4.2.3. 1 Lincang-Chiang Mai Island Arc Metallogenic Belt (Ⅲ1)
The metallogenic belt is located in the volcanic island arc superimposed belt, with large extension scale, many active periods and long duration. The total extension center line of volcanic island arc is about east longitude 100, north-south length 1000km, and east-west length 50 ~ 150 km. The western boundary is adjacent to Baoshan-Shanbang back-arc basin, and the eastern boundary is the boundary between Lancang-Nanbang fault and Wuliangshan-Nanbang metallogenic belt. The island arc was formed in Neoproterozoic, and has been active for at least two times since then. The first subduction occurred in the middle Variscan, between Devonian and Carboniferous (35 1 ~ 280Ma), during the fault activity before the Taidianma plate separated from Gondwana mainland. The subduction zone is located in the Lincang granite intrusion zone in the west of Lancang River, indicating that there is a glaucophane high-pressure and low-temperature zone in the north of Lincang, and the Rb-Sr isotopic age of polycrystalline muscovite associated with glaucophane is 260. There are variscan mafic ultrabasic rocks, ophiolite and gneissic porphyritic biotite granite belts in Chiang Mai, which intrude into Ordovician-Devonian shallow metamorphic rocks and are covered by upper Carboniferous conglomerate unconformity. The second subduction occurred during the collision and suture between Matai plate and Indosinian plate in Yunnan during the late Variscan to Indosinian period, and the subduction zone was on the east bank of Lancang River. It may be due to the westward subduction of the eastern oceanic crust near the Qinglai fault zone after the late Mesozoic strata were covered, and the Lincang area in the northwest was invaded by a large area of deep granite (244 ~ 2 100 Ma). The anatexic "S" granite in central Thailand was also formed at the same time. The main granite foundations in Chiang Mai area were formed in 240±64Ma (Fenxian-Ye Lian rock mass) and 237±25Ma (Tonglang rock mass). The granites seen in the two areas constitute the main tin-bearing granite belt in the island arc belt, and the initial value of 87Sr/86Sr is relatively high, mostly between 0.722 and 0.733, belonging to the low-pressure and high-temperature magmatic belt formed during the orogeny. After that, there were Permian volcanic rocks, and their time limit was extended to Yanshan period. The eastern part of Lincang area in the northern section is an excellent geosyncline sedimentary area, which belongs to the deep metamorphic migmatite belt, called Lancang Group, and contains sedimentary-metamorphic iron deposits related to volcanic rock series and genesis. The western part of Lancang is an geosyncline sedimentary area with a set of shallow metamorphic geosyncline deposits, which are called Menghong Group and Ximeng Group, and the age includes Sinian to Early Cambrian strata. Chiang Mai, located in the middle of the island arc, is a set of migmatized biotite gneiss and eyeball gneiss. The metamorphic degree is obviously higher than that of the early Paleozoic strata with fossil basis, and there is no direct basis for the exact age. It is generally called Trutao Group (a part of Myanmar is called Paleolithic Group).
Minerals in this zone are related to granites from late Variscan to early Indosinian, among which tin and tungsten mineralization are one of the important mineralization, mainly distributed in Chiang Rai, Chiang Mai and Dafu in northern Thailand, and extending northward along the Mekong River to Lincang area in China. The known granitoids are Maoxian-Suye (isotopic age 240±94Ma), Banhong (isotopic age 242 9ma), Tongluang (isotopic age 234±25Ma), Sha Meng (isotopic age 2065±438±0.22Ma) and Dui Shojaei-Babao (isotopic age 242 9ma). Indosinian granitoids such as Dafu (isotopic age 2 13 10Ma) and Kundan (isotopic age 2 12 12MA). Granite in this zone belongs to S-type granite zone, with deep melting and high collision emplacement. Granite is mainly porphyritic biotite muscovite adamellite. The types of tin deposits in this belt are mainly pegmatite and fine-grained rock. The average tin content of granite in this belt is 64× 10-6, which is 20 times of the world average tin content (3× 10-6, Ture Kian and Wedepohl, 196 1).
There is also antimony mineralization in this zone, which is mainly cystic metasomatic veins, occurring in clastic rocks and granite. The main minerals are stibnite and stibnite. Ore minerals and timely or chalcedony occurring in host rocks and granite. Some antimony veins are related to fluorite, barite and sulfide. The granite related to mineralization is mainly amphibole biotite granite from Triassic to Jurassic. In addition, there are tin, molybdenum and copper mineralization in the late Yanshanian-Himalayan granite in the Chiang Mai island arc belt. The only meaningful contact metasomatism tungsten deposit in Thailand occurs in this zone. The gold mineralization is distributed in the NE direction and squeezed by the suture line in front of the arc in Chiang Mai Island. It is distributed in the ophiolite belt and its east side of the Chiang Rai-Dafu suture zone. Gold deposits are mainly exposed in areas where sychrosite syenite porphyry, sychrosite diorite porphyry and granite intrusions are developed, but their surrounding rocks are mostly Paleozoic metamorphic rock series and volcanic rocks. The emplacement of biotite granite plays an important role in gold enrichment. Alteration zones such as crushed compression zone, silicification, pyritization and chloritization are favorable places for gold mineralization and enrichment. The main types of gold mineralization in this zone are deposits related to granite fracture alteration zone, which are mainly distributed in Lancangjiang island arc and its west side. Basic volcanic rocks related to ophiolite and gold deposits related to ultramafic are distributed along the Lancang fault zone.
Wuliangshan-Nanbang metallogenic belt in 4.2.3.2 (Ⅲ 2)
This belt is located in the wedge-shaped Mesozoic marginal depression pushed by the Indo-China plate to the northwest, adjacent to the Lancang-Chiang Rai fault zone and the Lancang-Chiang Mai island arc in the west, the Jiujia-Mojiang fault zone in the east and the Dean Bianfu-Nanjiang-Cheng Yi suture zone in the south. There is no obvious basement in the depression, and the Variscan period may be above the ocean crust. Since the late Paleozoic, the Tethys oceanic crust began to subduct eastward and westward, and volcanic rocks and pyroclastic rocks were deposited in the trench before subduction. These island arc volcanic rocks are intermediate-basic andesite and basaltic andesite on the east bank of Lancang River. A complete set of Permian volcanic aggregates, tholeiite, andesite, tuff and rhyolite sequence containing fusiform limestone have been found between Nanbang and Fu Nan. There are similar volcanic rock assemblages in Carboniferous sediments. The upper Paleozoic in the middle of the depression is mainly carbonate geosyncline deposits, and the eastern part is close to the Mojiang-Heishuihe island arc belt, with deep-sea graptolite shale of the SILURIAN-Devonian system, and the Devonian pseudo-conformity contact zone is above it.
From the late Variscan to the late Indosinian, with the collision of plates, the oceanic crust closed and the Paleozoic strata folded. Subsequently, the fault block activity formed a graben depression basin deposited by Triassic-Paleogene cover with a thickness of more than 10 thousand meters. After the orogenic period, the basin depression began to deposit in the early Indosinian period, and the northern Lanping-Simao area rose in the early and middle Indosinian period, lacking early-middle Triassic deposits. The subsidence of the basin began from Latin period to Carney period, forming the Gabila Formation. It is worth noting that there are still many volcanic tuffs sandwiched in flysch deposits in the middle and lower part of Nanbang Group and Jiapila Formation near the island arc belt on the west side of the depression, but their properties are different from those of volcanic rocks in the active island arc period. The former belongs to non-geosynclinal volcanic rocks erupting along cracks and gullies after the collision period, which is different from the early island arc volcanic rocks.
Yanshan period is the main period of depression subsidence and deposition. The thickness of Jurassic-Cretaceous continental red beds (with marine interlayer in the south) exceeds10000m. From Late Cretaceous to Paleogene, salt-bearing rock series was widely distributed, and potassium salt was contained in the south.
At the beginning of Himalayan period, the depression was in a contraction period, and after Eocene, the basin was closed, and then folds were uplifted to form mountains. After Miocene, the pearl intermountain basin developed along rift and gully, and lignite and hydrocarbon minerals were found in lacustrine sediments. In the southern coastal areas, seawater intruded northward along the Chao phraya River fault depression zone, forming a Cenozoic gully basin with an alternating deposition of marine facies and modern delta facies of more than 3,000 meters.
During the synorogenic and pre-orogenic periods from early Paleozoic to early Mesozoic, there was extensive intrusion of granite magma on the continental margin. Tungsten-tin mineralization related to these intrusive rocks, especially Permian-Jurassic granite intrusions, is not obvious, but mainly develops gold mineralization. There are more than 30 known gold deposits in Nanfu-Nalati gold belt in Thailand, which are mainly distributed near alkaline porphyry, Yanshanian granite and ultrabasic rocks on the west side of Nanfu-Cheng Yi ophiolite belt, and extend northward along Mekong River to Laos and northwest Yunnan. In addition to gold deposits, there are lead, zinc, copper and molybdenum mineralization in this area. Multi-stage basic-neutral-acid volcanic eruption and overflow activities are also widely developed in this belt, especially the metallogenic environment and characteristics are similar to the large Carlin-type gold deposits related to volcanic activities widely developed in Cenozoic in great basin, western United States. The gold mineralization is related to stibnite mineralization, fluorite mineralization and barite mineralization. In Thailand, the central belt of the tin ore belt in Southeast Asia consists of the Zhuangtabuli tin-antimony-gold-copper-gem ore belt, the Chiang Mai tin-antimony-gem ore belt, the Nanbang antimony-tin-jade ore belt and the Panxian tungsten-tin-antimony ore belt. Mainly developed in the Mesozoic depression in Lanping-Simao area. As the Lanping-Simao Pb-Zn-Ag polymetallic metallogenic belt, the ore-forming area entered the south of Yunnan, China. Large and medium-sized deposits such as Jinding lead-zinc mine in Lanping, Caizidi lead-zinc mine in Lanping, Nanzuo lead-zinc mine in Deqin and Baiyangchang copper mine in Yunlong have been discovered in Yunnan.
In addition, stratabound low-temperature mercury antimony deposits are also extremely important in Simao area, such as Shimen antimony deposit in Weixi, Wenshuihe mercury deposit in Lanping, Shiyacun antimony deposit in Weishan, Bijiashan antimony deposit and Heilongtan mercury deposit. Due to the suture zone of Lancang River, the ore belt near Deqin may shrink northward. And then returned to the north with the Yulong mining belt in China. In addition to metal deposits, Paleogene basins such as Mengla, Jiangcheng, Jinggu and Lanping also develop rock salt, gypsum and potassium salt deposits.
Zhenyuan-Jinping fault-fold metallogenic belt in 4.2.3.3 (Ⅲ 3)
The ore belt is located on the west side of the Yangtze block, and is controlled by the northwest extending Ailaoshan metamorphic terrane sandwiched by Honghe fault formed in Jinning period, Ailaoshan fault formed in late Variscan-Indosinian period and Jiujia-Mojiang fault (Luo Junlie et al., 1994). The deep metamorphic rock series between Honghe and Ailaoshan fault was originally the basement of Yangtze block, which was exposed to the surface under the action of nappe structure, and may be Archean-Proterozoic geosyncline deposition. The shallow metamorphic rock series sandwiched by Ailaoshan fault and Jiujia-Mojiang fault is a volcanic-sedimentary rock series dominated by Late Paleozoic. The regional structure has experienced a series of geological tectonic processes, such as Caledonian-early Variscan continental margin deposition, middle-late Variscan paleo-Tethyan tension, late Variscan-early Indosinian compression and so on. By the middle of Indosinian, the Paleotethys developed into an island arc on the continental margin, and the late Indosinian orogeny formed the basic structural pattern today. During Himalayan period, the belt experienced nappe and ductile shear transformed into strike-slip and other structural processes. Tectonic magmatism and tectonic fluid caused by the above geological processes provide sufficient and necessary conditions for gold, copper (lead and zinc) and nickel mineralization in this area. Typical deposits are Laowangzhai Gold Mine in Zhenyuan, Mojiang Gold Mine, Daping Gold Polymetallic Mine in Yuanyang, Baimazhai Copper-Nickel Mine in Jinping and Chang 'an Gold Mine.