Non-orogenic mantle-derived magma mineralization in orogenic belt: A case study of Maoniuping

1. Geology of Maoniuping deposit and tectonic significance of magmatic carbonate rocks

Maoniuping rare earth deposit in Sichuan is the largest rare earth deposit discovered in China in recent years, ranking third in the world among similar deposits (Yang Zhengxi et al., 200 1). Because there is no shortage of rare earth resources in China, the country has realized protective mining, and the investment in geological work is extremely limited, so the drilling work is generally only controlled to a depth of 400m, but most ore bodies actually continue to extend downward, and some new ore bodies have appeared. Therefore, Maoniuping is a very potential deposit. The deposit has a high grade, rare earth elements are mainly concentrated in a few minerals such as bastnaesite, and the mineral size is large, which is easy to be mined and selected, and the economic benefits of the mine are very remarkable. This large-size bastnaesite is often distributed in carbonate rocks rich in rare earth elements. However, the origin of carbonate rocks has been debated for more than ten years. One view is that carbonate rocks are of magmatic origin (Pu Guangping, 1988, 1993, 2001; Chen Congde et al.,1991; Jiang,1992; Niu Cai He, 1994), another opinion is that it belongs to hydrothermal origin, and carbonate rocks should be called calcite veins (Yuan Zhongxin et al.; 1995)。 In recent years, it has been found that scattered ore-bearing calcite veins on the surface often penetrate each other in the deep, with a width of 40m and 50m, and locally invade alkaline syenite. However, due to multi-stage mineralization and intense hydrothermal superimposed transformation, it is often difficult to distinguish magmatic carbonate from hydrothermal veins at a glance (which may be one of the reasons for different understanding of calcite veins). However, it can be clearly seen that carbonate rocks were cut by alkaline granite porphyry and ore-bearing quartz veins, indicating that carbonate rocks were formed before porphyry and hydrothermal time veins.

According to the geological characteristics of the mining area and the region (there are also magmatic carbonate rocks in continental trough and other places), considering that carbonate rocks are mainly composed of calcite, accompanied by silicate minerals such as sodalite, potash feldspar, aegirine-aegirine and rare earth minerals such as bastnasite, they belong to typical coarse-grained calcite carbonate rocks (S? Viet), and the content of rare earth elements is high, the enrichment of light rare earth elements is strong, and the Th/U ratio and carbon-oxygen isotope composition are high (δ13cpdb =-6 ‰ ~-7 ‰; δ 18 osmow =+7‰~+8‰) is the same as magmatic rocks, but different from hydrothermal calcite (Wang et al., 200 1). Combined with the comparison of other carbonate rocks at home and abroad, it is considered that this area belongs to magmatic carbonate rocks. This kind of magmatic carbonate rocks generally appear in continental rift areas, such as the East African Rift, corresponding to the thinning of the crust. Maoniuping carbonate rock is located in Longmenshan-Jinping mantle slope area (Figure 6- 1), which belongs to Cenozoic orogenic belt, and its formation age is completely consistent with that of orogenic belt, belonging to synorogenic magma carbonate rock. In addition to carbonate rocks, there are a series of alkaline rocks related to rare earth mineralization (Figure 6-2), which are usually found in rift areas.

Figure 6- 1 Moho depth and deep structural division in Panxi rift zone

(According to Zhang Yunxiang et al., 1988)

Fig. 6- 1 Moho depth and deep tectonic setting of Panxi rift zone

1-Moho isobath; 2- deep structural zoning line; 3- the slowly inclined mantle slope in the west; 4- Longmenshan-Jinpingshan mantle slope gentle dip area; 5- Eastern mantle slope area; 6- Liangshan mantle depression area; 7- Kangdian mantle uplift area; 8— mantle depression in western Yunnan

Figure 6-2 Schematic Diagram of Geological Structure of Panxi Rare Earth Ore Belt

(According to Pu Guangping 200 1)

Fig. 6-2 Structural Schematic Diagram of Panxi Rare Earth Metallogenic Belt

1- rare earth-rich Shi Ying syenite and alkaline granite; 2- syenite; 3- Alkali-rich granite; 4- alkaline granite; 5- basement fracture; 6- fracture; 7-Mesozoic marine distribution area of continental margin; 8- Pre-Sinian ancient land distribution area; 9-Paleozoic and terrestrial Mesozoic distribution areas

In a word, the appearance of carbonate rocks and alkaline rocks in Cenozoic orogeny in Maoniuping and other places shows that the occurrence of carbonate rocks and alkaline rocks alone cannot prove that the tectonic environment at that time was a continental rift, let alone geochemical data of rocks. That is to say, the popular method of judging tectonic environment by geochemical map based on rock geochemical data is limited.

2. Isotopic geochemical characteristics of Maoniuping deposit.

Rare earth elements in Maoniuping deposit are mainly enriched in carbonate rocks. Macroscopically, carbonate rocks can be divided into reddish carbonate rocks and colorless carbonate rocks, both of which are mainly coarse calcite. In white carbonate rocks, calcite accounts for more than 95%, while in meat red carbonate rocks, the calcite content changes greatly (50% ~ 90%), and it also contains common minerals in typical magmatic carbonate rocks such as microcline, aegirine, biotite, sodium amphibole, sodium amphibole, xenotime and pyrochlore. The particle size of calcite is generally above 1cm, so it can be called coarse calcite carbonate (s? Vite). Other types of carbonate rocks, such as fine calcite carbonate (bauxite), ankerite carbonate (ankerite carbonate) and magnesium carbonate (Beverite), are still rare at present. Carbonate itself was cut by ore-bearing quartz veins in the later stage. The timely vein also contains useful minerals such as fluorite and bastnasite, but the distribution is limited, and the pulse width is only 20 ~ 50 cm.

The carbonate rocks exposed in the central Maoniuping mining area were sampled and analyzed. Each sample is about 1kg, which is first crushed and then screened, and then detected by Isotope Geochemical Center of China Geological Survey (Yichang). Isotopic data of lead, strontium and neodymium are listed in Table 6- 1 and Table 6-2. Among them, sample MN- 1-2 is fluorite-bearing bastnaesite carbonate rock, MN- 1-4 is sodalite carbonate rock, MN-2- 10 and MN-2-7 are white carbonate rocks, MN-2-6 is heavy crystal bastnaesite carbonate rock, and MN-40 is shallow. Figure 6-3 shows the isotopic composition of carbonate rocks in Maoniuping and compares it with carbonate rocks in other areas. As can be seen from Figure 6-3, the carbonate rocks in Maoniuping have a relatively uniform isotopic composition, which is similar to the mantle isotopic composition of the EMII endmember, but obviously different from the sodic carbonate rocks erupted in Oldoinyo Lengai 1993 in the East African Rift Zone in Uganda (from the HIMU-type mantle endmember) and the carbonate rocks in Amba Dongar, India. These differences also reflect the heterogeneity of mantle isotope composition.

Table 6- 1 Lead Isotopic Composition of Carbonate in Maoniuping, Sichuan

Note: Testing unit: Isotope Geochemistry Center of China Geological Survey (Yichang).

Table 6-2 Strontium and Neodymium Isotopic Compositions of Carbonate Rocks in Maoniuping, Sichuan Province

Note: Calcite samples were determined by Isotope Laboratory of Institute of Geology, former Ministry of Geology and Mineral Resources according to Yuan Zhongxin et al. (1995). The rest is the data of this paper, which was determined by the Isotope Geochemical Center of China Geological Survey (Yichang).

Figure 6-3 Isotope Diagram of Carbonate Rock in Maoniuping

Figure 6-3 Isotopic Composition Diagram of Carbonate Rock in Maoniuping

Hollow circle-Maoniuping; Solid circle -—Oldoinyo Lengai (Bell and Simoneti,1996); Triangle-AMBA Dongar (Simonetti et al., 1995) EMI-I enriched mantle; The mantle rich in Emii-II; Himu-high μ mantle; NMORB-normal mid-ocean ridge basalt; OIB-sea island basalt; Nhrl-reference line of the northern hemisphere

3. Discussion-Dynamic process of mineralization

(1) Tectonic setting of Maoniuping mining area

Alkaline magmatic rocks, including carbonate rocks, are generally formed in rift valleys and extensional environments (Baige et al., 1985). Maoniuping rare earth deposit is located in the middle section of Haha fault in the northern margin of Panxi rift, which is an intracontinental or continental margin rift developed on the basis of early Paleozoic platform. Rifting started at the end of early Paleozoic, developed in late Paleozoic and Mesozoic, and closed with Himalayan movement in Cenozoic. Therefore, the formation of carbonate rocks and rare earth deposits in Maoniuping has no substantial relationship with Panxi rift. According to the data of Yuan Zhongxin and others (1993), the diagenetic and metallogenic age of Maoniuping is 40.3ma ~ 12.2ma, which belongs to Cenozoic mineralization. During this period, a series of NE-trending faults and folds converge here, which is actually a compressive and torsional tectonic environment and belongs to a part of the Longmenshan-Jinping Cenozoic intracontinental orogenic belt (Luo Yaonan et al., 1998). Geophysical data show that the Maoniuping area lies in the transitional zone between the Kangdian mantle uplift and the Longmenshan-Jinping mantle slope (Zhang Yunxiang et al., 1988).

(2) Comparison of diagenetic environment between Maoniuping and other carbonate rocks.

Carbonate rocks are mainly formed in stable platform areas, but they can also appear in orogenic belts or even island arc areas, so the geological environment is varied, but most of them are formed in the process of rift extension (Feng Zhongyan, 1985). More than 500 carbonate-alkaline/alkaline rock complexes in the world are mainly concentrated in the East African Rift Zone, northern Scandinavia-kola peninsula, eastern Canada and southern Brazil, and are structurally restricted by deep faults in and around the mainland (Kamitani and Hirano, 1990). The most typical is a series of Cenozoic carbonate-alkaline rock complexes in the rift zone of East Africa. Although Maoniuping is located in the north of Panxi Rift Zone, it has been in the stage of intracontinental evolution since Cenozoic, especially in the stage of mineralization, and belongs to a part of Longmenshan-Jinping Cenozoic intracontinental orogenic belt (Luo Yaonan et al., 1998). Carbonate rocks and alkaline rocks themselves have some geochemical characteristics of intracontinental extension and orogenic belts (Figure 6-4), which actually reflect the complexity of orogeny, or that the crust is thickening and mantle materials are rising. Zheduoshan granite, located in the northern part of the same structural belt (Xianshuihe large strike-slip structural belt), has an exposed area of 800km2 and intruded into 12.8Ma (Xu Zhiqin et al., 1997), which also shows that the Maoniuping area was in the orogenic uplift stage at that time, not the continental rift.

Figure 6-4 Structural Diagram of Trace Element Pearl in Carbonate Rock and Its Associated Feldspar Flint

(Some data are quoted from Yuan Zhongxin and others, 1995)

Figure 6-4 depicts the carbonate rocks and nordmarkite samples in the pearl trace element structure map.

Vag-volcanic arc granite; Wpg-intraplate granite; Sulfur-green syncollision granite; Org-Mid-ocean ridge granite; Abnormal mid-ocean ridge granite

(3) Geodynamic mechanism of diagenesis and mineralization of carbonate rocks in Maoniuping.

It can be seen from the above that rift extension environment is not a necessary condition for carbonate rocks and related mineralization, but the existence and effective upwelling of highly active mantle materials are the key to the formation of Maoniuping rare earth deposit. Considering that small-scale alkaline rocks are not easy to penetrate the increasing crust, it is speculated that there may be strong alkaline magma chamber or enriched mantle in the deep, which can effectively invade along deep faults and form a mineral-bearing carbonate alkaline rock zone. At this time, even if most of the magma from the mantle can't easily penetrate the thickening crust, some magma-fluids with great fluidity will still rise to the near surface along the deep fault. It is under such conditions that the carbonate-alkaline complex in Maoniuping area finally formed a rare earth metallogenic belt related to the carbonate-alkaline complex on the west side of the Anninghe fault zone. 1994, the second large-scale rare earth deposit was discovered in the continental trough about 130km south of Maoniuping. In other words, it is no accident that more rare earth deposits were discovered after Maoniuping.

It should be noted that the isotopic composition of Maoniuping carbonate rocks has the characteristics of EMI-type mantle endmembers (Figure 6-3), while EMI-type mantle is an enriched mantle, which cannot be the product of ocean crust or sediment recycling (Hart, 1988). The reason for its enrichment is small-scale melt and metasomatic fluid (Richardson et al., 1982). Typical examples of EMI are Walvis Ridge and Hawaii (White, 1985), which are mantle plume products with the lowest radioactive lead content. According to the isotopic composition of carbonate rocks, the carbonate rocks in Maoniuping come from the enriched mantle and are not obviously polluted by crustal sediments (Figure 6-3). The helium isotope test of the time-dependent pulse of the late-stage intrusive carbonate rocks in Maoniuping mining area shows that the highest 3He/4He ratios are 1.95× 10-4 and 3× 10-4, respectively, and only in the deep mantle can such a high ratio be obtained. Therefore, the carbonate rocks (and other associated alkaline magmatic rocks) in Maoniuping mean that it is a window to investigate the Cenozoic mantle material activities.

Considering that the carbonate rocks and other associated alkaline magmatic rocks in Maoniuping were emplaced when the Longmenshan-Jinping orogenic belt was formed, strong surface compression only allowed the mantle materials with strong fluidity to reach the upper crust quickly, thus avoiding the obvious pollution of the crust materials, and at the same time helping the ore-bearing magma-fluid to gather together and form a large ore concentration area without dispersion. Therefore, without extrusion, ore-forming materials may be difficult to concentrate on mineralization. Therefore, the rapid rise of fluid-melt in deep mantle and the simultaneous formation of surface orogenic belt (coupling event) restricted the formation of carbonate rocks and rare earth deposits in Maoniuping.