Metallogenic sequence in Sanjiang area, southwest China

The "metallogenic sequence" here refers to the metallogenic concentration period arranged in turn according to certain laws. The metallogenic concentration period refers to a specific metallogenic era, and the metallogenic sequence emphasizes the regularity of a series of metallogenic concentration periods in geological history. According to the data of tectonic evolution, magmatic-volcanic activity, metamorphism and endogenetic mineralization, the western margin of Sanjiang and Yangtze platform have experienced six main metallogenic stages since 65Ma, which constitute the following metallogenic sequence of endogenetic mineralization in Sanjiang area in southwest China (Luo Junlie et al., 2000).

Time series 1: Paleocene-Early Eocene (equivalent to Danian -Ypresian period). The crust-type granite magma mineralization, which has continued from the late Yanshan period to the present, is distributed in the Binglangjiang area west of Nujiang River and belongs to the westernmost branch of the tin ore belt in western Yunnan-Southeast Asia (from Tengchong to Phuket, Thailand). Granitoids include 3 superunits and 8 units, and their isotopic ages range from 59.8 to 51.1ma. Among them, large tin mines (Lailishan, etc. ) is related to early Eocene biotite feldspar granite; Small tantalum-niobium ore and rare earth ore (Dayangtian, Baihua Nao, etc. ) is related to the last muscovite albite granite. There is a double metamorphic belt on the border between China and Myanmar. The famous jadeite deposit in Myanmar is located in the high pressure zone, and the tin-bearing rare earth granite in Yunnan is located in the high temperature zone. There are also a large number of polymetallic mineralization such as silver, tin, lead, zinc, tellurium and gold that evolved from Mesozoic to Cenozoic in western Sichuan, and it is also closely related to crustal granite.

Time series 2: between the middle Eocene and the late Eocene (equivalent to Barton period and Priabo period). It is the main act (the first act) of Himalayan movement in Sanjiang area, and is rich in mineralization, mainly distributed along Lanping basin on both sides of Jinsha River-Ailaoshan fault. Lanping-Simao basin was inherited from Mesozoic, but it may be affected by large-scale strike-slip and shear after Paleocene. The ore-forming time may extend from the late Cretaceous to the early Tertiary. Jinding super-large lead-zinc deposit in the eastern margin of Lanping basin is controlled by Tongjiang syngenetic fault and is formed by the following three special geological processes. Firstly, the fluvial-lacustrine delta facies of the sliding front constitutes two kinds of lead-zinc deposits: sandstone and limestone breccia. Second, there is a huge weak plane between the original system (dome structure) and the foreign system (nappe inversion anticline); Thirdly, syngenetic faults control lake basin deposition. Since Paleocene, there has been a heat flow that extracted basement ore-forming materials and rose and overflowed, which has a long initial enrichment history. The age of the lead isotope model obtained in the early stage changed greatly (22 ~ 275 Ma). In recent years, the Re-Os isochron age of pyrite measured by Re-Os method is 72Ma (Wang et al., 2005), but macroscopic geological observation should mainly occur between the middle and late Eocene.

Time series 3: Late Eocene-Oligocene (equivalent to Puri Abboni-Chater period). Structurally located between the first act and the second act of Himalayan movement, it is the most important metallogenic period of crust-mantle mixed source in the western margin of Sanjiang and Yangtze platforms. It is mainly manifested in gold deposits related to potassium-rich lamprophyres, rare earth deposits related to alkaline complexes and copper, gold and polymetallic deposits related to alkaline porphyries. In space, it is rich in Ailaoshan, Mianning-Dechang, Zaduo-Jiangda-Mangkang and Yanyuan-Heqing-Jinping.

There are more than 65,438+000 hypabyssal porphyry bodies in the whole region with complex lithology, but the best ore-bearing ones are syenite porphyry, adamellite porphyry and syenite porphyry. Different rock groups have different metallogenic specificity. Eastern Tibet-southern Qinghai is dominated by Cu, porphyry and skarn type, with slightly deep diagenesis and mineralization, and most of them are large deposits. Heqing, Chuxiong and Yanyuan areas are dominated by gold, lead and zinc, followed by copper and silver, and there are many medium-sized deposits. The Jinping area is dominated by copper and molybdenum, both of which are very small. Although the isotopic age of porphyry varies from16.5 to 67.9 Ma, the mineralization period is relatively concentrated, and the ore-bearing rocks in western Yunnan are concentrated in 33 ~ 36.3 Ma (individual 4 1 ~ 52.76 Ma). The ore-bearing rock mass in western Sichuan is 32.2 ~ 34.6ma；; The ore-bearing rocks in eastern Tibet are 34.9~43.2Ma (a few are 43.5 ~ 57.9 Ma). The ore-bearing rock mass in southern Qinghai is 33.9~49.4Ma (individual 22Ma).

Relatively speaking, the Jinsha River-Honghe metallogenic belt, represented by Yulong super-large copper mine, Ma Changqing, Tongchang, Tibet and other small and medium-sized porphyry copper-molybdenum mines, was formed in a concentrated era, and the mineralization of Ailaoshan metallogenic belt on its south side may start early and end late, while the porphyry copper-gold deposits, lead-zinc deposits and rare earth element deposits on its north side formed late.

Gold deposits related to K-rich lamprophyres occur in Ailaoshan ductile shear zone. It is a continuation of Indosinian (silicified altered rock type, Mojiang Jinchang) and Yanshan (sericitized altered rock type, Zhenyuan Kudumu) in Cenozoic, including carbonate altered rock type (Zhenyuan Laowangzhai) and chronological pulse type (Yuanyang Daping). The isotopic age of gold-bearing and potassium-rich lamprophyres ranges from 35.6 to 43 Ma, and in some cases it is extended to 28.2 1Ma, but a batch of age data of about 60Ma are obtained in this study (see Chapter 4).

Alkaline complexes are distributed in Yongping, Mianning and Dechang, forming rare earth deposits. There are two types: fine-grained disseminated bastnaesite (Mianning Maoniuping) occurring in alkaline pegmatite, calcite carbonate and aegirine alkaline granite; Fluorite-barite-celestite barium-type bastnaesite deposit occurs in aegirine syenite (Dechang Mainland Township). The isotopic age is 27.8~40.3Ma, which is completely consistent with K-rich lamprophyres. Compared with porphyry, its time limit range is relatively concentrated.

Time series 4: between Oligocene and Miocene (equivalent to Chatian and Aquita). Sanjiang area experienced the second act of Cenozoic movement. The metamorphic terranes such as Ailaoshan and Gaoligongshan are superimposed by migmatization and mylonitization, and granite pegmatites and chronological veins appear in groups, forming high-grade gems concentrated in Honghe-Jinping and Huanglianshan areas, mainly including corundum ruby, sapphire and beryl aquamarine. The isotopic age of ore-bearing pegmatite in Ailaoshan metallogenic belt is 25.6～28.7ma, and the Daduhe metallogenic belt and Beiya gold deposit also have a metallogenic record of about 25Ma.

Sequence 5: Neogene (Miocene to Pliocene, including Aquilaria to Pliocene). Different from the strong orogeny of Himalayas, the western margin of Sanjiang and Yangtze platforms is mainly characterized by faulted intermontane basins, and the mineralization is mainly exogenous lignite, diatomite, kaolin and germanium ore. Endogenous metal mineralization can be divided into the following three types:

First, large and medium-sized silver-rich polymetallic deposits (Baiyangping), copper-cobalt deposits (Yongping), antimony deposits (Bijiashan), gold deposits (Zhacun) and arsenic deposits (Shishuangchang) were formed along the linear thrust belt and interlayer fault zone in the west and south of Lanping Basin, with a large number of deposits, rich grade and complex composition. The highest ore-hosting horizon is the Oligocene Baoxiangsi Formation, which lacks reliable ore-forming isotope age data.

Second, the time-dependent gold deposits (Kangding Huangjinping, Shimian Pubugang, etc.). ) along the ductile shear zone, mylonite zone and interlayer detachment zone of Xianshuihe fault and Jinpingshan fault. There are many occurrences, rich grades, but small scale. The isotopic age of mineralization is 65438±05.4～26.9Ma Ma (Wang, et al., 2000).

Thirdly, tungsten, tin, polymetallic and rare earth mineralization (spots) were found in Zheduoshan granite body in Sichuan. The isotopic age of granite is 9.9 ~ 15 Ma, and its mineralization intensity is much less than that of crust-derived granite with time series of 1, and its tectonic position is also different.

Time series 6: Quaternary (Pleistocene to Holocene). It is the main uplift period of the Qinghai-Tibet Plateau, and the tectonic magmatism in the western margin of the Sanjiang and Yangtze platforms is mainly manifested in the middle-basic volcanic eruption that began in Pliocene, with concentrated hot spots and all concentrated eruptions. The most developed Tengchong area in Yunnan covers an area of 724km2, with *** 19 volcanic cone groups and 79 modern hot (gas) springs. Volcanic eruption is dominated by basalt and andesite, which is divided into two cycles, * * * four eruption periods and 10 sub-period (including N2, Q 1, Q3 and Q4). Endogenous deposits are mainly called modern hot spring gold deposits (Lianghe, Litang and Batang areas in Tengchong, western Sichuan). In other areas, such as Jianchuan, Yongde, Maguan and Pingbian, trachyte and alkaline volcanic rock pipes have appeared, but no industrial deposits have been found.

As can be seen from the metallogenic sequence:

1) The time series evolution of Cenozoic endogenetic deposits in the western margin of Sanjiang and Yangtze platforms is relatively complete. On the one hand, it inherited the evolution of the Indosinian and Yanshanian ductile shear zone gold deposit (Ailaoshan) and the shell granite tungsten-tin deposit (Tengchong). On the other hand, it has bred several major metallogenesis, among which the stratabound polymetallic deposit in the heat flow basin (Lanping Jinding), the mantle-crust mixed source porphyry copper-gold deposit (Jiangda Yulong), the mantle-derived alkaline rock rare earth deposit (Mianning Maoniuping) and the ultra-deep metamorphic belt gem deposit (Yuanyang and other places) are the most important.

2) The Cenozoic endogenetic mineralization on the east side of Qinghai-Tibet Plateau was the strongest before Oligocene, which roughly corresponded to the first and second intracontinental orogenic cycles. After that, the mineralization intensity decreased obviously.

3) The distribution of Cenozoic ore belt is controlled by tectonic-magmatic belt, but it is most abundant between 24 and 32 north latitude. The deep crust thickness in this area is moderate (about 40 ~ 60 km). Among them, the deposit development area related to mantle source and mantle-crust mixed source is the abnormal mantle with P wave velocity of 7.76 ~ 7.84 km/s in the deep crust profile, and there is a low-velocity layer (wave velocity of 5.54 ~ 5.84 km/s and thickness of 8 ~ 12 km) in the crust at the buried depth of 7 ~ 2/km.