Phase Field In The Western Mountain Mineralization Mechanism Research Of Uranium And Thorium Deposits

Depending on the project--“The investigation and appraisal of thorium resourcesin China”, taking the volcanic hydrothermal type deposits of western Xiangshan orefield—Julongan deposit, Lijialing deposit and Zoujiashan deposit as the researchobjects, and summarizing a large amount of foreign and domestic research andanalyzing materials, this dissertation does the analysis on the following aspects:alteration petrology, uranium-thorium mineralogy, petrochemistry, geochemistry, fluidinclusion, isotope and chronology, finds out a series of conclusions about the thoriumalteration characterization, thorium mineral types and its existing occurrences, majorand trace elements’ geochemical characterizations of thorium mineralized samples,compositions, temperature, salinity, and density of thorium metallogenetic epoch fluidinclusions, the epoch of thorium metallization, origin of fluid and metallogenicmaterial in the mineralized process, and the differences between diagenetic andmetallogenic process, and discusses the evolution process and the thoriummetallogenic mechanism.Through the alteration petrology research of western Xiangshan ore field, itclarifies the alteration characters of this area：the alkali alteration phase developingcarbonation and albitizaiton is the the single uranium mineralization phase; the sagegreen hydromicazation and chloritization alteration zone spreads widely but alwayswith low grade mineralization; and the purple fluoritization and apatitization areintensely related with thorium mineralization. Studying detailedly macroscopicoccurrence and microscopic petrology characteristics of ore beraing thin sections, theauthor summarizes the occurrences of thorium in these deposits: existing asindependent thorium minerals-thorite, uranothorite, and thorianite; lying asisomorphous admixture in uranium minerals–pitchblende, uraninite, uranothorite,coffinite, and brannerite; occurring as microisomorphic form in accessory minerals-apatite, zircon, monazite, rutile, and xenotime.Comparing the analyzing material of the alteration samples, it indicates thaturanium and thorium rich minerals have the characters as the following: rich in CaO, and P₂O5，poor in SiO₂; high content of REE especially HREE, low ratio ofLREE/HREE-high differentia degree of LREE and HREE; rich in high field intensityelements such as Zr, Hf, Sm, Y, and Yb, poor in large ion elements such as Rb, andBa, but due to the carbonation and fluoritization with relatively high content of Sr.Sulfides and accessory minerals are the main cause of the enrichment of some relativerare elements.Testing the homogeneous temperatures and the freezing temperatures of the fluidinclusions in different metallogenetic phases, together with the components of thefluid inclusions testing by Laser-Roman spectrometry, the results show that the gasphase of fluid inclusions are composed of N₂,CH₄, H₂, CO₂and so on, which are thetypical components of mantle; the metallogenetic fluid is low-middle temperature,low-middle salinity, and low density; the temperature of metallogenetic phase is lowerthan the temperature of prior metallogenetic phase, but the salinity of metallogeneticfluid is higher than the prior metallogenetic fluid; and the temperature of thoriummineralization is190℃²00℃。.Using LA-ICP-MS to testing the chronology data of uranium and thoriumminerals and the accompanied apatite, this paper obtain97.5Ma Th-Pb age ofmetallization for the first time which is similar to the second phase of Umetallization-99.0Ma. Combining with the previous isotopic research material ofRb-Sr、Sm-Nd、Pb, it proves that the volcanic-subvolvanic rocks of Xiangshan isderived from the part melting of low crust and early-middle Proterozoic metamorphicrocks, besides some mantle magma; at the mean time both the Sm-Nd isotopic dataand the fluid inclusion components of metallogenetic fluorite indicate themetallogenetic thermal fluid is partly composed of mantle substance.The ore-forming substances partly are derive from mantle fluid, and partly comefrom wall rock which the fluid goes up through, abundant incompatible elements areextracted. Mineralized samples are richer in U,Th and other incompatible elementsthan normal crust and mantle.Alkali thermal fluid come from mantle with plenty mineralizer （CO₂、CO、CH₄、 H₂S、H₂and so on）, can promote the microcirculation of matter movement betweenthermal fluid and wall rock, and extract the incompatible elements from wall rock.when the mantle fluid uprising, the evolution of the mantle fluid also is the process ofthe enrichment of U, Th, REE and other incompatible elements, and finally forms theore-bearing thermal fluid rich in U, Th, and REE. In the process of metallogeneticfluid moving, the change of temperature, salinity, compositions and the PH is theimportant conditions, in which U,Th can precipitate separately. Alkali thermal fluidwith CO₂, is benefit for the migrating of U, but the thorium; in the other side acidicthermal fluid with F-is favorable for the migration of both uranium and thorium,therefore uranium and thorium can form ore-veins separately, and the thermal fluidrich in F-can enlarge the solubility of silicate in water solutions and fluids, leading thegrowth of thorite （not the thorianite） around the apatite in the alteration zone offluoritization.