A Study On The Characteristics Of Fluid Inclusion In Zoige 510-1 Uranium Deposit, Sichuan Province, China

In this thesis, the 510-1 uranium deposit of Zoige area is studied as the representative, the characteristics with respect to petrography and microthermometry, inorganic gaseous components and liquid anion and cation components of fluid inclusions in quartz and calcite(which are closely related to mineralization) are highlighted, the physical-chemical mineralization conditions are summarized, and the source of ore-forming fluid is explored by combining with the features of C and O isotopes, to discuss the reasons for the formation of ore deposits. The discoveries are as follows:1. Many primary or pseudosecondary inclusions were developed in quartz and calcite samples. These fluid inclusions are mostly clustered and banded, occasionally scattered. They are colorless, gray or dark gray. Most of them are in regular shape, with the size ranging from a few microns to tens of microns, and the gas-liquid ratio generally between 5% and 30%. According to the phase characteristics of fluid inclusions, they can be divided into three types:(1) the gas-liquid phase inclusions: consists of gas and liquid phase composition, colorless or gray, including liquid-rich inclusions and gas-rich inclusions.(2) pure gas inclusions: a dark grey, all for the gas phase composition;(3) the pure liquid inclusions: colorless at room temperature, transparent, all for the liquid phase composition.2. Microthermometry results of fluid inclusions show that: From the pre-ore period to the post-ore period, the homogenization temperature of fluid inclusions in quartz and calcite presents an obvious decrease tendency. The range of homogenization temperature of fluid inclusions in the pre-ore quartz is mainly concentrated from 290 ℃ to 370 ℃ with the average of 329℃; The range of homogenization temperature of fluid inclusion in the main-ore quartz is mainly concentrated from 230 ℃ to 330 ℃ with the average of 288℃; The range of homogenization temperature of fluid inclusion in the post-ore quartz is mainly concentrated from 178 ℃ to 303 ℃ with the average of 213 ℃. The range of homogenization temperature of fluid inclusions in the pre-ore calcite is mainly concentrated from 93 ℃ to 380 ℃ with the average of 227 ℃; The range of homogenization temperature of fluid inclusion in the main-ore calcite is mainly concentrated from 110 ℃ to 230 ℃ with the average of 174 ℃; The range of homogenization temperature of fluid inclusion in the post-ore calcite is mainly concentrated from 99℃ to 286℃ with the average of 144℃. The results also show that the deposit belongs to the medium-low temperature hydrothermal deposit.3. The experimental results of fluid inclusion show that: All salinity of fluid inclusion is far less than 30wt% NaCl eqv. The range of salinity of fluid inclusions in the pre-ore quartz is mainly concentrated from 6.4~8.8wt%NaCl eqv with the average of 7.11wt%NaCl eqv; The range of salinity of fluid inclusion in the main-ore quartz is mainly concentrated from 4.0~8.8wt%NaCl eqv with the average of 7.17wt%NaCl eqv; The range of salinity of the fluid inclusion in the post-ore quartz is from 4.96~5.11wt%NaCl eqv with the average of 5.04wt%NaCl eqv; The range of salinity of the fluid inclusions in the pre-ore calcite is mainly concentrated from 2.4~5.6wt%NaCl eqv with the average of 4.18wt%NaCl eqv; The range of salinity of fluid inclusion in the main-ore calcite is mainly concentrated from 0.8~7.2wt%NaCl eqv with the average of 4.47wt%NaCl eqv; The range of salinity of fluid inclusion in the post-ore calcite is from 0.88~8.41wt%NaCl eqv with the average of 4.28wt%NaCl eqv; Above of all analysis, the salinity of inclusions in the quartz is relatively stable at 7.2wt % NaCl eqv and the salinity of the inclusions in the calcite is relatively stable at 4.0wt%NaCl eqv. The inclusions salinity in calcite has the obvious lower characteristics than that of quartz inclusion. Overall the inclusions in quartz and calcite have a low salinity characteristics, so that 510-1 uranium deposit belongs to the low salinity fluid.4. By formula of inclusion density in previous studies, the inclusions density in quartz is 0.52~0.97 g/cm3, with the average of 0.76 g/cm3. Inclusions density in calcite is 0.56~1.02 g/cm3, with the average of 0.90 g/cm3. On the whole, the density of fluid inclusion in calcite should be slightly higher than that of quartz, but all of them also belongs to the low density fluid. It illustrated that the deposit is a low density fluid. According to the relevant formula calculation, the average pressure of inclusions in quartz from pre-ore to main-ore and post-ore stage were 90.3MPa, 77.4MPa, 49.2MPa, mineralization depth corresponding respectively to 3.01 km, 2.58 km, 1.64km;The average pressure of calcite inclusions were 59.1MPa, 45.7MPa, 37.5MPa, which corresponds to a mineralization depth of 1.97 km, 1.52 km, 1.25 km. Overall, the mineralization is a gradual reduction process in pressure and depth, and the fluid of the deposit is mainly formed under the conditions of the medium-low pressure, medium-low depth.5. The results of Micro Laman Spectroscopy Analysis to single inclusion show that : Six kinds of gases have been found in the fluid inclusions of 510-1 uranium deposits including CO2, N2, H2 S, and CH4, H2 O, H2. N2, CH4 and CO2 are the main components in the fluid inclusions followed by H2 S and H2 O. H2 only found in few inclusions. The groups inclusion analysis indicates that : The average contents of gas components decrease as following order: CO2>N2>H2>CO>CH4, The analytical results are basically in accordance with the qualitative analysis results by microscopic laser Raman spectroscopy. both of them contains some reducing volatile(H2, CO, CH4), especially riching in the mantle components(N2 and CO). Overall, the gas phase constitutes the C-H-O-N-S system. The volatility of the C-H-O-N-S system is similar to that of mantle fluids, and also has a certain similarity with the inclusions in the typical mantle rocks from East China, indicating the formation of fluid inclusions from the mantle.6. The result of ion chromatographic analysis in fluid inclusions show that : The cation of fluid inclusions is composed by Na+, K+, Mg2+, Ca2+; The cation is given priority to Ca2+, ranging from 2.40 to 224.10 ug/g, with the average of 30.07 ug/g, followed by Na+(0.28~2.14 ug/g) and Mg2+(0.14~3.82 ug/g). The anion of fluid inclusions is composed by F-, Cl-, NO3-, SO42-, HCO3-, CO32-, The anion is given priority to HCO3-, ranging from 7.22 to 217.20 ug/g, with the average of 30.07 ug/g, followed by SO42-,content from 1.55 ug/g to 484.60 ug/g, with a mean of 36.33 ug/g. As a whole, the ore-forming fluid of this deposit should be the Ca2+-Mg2+-Na+-HCO3--SO42--Cl- system. In addition, after the analysis of main ion composition of the inclusions, the SO42- and Ca2+ content of the pre-ore period is distributed substantially in low level range of 1~10μg/g; Most of SO42- and Ca2+ content in the main-ore and post-ore period is in the high level range of 10~100 μg/g; It indicated that the late hydrothermal mineralization has been influence by the hydrothermal of enriched SO42- and Ca2+.7. Combining with the analysis of Na+/K+ ratio, Cl-/F- ratio and(K++Na+)/(Ca2++Mg2+) ratio of ions component, we can infer that the ore-forming fluid is mixed by meteoric water or geothermal brine in the late stage hydrothermal; The meteoric water may be the more possibility of sources, and changed the nature of the ore-forming hydrothermal, but have a little impact on the mineralization.Through the systematic study of the characteristics with respect to petrography and microthermometry, gas and liquid components of fluid inclusions in 510-1 uranium deposit, combining with previous studies to the features of C and O isotopes of calcite in zoige uranium orefield, it suggests that the ore-forming fluid is from the deep mantle, and the deposit is formed in the environment of a medium-low temperature, a low salinity, a low density, a medium-low pressure, a medium-low depth, the middle-late stage of the deposit have been added by a portion of meteoric water. All of achievements has important significance for deeply understanding to the genesis and exploration of uranium deposits in the Zoige region.