| The Gaudeanmus area, Namibia, located to the southeast part of Rossing deposit, to thesouthwest of Valencia deposit and in the central belt of the southern part of Damara orogenic belt,is a U-prospective exploration area. Further systematic study on the Gaudeanmus area has beencarried out to analysis:(1) the origin, characteristics, transportation and evolution ofmineralizing fluids;(2) the mechanism of uranium precipitation and concentration and (3) themetallogenetic model of alaskite-type uranium deposit with respect to regional geology, mineraldeposit geology, uranium mineralogy, geochronology, metallogenetic epoch, geochemistry ofmetallogenetic elements, and multivariate isotopic elements analysis.In this area, the main ore-controling factors are structure, lithology, stratum, basic rock veinsand late hydrothermal superimposition. The ore bodies mainly occur in ductile shear zone withthe predominant NNE direction, turning of folds, margin of dome and structural variationposition (twist or expand), which provide abundant space for uranium mineralization. At thesame time, the Welwitschia regional faults are the transportation channel for uranium.Mineralized alaskite has metallogenetic specialization that means D, E-type alaskite aremineralized and other four types of alaskite and Salem-type granite are not mineralized as aresult of the differences of their formation processes. D, E-type alaskite are U-rich because theD3deformation associated with dome formation resulting from partial melting of a pristinesection of pre-Damaran basement which was not affected by earlier appreciable melt extraction.However, unmineralized alaskite and Salem granite was formed before D3，and the materialsources are different either. In terms of remelted strata, mineralized alaskite occurs as veins andstockwork intrud into Rossing Formation, Khan Formation, Chuos Formation, KaribibFormation (in case of the absence or the thickness reduction of Rossing Formation). The reasonis decarbonation reaction between marble and magma leading to the increase the contents of CO2concentration in ore-forming fluids, and boiling of magma, thus resulting in precipitation ofuranium when the magma contacts Rossing Formation or Khan Formation. The latehydrothermal superimposition occurs in fault fractures making mineralized alaskite stronglyaltered and apparently increasing ore grade. This stage of uranium mineralization is closelyrelated to the intrusion of Jurassic basic magma.Detailed research was carried out to study the alteration types of U ore of Gaudeanmus area. Three stages could be generally observed:(1) the first stage is late magmatic autometasomatismalteration, including albitization, little potash feldspathization, muscovitization, silicification andlittle fluoritization, associated with the formation of idiomorphic coffinite and uranothorite;(2)The second stage is mesothermal-epithermal alteration, mainly including chloritization,kaolinization, sericitization, pyritization, silicification, illitization and hematitization, closelyassociated with the formation of vein coffinite and pitchblende and (3) the third stage issupergene oxidation alteration, including limonitization, tightly associated with uranotile,carnotite and other secondary uranium minerals.Through microscopic observation and electron probing analysis, it is revealed that uraniummainly exists in the form of independent uranium minerals, some uranium exists in isomorphicform in Th-bearing minerals, zircon, titanite, monazite and apatite. The ore-forming process canbe divided into three stages: magmatic stage, hydrothermal superimposition reformation stageand supergene infiltration stage. Magmatic stage is the main ore formation stage and uraninite,thoruraninite, brannerite, idiomorphic coffinite, uranothorite are formed, and the ore-forming ageis502±3Ma. In hydrothermal superimposition reformation stage, uranium minerals contain veincoffinite and pitchblende, and the ore-forming age is153.5±6.5Ma. In supergene oxidation stage,uranium minerals mainly contain uranophane and carnotite, with the ore-forming age of11.0±3.2Ma. Thus, the alaskite-type uranium deposit of Gaudeanmus is a comprehensive productof crystallization differentiation of protomagma, superimposed reformation of late hydrothermalfluid and supergene infiltration.Study on Sr-Nd-Pb isotope, stable isotope and fluid inclusion geochemistry shows thaturanium mainly comes from the pre-Damaran uranium-rich basement in the main metallogeneticepoch of alaskite-type uranium deposit. Uranium migrates in forms of UO4-2nn, UCl4and UF4. Thetemperature of main metallogenetic epoch ranges from470℃to530℃, and salinity ranges from3.55%to9.60%NaCleq (the average is6.14%NaCleq), and pressure ranges from247MPa to284.5MPa (the average is257.3MPa), ore-formation depth ranges from8.05km to9.27km(theaverage is8.38km). The late hydrothermal fluids are originated from the combination of deepfluids and atmospheric water. Uranium mainly comes from the remobilization of former uraniumminerals and is transported in forms of UO2(CO3)22-and UO2(CO3)34-. The temperature, salinity,pressure and depth of hydrothermal superimposition reformation is150℃-220℃,4.65%-19.05% NaCleq (the average is11.5%NaCleq),38.6-267.6MPa (the average is172.2MPa),1.26～8.72km (the averge is5.61km), respectively. |
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