Shui Yuan, which originates from Wuyun Mountain in the southeast of Guizhou, flows through Jianhe and Jinping in Guizhou, Hongjiang, Anjiang, Chenxi, Yuanling, Taoyuan and Changde in Hunan, and flows into Dongting Lake, with a total length of 1.047 km and a catchment area of about 80,000 square kilometers. Large areas of gold-bearing strata in the basin-Mesoproterozoic Lengjiaxi Group (now changed to Jixian System) and Banxi Group (now changed to Neoproterozoic Qingbaikou System) have been eroded, and Shui Yuan is rich in placer gold and has a long history of gold prospecting. Gold prospectors recycle the discovered diamonds as by-products, so Shui Yuan has a long history of discovering diamonds, which can be traced back to at least 100 years ago. According to Taoyuan County Records, during Daoguang period of Qing Dynasty (182 1~ 1850), villagers discovered diamonds one after another. Gold diggers distinguish diamonds according to their colors and crystal forms, which are called sapphires, star anise seeds and diamonds. 1940, Hu Bosu, Bureau of Gold Exploration, Ministry of Economic Affairs of the National Government, published the article "Diamonds in Shui Yuan, Hunan" in the fifth volume of Geological Review. According to the records, Shui Yuan is below the Liulin branch of Yuanling, and the proven diamond producing areas are Bajiaozhou, Luojiawan, Gaoduyi, Fuling Creek, Shen Xi, Baiyang River, Gangkou and Wuquanshan, Pailoupo, Dingjiagang, Ma Yiping and Shuibaoshan in Changde County (now Dingcheng District). In the same year, in the article "Overview of Quaternary Glaciers in Western Hubei, Eastern Sichuan and Northern Guangxi", Li Siguang also described the diamonds found by gold prospectors in the Shui Yuan gravel layer from Jiangxi Street to Tuokou in hongjiang city, Hunan. 1949, Yu Deyuan also mentioned the original diamond in water in his book "Geology of Gold Deposits in West Hunan and East Guizhou". Li Yuyao, the first director of Hunan Geological Survey, once bought 1 diamond weighing 70 carats in Taoyuan Port on the west coast of Shui Yuan. Many previous works have revealed the original rich diamond resources of water.
1.2.2 took the lead in finding diamonds, and placer exploration was fruitful.
4 13 geological team first conducted a general survey of Quaternary geology and geomorphology and a large number of ore reports in the mainstream valley of Shui Yuan, Hunan Province, and found that the Quaternary system in Hongjiang Town to Anjiang Town and Taoyuan to Changde was well developed and the accumulated geomorphology was well preserved. Through the visit, it is preliminarily counted that more than a thousand diamonds have been mined in Xiaohe and Yadou, Dingjiagang, Dingcheng District, Changde City, and the gold miners have summed up the regular understanding of "where there are seeds, there are drills". The seed crystal is diaspore, because the specific gravity and dehydration are similar to those of diamond, so they are closely associated. According to the investigation, it is confirmed that Anjiang, Dingjiagang and Taoyuan suburbs all have the preconditions for prospecting, and it is decided to conduct a general survey of Dingjiagang area, and strive for a breakthrough and launch in an all-round way after gaining experience. After dozens of days of hard work, Xia Deqing's team finally found a transparent and shiny diamond in Luojiawan, a small river in Dingjiagang, which strengthened their confidence in prospecting in Dingjiagang. However, due to the general survey method of placer gold in the early stage, it is wrong to dig shallow wells perpendicular to the trend of Quaternary sediments to take small samples and use gold mining basin for mineral processing, and the effect is not great. The main technical person in charge has realized the problem, and it is futile to continue. At this critical moment, the Ministry of Geology quickly dispatched a working group and a Soviet expert, Iskov, to inspect and guide, brought a manual jig and oil separation equipment, held a training course on placer geology and mineral processing technology, purchased new X-ray mineral processing equipment from Shanghai, and established a small mineral processing plant, thus promoting the progress of the whole census. In the field geological work, according to the stepped Quaternary surface elevation and bedrock elevation, combined with the color, composition, structure, structure and other characteristics of loose deposits, I ~ VII internal overlapping terraces are divided. Dendritic gully sediments distributed in terrace distribution area are a new type of accumulation geomorphologic unit, which is called "fine valley" because terrace sediments suffered from later erosion and denudation, and were transported, sorted and re-deposited on terrace bedrock. The earliest formed fine valley is called "fine valley terrace", which is the most abundant type of diamond placer, so it is necessary to fill in the Quaternary geological and geomorphological map, which is an essential and extremely important basic map for diamond placer exploration, as shown in figure 1-2.
At the initial stage of placer general survey, each accumulation geomorphic unit perpendicular to strike in the mining area is sparsely arranged with 1~3 exploration lines, and the spacing of exploration work (shaft, shallow well and exploratory trench) is 20 ~ 40m. Because the content of diamond in the ore-bearing gravel layer is extremely low, in order to find 1 diamond, it is necessary to take a large volume gravel sample (10 cubic meter). After regular survey, it is basically found that diamonds are mainly concentrated at the bottom of gravel layer, which requires the exploration project to excavate to 5~ 15 cm bedrock, and the stratified sampling recommended by experts is changed to 2 meters. According to the volume requirements and layered thickness of each sample, determine the size of the section specification of the prospecting project to prevent artificial dilution. For shaft exploration, the section size is usually 3.5m× 1.5m, and its long side is perpendicular to the placer strike. After half a year's general survey and evaluation, it is basically found that the probability of fine valley seeing ore is one to several times higher than that of terrace. At this point, it is preliminarily considered that Dingjiagang mining area has a good prospect for mineralization and prospecting, and it was converted to exploration on 1955; The general idea of "fine valley first, terrace later, main valley first and branch valley later" was established. According to the analysis of metallogenic prospect, the Ministry of Geology requires the 4 13 geological team to submit the diamond placer industrial reserve ××× 10000 carats * during the First Five-Year Plan period. In order to speed up the exploration and ensure the completion of the reserve task, the superior decided to transfer Jiang Shichang as the team leader, and the geological team quickly expanded to more than 600 people, and launched a large-scale diamond placer prospecting work.
Figure 1-2 Quaternary geological and geomorphological map of Taoyuan diamond placer mining area in Taoyuan County, Dingjiagang, Changde City
In order to find ore in Quaternary sediments, it is necessary to drill wells and extract a large number of samples for mineral processing. At that time, there was no excavator. We dug manually with picks, two-tooth hoes and shovels. If there is no mechanical equipment for hoisting, hoist it with derrick and hoist it with winch; Without a car, the samples were pushed by carts and carried by shoulders, which carried forward the spirit of hard work, hard work and determination to start a business for geological team members to find a "competitive mine".
With the deepening of prospecting in Dingjiagang mining area, the placer exploration in Taoyuan and Anjiang was included in the "First Five-Year Plan" by further summarizing the distribution and enrichment law of placer. To ensure the completion of the reserve task. The Ministry of Geology sent Soviet mineral processing expert Sulovisky to inspect and guide the work. Experts pointed out that-16+0.2 mm sand samples should not leak during transportation, and the samples must be strictly handed over when entering the concentrator; In the process of screening-jigging-oil separation -X-ray separation, in order to ensure the quality of mineral separation, fluorite minerals and even diamonds must be added to the original samples for quality inspection in each process of mineral separation, and the quality should be strictly controlled, and 100% should be recovered, otherwise, it will be reworked, re-selected or re-sampled. All prospecting work has strict rules and regulations, working methods and quality requirements, which are the most important and basic requirements for doing a good job in diamond prospecting.
1in March, 958, the Final Report on Geological Exploration of Diamond Placer in Dingjiagang Mining Area of Changde County and Taoyuan Mining Area of Taoyuan County, Hunan Province was submitted, which was the first large-scale geological exploration report of fine-grained diamond placer in China and filled the blank of diamond mineral reserves in China. According to the submission, the prospective reserve of diamond industrial reserve is ×××10,000 carats, and the gold reserve is 465,438+06.67 kilograms. It was rated as a high-quality report by Hunan Mineral Reserves Committee. The exploration work * * * has been put into geological and geomorphological mapping 1: 1 000,000 square kilometers, geological and geomorphological mapping 1: 1 000 square kilometers, exploratory trench 1 000 cubic meters and shallow well 72/kloc. Searching for 1 carat diamonds only takes 4.2~7.8 yuan, and this kind of low cost and high return is obtained by all the staff of the 4 13 geological team working hard to find a "competitive mine" for the construction of new China, racing against time, racing against time, racing against time, racing against time. Because the country is in urgent need of diamonds, 60 1 mine was established in 1959.
After submitting the above report, under the leadership of the team leader and chief engineer Lu He, the exploration of bench placer and fine-grained placer in Dingjiagang and Taoyuan mining areas was started, and the general survey and exploration tasks of placer in Anjiang mining area and Yaotou mining area in Yuanling were completed. By the end of 1964, the proven diamond reserves in four major sand mining areas in Shui Yuan were * * * million carats, of which the in-table reserves accounted for 92.12%; In addition, there are placer gold reserves of 639.874 kg and zircon geological reserves of 1929.8 tons. During the period of 1970, Hunan Geological Bureau held the annual geological work plan meeting. Comrade Bai Yulan, director of the Provincial Planning Commission, made a report at the meeting and mentioned that your bureau has made contributions to the country. On April 24th this year, China successfully launched its first artificial earth satellite, which contained high-precision components made of diamonds from Shui Yuan, Hunan. The players cheered: "The East is red, and the diamonds in Shui Yuan are blue. The people of China rejoiced and urged me to go forward and find the mine source. "
In order to expand the reserve foundation of placer exploration and further search for primary ore, according to regional development, we will vigorously strengthen the general survey deployment and carry out exploration work in new areas. Using the method of placer mineral processing, the main stream of Shui Yuan and its seven tributaries, Qingshui River in Guizhou, Duliujiang River in northern Guangxi and Zishui in Hunan, were systematically investigated successively in the Bagua River in Jinping, Huangzhong Dengzhai in Liangjiang, Shidongkou in Taijiang and Duliujiang River basins in Guizhou. Although no industrial placer has been found, the outline of the diamond primary ore prospecting area has begun to appear, indicating that the Yangtze platform is an important diamond metallogenic area.
Through the geological mapping of 1: 50000 in the periphery of Shui Yuan placer mining area, some new understandings of regional geology have been generated. In Yuanling-Changde Huangtudian area, which was originally classified as Banxi Group, regional high-angle unconformity was found, with Lengjiaxi Group below and Banxi Group above, and Wuling orogeny was found. At the same time, a number of basic and ultrabasic rocks were discovered in Xuefeng period, which broke the traditional understanding that there was no basic ultrabasic magmatic activity in Jiangnan platform uplift. In the mapping of the east side of Anjiang mining area, basic and ultrabasic rock belts in the Xuefeng period of NNE, 25 kilometers long, were also found in the area of Huaihua Aikou-Anjiang-Hongjiang Town. Through the mapping work, the regional basic geological data are enriched. According to the current viewpoint, the discovery of folded basement reveals that the Yangtze platform is a P-type craton with the occurrence conditions of K-Mg lamprophyre, which provides the most basic geological background for the mineralization of diamond primary ore.
1.2.3 distribution and enrichment law of diamond placer in Shui Yuan
Tuokou-Changde section, which is 567 kilometers long, is the main reach of Yuanshui River, and the valley and wide valley are alternately distributed, which controls the staged enrichment law of diamond placer (Figure 1-3). The Green yingtan Gorge between hongjiang city and Hongjiang Town, the Huangshi Cave Gorge between Anjiang and Chenxi, and the Wuqiangxi Gorge between Yuanling and Taoyuan have all been eroded and cut into Proterozoic strata. The rapid water flow, strong bottom erosion and handling capacity, strong river scouring force and many shoals are not conducive to the deposition, enrichment and preservation of diamonds. When Shui Yuan stepped out of the canyon and entered the Meso-Cenozoic red bed basin, the meandering river developed abnormally (the bending index was generally greater than 1.5), and the carrying capacity of the river decreased rapidly, and the lateral erosion and accumulation kept pace. In addition, the separation of lateral circulation in the channel bay area is strengthened, and heavy sand minerals such as diamond in deep water area are continuously pushed to the river bottom shoal facies in shallow water area, which promotes the mineralization process of accretion, separation and re-enrichment.
Shui Yuan diamonds also have obvious distribution characteristics of rich main stream and poor tributaries. In seven tributaries, hundreds of cubic meters of samples were taken from mineral processing, and only a handful of diamonds were seen, and no sand-rich ore near the source was found. The main reason is that the longitudinal slope of the valley is close to or greater than 1‰, and the river has strong carrying capacity, which is not conducive to diamond deposition and enrichment and preservation. Therefore, it is not appropriate to generalize that near-source placers will be rich. According to the prospecting experience at home and abroad, only the longitudinal slope of the valley is less than 0.6‰ can a near-source sand-rich deposit be formed. The four diamond sand mining areas in Shui Yuan are all located in the wide valley area of Mesozoic and Cenozoic red basin area, and they are all distributed in the initial area where the longitudinal slope of the valley drops to 0.34‰, from steep to slow, and the accumulation landform is abnormally developed.
Large diamonds are found in all the wide valleys of Shui Yuan. People in the suburbs of Hongjiang reported that they saw a complete diamond crystal, weighing 13.825 carats. I once saw a big diamond weighing 43. 15 carats in the sand mining mine of Maocao Street in Taoyuan. When mining diamond placer in the lower part of Dingjiagang fine valley in 60 1 mine, diamonds weighing more than 5 carats are common, and the largest one weighs 18.9 carats. The excavation of these large diamonds not only shows the quality of placer, but also implies important information about the origin of diamonds.
Figure 1-3 General situation of diamond distribution in Hunan Province
Quaternary diamond placer types in Shui Yuan include terrace alluvial placer, fine valley terrace and fine valley slope alluvial placer and modern riverbed alluvial placer. Terrace sand ore bodies mostly occur in the shallow water area of Guhechi Bay or Wanji, and the deep water area is the middle and front part of the terrace, only the head of the ancient sandbar is mineralized. Fine-grained placer is a new type of placer, which is distributed in dendritic streams in terrace area, thrusting the ore-bearing gravel layer of terrace, cutting into bedrock, transporting, sorting and redepositing terrace sediments. Therefore, the grade of fine-grained placer is several times or even ten times higher than that of terrace placer as its ore source, and it is the main type of placer with proven reserves in Yuanshui River Basin. Orebodies rich in fine grains are mostly distributed in the central axis area, but their grade is reduced due to the mixing of terrace slope deposits on both sides. The fine valley terrace is the old fine valley in the terrace area, and its metallogenic conditions are the same as those of the new fine valley. Among the modern riverbed placers in Shui Yuan, Chenxi-Luxi reach has the highest diamond content, and the river bending index is ≥3, which is one of the most favorable reaches for diamond enrichment. The content of brown corundum here is 42.4%~45.2%, which is 1.7~ 1.9 times higher than that in Anjiang, Dingjiagang and Taojiang mining areas, highlighting the source of lateral supply.
1.2.4 crystal characteristics of placer diamonds in Shui Yuan
There are more than 40 kinds of diamonds in Shui Yuan, mainly including red, yellow, blue, green, orange, brown, gray, black and colorless, among which light yellow, green and brown diamonds are the most, accounting for 70%~90%.
Shui Yuan's diamond crystal is 98% single crystal, and the highest integrity is 79%~90%. The crystal forms are various, including octahedron, rhombic dodecahedron, planar polyhedron, curved polyhedron and planar polyhedron, as shown in figure 1-4. According to the crystallographic principle of equiaxed crystal system, diamond should be tetrahedral, but it is rare in nature. A diamond was found in Dingjiagang mining area.
Shui Yuan diamonds are large in size and good in quality, and the gem-grade diamonds reach 63%. In the four placer mining areas, the proportion of coarse-grained diamonds of 1~4 mm is relatively high, accounting for 55%~79%. The average weight of diamonds is 25. 1mg in Anjiang, 13.5mg in Yaotou, 14.8mg in Taoyuan and 9.2mg in Dingjiagang, all of which indicate that diamonds are rarely seen in mining areas due to their long handling distance and strong sorting.
The original 15% diamonds have spots (or freckles), and their colors are brown, green, yellow, brown and black (Figure 1-5). Among them, brown diamonds account for 10%, green dots account for 3%, and other dots account for 2%. There are different opinions about the causes of pigmentation. Orlov believes that brown spots are typomorphic features of ancient diamonds, which are transformed from green spots through regional metamorphism and can be used as geological thermometers. But he can't explain the phenomenon that a diamond in Hunan has brown-green spots. Valenkin believes that ancient diamonds must have brown spots, wear, fracture and impact marks in order to accurately locate them, because the diamonds in the crater stage will also form brown spots due to leaching and corrosion of plant acids on the surface.
There are more than ten kinds of inclusion minerals in Shui Yuan diamonds, as shown in Figure 1-6. Diamonds containing forsterite, enstatite, diopside, garnet and S 1-S3 chrome spinel belong to peridotite type (P type). Diamonds containing omphacite, Fe-Al-Mg-Al garnet and kyanite belong to eclogite type (E type). There are also transition diamonds combined with P and E minerals, coesite, nepheline and moissanite. There are phlogopite, calcite, galena, diamond, ferrosilicon and aluminosilicate particles in diamonds, but graphite is the most common mineral in Shui Yuan diamonds. Diamond inclusion minerals provide a lot of geological prospecting information. Modern diamond mineralization theory holds that eclogite diamond is related to plate subduction in the active belt of ancient craton margin. According to the mantle rock minerals captured in Hunan diamonds, we infer that the ore-bearing surrounding rocks are mainly olivine-potassium-magnesium lamprophyre. Therefore, we should pay attention to the exploration of olivine-potassium-magnesium lamprophyre in the Yangtze platform area.
Figure 1-4 Morphological Characteristics of Shui Yuan Diamonds
Figure 1-5 Shui Yuan Diamond Point
According to nitrogen content and infrared and ultraviolet absorption spectrum characteristics, Shui Yuan diamonds can be divided into Ⅰ, Ⅱ and transitional diamonds. 81.6% of type I diamonds with nitrogen content greater than one ten thousand; Type ⅱ diamond with semiconductor properties and nitrogen content less than one ten thousand, accounting for 6.3%; Transition diamonds account for 12. 1%. Among them, type I diamond has the highest hardness, and octahedron can drill dodecahedron and hexahedron. The drawing die made of raw water diamonds can reach13 ~160,000 meters, which is several times higher than that of diamonds from other places. Therefore, Shui Yuan diamonds are favored by customers.
There are many kinds of etching images on the surface of Shui Yuan diamond, including stacked inverted triangular growth etching images (Figure 1-7) and regular triangular dissolution etching images, as well as square stacked etching images and orderly strip, drop and tumor etching images. This microscopic crystal feature is very common in placer, indicating that diamonds will not be transported far away. In addition, this feature is also one of the important signs to distinguish genuine and fake diamonds.
Figure 1-6 Diamond Inclusions in Shui Yuan
Figure 1-7 Etching Characteristics of Shui Yuan Diamonds
1.2.5 a new round of primary diamond ore survey (1964~ 1984)
According to the progress of proven diamond industrial sand deposits in Hunan and Shandong, the Ministry of Geology and the Ministry of Construction formed an investigation team to Tanganyika in August 1963 to investigate the diamond prospecting methods and experience. 1964165438+1October/kloc-0 ~17 the national conference on diamond geology was held in Linyi, Shandong province, which summarized all the prospecting achievements in China, introduced the methods and experiences of foreign primary ore exploration, discussed the prospecting situation and tasks, and formulated the plan and objectives. In the past 20 years, Team 4 13 has been fighting in the south-west of Hunan, completed the general survey of an area of nearly 40,000 square kilometers, and found more than 50 abnormal points of diamond and garnet. During the period of 1965, according to the regional investigation data, a large number of chromium-containing garnets, chromium diopsides and chromium spinels were found in the basic ultrabasic volcanic rocks in Baoan Xu, ningyuan county, southern Hunan. There are deep xenocrysts of dunite and spinel lherzolite in the rock tube, but no diamond is found in mineral processing. A general survey of placer mineral processing was carried out in the surrounding water system, and two diamonds were found in Leiyang, Xiangjiang River. So far, diamonds have been found in four major river systems in Hunan.
1965 ~ 1968 carried out a large-scale investigation in southwest Hunan, and found garnet or diamond in Potian, Duihe, Bamen River, Shuping Shuangchaxi, Huitong Cretaceous basin and Wushui main stream in the south of Anjiang. It is particularly worth mentioning that there are blue-purple or purplish-red garnets in the mainstream of Hunan dancing water, but in the toilet west of Zhijiang, the diamonds along the river are broken. In the lower reaches of Yuanshui River, anomalies of diamond and garnet were also found in Fushangang, Taoyuan and Yalinqiao in Linli. In order to speed up the prospecting and find the primary ore as soon as possible, the prospecting target area was optimized while the general survey was carried out.
During the period of 1966, a diamond was selected in Ou Tuan area of Jingzhou, and a diamond was selected in Gongdongxi and Kangtouzhai River in this area, with a water collection area of 90 square kilometers. If the primary ore breaks through the Neoproterozoic strata and Sinian strata in South China, it will promote the prospecting work in Liangjiang area of Guizhou. However, through the geological observation of 1: 1 000, mountain engineering exposure and regional heavy sand sampling, no garnet or rock mass was found, and only the exotic residual gravel layer was found in the depression at an altitude of 800 meters at the top of the mountain. Therefore, the source of diamonds in this area has not been identified.
From 1969 to 1972, Shuping area on the south side of Anjiang River was selected as the prospecting campaign, with the participation of regional geological survey, geophysical exploration team and diamond group of Central South Geological Institute. In the range of 36~40 square kilometers, 1: 1000 geological, heavy sand, geophysical and geochemical exploration and drilling work were carried out, and more than 300 plagioclase and amphibole lamprophyre veins were found. Nine diamonds were found in the water system, but no garnet containing chromium, magnesium and aluminum was found, and no rock mass containing diamonds was found, so the work was temporarily ended.
During the period of 1972~ 1973, 45 garnets and 13 chromite spinels were found through heavy sand sampling in water system and artificial heavy sand sampling in the formation at the foot of Feishan Mountain in Jingzhou. The refractive index of garnet is1.762 ~1.712, with purple series accounting for 68.9% and the rest being eggplant color. The abnormal area of heavy sand passes through 1: 1000 geological observation, heavy sand sampling and geophysical and geochemical exploration, and no rock mass is found. After the artificial heavy sand sampling of the second lithologic group of Jurassic red beds in the Upper Triassic, a fine diamond and several garnets were found, belonging to intermediate reservoirs. Based on the preliminary study of lithofacies paleogeography of red beds, it is considered that Feishan red beds belong to alluvial deposits of ancient rivers on the north side, and the prospecting work should be promoted northward. According to the analysis of existing data, the east-west fault on the north side of Huitong may be the eastward extension of Maping ore-bearing rock belt that controls Zhenyuan. The olivine cloud Huangyan seen by Ruoshui, a tributary of Wushui on the east side of the north side, was later identified as leucite K-Mg lamprophyre by He Ansheng. There are abundant garnet horizons in the Cretaceous basin in the south of the east-west fault, so it is necessary to further carry out prospecting work in Feishan-Huitong area of Jingzhou.
18 garnet was found in the water system of qinban area in Huitong in 1973. In the artificial heavy sand of Jiangkou Formation in South China, * * 379 garnet and 1 diamond particles were found. The refractive index of garnet is 1.746~ 1.733, and there is no purple series, mainly orange and eggplant series. At the same time, in the general survey of Tang Ye and Broken Houses in Jingxian County, there are 1 12 garnet in heavy sand of water system, and 15 garnet in South China stratum, with refractive index of 1.746~ 1.730, which is mainly orange series.
1968 A general survey was carried out in Diling Basin, southwest of Huitong, and four garnet crystals were found in the water system. After many times of work, 657 garnet crystals were found in the second and fourth members of Cretaceous red beds, and 59 garnet crystals were found in heavy sand of water system, with refractive index of 1.770~ 1.742, and purplish red series accounted for 58%.
Based on the above, there are similarities and differences between the South China system and the Lower Jurassic and Cretaceous red beds of the Upper Triassic. The diamonds contained in (1) are all of particle size, and the weight is less than1mg; ② The South China and Sinian garnets are single in color, mainly eggplant and orange series, with refractive index less than 1.746. Microscopic analysis shows that the content of chromium is low, and the minerals contain more inclusions and impurities. The compositions of Cr2O3 and CaO determined by electron probe are projected on Soxhlet Polev discriminant diagram, and both fall in non-kimberlite areas. However, the garnet in the red bed area has various colors, mainly purplish red, with high and low refractive index, and the high one is 1.770~ 1.760, which is obviously different from the South China and Sinian systems, indicating that they are not from the same source and have different occurrences. There are multi-layer intermediate reservoirs containing diamond and garnet, which reveals the multi-stage nature of diamond primary minerals.
In the lower reaches of Yuanshui River and Lishui River, after years of heavy sand investigation and mineral processing method search, there are always some rare diamonds and magnesium aluminum garnet distributed in the area where the two lakes are interrupted (Dongting block), the Dietailong in the south of the Yangtze River (Xuefeng block) and the fold belt of the upper Yangtze platform (Wuling block). This NNE-trending or nearly NNW-trending weakly distributed structural belt extends all the way to Huangling anticline, where diamonds are found. It should be a metallogenic belt dominated by primary diamond minerals, and primary ore exploration should be carried out in depth.
In the northeastern region of Xiangxi Tujia and Miao Autonomous Prefecture and Zhangjiajie City, in the 1970s and early 1980s, after large-scale investigation and reconnaissance of mineral processing points distributed in various counties, 56 diamonds were found only in the modern water system and karst fissures and funnel-shaped sandy cobble layers at an altitude of 720m in the Yellow River cloud area, but no anomalies of chromite and magnesium aluminum garnet were found. Therefore, it is necessary to carry out special regional geomorphology survey and high-precision aeromagnetic survey to guide ore prospecting.
In the process of searching for primary ore, only a small amount of garnet containing magnesium and aluminum was found in Nanhua, Sinian, Upper Triassic-Jurassic, Cretaceous and Tertiary, and five tiny diamonds were found in Nanhua, Jurassic and Cretaceous. Such rare diamonds are difficult to explain the rich placer accumulation in Shui Yuan Basin, so they are not the main source of Quaternary placer deposits in Shui Yuan, but only show the multi-stage mineralization of regional diamond primary ore.
Looking back today, it is strange why primary diamond mines have not been found in the past twenty years. There are many reasons.
First, heavy sand prospecting only pays attention to garnet, and chrome spinel is not used as an indicator mineral, so there is a great possibility of ore leakage. For example, in Australia, a company once conducted heavy sand exploration for Mg-Al garnet, but found no abnormal heavy sand, leaving out the big rock pipe in argyle. Later, another company came to look for mines. According to a large number of chrome spinels closely associated with diamonds in the water system, micro-diamond and chrome spinels are used as indicator minerals for heavy sand prospecting, and world-class diamond-bearing rock pipes are quickly found in argyle. We must draw lessons from this valuable exploration experience. At present, it is urgent to re-develop100000 heavy sand samples and the original data of previous heavy sand identification, delineate the anomalies of chromite spinel, and conduct internal inspection and re-examination on the old samples. Anyone who sees the mixed particle size of chromite spinel, including automorphic, semi-automorphic and heteromorphic, with potholes on the surface and banded structure on the mineral profile, needs further study. In particular, it is necessary to use electron probe to determine the chemical composition, find the abnormal indicator mineral chromite, and achieve the purpose of optimizing the prospecting target area and discovering primary ore. According to the contemporary theory of diamond mineralization, the Archean A-type craton (>: 2.5 billion years ago) is dominated by kimberlite, while the Proterozoic P-type craton (2.5 ~/kloc-0.6 billion years ago) is rich in K-Mg lamprophyres. The indicator minerals of the two are different, and the kimberlite type is composed of garnet, chrome diopside and ilmenite. However, the K-Mg lamprophyre type takes diamond and chrome spinel as indicator mineral combinations. Special attention should be paid to the second mineral assemblage in prospecting on the Yangtze platform.
Second, the vegetation in Hunan is developed, the weathered soil is thick, and the mountain is surrounded by terraces, which prevents the erosion and diffusion of rock mass materials and cannot form the anomaly of heavy sand prospecting in water system. Especially in limestone areas, karst depressions are abnormally developed, and the Quaternary overburden is thick, so it is difficult for weathered rock to spread outward. Therefore, it is imperative to use high-precision aeromagnetic exploration methods. In the red bed area of Hunan, the mountain vegetation is undeveloped, and the soil layer is not thick or even without soil layer. Therefore, as long as the red bed contains garnet, it will easily spread into the water system and be discovered by heavy sand method. However, in the past, there was a lack of in-depth study on the mineral surface characteristics of the abnormal zone of Mg-Al garnet replenished by secondary sources, so it was impossible to judge the transportation distance. For example, if you see garnet with ROK characteristics with altered crust, it is generally within 10 km from the original source area. It is difficult to judge the direction of erosion source area because the composition of secondary gravel and heavy sand is not deeply studied.
Third, in the prospecting work, the workload is re-completed, while the research on geological achievements is neglected, so we are eager to achieve success, and we have repeatedly made breakthroughs in secondary sources, which consumes manpower and material resources and prolongs the time needed for prospecting.
1.2.6 New progress in searching for primary diamond deposits
1989, according to the development of two groups of large faults in Ningxiang area, the ring structure on satellite images is very clear, and the basic and ultrabasic rocks are in multiple stages, and garnet was found in Daolin, Changsha. Team 4 13 of Hunan has demarcated an area of 1000 square meters for the general survey of primary diamond mines. Based on the anomaly of chromite spinel and its mineral crystal characteristics, a new type of diamond-bearing rock-olivine K-Mg lamprophyre was discovered in 1990. Six rock pipes are over 654.38+00000 square meters, 200-500 meters long and 654.38+020-200 meters wide. Rock pipes are distributed in the upper Devonian Yuanguyu Banxi Group and Xikuangshan Formation in the north-south direction (Figure 1-8 and Figure 1-9), and the intrusion time is late Paleozoic Hercynian.
Figure 1-8 Geological Schematic Diagram of Yunyingwo, Wuhui Township, Ningxiang County, Hunan Province
Figure 1-9 Section of Yunyingwo 23 Exploration Line in Jiuwu Township, Ningxiang County, Hunan Province
The important discovery of olivine-potassium-magnesium lamprophyre in Ningxiang has promoted the diamond prospecting in Hunan Province and even the Yangtze Platform. According to the new progress of prospecting in Ningxiang area, a general survey of Changsha-Taojiang area was carried out in 1992, and diamonds were found in 7 places. In 2007, artificial heavy sand samples were collected from glass-based olivine pyroxene in Luotou Mountain, Loach Pond, Wangcheng County, and 1 particles with particle size less than 1 mm were found, adding a new type of rock containing diamond.
Based on the favorable metallogenic conditions of diamonds in Hunan, the wide distribution of diamonds and their indicator minerals, and the deepening of Sino-foreign cooperation, Eyston Company, Guangta Company and Belgian Sibeck Company have successively cooperated with Xiangxi 4 13 team. At present, we are cooperating with Australia's Baolan Nomiri Company to find diamond mines and set up Changde Western Australia Diamond Mining Co., Ltd.
Discussion on several problems of diamond prospecting in the future.
(1) About what type of primary diamond mine to look for.
According to modern diamond metallogenic theory, Xiangxi area belongs to P-type craton. According to the division of Hunan microplate by geophysical data, Xiangxi area is located in the upper wall of subduction zone. These two conditions are consistent with the metallogenic characteristics of olivine-potassium-magnesium lamprophyre. All the diamond-bearing rocks found in Hunan and Guizhou belong to this kind of rock. Therefore, the search for K-Mg lamprophyre should be the basic direction for Hunan to achieve a breakthrough in primary diamond prospecting.
(2) About when to find primary ore.
Diamonds in the mantle need a stable growth period of 500 million ~ 654.38+0 billion years. Because the Mesozoic and Cenozoic red beds in western Hunan generally contain indicator minerals of diamonds, Professor Chen once pointed out that diwa period is the main metallogenic period of primary diamonds. Therefore, it is necessary to strengthen the primary ore prospecting work in Indosinian period and early Yanshan period in western Hunan. Diamonds and their indicator minerals are widespread in red beds, and the erosion source area is the main area to break through primary ore, so high-precision aeromagnetic exploration should be strengthened.
(3) Determination of indicator mineral group for heavy sand prospecting.
According to the results of deep geophysical research in Hunan, spinel lherzolite is found in the range of 40~80 km from the surface to the deep, and garnet lherzolite is found in the range of 80~ 1 10 km. The ore-bearing host ultrabasic magma intrudes from the deep mantle and must pass through these two mantle layers. According to the accessory mineral characteristics of these two types of deep rocks, there must be a large number of chromite spinel and garnet, so diamond+chromite spinel+garnet should be a reasonable indicator mineral combination. The contents of chromite (S 1, S2, S3, S7) and garnet (G 10) with low calcium and high chromium are the main indicators to judge the ore-bearing property of rock mass.
According to preliminary estimation, there are at least180,000 carats of diamonds in the main reach of Yuanshui River. It is difficult to explain why so many diamonds are provided by only a few intermediate reservoirs with a small amount of diamonds to the mainstream placer in Shuiyuan. In addition, each wide valley area has a large number of diamonds. Therefore, there must be a large number of abundant primary ore sources to form a huge secondary diffusion field of mainstream diamond placer in Shui Yuan. Maping primary mine in Guizhou in Yuanshui River Basin belongs to Class II diamond minerals, which cannot explain the origin of Class I diamond in Yuanshui River. Statistics show that only 2%~3% of kimberlite-type rock mass meets the requirements of industrial mining, and K-Mg lamprophyre is even lower. Therefore, it is normal to find some rock bodies with no ore or poor ore content, and it cannot be concluded that a kimberlite field or a K-Mg lamprophyre field is ore-free. Insisting on finding ore-bearing rock pipes from a large number of ore-free rock pipes is one of the basic ideas for primary diamond prospecting.