Magnetite in Lugu Iron Mine, Mianning County, Xikang

Introduction

Lugu Iron Mine has become world-famous since it was surveyed by Chang Longqing. Due to its high quality and large quantity, it has been rated as the first iron mine in Southwest China. Since the Anti-Japanese War, many geologists, mining and metallurgists have visited for inspections. However, due to time constraints, most of them have not been able to do detailed work. As for the origin of the deposits, there are many speculations, and the estimates of the ore quantities have also been underestimated. Their reports They are not sufficient reference for actual development. In view of this, we ordered Qi and others to conduct a more detailed geological survey of the mine, so as to have a more reliable idea and record of the origin of the iron ore and its reserves.

This work started on November 1, 28, and was completed on the 11th. A geological sketch map of one-fifty-thousandth of the Lugu mine and one-two-thousandth of the mine were completed. A detailed topographic and geological map delineates the boundaries of the state-owned mining area. Workers were hired to dig twelve exploratory pits to understand the distribution and extension of the ore body and to facilitate the estimation of ore volume. The field investigation in the mining area uses the "outcrop mapping method". After the work was completed, Zeng Cao immediately made a report in Xichang, briefly describing the geological deposits and mineral quantities in the area, and attaching a drilling plan.

This mine is not only economically valuable, but its geological deposits are of particular interest in academic research. Due to time constraints, this report is written purely based on the iron ore itself based on materials obtained in the field and preliminary observations under a microscope. As for the detailed contact metamorphism of the granite and its surrounding rocks in this area, we will discuss it in detail when we have the opportunity.

This investigation is due to Commander Deng Xiuting, Xichang Xingyuan Director Zhang Dulun, Team Leader Zhang Huachu, and Commissioner Long Qing for their care and assistance, so that the work can be carried out smoothly. I would like to express my sincere gratitude.

Transportation and terrain

Lugu is located 70 miles southeast of Mianning City and 120 miles north of Xichang City. It is located in the upper reaches of the Anning River, between Mianning Xichang and Fulin Xichang The place where the two major thoroughfares meet, the traffic is convenient (Plate 2). After the Lexi Highway is completed, passenger and freight transportation will be more convenient. The ore producing area is commonly known as the iron ore mine, about 20 miles southeast of Lugu.

The highest point of the iron ore outcrop in the mine is about 2,400 meters above sea level, which is more than 700 meters higher than Lugu. From there, there are two small roads to Lugu, making transportation difficult. The first road goes west for about four miles to Miaowan, descends 500 meters along the slope, then turns north and northwest, enters a deep valley, goes down for more than ten miles to the mouth of Nanchong Gorge, where a flat land is in sight, and then reaches Lugu two miles further. The second road heads north-northeast, along the slope and down the ditch, for seven miles to Yanjin ditch, which is about 400 meters lower than the mine. From there, it turns north-northwest, and the valley is still flat and wide, descending 200 meters for eight miles. It reaches Gaoshi Bridge; then it turns southwest and enters a larger river valley. The road follows the stone wall, exits the mountain pass for seven miles, and reaches Lugu for another mile, descending 100 meters. When the nearby Yi people go down the mountain, they usually take the first road. There is low-quality anthracite coal five miles southeast of the mine, which is now mined by Han people and can be used as household fuel.

The terrain in the mining area (Plate 1) is simple and gently undulating, but erosion is more severe in the north and the slopes are quite steep. To the north of the district, next to the Second Road to Lugu, the terrain is low and the mountains are steeper. As far as the whole area is concerned, it should be that the topography rose in its prime and then experienced more severe erosion in modern times.

Strata

The strata in Lugu and the iron ore mine area have all deteriorated, and fossils have never been found. It is difficult to determine their age. This will be discussed later.

1. Pre-Permian (?) metamorphic volcanic rock system

This system is rhyolite volcanic rock, which undergoes dynamic metamorphism to form a variety of different rocks. Its distribution area is limited to both sides of the southern gully, both tilting to the southeast or east-southeast. The sections seen in the trench are listed here to show the replacement of the sequence and the changes in lithology.

Xin. Rhyolite that has undergone slight dynamic metamorphism, about 540 meters thick. Most of them are green and gray-green rocks, containing many quartz and feldspar phenocrysts up to three millimeters long, sometimes with clear ripple structures. Observed under a microscope, the quartz phenocrysts have been corroded and show a strain-extinction shadow; the feldspars are mainly microcline feldspar, and there are also a small amount of albite. The stone base is composed of microcrystalline quartz, potassium feldspar, acidic plagioclase, and a small amount of chlorite. The metamorphic ones show an obvious pleat-like structure (approximately parallel to the original rhyme structure), and the phenocrysts are elongated into strips or lentils, with a silky luster on the surface.

The quartz and feldspar in the thin slices all show evidence of being squeezed by force; the "stone base" contains a lot of sericite, sometimes forming corrugated strips, as well as calcite, green hornblende, sphene and zirconium. Stones, etc., all of which are generated by dynamic metamorphism of feldspar and other primary minerals and impurities. The upper part of this layer is in fault contact with Permian marble and limestone.

Gen. It intrudes into the gray-white coarse-grained granite (older granite) of the volcanic rock system. The outcrop is about 1,700 meters wide. Its nature and occurrence can be seen in the intruded rock sections.

This is a slightly dynamically metamorphosed rhyolite, about 270 meters thick. Its rock properties are similar to those of the Xin layer.

E. Green-purple quartz sericite schist and yellow-green phyllite shale, 50 meters thick. This layer is mainly schist, which is light green to silvery gray-green flake rock, with occasional purple layers, containing many pod-like white particles. The joints and polished surfaces that are approximately perpendicular to the schistose have a primary rhyme structure. Sometimes faintly discernible. Observed under a microscope, it is similar to the folded leafy metamorphic rhyolite in the symplectic layer, but its degree of dynamic metamorphism is deeper, and it occasionally has the rudiment of mylonite; the white spots are the elongated and strained quartz and feldspar (based on potassium Mainly feldspar, there are also striated feldspar and albite), or small particle aggregates of the two; the composition of the "stone base" is also similar, but sericite is larger, and there is a small amount of chlorite. exist. The detailed rock description is omitted.

Ding. Green-gray quartz porphyry intrudes into the rock formation, about 18 meters thick.

C. Green quartz sericite schist, about 540 meters thick. Its rock properties are similar to the schist in the E layer, and it is also formed from rhyolite volcanic rocks.

B. Green-gray quartz porphyry intrudes into the rock formation, with the thickest point reaching 130 meters. This kind of rock has a dense structure and contains quartz and feldspar phenocrysts. Its upper part is close to the schist and has a slightly pleated structure, which is almost parallel to the schistosity of the latter. The nature of the rock is shown below.

A. Green quartz sericite schist and yellow-green-white phyllite shale. The exposed area is more than a hundred meters thick, and the bottom is covered by alluvium. This layer seems to be dominated by shale.

According to the above sections (excluding intrusive rocks), the total thickness of this system is more than 1,000 meters. However, it is difficult to say whether there are repeated exposures due to structural relationships. The degree of metamorphism is deeper in the lower part than in the upper part. The main cause of metamorphism is the active dynamic force, which is related to a certain orogeny. In terms of the vast scope of its influence and the types of new minerals it produces, it has almost entered the realm of the lowest-level regional metamorphism.

From the Lugu Shun Anning River Valley southward to Huilijing Yongdingying, the older and modern alluvium at the mouth of the eastern tributary of the valley often contains a lot of large and small gravels of this series of rocks, half of Zhanying There appear to be outcrops of purple and green rhyolite (slightly metamorphosed) nearby, which shows that this series is widely distributed in the Anning River Basin, and its age will be discussed later.

Permian marble and limestone

This layer is widely distributed from Miaowan to Iron Mine. Those close to the granite are medium-grained marble, and those slightly further away are Fine-grained marble, and further away locally crystallized limestone and limestone. The marble inclusions in the granite at Wuli southeast of Lugu must belong to the same layer. Most of them are pure and only some contain magnesium. The contact metamorphism and hydrothermal corrosion will be discussed later. The layer is quite thick, and there are sometimes small elliptical spheres protruding from the weathering surface, which look like flint nodules commonly seen elsewhere in the Qixia Formation of the Permian, but have been transformed into minerals such as calcite, diopside, and plagioclase. The full layer thickness is at least 300 meters. Its lower part seems to be in fault contact with the metamorphic volcanic rock system.

Third, Triassic (?) Quartz Sandstone Layer

This layer is located on marble and limestone. It is about 400 meters thick. It is mainly dark green gray quartz sandstone and is weathered. It turns light yellow later, occasionally gray-white, with multiple layers of green-yellow phyllite shale at the bottom and middle. Most of the quartz in the quartz sandstone has undergone recrystallization, and is produced with a small amount of transformed biotite and chlorite. The rock near the iron ore is brecciated, and the quartz has a "strain extinction shadow", indicating that it has been subjected to Squeeze hard. The reason why the rocks in this layer are slightly metamorphosed is partially due to the high temperature during the intrusion of younger granite, or partially due to regional metamorphism related to orogeny. It is difficult to tell before and after the two occurred.

4. Jurassic Coal Measures

Above the quartz sandstone layer is the coal measure, which has undergone very slight regional metamorphism and is exposed in the southeast of the iron ore mine. It is composed of yellow-green hard sandstone and long It is composed of stony sandstone and black shale or tabular shale, with a layer of anthracite ranging from thirty centimeters to one meter thick.

Discussion on stratigraphic age

The above strata have all undergone metamorphism and no fossils have been found, so the age is difficult to determine; volcanic rocks and marble are in fault contact, and it is difficult to determine the time before and after the formation of the two. assertion. Ignoring contact metamorphism and focusing on regional metamorphism, the lower part of the metamorphic volcanic rock system is more deeply metamorphosed than the upper part, and the degree of metamorphism of the contained clay rocks is also deeper than that of similar rocks east of the fault line near Miaowan. Therefore, There is no doubt that the volcanic rock system seems to be the oldest formation. Tan Xichou and others called the rocks between Lugu and the Iron Mines the Permian Mahal System, including schist, phyllite and marble, while Chang Longqing called the rocks in Nanchong Valley Jurassic schist. Comparing with nearby places, the coal series may belong to the Jurassic (or Upper Triassic), and the marble and limestone may belong to the Permian. Therefore, the quartz sandstone is temporarily classified as the Triassic, and the volcanic rock series is assumed to be the Pre-Permian. Or the Permian. The Paleozoic acidic volcanic rock system was discovered for the first time in my country. Because the eruption process has not been studied in detail and the era has not been determined, it has not been named.

Structure

1. Folds

The rhyolite layer of the rhyolite in the metamorphic volcanic rock system is approximately parallel to the secondary fold pages and sheet layers. , roughly tilting to the southeast or east-southeast by more than 30 degrees or even more than 70 degrees. Near the iron ore mine, the marble and quartz sandstone (Plate 1) have a structure of one syncline and two anticlines, with the axis running roughly northwest and southeast. They all dip toward the southeast, affected by the intrusion of granite, and their shapes are no longer complete. . The anticline in the northeast extends to the southeast to the end of the extension ditch, where coal measures are exposed.

2. Faults

The direction of the aforementioned faults near Miaowan is approximately north-northeast, south-southwest, and the west line seems to be on the upward side. The inclination direction of the fault plane cannot be inferred. There seems to be a fault between the marble inclusions in the younger granite and the Nanchonggou volcanic rocks and older granite, and it is on the extension line of the above fault. There is a small north-south fault near the iron ore mine (Plate 1). The west side of the fault line is the upward side, and the fault plane is almost vertical. These faults all occurred before the intrusion of younger granite.

Intrusive rocks

There are three kinds of intrusive rocks in the Lugu Iron Mine, all of which are acidic. They will be described below according to the order of their times.

1. Quartz porphyry

As mentioned before, there are two quartz porphyry intrusions into the metamorphic volcanic rock system, one is 18 meters thick, and the other is 130 meters thick; both are It is a green-gray dense rock containing quartz and feldspar phenocrysts one millimeter wide and two millimeters long. Under the microscope, the quartz phenocrysts have been corroded and show a strain phenomenon. The feldspar phenocrysts are mainly orthoclase and striated feldspar, and the stone base is microcrystals of quartz and feldspar (including potassium feldspar and anandite). The aggregate contains a small amount of sericite (partially evidence of substitution of feldspar) and trace amounts of sphene and chlorite. Its composition is similar to that of rhyolite and its source must be the same. It is believed to be an intrusion at the end of the volcanic eruption. The body is indispensable. Due to its thick layer, the middle part is not susceptible to dynamic extrusion and obvious metamorphosis, and its traces can only be seen in thin sections. However, its edges often have a slightly pleated structure, making it the oldest intrusive rock in the area.

2. Older granite (coarse-grained granite)

There is gray-white coarse-grained granite between Gao Shiqiao and Lugu, with potassium feldspar phenocrysts up to two centimeters long, which are consistent with the black color in the rock. Minerals, etc., are arranged approximately in parallel, and are partially "gneiss-like", which seems to be the original flow layer (fluxion banding). Tan and Li Yizhi belong to Kangding gneiss. The granite intruded into the volcanic rock system in the middle section of Nanchong Valley is coarse-grained and gray in color, and belongs to the same type of rock. The upper and lower parts are in parallel contact with the surrounding rock. The edge part (the upper and lower widths are two to three hundred meters each) is more than one centimeter wide and two and a half centimeters long. It contains potash feldspar phenocrysts (sometimes with Carlsberg twin crystals) and biotite. etc. are arranged slightly in parallel. This parallel structure should be called a flow layer. In fact, it has partially inherited the original structure of the surrounding rock and covered it in the contact zone. It has the characteristics commonly seen in the injection complex. Laminar intrusion phenomenon (Plate 4, Figure 1).

The middle part also occasionally contains smaller potassium feldspar phenocrysts, and biotite is mostly in agglomerates with no orientation. Observed in the thin section, the minerals in the stone base are mainly quartz and orthoclase, showing a texture, the biotite is slightly altered, the quartz shows a strain extinction shadow, and there is also antiperthitic feldspar. The surrounding rocks do not appear to have undergone significant contact metamorphism.

3. Newer granite (pegmatized tourmaline granite)

This type of rock is widely distributed in the mountains north of the iron ore mine, intruding into volcanic rock series, marble, and quartz sandstone. And older granite is easily weathered into sand. This is a grey-white medium- to coarse-grained deep-formed rock, sometimes in the form of pegmatites, occasionally containing potassium feldspar phenocrysts with a volume of 6 mm × 10 mm × 25 mm. In addition to feldspar and quartz, there are also many fine black tourmaline pillars, the longest of which is seven or eight millimeters long. They are unevenly distributed and often form clumps. There are also a small amount of biotite or muscovite, and occasionally small light yellow-green topaz particles. or cylinder. Observed under a microscope, the potassium feldspar phenocrysts show a "patch and vein perthite" structure (patch and vein perthite), which proves that after crystallization, it was affected by high-temperature fluids containing sodium, resulting in local exchange. The andesine in the "rock base" is mainly composed of slightly oblique striped feldspar, and there are also potash feldspar and anandite. Part of the quartz and tourmaline (blue and light blue pleochroism) are exchange feldspar. It is formed by stone or biotite, and these two minerals are often partially exchanged with feldspar, biotite, topaz, etc. for muscovite or sericite; fresh biotite is rare, and occasionally turns into chlorite; topaz is sometimes accompanied by Born from tourmaline. It can be seen that this rock is originally a medium-grained to coarse-grained biotite granite. When it is about to solidify, it is dissipated and exchanged by pegmatite granite magma, and contains coarse-grained striated feldspar, quartz (this kind of quartz sometimes has a local crystal form) and topaz. A small amount of quartz and tourmaline crystallize later. Later, under the influence of hydrothermal fluids, the leading minerals were replaced by muscovite (or sericite) and a small amount of chlorite.

Contact metamorphism

The surrounding rocks close to the granite bodies in the iron ore mine area were all metamorphosed by the high temperatures during the invasion; the surrounding rocks elsewhere should also be subject to contact metamorphism. The details are unknown without careful observation. The collected contact metamorphic rocks (their origins are all within Plate 1) are summarized below:

A. Marble pure limestone metamorphoses into white marble, with almost no trace except calcite. Other minerals; those with dolomite quality become serpentine marble (originally forsterite marble). Those far away from the iron ore are white fine-grained or medium-grained rocks, or contain an indefinite amount of yellow-green or dark green elliptical grains or agglomerates, and occasionally magnetite microcrystals or muscovite mica flakes. Observed under a microscope, calcite is formed into irregular grains, slightly strained. The green grains are serpentine, often containing iron ore particles, and occasionally small pieces of calcite and forsterite residual grains, close to this complex combination of calcite. , with partial crystalline shapes. In one specimen, long cylindrical (the longest can reach about two millimeters) hornblende and residual forsterite occur closely together. In addition, there are irregular pillar stones (tachylite or mesalite) or flaky muscovite aggregates, which contain a few magnetite grains and scattered calcite rhomboids. The calcite connected to them often shows crystal edges with attached There is brownish-yellow amorphous iron (Plate 4, Figure 2B). Among the above-mentioned minerals, forsterite, amphibole and irregular calcite are all products of contact metamorphism; pillars, muscovite and magnetite crystal pillars are formed by the later exchange of hydrothermal fluids and marble (exchange The traces are quite obvious in thin sections), and the residual calcite close to it is recrystallized by the influence of hydrothermal fluids, showing the crystal shape. It is difficult to determine the period when serpentinization occurred, or whether it coincided with the hydrothermal period. Serpentine marble is subject to extremely obvious hydrothermal action. Microcrystalline serpentine is often aggregated into irregular lamellae and contains light yellow-green fibrous serpentine (asbestos) veins.

Marble adjacent to iron ore bodies often contains brown-black spots and clumps. Observation under thin sections is quite similar to the above, except that the degree of strain of the calcite particles is deeper; the brown-black part is the above-mentioned muscovite or pillar aggregates, but it often contains more magnetite particles, including calcite on both sides. The crystal form is more complete (Plate 4, Figure 2A), and mostly without strain, and the attached amorphous iron is also denser.

It can be seen that the main period of extrusion of marble is after contact metamorphism and before hydrothermal alteration. The degree of extrusion and hydrothermal alteration depends on the distance from the iron ore body.

B. Lime-silicate hornfels is located about 280 meters west of the southern end of the C ore body in the iron ore mine (about 90 meters away from the granite contact), in the marble There is a layer of "clay limestone", which has now turned into calcosilicate angular shale, and the original layer is still roughly distinguishable. The thicker tissue is a layer richer in calcium, which is green-gray in color and contains rhombic dodecahedron garnets. The longest axis diameter reaches one centimeter. Under the microscope, there are two combinations: one is calcium-aluminum garnet-Fushanite, which contains a small amount of acidic plagioclase, and V.M. Goldschmidt's Type 10 and Type 8 angular sheets. The other rock is similar to the garnet-diopside-Fushan stone, with trace amounts of calcite, and belongs to the branch of Gao's Type 10 angular shale. Garnet is heterogeneous and has lamellar twin crystals, so the temperature when it is formed is below 800°C.

Those with finer tissues contain less calcium and are gray-green and green-gray dense rocks that are slightly thin-layered, which are the so-called calflintas. The layer with coarser particles is dominated by diopside, followed by epidote, and there is also a small amount of acidic plagioclase or orthoclase; the finer layer is dominated by fine diopside particles, followed by acidic plagioclase. Feldspar with a little quartz. Both belong to the eighth branch and seventh category of Gao's family. Other less common combinations will be omitted.

C. Metamorphic "flint nodules" The "flint nodules" in the serpentine marble have now turned into light gray-green and gray-white layered individuals. According to observation under a microscope, the following four layered combinations can be seen:

(1) Large pieces of diopside.

(2) Small grained diopside, neutral plagioclase and trace amounts of chlorite.

(3) Neutral plagioclase, calcite and trace muscovite.

(4) Calcite, trace amounts of diopside and plagioclase.

Comparing it with the original chemical composition of flint, we know that when metamorphism proceeds, a large amount of silicate moves out of the nodules, and compounds such as calcium carbonate and magnesium oxide are added, causing this A rock similar to calcium silicate horny shale.

D. Quartz Sandstone The transformation of sandstone into quartz sandstone in this area is partly related to the intrusion of tourmaline granite.

The issue of the age of intrusive rocks

The intrusion of quartz porphyry occurred at the end of the accumulation of volcanic rock systems, around the end of the Paleozoic era; the solidification of tourmaline granite occurred in the Jurassic (? ) after the coal measures were folded, as early as the middle of the Mesozoic Era. The age of the older granite is difficult to estimate. In terms of its occurrence (its edges are interlayered and intruded into the surrounding rocks), it was approximately at the end of the initial folding movement of the metamorphic volcanic rock system.

Ore deposits

1. Ore bodies

There are three ore bodies near the iron ore mine, with irregular shapes and not far apart. The rocks are separated by quartz sandstone and marble, not far from the contact with granite. Its extension direction forms an intersection angle of 50° to 80° with the syncline axis formed by the quartz sandstone layer, or coincides with the fracture zone formed at the end of the granite intrusion. Each ore body has three or four well-developed joint surfaces, which is very convenient for mining. Therefore, it is very easy to weather, and the parts near the surface are extremely weathered, often broken into small pieces of poor quality. After being moved by external forces, its mineral quality is also poor; the lower part is a solid part, slightly weathered, with fewer joint surfaces and the best mineral quality; therefore, in the better "outcrops", it can often be divided into three zones according to the degree of weathering. As shown in Figure 3 of Figure 4.

The A ore body is the largest, more than 300 meters long, and more than 50 meters wide at its widest point, with a steep slope to the south and southeast. Most of it is born in quartz sandstone, and the two ends are in marble. There are two widely distributed joint planes, the inclinations of which are S15°±E∠40°～45° and N—N10°W∠50°～65°. There are many localized joint planes.

Most of them are solid massive magnetite, with a few small crystals, and occasionally hematite and mirrorite. Small quartz grains are sometimes widely distributed, and sometimes weather into limonite in small holes; there are more quartz near the edges. It may turn into clumps and turn red when mixed with trace amounts of iron oxide. Quartzite and quartz breccia fragments are also common.

The B ore body is smaller, more than 200 meters long, and more than 50 meters wide at its widest point. It also dips steeply to the south and southeast. Most of it is born in quartz sandstone, and its eastern end is in Dali. Iwauchi. There are two commonly distributed joint planes, with inclinations of S60°E∠45°～50° and S60°～70°W∠40°±. The types and changes of its constituent minerals are similar to those of ore bodies.

The C ore body is the smallest, about a hundred meters long and more than 30 meters wide at its widest point. It slopes to the east and southeast and was born in quartz sandstone. There are two widely distributed joints, the inclinations of which are N20°～30°W∠60°～70° and N60°W∠50°±. It is also mainly composed of massive magnetite and contains more quartz grains, so its quality is poor.

On the hillside to the west and north of the three ore bodies, there are hillside accumulation layers composed of solid iron ore blocks (the largest volume is more than 23 cubic meters), and occasionally contain quartz sandstone, shale and other debris. Blocks, mixed with a small amount of soil, ranging in thickness from half a meter to three or four meters, are floating skin fragments of iron ore layers.

2. Surrounding rock and hydrothermal alteration

The surrounding rock of the iron ore body is mainly quartz sandstone. Those close to the ore body are sometimes brecciated and mostly affected by iron. Dissipation and exchange, or in the form of veins or blocks; this can be contrasted with the fact that the edges of iron ore bodies contain quartzite or brecciated quartzite fragments. Observed in thin sections, quartz is aggregated into various irregular clumps, showing obvious strain phenomenon, which are "broken rock fragments"; magnetite also has a tendency to agglomerate, interspersed with quartz and slightly altered brown and black. Mica, and occasionally muscovite, seem to occupy the "dust" part among the "pieces". It can be seen that the quartz sandstone is in the breccia shape after contact metamorphism (otherwise the strain phenomenon of quartz cannot be preserved) and before the iron ore is formed. Biotite seems to be a product of contact deterioration. The deterioration and the formation of muscovite are related to the activity of mineral-containing hydrothermal fluids.

The transformation of the marble next to the iron mine has been described in the "Contact Metamorphism" section of the "Intrusive Rocks" section, so we will not go into details here. Close to the ore body, the strain degree of calcite is much deeper than elsewhere. This phenomenon is quite comparable to the breccia shape of the quartz sandstone next to the mine. Although the two respond to dynamic forces in different ways, they are both the same. There is no doubt that it was squeezed by it (after contact and deterioration, but before hydrothermal corrosion). We can infer that the location of the ore body is approximately consistent with several fracture zones, and the driving force that promoted such fractures may still be related to the intrusion of tourmaline granite, but its termination was later.

In summary, although it is difficult to identify the hydrothermal alteration of the surrounding rocks of the ore body during field work, it is quite obvious when observed under a microscope. The transformation of the biotite in the brecciated quartz sandstone into "green biotite" and the production of muscovite are all due to it. Magnetite pillars or magnetite muscovite agglomerates commonly found in marble often have calcite crystallized next to them, which proves their wide range of influence. The secondary muscovite in the granite is probably a product of the same period. The degree of alteration is closer and deeper to the iron ore body (see above). The minerals formed by the alteration are closely related to the irregular magnetite particles, which clearly shows that this kind of alteration is closely related to the mineralization. , and is actually a by-product of the latter.

3. Minerals

The ore body is almost entirely composed of magnetite, and contains a small amount of hematite, which may be generated at the same time. The formation of specularite is relatively late, which may be related to the later and lower-temperature hydrothermal fluids. Limonite is a weathering product. The temperature and pressure conditions when the pillar stones and muscovite in the surrounding rocks are generated are similar to those of magnetite. It is unknown whether most of the quartz in the mine is derived from quartz sandstone, or whether part of it comes from rising ore liquid.

4. Origin

The three ore bodies in the area are not far from the contact with tourmaline and granite. Even without detailed study, it can be inferred that the two are closely related. Tourmaline and topaz formed by pegmatization of granite, and pillar stones produced by hydrothermal erosion, are both compounds of volatile elements, so the connection between the two is more certain.

The ore body is generated along the fracture zone, and its extension direction is approximately parallel to the granite contact zone. How can it be said that it is not affected by intrusive rocks?

No contact has ever been seen in the mine. Minerals, according to various studies, were formed later than the contact metamorphism of granite and the "dynamic metamorphism" produced during the invasion, so they can never be regarded as contact metamorphic deposits. The ore body is almost entirely composed of magnetite, and the hydrothermal minerals connected to it include pillars and muscovite. The edges contain quartz sandstone residues and quartz agglomerates. The surrounding rock was once dissipated and exchanged by molten iron, so it is a deep mine. There is no doubt that the hydrothermal metasomatic deposit is comparable to the Caizigou Iron Mine in Daofu County.

In summary, we can summarize the recent history of magma activity and deposit formation as follows: after the middle Mesozoic Era, granite magma invaded the limestone and sandstone north of the iron ore mine. Affected by its high temperature, it turns into marble and quartz sandstone. The maximum temperature when metamorphic minerals are generated is below 800°C. The pegmatite granite fluid from the same source rises and exchanges with the granite to form tourmaline, topaz, quartz, etc. When the intrusive rock gradually cools, the surrounding rock also shrinks, cracks appear, and small-scale dislocations occasionally occur along this weak zone, causing the quartz and calcite in the rock to become strained. After the acidic magma and volatile fluids rise, the iron in the magma deep in the earth's crust is concentrated. Later, due to opportunities and residual volatiles, they rise to the surface along with the hydrothermal fluid. The iron condenses into minerals along the weak zone (rupture zone). body, and other impurities are scattered with the liquid in the surrounding rock and granite. Due to the corrosion of the two, the degree of corrosion is closer and deeper to the ascending channel. The temperature of the granite magma is quite high, and a variety of high-temperature minerals can be generated in the surrounding rocks. As the molten iron rises close to the ground, the temperature is already low, but there is still considerable heat, which is enough to "assimilate" and exchange a huge amount of surrounding rocks.

5. Minerals

The iron ore minerals in this area are quite pure, but often contain small quartz aggregates. There are angular and round quartz sandstones and breccias near the edge of the ore body. There are scattered pieces of quartz sandstone, especially propylene ore bodies. The specimens collected this time were analyzed by Messrs. Xiang Sida and Wang Maoqian in our laboratory. The results are listed below to show their composition:

Selected Geological Writings of Cui Kexin

The average iron content of the A ore body is 67.07, and the insoluble matter (mostly silica) is 5.55. Only two of the thirteen analyzes (the specimen at the extreme west end of the ore body) have an iron content below 60, and the insoluble matter is 10 or more. The average iron content of B ore body is 69.59, and the average amount of insoluble matter is 3.64. Among the seven analyses, the lowest iron content is 67.15, and the highest insoluble matter amount is 9.50. This specimen was collected from the edge of the ore body. Excluding iron quartz breccia, the C ore body specimens have a maximum iron content of 63.50 and an average of 60.26, and a minimum insoluble content of 5.00 and an average of 6.74. Therefore, based on the above comparison, the composition of ore body B is the best, ore body A is the second best, and ore body C is the second best. The average iron content of the iron ore in this area should be well above 65, and the average number of insoluble matter should not exceed 5. The composition is very good. According to Chang Longqing's report, the average iron content in the middle of the ore body (A ore body? B ore body?) is 65.85%, 4.90% silica, 0.05% phosphorus, and only has traces of sulfur. The first two figures are consistent with the results of this analysis. , and the traces of phosphorus and sulfur in the mine can also be seen from this.

6. Mineral Quantity

There are not many iron ore outcrops, most of which are found in mines opened by predecessors. In order to determine its distribution and extension in order to estimate the mineral volume, Workers were hired to dig twelve exploratory pits. Its thickness varies greatly and its shape is irregular (Plate 1). The depth and changes in depth are difficult to measure; however, the height difference between the outcrops of the same ore body can reach 65 meters (the first ore body), and its volume is quite large, extending from dozens or even hundreds of years below the lowest outcrop. For the remainder of the meter, its thickness may have a tendency to gradually decrease, and the ore volume is estimated based on this principle.

There are three standards for calculating the ore volume this time: ① The specific gravity is calculated as 4.2; ② The three ore bodies are estimated in sections, and then the total volume is combined; ③ The ore volume is estimated in three times, and the first number is the amount above the lowest outcrop horizontal line of each section. Reliable reserves, the second number is the total reserves assuming that the ore body reaches a horizontal level of 30 (C ore body) or 50 (A, B) meters below the lowest outcrop, which is a possible reserve, the third number assumes that the ore body depth is deeper than The second number is added by adding 30 (C), 50 or even 60 meters, the length remains unchanged, and the thickness is reduced by about a quarter compared with the previous one, which is the approximate reserve. The individual and total ore quantities of the three ore bodies are now listed in the table below. As for the material figures on which the estimates are based, we will omit them due to space limitations.

Cui Kexin's Selected Geological Writings

The distribution area of ??the floating surface ore accumulation layer is approximately rectangular, 380 meters long and 140 meters wide. Its average thickness is tentatively estimated to be 1.5 meters, with three One-third is composed of loose rocks and soil, with a specific gravity of 4.2. The total amount of iron ore is

Cui Kexin Geological Wenxuan

The total reliable reserves in the entire region are more than 2 million Metric tons, the total possible reserves are more than 5 million metric tons, and the total reserves are approximately 7.7 million metric tons.

The above estimates are based only on ground observations and rough test results. More precise calculations of ore volume can only be started after drilling in the future. The drilling plan in this area has already been seen. In the briefing paper written by Qi et al., we will not go into details here.

The reserves estimated by previous people are much larger than those calculated this time. Due to time constraints, no more detailed investigation was conducted, which led to the misconception that the area where the iron ore is distributed is a huge ore body. .

Mining and Development Prospects

This mine was mined very early, and there are many pits nearby. During the investigation, Commander Deng Xiuting and his wife sent two workers to excavate in the middle of the middle section of the A ore body. Ma transported about 1,000 to 2,000 kilograms of ore to Lugu Huaxing Iron Works for smelting. The factory was established not long ago and has a smelting furnace with a daily output of 700 to 1,000 jins of iron.

The iron ore in this area has a very good composition, with possible reserves of more than 5 million metric tons, and estimated reserves of 7.7 million metric tons. In our country, which is poor in iron ore, it has been called a big mine. Located next to a transportation road, the upper part of the ore body can be excavated in the open, making mining, transportation and marketing easy as expected. However, iron smelting mainly relies on coke, and there is no coking bituminous coal that can be mined within hundreds of miles from the north to the south. Therefore, the large-scale development of this mine may not occur until the railway line is completed. In order to relieve the iron shortage during the national crisis, small-scale mining can also be carried out in advance, and a number of improved small iron-making furnaces can be set up in Lugu to increase iron production. Unfortunately, there are few forests nearby, so the problem of charcoal supply may not be easily solved satisfactorily.

Explanation of Plate 4

Figure 1. Natural section 800 meters northwest of Miaowan, showing the contact between coarse-grained granite and rhyolite. Rh. Rhyolite that has been dynamically metamorphosed is formed into layered inclusions, partially thrust by granite magma; Gr. Coarse-grained granite has a flow layer structure.

Figure 2. Marble altered by hydrothermal fluid, magnified 24 times. A. A conglomerate containing magnetite, muscovite and rhombohedral calcite (M-M). The calcite next to it is partially crystalline, with yellowish-brown amorphous iron. B. Contains magnetite (M), pillar stone (an aggregate of S and calcite, with yellowish-brown amorphous iron attached to the calcite next to it, and some with crystalline edges).

Figure 3. Damingtrough section in the middle of the A ore body, showing the weathering condition of the iron ore and its joints. 1. Solid ore that has been slightly weathered and has joint surfaces; 2. Deeply weathered ore with many joints that has been slightly moved; 3. Crushed iron ore; 4. Surface cover, containing a lot of iron ore fragments.

Plate 1

Plate 2

Plate 3

Plate 4