The Geological Characteristics Of “Da Changceng” In Southwest Guizhou And Its Controlling Effect On Gold Deposits

Carlin-type gold deposits are the most important sources of gold in the world, and also the second biggest type of gold in China. The southwest of Guizhou province is one of the Carlin-type deposits concentration areas in China and the main part of “Dian-Qian-Gui” gold concentration areas. There are some super-large, large and mid-large gold deposits in this area. Lots of study works have been done by many researchers, and they have got many achievements.This thesis is focusing on the geological characteristics of ore-controlling tectonic which having close relationship with gold: “Da Changceng” and high-angle fault. We choosed two typical deposits(Shuiyindong deposit and Lannigou deposit) and did some works on ore-bearing layer, fluid and stable isotope characteristics. We described the distribution of “Da Changceng”, outcrop characteristics and the relationship with gold, as well as compared with the similarities and differences of deposits in basin facies and plateau facies area on ore-controlling tectonic and ore-forming processes.According to the study, the main conclusions are:(1) There is an obvious tectonic framework of plateau-basin, which formed in Indosinian-Yanshanian. A face transformation line exists between them. It controls the distribution of carbonatite and clasolite. Their relationship is mutation. In fact, this face transformation line is a large thrust-nappe structure, a fracture phenomena exists between these two faces.(2) “Da Changceng” is composed of a suite of siliceous rock with breccia which exists between Maokou formation and Longtan formation or Emeishan basalt formation. The breccia structure and siliceous alteration are the most important characteristics. This layer exists underground in some deposits such as Shuiyindong, but in Nibao deposit and Getang deposit is outcropping.(3) The interface of the study area formed after the deposition of stratums. Because of the sub-tectonization, the attitude of “Da Changceng” and high-angle fault is different. These two tectonics have clear function in controlling gold-ore. After the comparing of ore-bearing layer in basin facies area and plateau facies area, the author thinks that there is a pattern of ore-bearing layer in the southwest of Guizhou: it forms at the interface between brittle rocks and plastic rocks. The reason is interlayer sliding likely to happen between them. The porosity of the top and the low layers is mutation, it generates the pressure of fluid, breaks the host-rock and sediments the ore-forming material. So, this paper considers that the definition of the conventional “Da Changceng” needs to be expanded: the interface between brittle rocks and plastic rocks which include the conventional “Da Changceng” and the high-angle fault.(4) The study of fluid inclusions shows that ore-forming fluid in these two structures is both low in temperature and salinity. Some kinds of gas such as CO2, CO and CH4 exist in “Da Changceng” which is different from high-angle faults. The test of fluid composition shows that main kation in these two strctures is Na+, K+ and Ca2+, and the anion is Cl- and SO42-. The analysis of stable isotope shows that the main source of “Da Changceng” is the interaction of magmatic water and atmospheric precipitation whereas the the main source of Lannigou deposit is the combination of metamorphic water and atmospheric precipitation.(5) Before the mineralization, the source of fluid is magmatic water, it was transported to the ore-bearing structures through the large thrust-nappe structure. Atmospheric precipitation generates the combination of fluid as well as the deposition of ore-forming materials. Besides, when fluid was transported to the ore-bearing structure, the pressure of fluid suddenly decrease, boiling effect happens. It causes the losing of H2 O and CO2 and the concentration of ore-forming materials. Gold moves by the complex of Cl and Si in the “Da Changceng” and by the complex of S in high-angle fault. Metamorphic water is the main source because of the strong tectonism. Moreover, the boiling effect is becoming more and more stronger from “Da Changceng” to high-angle fault.