LANPING Cenozoic Basins Forming Fluid Characteristics, Origin And Evolution Of Research

Lanping basin is located in the middle of the three-river area in western Yunnan, which is also between the Jinsha River-Ailao Mountain fracture and the Lancang River fracture. Being formed after a long tectonic evolution, with development of various of tectonic phenomenon and production of abundant deposit, it is an important polymetallic ore zone of our country. The large-scale mineralization of the basin occurred in the Meso-Cenozoic sedimentary strata, however, the large-scale deposits can not analogy with the typical MVT deposit, SST deposit and the Sedex type deposit. Lanping basin deposit has significant epigenetic and hydrothermal ore-forming characteristics, and the key problem of the deposit formation mechanism is the problem of the fluid. Jinding and Baiyang Flat deposits are two typical deposits in Lanping basin, which have highly representativeness. Taking the Meso-Cenozoic ore-forming fluid of Lanping basin as the research target, this paper studies and comparatively analyzes the fluid inclusions and isotopic of Jinding and Baiyang Flat mining area, the two typical mining area of Lanping basin, to come to the cognition of the characteristics and the source of the ore-forming fluid of Lanping basin. Combining with the geological structure evolutionary background, this paper also does a micro analysis and research of tectonic evolution of Meso-Cenozoic ore-forming fluid in Lanping basin, to obtain the fluid evolutionary concept model. The main cognition and conclusions are as follows.1. The main ingredient of the ore-forming fluid in Lanping basin is H2O, and a small amount of CO2and CH4. The main temperature of the ore-forming fluid is about150℃～350℃, which belongs to low temperature fluid. The main salinity of the ore-forming fluid is about6～25wt%NaCleq, which belongs to high salinity fluid. The density of the ore-forming fluid is about0.7697～1.12g-cm-3, which belongs to low density fluid. The pressure of the ore-forming fluid spreads extensive, and the depth is shallow——depends on specific ore-forming conditions, which needs consideration of various of conditions and causes.2. The sulfur in the ore-forming fluid in Lanping basin comes from the biological sulfur of sedimentary origin and strata cream salt sulfur and may also from the mantle sulfur. The lead in the ore-forming fluid is mixed with the lead comes from upper crust, lower crust and mantle, which suggests that the fluid most comes from the regional strata. The water in the ore-forming fluid mainly comes from the meteoric waters, and maybe mixed with modicum metamorphic water and magmatic water. The carbon in the ore-forming fluid has a wide source. It may be made by the oxidation and dehydroxylation of sedimentary organic matter, the extraction of ancient precipitation, igneous carbonate and mantle inclusion which caused by the meteoric water, or even the dissolution of marine carbonate rocks.3. The ore-forming fluid in Lanping basin is mainly mixed by three kinds of fluids: fluid rich in organic matter formed by porewater from the early stratum which is oppressed by large amount of deposit therefore with organic matters; high mineralized fluid made by the extraction that the ancient meteoric waters infiltrate down fracture and gypsolyte that made to the ancient precipitation; and deep-focus or mantle-derived fluid which with various of metallogenic material ascend by fracture structure. The ore-forming fluid in Lanping basin has different forming and migration ways in different stages, mainly mixed by the three kinds of fluids above. In the progress of forming, evolution, migration and compounding, the fluid absorbs Pb、Zn、Cu、Ag、Co, etc. from the deposit stratum and the basement rock, and come into a kind of high salinity and low temperature ore-forming fluid.4. Meso-Cenozoic ore-forming fluid in Lanping basin experienced three main evolution periods. The first period is the ore-forming fluid forming period (T2-J2), during which, with the deposition effect of basin and squeezing effect that overlying strata made to underlying strata, the porewater carried with various of materials formed in interlayer fluid rich in organic matter, which then continuously absorbed metallogenic material as centrifugal flow to become the early ore-forming fluid under the squeezing effect of basin. The second period is the ore-forming fluid converge and storage period (J3-K2). In this period, basin upraised entirely, salt brine formed by meteoric waters extracted ore-forming materials in stratum, and then mixed with the early-formed ore-forming fluid. Under the effection of basin structure, some of the deep source fluid and the mantle fluid came up and mixed with the fluid above to come into a more mature ore-forming fluid, which then was separated by tectonic actions and saved in stratum. The third period is the ore-forming migration and enrichment period (E1-E3). In this period, the strike-slip pull-apart structure lead the ore-forming fluid of the basin to large-scale migrations, and then precipitated and enriched into ore in proper places.