies occur in the contact zone between carbonate rocks and Yanshanian magmatic rocks and are controlled by it. The widely exposed middle Ordovician carbonate strata, strong Yanshanian intermediate acid magmatism and superior tectonic environment in Hanxing area have created ideal metallogenic geological conditions for the formation of Hanxing-type iron deposits.
1. Strata
The exposed area of Middle Ordovician strata in this area is more than 1400 square kilometers, mainly distributed in Wu 'an fault depression, both sides of Cunsyncline and Shexian syncline. According to statistics, 90% of Yanshanian magmatic rocks in the total area of Hanxing area are distributed in the middle Ordovician strata, and Hanxing-type iron deposits with industrial significance are also distributed in the middle Ordovician strata. The main reason is that there are three layers of breccia limestone in the Middle Ordovician strata, which constitute a weak zone between layers and are easy to be invaded by magma, which is a good rock storage space. The Hanxing-type iron deposit in this area is located in the uplift part of the rock mass at the interface between the breccia limestone and the overlying strata, forming a layered stack.
Due to the difference of formation thickness and surrounding rock composition, the ore-bearing rate of different strata in Middle Ordovician is obviously different. Shangmajiagou Formation (Cixian Formation) is the best ore-bearing stratum of Hanxing-type iron deposits in this area, and large and medium-sized deposits such as Guanzhong, Xishimen, Wang Yao and Beiminghe are all produced in this formation. The ore-forming conditions of Xiamajiagou Formation and Fengfeng are poor, and most of them are small and medium-sized deposits. The former is like Fushan No.4 Mine, No.1 Mine, Guzhen Mine and Majinao Mine. , and the latter, such as Kuang Cun and Zhuang. According to the statistics of proven iron ore data, Shangmajiagou Formation accounts for 79.7% of the iron ore resources in the whole region, Fengfeng Formation accounts for 1 1.8%, and Xiamajiagou Formation accounts for 8.5%.
2. Structure
In the ore field, the structures that control ore deposits and ore bodies are various types of contact zone structures and interlayer cracks and structural cracks near the contact zone.
The contact zone between the top surface of magmatic uplift and carbonate surrounding rock controlled by small anticline structure is the most favorable ore-controlling structure, and some large and medium-sized deposits in the area are mostly controlled by this structure. Most ore bodies are distributed on one side of the upper Longyan body, and some are distributed on the top of the upper Longyan body. The other is a complex steep contact zone formed by intrusion of fault rock mass. The rock mass near the contact zone is serrated and dendritic, and the contact zone is irregular. The ore bodies in the contact zone are small in scale, numerous and complex in shape.
There are interlayer cracks and structural cracks near the gently inclined and steeply inclined contact zones, and the ore-controlling cracks are mostly distributed in the range of 100 m from the contact zone. The ore bodies in interlayer cracks are rich in grade, mostly dense massive ores, and some contact zones are undeveloped, mainly interlayer ores, such as the mining areas in the river bottom and Guzhen area, but the scale is small. Vein magnetite in structural fractures is far away from the contact zone, and rarely forms industrial ore bodies, but it can be used as a sign of deep mineralization.
Due to the metasomatism of magmatic hydrothermal solution, iron ore is formed around the carbonate xenoliths invading the upper part or near the top surface of the rock mass, with irregular distribution, some of which are the remnants of denudation of the upper surrounding rock, and there are ore bodies around the remnants.
3. Magmatic rocks
Neutral magmatic rocks invaded in Yanshanian period are the main source of ore-forming materials for Hanxing-type iron deposits. Magma invaded many times, and the iron ore was formed in the stage of large-scale magma intrusion. Yanshanian magmatic rocks in Hanxing area are invaded in three stages, diorite invaded in the second stage is closely related to mineralization, followed by amphibole and diorite invaded in the first stage.
After the magma rose along the basement structure in the middle Yanshanian period, it invaded along the brittle zone of the Middle Ordovician limestone, forming a multi-layered branched layered rock mass. In the first stage, the intrusive horizon of rock mass is low, mainly invading the horizon of Xiamajiagou Formation, such as hornblende diorite in Fushan and Qicun rock mass, which is layered, with flat bottom and complex top with small bumps and depressions. The thickness can reach more than 500 meters (floating mountain). Local rock mass intrudes into the horizon of Shangmajiagou Formation, and iron ore is formed in the contact part between the top surface of layered rock mass and Xiamajiagou Formation and in the interlayer cracks near the contact zone. The rock mass invading the upper layer of the Xiajiagou Formation in the form of rock strands forms a complex steep contact zone with surrounding rocks, which inclines to the contact zone on one side of the rock mass, which is beneficial to mineralization. Such as Fushan No.1 Mine and Broken Mountain.
The second diorite is the most important metallogenic intrusion in this area, and it is also the period of the strongest magmatic intrusion. It mainly intrudes into Shangmajiagou Formation and a little into Fengfeng Formation, forming diorite-monzonite series intrusions, which are the main ore-forming parent rocks of iron ore in this area, that is, the main second mineralization period in this area. Local magma intruded into Carboniferous-Permian strata, forming diorite porphyrite. In the process of magmatic intrusion, due to the differentiation and evolution, mineral liquid and volatile matter gather in the upper part of the rock mass, so the small rock mass at the top of the rock mass and the front of magmatic intrusion, or arc-shaped and columnar uplift, is conducive to the formation of iron ore. On the one hand, the contact between magma and carbonate rocks produces metasomatism and precipitates into minerals; On the other hand, due to the high internal pressure at the top of the magma, the ore liquid migrates to the surrounding rock and precipitates and mineralizes in the contact zone and cracks in the surrounding rock. Therefore, the front of magmatic intrusion and the protruding part of the top surface of rock mass are the most promising metallogenic areas. The diorite body gradually transforms into syenite diorite in the deep or inside of the rock mass, and the metallogenic conditions are poor in the areas where syenite diorite bodies develop. The contact zone between small rock mass and surrounding rock with arc uplift is beneficial to mineralization, and the gentle slope and concave part of uplift are the main parts of ore body. The ore bodies in the contact zone with complex shape are small in scale and complex and scattered in shape.
In the third stage, the rock mass invaded the upper Permian Shangjiagou formation, forming Hongshan alkaline syenite body.
Judging from the rock composition, amphibole diorite has high iron content, and the dark mineral amphibole has high content, so the metallogenic conditions are poor. Diorite has a low content of dark minerals and a high degree of iron oxidation, which is beneficial to mineralization. The chemical composition of ore-forming diorite is rich in sodium and saturated with silicic acid. Albitization or sodium zoisite produced by self-metasomatism enriched iron ore. Therefore, the sodium mineralization of magmatic rocks is one of the main signs of ore-forming rock bodies.
Second, the geological characteristics of the deposit
Hanxing iron mine in this area belongs to Yanshanian product. In space, the deposit is closely associated with magmatic rocks, forming five ore fields: Qingcun, Kuangcun, Wu 'an, Gushan and Fushan. Generally, there are many magnetite deposits of different sizes in each ore field, but they are concentrated in Qicun and Kuangcun ore fields, accounting for 74.4% of the total resources. More than 95% of ore bodies occur in the contact zone between middle Ordovician limestone and intermediate-acid magmatic rocks, and the shape, quantity and scale of ore bodies are diverse. The ore grade is mainly rich (TFE > 45%), accounting for 55.2% of the total ore. The sulfur content of the ore is high (the sulfur content in the sulfur-rich ore of Guanzhong blast furnace is 3.63%), and the high-sulfur ore accounts for 92%, accompanied by cobalt, which can be comprehensively utilized.
According to the statistics of the relationship between ore-controlling surrounding rocks, magmatic rock conditions and resource reserves, the ore-forming surrounding rocks are mainly Shangmajiagou Formation (Cixian Formation), and the ore-forming magmatic rocks are mainly diorite invaded in the second stage of Yanshanian period.
(1) ore-controlling contact zone and ore body shape and scale
The ore bodies in this area mainly occur in the contact zone between Yanshanian complex and Middle Ordovician carbonate rocks. The occurrence, scale and shape of ore bodies are strictly controlled by contact zones. According to the statistics of 1 1 large and medium-sized iron mines in Hanxing area, 95.4% of the ore occurs in the gentle contact zone, 3.7% in the interlayer fracture zone of surrounding rock outside the contact zone, and 0.9% in the contact zone related to xenoliths. Generally speaking, if the structural form of the contact zone is simple, the occurrence of ore bodies is relatively stable and large in scale, and most of them are layered and lentil-shaped, such as Guanzhong, Xishimen and other large and medium-sized deposits (Figure 6-5 and Figure 6-6); If the structural form of the contact zone is complex, the occurrence of the ore body changes greatly, the scale is small, and the shape is complex and diverse, and it is lenticular, bifurcated, cystic, veined or irregular along different parts of the contact zone, such as Xihaozhuang and Chongyi.
Figure 6-5 Geological Map of Xishimen Iron Mine
The deposits in this area are rarely composed of a single ore body, but most of them are composed of a group of overlapping ore bodies dominated by one ore body. The ore bodies are mainly small and medium-sized, with only four large ones, but their resource reserves account for 40.80% of the whole region, 26 medium-sized ones account for 45.04%, and 46 small ones only account for 14. 17% (data as of the end of 2008). The scale of ore bodies is generally tens to hundreds of meters long, several kilometers or even more, and some can reach 5020 meters (Xishimen); The thickness of ore bodies varies from several meters to tens of meters, and in some cases the thickest can reach 190 meters (Beiming River).
Figure 6-6 Schematic Diagram of Alteration Zoning of Deposit
(2) Composition and enrichment law of minerals
The mineral composition of the ore is relatively simple, with magnetite as the main metal mineral, followed by pyrite, pseudohematite, chalcopyrite and limonite. Gangue minerals are mainly diopside, hedenbergite, tremolite and phlogopite; Followed by calcite, serpentine, actinolite, epidote, hedenbergite, andalusite, pumice stone, dolomite and chlorite. Because of the different lithology of ore-forming parent rock and surrounding rock, the ore-forming temperature is different, and the main mineral assemblages in some mining areas are also different. The spatial distribution of different mineral combinations of ore has certain rules, some of which are obvious in mining areas and some are not. Generally speaking, diopside-magnetite assemblage is close to albite contact zone, while phlogopite, tremolite-magnetite and tremolite-magnetite are developed near carbonate contact zone.
The main mineral is magnetite, with a content of 30 %~ 90%, generally 60 %~ 80%, which is mainly semi-autogenous granular, followed by octahedral and cubic autogenous crystals, and occasionally flaky crystals with a particle size of 0.0 1 ~ 5 mm, generally 0.01~ 2 mm. Magnetite is formed in at least two generations.
(3) Ore structure, structure and ore industry type
The ore structure is dominated by authigenic-semi-authigenic-special-shaped particles, and other structures include residual, skeleton, sieve, pseudocrystal, crushing and solid solution separation. The metasomatism in the ore is obvious. The ore is mainly disseminated (dense disseminated and sparse disseminated), banded and dense massive structures, followed by punctate and breccia structures.
According to mineral assemblage and ore structure, the natural types of ores can be divided into dense massive magnetite, magnetite hematite, pyrite magnetite, banded and disseminated skarn magnetite, etc. Blast furnace is dominated by high sulfur and rich ore, accounting for about 80%.
(4) Chemical composition and associated beneficial components of ore.
The chemical composition of ore is mainly iron, followed by calcium, magnesium, silicon, aluminum, sulfur and phosphorus, and a small amount of trace elements such as cobalt, nickel, copper, selenium and tellurium.
Iron is the main component of ore, mainly in the form of iron oxide, followed by sulfide. The average grade of iron in the deposit is 35.02% ~ 56.09%, generally around 45%.
The associated elements in the ore are sulfur, cobalt, copper, nickel, selenium, tellurium, gallium, vanadium, titanium, germanium and indium. The occurrence state of associated elements, except that copper exists as an independent mineral and closely coexists with pyrite, cobalt, nickel, selenium and tellurium mainly exist in pyrite as isomorphism.
Cobalt is the most important beneficial associated element in skarn iron deposits in Hanxing area. No independent minerals were found except cobalt-bearing hard manganese ore (Yunjialing), sulfur cobalt ore and hard cobalt ore (Guanzhong Hezhuang) in some mining areas. Cobalt in ores mainly occurs in pyrite, and it is also commonly found in magnetite and gangue minerals, but the content is very low.
Sulfur is an important associated component in iron ore, mainly in the form of pyrite, and its content varies greatly, with an average grade of 0.011%~ 2.17%. The average grade of sulfur in most large and medium-sized deposits is more than 0.3%. Among the ore bodies, thick ore bodies and ore bodies near carbonate have high sulfur content. The important associated beneficial elements content is often directly proportional to the sulfur content. At present, the associated sulfur in iron ore in this area has not been well recovered.
The content of phosphorus is generally lower than 0. 15%, and it exists in the form of apatite. The distribution space of phosphorus is similar to that of sulfur, with high content in the upper part of ore body and low content in the lower part, high content in the near contact zone and low content in the far contact zone.
(5) Wall rock alteration
The wall rock alteration of Hanxing-type iron ore body in this area is common and varied, including albitization, skarnization and various hydrothermal alteration. Among them, albitization and skarnization of inner and outer contact zones are most closely related to iron ore mineralization, and the width and intensity of alteration are obviously positively related to iron ore scale.
Albitization (albitization) is developed in amphibole diorite, diorite and other neutral complexes near the mine, which is the product of autometamorphism in the late stage of magmatic activity. It is characterized by the fading of rocks and the precipitation of iron, which is beneficial to the enrichment and integration of iron. The extension direction of sodium mineralization is generally almost parallel to the contact zone or ore body, and its alteration intensity tends to weaken from the contact zone to the rock mass. Due to the continuous precipitation of iron components in the process of sodium mineralization, it provides a material source for the formation of magnetite bodies near the contact zone. Therefore, the more the sodium mineralization zone develops, the larger the iron ore bodies are, and there is a positive correlation between them.
Skarnization is another important alteration type related to mineralization. Skarns in the area are mainly calcium-magnesium type, which can be divided into diopside skarn, phlogopite diopside skarn and garnet skarn according to mineral assemblage. Among them, diopside skarn and phlogopite diopside skarn are most closely related to iron ore, and they often control large deposits. In addition, garnet skarn controls some ore bodies with small scale, high grade, low sulfur content and complex shape. Mineralization is monotonous, mainly magnetite mineralization, and large, medium and small deposits are often formed separately.
Three. Genesis and metallogenic model of ore deposits
1. Metallogenic age
The metallogenic age of the deposit is early Yanshanian, about late Jurassic.
2. Sulfur isotope composition
The sulfur isotopic compositions of pyrite and anhydrite in different occurrences in Hanxing area show that pyrite δ 34s = 2.5 ‰ ~ 15.6 ‰ in magmatic rocks, and most of them keep the characteristics of deep magma sulfur, while anhydrite δ 34s = 24.0 ‰ ~ 29. 1 ‰, which is different from typical evaporated sulfur (δ 34s = 20 ‰ ~)
3. Hydrogen, oxygen and carbon isotopes
According to the hydrogen, oxygen and carbon isotopic compositions of the main minerals and their inclusions in Hanxing Iron Mine, on the δ D-δD-δ 18O diagram, magnetite falls in the magmatic waters, while diopside and garnet fall near the magmatic waters, indicating that the ore-forming fluid is mainly magmatic water mixed with atmospheric precipitation.
4. Metallogenic temperature
The statistics of ore-forming temperature show that the main alteration temperature is 290 ~ 400℃ and the ore-forming temperature is 258 ~ 356℃, which belongs to high temperature gas-forming conditions.
5. Metallogenic environment and pressure
The ore-forming parent rock of the deposit is located in the Ordovician strata, and the overlying strata are 1.5 ~ 2.5 km thick, which is in a shallow environment and conforms to the fine-grained porphyritic structure characteristics of diorite. According to the surface pressure gradient of 275× 105 Pa/km, the rock static pressure of the metallogenic system is 410×105 ~ 690×105 pa, which is close to the altered ore pressure calculated by Feng Zhongyan and others, indicating that the metallogenic system is synchronous with environmental conditions.
6. Inclusion characteristics
Diopside inclusions in Hanxing iron mine mainly include Na+, Mg2+, Ca2+, Cl-, CO2, H2O, CO, etc. Magnetite inclusions are mainly Na+, Mg2+, Cl-, CO2 and H2O. Obviously, the ore-forming fluid is rich in calcium and sulfate, which is not unrelated to the intrusion of parent rock into gypsum-bearing carbonate formation.
7. Genesis of the deposit
Based on a large number of examples of ore deposits in this area, the geological conditions and deposit characteristics of Hanxing-type iron deposits are comprehensively analyzed, and it is considered that Hanxing-type iron deposits are typical skarn-type iron deposits. The main basis is as follows:
1) iron ore body mainly occurs in the contact zone between Yanshanian intrusive complex and gypsum-bearing carbonate rock of Middle Ordovician.
2) Iron ore mainly comes from nearby rock mass (sodium plays an important role in the process of iron removal).
3) metasomatic texture and metasomatic residual structures are widely developed in the ore, and common magnetite metasomatic skarn and carbonate surrounding rock. The mineral composition, chemical composition and structure of ore are closely related to metasomatic carbonate rocks. Magnetite and skarn, especially diopside skarn and phlogopite skarn, are closely related in both space and formation time, accounting for diopside and carbonate minerals in surrounding rocks.
4) The mineral assemblage and metallogenic characteristics of magnetite reflect that magnetite was mainly formed in the gas-high temperature hydrothermal stage after magmatic stage.
5) The shape of sediments varies greatly. Generally, large and medium-sized deposits are composed of layered and lenticular ore bodies with relatively simple shapes, while small deposits are mostly composed of irregular bodies with complex shapes such as capsules, cakes and dendrites. The same ore body can be in contact with different formations of the Middle Ordovician.
Compared with other skarn-type iron deposits in China, Hanxing-type iron deposit has its own characteristics: (1) The ore-forming parent rock in this area is a neutral magmatic complex system with low acidity, and the lithology is mainly diorite-monzonite series; (2) The mineralization type is mainly single magnetite mineralization; (3) Pyrite is rich in heavy isotopes of sulfur; (4) The common complex layered intrusions in the Middle Ordovician strata are controlled by the breccia layer in the Middle Ordovician strata, forming a multi-layer contact zone, resulting in the overlapping of multi-layer ore bodies; (5) The alteration of surrounding rock is characterized by albitization near the edge of ore body.
The metallogenic model of Hanxing type iron ore is shown in Figure 6-7. Diorite intruded into the limestone of Majiagou Formation of Ordovician, and skarnization occurred in the contact zone. Subsequently, the ore-bearing hydrothermal solution from magma replaced the surrounding rock, transformed the skarn, and formed the skarn type iron deposit.
Figure 6-7 Metallogenic Model of Hanxing-type Iron Ore