Magmatic rocks in mining area

Gejiu mining area, located in the southeast of Yunnan, is the center of multi-stage and multi-cycle magmatic activity. It can be divided into Hercynian cycle, Indosinian cycle and Yanshan cycle in chronological order. The Hercynian cycle and Indosinian cycle are dominated by volcanic eruption-overflow, while the Yanshan cycle is dominated by acidic and alkaline magma intrusion. The volcanic rocks of Hercynian cycle include Carboniferous and Permian volcanic rocks, of which Permian volcanic rocks are distributed around Gejiu mining area along the north, west and southwest edges. The volcanic rocks of Indosinian cycle occur in the Middle Triassic horizon, which can be divided into three periods from morning till night, namely, the Anne period, the early Latin period and the late Latin-Nolik period, forming three northeast volcanic belts distributed in the southeast of the mining area; Yanshan cycle is dominated by acidic and alkaline rocks, forming equiaxed complex with an exposed area of about 320km2, which is distributed in the center of the mining area.

Fig. 3. 1 Geological Schematic Diagram of Old Ore Field

Q. Quaternary humus soil; Paleogene-Neogene gravel layer in the northeast; T3。 Upper Triassic shale, sandstone mixed with conglomerate and coal seam; T2β。 Indosinian volcanic rock series; T2f。 Argillaceous clastic rocks and argillaceous limestone of Falang Formation of Middle Triassic; T2g。 Middle Triassic Gejiu Formation limestone; T 1。 Lower Triassic sandstone shale and argillaceous limestone; Afternoon. Permian Maokou Formation limestone; Pβ。 Late Hercynian volcanic rock series; C. carboniferous limestone; Devonian limestone is mainly sandy shale. Late Yanshanian alkaline rocks; Middle-fine grained granite in late Yanshanian; Middle-coarse grained granite in late Yanshan period; Late Yanshanian porphyry granite; Late Yanshanian porphyry granite; Gabbro-monzonite in early Yanshanian or late Indosinian.

1. Medium and coarse grained granite; 2. Porphyry granite; 3. Nepheline syenite alkaline rocks; 4. Gabbro-monzonite; 5. Metamorphic gabbro; 6. Basic volcanic rocks and metamorphic diabase; 7. Fault of measurement and speculation; 8. Synclinal axis; 9. Anticline axis; 10. Gejiu ore field polymetallic ore concentration area; 1 1. Geological boundary

Among the above magmatic activities, Indosinian submarine basic volcanic eruption and Yanshanian granite magma intrusion are most closely related to mineralization.

(1) Indosinian basic volcanic rocks

Indosinian basic volcanic rocks belong to submarine volcanic rocks and erupted many times in the Middle Triassic.

Anne volcanic rocks are mainly distributed in Songshujiao, Qilin Mountain, Laochang, Kafang and Tian Xin Dam. They occur in the limestone or shale stratum in the lower part of the middle Triassic Kafang section (T2g 1) as a whole, and consist of one or more layers, with the thickness ranging from several meters to 100 meters, generally 20 ~ 50m. Rocks often suffer from intense alteration, with phlogopite (biotite) being the most intense alteration, while actinolite, chloritization, fluorite, sericite and carbonate alteration are also common. The shallow alteration is mainly olivine basalt with porphyritic structure and almond-shaped structure. The porphyritic crystals are mainly olivine, followed by pyroxene and plagioclase. The striped microcrystals of plagioclase are irregularly arranged in the matrix, and other granular augite and chlorite are distributed among them. The deeper metamorphic rocks are phlogopite basalt, actinolite basalt and their mixture, and the deeper metamorphic rocks are phlogopite or actinolite. Anni volcanic rock is the main host rock of metamorphic volcanic rock deposits.

Volcanic rocks in early Latin are mainly composed of massive metamorphic tuff and amphibole metamorphic tuff, which are integrated in marl stratum of Fran Formation (T2f). Volcanic rocks in the late Latin-Nolik period are distributed in Tabai area in the west of Gejiu, mainly composed of basalt and tuff, in which basalt occurs in the strata between Falang Formation (T2f) and Falang Formation and Huobachong Formation (T2h) in an integrated form.

The relationship between Indosinian basic volcanic rocks, especially annihilated metamorphic volcanic rocks, and mineralization has always been controversial. Zhuang et al. (1996) believe that the genesis of metamorphic volcanic deposits is based on the basic volcanic rocks (subvolcanic rocks) of Indosinian cycle, which was transformed by hydrothermal solution of biotite granite after the late Yanshanian period. However, the author finds that Gejiu metamorphic volcanic deposit retains the typical characteristics of VHMS deposit, and thinks that the formation of metamorphic volcanic deposit is directly related to the activity of altered basic volcanic rocks in Middle Triassic and submarine jet deposition and mineralization.

(2) Yanshanian granite

Yanshanian granitoids are widely distributed in Gejiu area, and most of them are exposed to the surface, covering an area of about 320km2. The main rock masses are Longchahe rock mass and Shenxianshui rock mass. Most rocks in the eastern region are buried 200 ~ 1000 m underground, and the main rocks are Baishachong rock mass, Malag-Songshujiao rock mass, Laochang-Kafang rock mass and so on. It is closely related to the main mineral deposits in the eastern region.

As for the genesis of Gejiu granite, many researchers tend to think that the granite in Gejiu area was formed by remelting and consolidating magma from Proterozoic crystalline basement to a certain depth. However, on the classification map of S-type, I-type and A-type granites, the distribution of Gejiu granite samples is very complicated, mainly falling within the scope of A-type and I-type granites, indicating that the origin and formation process of granites are very complicated.

According to isotopic age, Yanshanian magmatic rocks in this area are the products of Yanshanian multi-stage tectonic activity and magmatic emplacement.