Corundum primary deposits can be divided into magmatic deposits (Figure 4- 19) and metamorphic deposits (Figure 4-20) according to surrounding rocks and crystal types.
Magmatic deposits can be subdivided into alkaline basalt, lamprophyre, syenite and porphyry deposits according to different types of surrounding rocks. Among them, there are most alkaline basalt corundum deposits, such as basalt sapphire deposit in Changle, Shandong Province, China and basalt corundum deposit in Duonong Province, Vietnam. The corundum crystals in this kind of corundum deposit are usually blue, yellow and green.
Figure 4- 19 Classification of Primary Magmatic Corundum Deposits
Figure 4-20 Classification of metamorphic corundum primary deposits
Metamorphic rock deposits can be subdivided into those formed by acid magma invading basic-ultrabasic rocks, those in silicon-poor pegmatites formed by rhyolite metasomatism [Umba deposit in Tanzania], skarn deposits [Andranandambo deposit in Madagascar], biotite deposits (Sahanbanu deposit in Madagascar) and cordierite. In addition, there are migmatite deposits (such as Morogoro deposit in Tanzania), gneiss deposits (such as Mysore deposit in India), granulite deposits (such as Ratnapura deposit in Sri Lanka), marble deposits (such as Gumu deposit in Myanmar) and basic-ultrabasic rock deposits.
Geological characteristics and genesis of (1) magmatic corundum deposit
With regard to the study of corundum deposits, people have studied the geological conditions, formation mechanism and age of magmatic deposits in Asia and Oceania, especially basalt deposits, and put forward some possible metallogenic models. However, due to various reasons (such as government policies or military conflicts), many famous ruby and sapphire deposits have neither carried out systematic geological prospecting activities nor systematically studied the corresponding deposits.
Magmatic corundum deposits can occur in plutonic and hypabyssal rocks. In plutonic rocks, corundum is related to silica-poor rocks and pegmatites (especially syenite and nepheline syenite). Corundum can be formed by direct crystallization of magma melt, can also exist in the form of xenocrysts, and can also appear as an accessory mineral in porphyry copper deposits.
In shallow diagenetic rocks, sapphires and occasionally rubies usually appear in continental alkaline basalt ejecta in the form of xenoliths. Figure 4-2 1 shows the corundum metallogenic model of Cenozoic alkaline basalt. This kind of deposit has important economic value, and the color of sapphire is mainly blue, green and yellow. Figure 4-22 shows the distribution map of Neogene-Paleogene basalt corundum deposits in the world.
Fig. 4-2 1 schematic diagram of basalt corundum metallogenic model.
Figure 4-22 Distribution Map of Neogene-Paleogene Alkaline Basalt-type Corundum Deposits in the World
Sapphire crystal of basic basalt is characterized by "filiform" inclusions of iron and titanium oxides and obvious color bands. According to Pb-U isotopic age of zircon in corundum crystals, it is found that the age of corundum formation is related to the age of basalt magmatism in this area. The content of trace elements Fe and Ti in corundum is the highest, gallium (Ga) is slightly enriched and chromium (Cr) is low. The ratio of Cr2O3/Ga2O3 is usually lower than 1, while the ratio of Fe2O3/TiO2 is mostly between 4 and 800. The UV-Vis absorption spectrum shows the absorption peak related to the charge transfer of Fe2+/Fe3+.
1. Alkaline basalt corundum deposit
Blue-green-yellow-sapphire deposit has the most economic value, and the economic value of placer is even greater. These deposits are mainly distributed in the following areas:
(1) Oceania: A corundum belt in eastern Australia, extending from Queensland in the north to Tasmania in the south.
(2) Asia: China, Fujian, Shandong and Hainan; Pingshun, Tongnai and Anda provinces in southern Vietnam; Bailin, Datuk, Baihu and Denchai provinces, which are located on the border between Thailand and Cambodia.
(3) Africa: Cyangugu Valley in Rwanda, Kivu in Democratic Republic of Congo, Turkana in Kenya, Kaduna in Nigeria, Mamfe in Cameroon, Takkola in Algeria, Air in Nepal, Ambondromifehy in Anzilanana in Madagascar.
(4) Europe: Espa, German Eiffel Plateau, Skaluka, Czech Republic, Warsaw, Poland.
(5) Mecaderes-Riomayo, Colombia.
The characteristics of the above deposits are related to ultrabasic xenoliths of alkaline basalts in the plate. Corundum generally coexists with pyrrhotite, clinopyroxene, zircon and iron-rich spinel. Mineral inclusions in corundum crystals mainly include rutile containing niobium, ilmenite, niobite, niobite, niobite and apatite. These mineral inclusions are usually rich in trace elements such as zirconium (Zr), niobium (Nb), hafnium (Hf), tantalum (Ta), uranium (U) and thorium (Th).
The formation mechanism of basalt corundum deposits has been studied by predecessors, and it is considered that corundum and some solid inclusions (zircon, tantalite, pyrochlore, etc.) are formed. ) cannot be crystallized from basalt magma. The crystal growth of sapphire needs a geochemical environment rich in thorium, zirconium, niobium, tantalum, vanadium, sodium, potassium, iron, aluminum and volatile components. They believe that corundum was formed in the upper mantle. When alkaline basalt magma intrudes into host rocks, mineralized xenoliths or sapphire xenoliths are transported to the surface by basalt magma.
2. corundum deposits in lamprophyre veins
The typical lamprophyre corundum deposit is a sapphire deposit in Montana, USA. The appearance of secondary deposits is related to Missouri River, Gancatonwood Bay, Locke Bay and Jorge Canyon. The ultrabasic rocks lamprophyre and breccia formed in Eocene are the host rocks of sapphire. Sapphire-containing dikes are 8 kilometers long and 2 meters thick, and the surrounding rocks are limestone and shale. The host rocks are mainly composed of coarse-grained phlogopite, pyroxene, calcite, zeolite, magnetite, apatite and diopside. Gemstone minerals are produced as accessory minerals, mainly spinel and sapphire. At present, it is generally believed that the genesis of corundum in lamprophyre is related to magma intrusion into metamorphic rocks, but the specific genetic model is still inconclusive.
3. corundum deposits in syenite and monzonite
The corundum in syenite is mainly sapphire, which is a typical pegmatite mineral. Other pegmatite minerals include feldspar, sphene and Nb-a minerals. Sapphire crystal particles are large, mostly opaque, and the blue tone is obvious. Sapphire in syenite is only produced in a few mining areas, such as Urals mining area in Russia. The corundum deposit found in monzonite is only Duxi mining area in Kenya. Dusi mining area is located in the middle of Kenya. It is a monzonite body composed of potash feldspar and albite, which extends horizontally for several kilometers and has a thickness of several meters. Zircon inclusions can be seen in feldspar and corundum. According to zircon U-Pb chronology, monzonite was formed about 580 million years ago.
The predecessors put forward several hypotheses and inferences about the formation mode of magma corundum. It is generally believed that corundum is crystallized in high-temperature unsaturated alkaline magma, but the specific formation mode of corundum varies with the geological conditions of mining areas, which is complex and changeable. Table 4-4 lists the main deposit types, typical deposits and biominerals of magmatic corundum ore bodies.
Table 4-4 Main deposit types, typical deposits and biomineral assemblages of magmatic corundum
(2) Geological characteristics and genesis of metamorphic corundum deposits.
Corundum crystals exist in many metamorphic rocks. Although there are different opinions on the specific formation mechanism, the genetic theory of metamorphic rocks has been generally accepted. The metamorphism of corundum deposits can be regional metamorphism or skarn contact metasomatism metamorphism. According to the types of corundum surrounding rocks, metamorphic corundum deposits can be classified according to Table 4-5.
Table 4-5 Types, Typical Deposits and Mineral Assemblies of Major Metamorphic Corundum Deposits in the World
1. corundum deposits in marble
Gem-grade rubies are mainly produced in marble corundum deposits, and deposits with high economic value are mainly distributed in Central Asia and Southeast Asia. Marble ruby deposits in Southeast Asia mainly include Zadaleka in Afghanistan, Nagimali in Kashmir, Hansa in Pakistan, Kukut and Dura Kourouma in Tajikistan, Chuma and Yi Rui in Nepal, Namiasek in Myanmar, Lu 'an in Vietnam and Yunnan in China.
Marble ruby deposit has the following characteristics:
(1) occurs in carbonate metamorphic rock assemblage, and the surrounding rock of ore body is often accompanied by kyanite schist or gneiss.
(2) The strike of Dali rock stratum is parallel to the shear zone produced by Cenozoic Himalayan orogeny, and the thickness of the rock stratum is usually about 300m, sometimes the rock stratum contains igneous veins formed by the intrusion of intermediate-acid magma.
(3) Ruby deposits are mostly layered, which is consistent with the layered structure of marble. The seam thickness is usually between 0.5~ 10m, and it is often distributed along marble cracks in the form of veinlets. The biogenic minerals of ruby include spinel, corundum, diopside, phlogopite, garnet, chlorite, nacrite, tremolite, leek amphibole, edenite, calcium magnesium tourmaline and forsterite. Graphite, andalusite, anorthite, sphene, garnet, chromite mica, zoisite, potash feldspar, epidote, pyrite and pyrrhotite can also be found in such deposits in Afghanistan, Pakistan, Myanmar, Nepal and Viet Nam. The main components of surrounding gneiss or schist are biotite, garnet, sillimanite, andalusite, kyanite, amphibole and clinopyroxene.
Ruby crystals in marble usually have the following characteristics: ① They are dispersed in marble, and biogenic minerals are phlogopite, muscovite, andalusite, nacrite, spinel, sphene, pyrite and graphite. Typical examples are Zadaleka in Afghanistan, Hansa in Pakistan, Gumu in Myanmar and Su Meng mining area. ② It appears in the form of vein. Biogenic minerals include phlogopite, nacre, sphene, graphite and pyrite, and most ruby mining areas in northern Vietnam occur in veins. (3) Produced in capsules, the primary minerals are orthoclase, phlogopite, nacre, graphite and pyrite, such as mining areas in northern Vietnam.
Marble rubies in Southeast Asia and Central Asia usually have the following characteristics:
(1) Rock salt, pyrite and sulfate inclusions are common in rubies.
(2) The minerals related to rubies are often rich in Mg, Al and F (sometimes Cl), indicating that volatile substances are involved in the formation of rubies.
(3) Fluid inclusions composed of CO2-H2S-S8-AlO(OH), natural sulfur and diaspore inclusions are sometimes found in rubies.
According to the ruby formation mechanism of this kind of deposit, predecessors have done a lot of analysis and demonstration on geochemical metallogenic conditions, and put forward a series of theoretical speculations such as aluminum migration and enrichment and ruby crystallization. However, the research in this field needs to be further deepened.
2. Metamorphic corundum deposits related to desilicated pegmatite and granite.
Some corundum deposits in the world are closely related to desilicated pegmatite and granite, such as Umba mine in Tanzania, Sumjam mine in Kashmir, Mangari mine in Kenya and Urals mine in Russia. Corundum occurs in silicon-poor pegmatite or granite. When acidic magma or hydrothermal solution intrudes into ultrabasic rocks, due to the difference of lithology, desilication and aluminum-rich metasomatism occur at the contact metasomatism site, forming corundum crystals.
3. corundum deposits in gneiss
This kind of deposit is a newly discovered deposit type in Madagascar in recent 20 years, which is distributed in feldspar gneiss and cordierite gneiss. Typical examples are Sahambano and Zazafotsy corundum deposits in Madagascar; Ruby and colored sapphire are produced in cordierite gneiss. Sahanbanu produces colored sapphire crystals, but gem-grade ones are rare; Most Zazafuqi sapphires (blue, fuchsia, orange, pink, purple, lavender and brown) need heat treatment to improve transparency and color before they can reach the gem level. Iankaroka sapphire is brightly colored with obvious ribbons; Ambatomena ruby mine has high quality rubies.
4. corundum deposits in granulite
Granite is a metamorphic rock formed under high temperature and high pressure, with fine texture, and often contains syenite, feldspar, hypersthene, garnet, sillimanite, kyanite, rutile, cordierite and spinel. In Precambrian strata, granulites often coexist with perilla granite and gneiss. Corundum deposit occurs in Precambrian granulite and its existing rocks. Some primary deposits in Sri Lanka, India and Madagascar belong to this type.
5. corundum deposits in migmatite
The migmatite is a mixed product of granite and gneiss, and corundum deposits occur in these rocks with complex genesis. A typical example of this kind of deposit is Morogoro corundum mine in central Tanzania.
6. corundum deposits in ultrabasic rocks
Some ruby deposits in South Africa, Zimbabwe and New Zealand are mainly distributed in ultrabasic rocks. Chromite is a metamorphic rock rich in aluminum, chromium, boron, vanadium and arsenic as well as potassium, rubidium, nickel, antimony, bismuth and tellurium. In Zimbabwe, the common minerals in chromite are andalusite, chlorite, perlite, tourmaline, diaspore, rutile, bischofite and metal bismuth. Chromite in transvaal province often contains ferrochromium aluminum oxide, biotite, plagioclase, kyanite and rutile, but generally does not contain hydrated bauxite, tourmaline and perlite. Usually, the quality of this corundum is very poor, and it can't reach the gem level. Mainly used for craft carving.
(3) Secondary deposition of corundum
The secondary deposit (placer) of corundum is the gem deposit with the highest commercial value. According to statistics, most gem-grade corundum is produced in placer. The corundum crystals in the primary ore are denuded from the parent rock under the geological action of weathering, earthquake, etc., and transported and deposited under the geological action of flowing water, forming secondary deposits on the hillside or river bed far away from the primary ore (Figure 4-23). Corundum is usually distributed in sedimentary conglomerate in the form of river pebbles or abrasive crystals.
Weathering is the most important factor for the formation of secondary deposits. In tropical areas, due to the change of surface geochemical environment such as temperature difference, rocks are weathered, forming a series of clay minerals, iron and manganese oxides, hydroxides and so on. Corundum usually exists in soil and sedimentary conglomerate because of its stable chemical properties and weathering resistance (Figure 42-4). In Myanmar, these gem-bearing sediments are called "yellow brown soil with gems". Usually, there is a certain geographical connection between primary ore and secondary ore, but sometimes this connection is not obvious. 1979, a ruby pebble was found in a stream in Kimari, and its related primary ore was found in 1988.
According to the occurrence, secondary deposits can be divided into eluvial deposits and alluvial placers. Residual slope deposit refers to the deposit formed after the primary ore left on the hillside, karst cave, etc. is stripped. Alluvial placer refers to the ore stripped from primary ore, which is transported and washed under the action of flowing water geology and deposited in the area where the flow speed is reduced. Alluvial placers are mostly related to rivers, and secondary deposits in coastal areas are extremely rare. Sapphire discovered in Wangdai, France may belong to marine alluvial placer.
Alluvial placer can be divided into ancient alluvial placer and modern alluvial placer according to the formation age.
Ancient alluvial placers were formed in Paleogene, Neogene and Quaternary sedimentary rocks. Lithofacies are mostly related to carbonate or silicon-rich rocks. Madagascar's alkaline basalt corundum placer is a typical example. In Anziranana province, Madagascar, ancient alluvial placers are related to carbonate-karst breccia in Jurassic limestone caves, while in Irakaka area, corundum placers are mainly produced in three different gravel strata of Isaru sandstone alluvial terrace. The remounting and deposition of ancient alluvial placers can also form modern alluvial placers.
In developing countries, finding a new place may create employment opportunities for many local people. Miners use primitive tools such as shovels and axes to mine (Figure 4-25). Miners dug out pebbles containing gems from the pit, washed and sorted them in the river, and selected gem-grade corundum. This method of placer mining has not changed for hundreds of years.
Figure 4-23 Schematic diagram of the distribution of corundum secondary deposits located on the hillside and river bed respectively.
Figure 4-24 Yellow-brown soil containing corundum gems in Sri Lanka
Figure 4-25 Miners choose gem-grade corundum in rivers.
Important secondary gem-grade corundum deposits are mainly distributed in Australia, Madagascar, Tanzania, Myanmar, Vietnam, the United States and Sri Lanka. The secondary mines in Gu Mo, Su Meng, Myanmar and Lu 'an, Vietnam are closely related to the formation of karst landforms and the geochemical environment of marble. The types of secondary deposits distributed in Madagascar are complex, many of which have nothing to do with primary deposits.