Namibia is located in southwest Africa and very rich in mineral resources. At present, there are mainly four types of uranium deposit: intrusive, Surficial and sandstone-hosted. Rössing uranium deposit is the famous leucogranite-type uranium deposits [1]. With the geological work progression, leucogranite-type uranium deposits were discovered in the surrounding areas, including Valencia, Rössing South, Ida Dome, Goanikontes, Gaudeanmus [1]-[11]. The geochronology of monazite U-Pb has shown that the U-Pb age of leucogranite is about 509 ± 1 Ma, consistent with the age of native uraninite (508 ± 2 Ma) [12] [13]. However, there are disagreements on the mechanism of leucogranite-type uranium deposit. Nex et al. concluded that leucogranite-type uranium deposits were merely related to the magmatic differentiation, didn’t experience hydrothermal alteration [13]; Herd found later fluids in the leucogranites, but only related to the formation of secondary uranium uranophane [14]; while Chen et al. found that the pitchblende which was in the uraniferous leucogranites, indicated that leucogranite-type uranium deposits formed by the original magmatic crystallization differentiation and later superimposed hydrothermal alteration [10] [11]. So the opinion may relate to the later hydrothermal alteration, which has not yet reached agreement.

REE, fluid inclusion and C-H-O stable isotope is an important geochemical tracer, a powerful tool for source and the forming mechanism of mineralization [15] [16] [17]. This paper presented the C-H-O stable isotope, elements and fluid geochemistry of the uraniferous leucogranites, discussed which specific trace elements have close relation with uranium mineralization and whether post- mineralized hydrotherm has superimposed transformation to the uraniferous leucogranites, and determined the stages of uranium mineralization as well. An essential evidence for metallogenic mechanism was proposed and new geochemical information to research on the metallogenic model and prognosis was provided.

The Damara Orogen is a part of the Late Precambrian (Neoproterozoic) and Early Paleozoic (650 - 460 Ma) system of the Pan-African mobile belt, which transverses the African continent. In Namibia, the Damara Orogen has been divided into a north-south trending coastal branch and a northeast trending inland branch which has been interpreted as a result of the collision between the Congo, Kalahari and Rio de la Plata cratons [18]. The inland branch has been divided into Northern Platform, Northern Zone, northern Certral Zone, southern Certral Zone, Okahandja Lineament Zone, Southern Zone, Southern Margin Zone and Southern Foreland by the lithostratigraphical, structural and metamorphic criteria [19] [20]. The boundaries between the zones are formed by major faults or linear structures with an aeromagnetic expression [21]. Uranium-bearing leucogranites in Namibia occur within the Central Zone of the Damara Orogen [22]. The Gaudeanmus locates in the southern Certral Zone (Figure 1). The distinguishing characteristics of the southern Central Zone are a number of granitoid intrusions and low-pressure high temperature metamor-

phism at upper-amphibolite to granulite facies [23].

The strata in the Gaudeanmus have been divided into Abbabis Complex, Etusis Formation, Khan Formation, Rössing Formation, Chuos Formation, Karibib Formation, Kuiseb Formation from bottom to top. Abbabis complex is largely composed of augen gneiss and quartzofeldspathic gneiss with a small amount of argillaceous schists, quartzites, marbles, amphibolites. Geochronological studies of the gneisses have resulted in various ages, including 2093 ± 51 Ma and 1038 ± 58 Ma (U-Pb SHRIMP) [24]. The Etusis formation lies unconformably on the Abbabis metamorphic complex, occurs as cores to several of the prominent domes or anticlines in the Gaudeanmus. It is mainly composed of metamorphic conglomerates, quartzites, arkoses, schists, gneisses, etc. In Damara Orogen, Rift-related acid volcanics close to the base of the Damara sequence have been dated at 746 ± 2 Ma by Hoffman et al. (1996) [25]. The Khan Formation is distinguished by the grayish-green rocks and numerous granitoid intrusions, and consists of pyroxene-garnet gneisses, schists, amphibolites. It is overlain unconformably by the Rössing Formation, which is dominated by the upper and lower sets of marble, cordierite gneisses and calcsilicates. The uranium-bearing leucogranites intrude mostly into this formation. Chuos Formation which is overlain by a major thickness of glacial diamictite containing a small quantity of calcsilicates and quartzites, and so on. These glacial deposits are interpreted as a result of the worldwide Sturtian glacial event by Hoffman, so Chuos Formation has been dated at 710 Ma [26]. Karibib is composed of thick-bedded marbles, calcsilicates, schists and gneisses, which the thick-bedded marbles developed strong joints. They are overlain by the pelitic schists, gneisses, migmatites, calcsilicates and quartzites of the Kuiseb Foramtion.

In Gaudeanmus, leucogranites are common as sheeted, net-veined, and stratiform-like dykes, and vary in width from 10cm to more than 200 m across. The uraniferous leucogranites intruded into the marble of Rössing and Karibib Formations, gneiss of Khan and Kuiseb Formations. Based on detailed field observations and comparative analysis of cores, we consider that the formation of the rich ores was significantly affected by the hydrothermal alterations, which included albitization (Figure 2(a)), silicification, muscovitization, illitization, kaolinization, sericitization, chloritization (Figure 2(b)), pyritization and limonitization.

The leucogranites are texturally variable: ranging from graphic, and medium-, or coarse-grained granitic to pegmatitic. They are distinguished by the smoke gray, or black quartz. The leucogranites have a simple mineralogy comprising quartz, plagioclase and K-feldspar with minor biotite and muscovite, and develop myrmekite and metasomatic edulcoration border texture (Figure 2(c)). Accessory minerals include zircon, apatite, monazite, epidote, rutile, sphene, pyrite (Figure 2(d)), galena, ilmenite, hematite and various uranium minerals. In majority of leucogranite samples, K-feldspar content is higher or slightly higher than that of plagioclase, therefore, the leucogranites are mostly Syenogranite and adamellite, a small number of alkali-feldspar granite, granodiorite and tonalite.

According to rock-mineral identification and electron microprobe analysis, In Gaudeanmus, the uranium of leucogranite-type uranium deposit mainly exists as independent uranium minerals, and partially exists in thorium minerals as isomorphic form. The uranium minerals are mainly uranium oxides, U-Ti- oxides and uranium silicates, in which uranium oxides including uraninite, pitchblende (Figure 2(e), Figure 2(f)) and thor-uraninite. Generally, the uraninite varies from a few micrometres to 5 mm in size (Figure 2(d)), and often occurs

within plagioclase, K-feldspar and biotite. There are many radioactive halos in the contact sites of the uraninite, biotite or feldspar. U-Ti-oxides are a complex minerals, including brannerite, betafite, and uranpyrochlore. The uranium silicates contain coffinite, uranophane (Figure 2(b), Figure 2(f)), and uranothorite.

The leucogranites have been divided into six different types based on field characteristics of petrology, mineralogy and radioactivity [13]. Only the leucogranites which contains smoke gray quartz is the uranium ore. With respect to the purpose of this study, a total of 13 representative samples were collected from the drilling in Gaudeanmus, including non-mineralized leucogranites (ZK11-11- 20, ZK0-11-9, ZK20-1-3), weak-mineralized leucogranites (ZK0-11-8, ZK0-11- 10, ZK80-9-2), and uraniferous leucogranites (ZK76-13-5, ZK76-13-6, ZK76-13- 8, ZK76-13-19, ZK80-9-4, LS-1, ZK15-2-7). The detailed sample locations are anomaly area 18 (Figure 1).

The concentrations of trace elements and REE in the samples were measured at the Analytical Laboratory Beijing Research Institute of Uranium Geology, using HR-ICP-MS (Element I) which manufactured by Finnigan-MAT in German, following the methods of the DZ/T0223-2001. The temperature and relative humidity of the laboratory is 20˚C and 30%, respectively. The contents of trace elements and REE are greater than 10 ppm, the relative error is less than 5%, while the contents are less than 10 ppm, the relative error is less than 10%. Detailed analytical procedures for trace elements and REE are described by Qu et al. [27].

Doubly polished sections were prepared for fluid inclusion optical examination and microthermometric analysis. Heating and cooling experiments were carried out using a Linkam THMS 600 programmable heating-freezing stage at the Key laboratory of Uranium Resource Exploration and Evaluation Technology, CNNC. The heating rate near ice point and homogenization temperature was 0.1˚C to 0.2˚C/min. The reproducibility of measurements was ±0.2 below +30˚C and ±2 above 100˚C, where the chips were centered in the specimen holder.

A systematic research on the ore-forming fluid, can judge the uranium mineralization stages and different uranium mineralization [31]. Therefore, a detailed study of the quartz inclusion in mineralization granite and quartz veins, found that there are two periods of ore-forming fluid. The primary inclusions of main metallogenic epoch are isolated or clustered, homogenization temperature concentrated in 470˚C - 530˚C (Figure 5), salinity is 3.55% - 9.60% NaCleq, mean 6.14% NaCleq. It belongs to magnatic residual high temperature and low salinity fluid. Another inclusions is secondary, distributed as linear or banded, homogenization temperature ranges from 150˚C to 220˚C (Figure 5), salinity is 4.65% - 19.05% NaCleq, average 11.5% NaCleq. Obviously, the late hydrothermal fluids for reformation is mid-low temperature, low salinity fluid [32].

1) The contents of total rare earth elements in the non-mineralized, weak-mi- neralized, uraniferous leucogranites are 222.02 - 377.26 ppm (mean 302.59 ppm), 385.06 - 1763.00 ppm (mean 878.07 ppm), 109.19 - 5994.28 ppm (mean 1925.79 ppm), respectively, showing that there is an obvious growth trend from non-mineralized to uraniferous leucogranites, it suggests that uranium mineralization presents synchronous increase trend with REE. δEu = 0.03 - 0.47, with an average of 0.22, indicating that magma experienced strong differentiation crystallization.

2) In Gaudeanmus, leucogranites show enrichment in large ion lithophile elements, especially Rb, Th, U, Pb, and relative depletion in Ti, P, Sr and Ba. In the uraniferous leucogranites, there is a high correlation between U and Pb, Th, Co, Ni, REE, the correlation coefficient is 0.793 - 0.997, indicating that U is highly homologous with Pb, Th and REE.

3) The leucogranite-type uranium deposite in Gaudeanmus can be divided into two stages: one is main metallogenic epoch, the representative uranium minerals of this stage formed by magmatic crystallization differentiation, which are uraninite, thor-uraninite, coffinite, uranothorite, brannerite and betafite. And the REE pattern is characterized by rightinclining type with obvious Eu negative abnormity and enrichment of LREE. The other is secundarii metallogenic epoch, secondary uranium minerals of this stage produced by later hydrothermal reformation, representing by pitchblende and veined coffinite. And this REE pattern manifests as leftinclining type with obvious Eu negative abnormity and enrichment of HREE.

4) The main mineralization temperature of uraniferous leucogranites concentrates on 470˚C - 530˚C, the salinity is 3.55% - 9.60% NaCleq, with C, H and O isotopic characteristics of typical magmatic source, −23‰ - −13.6‰, −53.3‰ - −46.4‰, 7.71‰ - 8.81‰, respectively, further revealing that the leucogranite-type uranium deposit in Gaudeanmus area was chiefly formed by magmatic crystallization differentiation. The temperature of superposed hydrothermal fluid centeres on 150˚C - 220˚C, the salinity is 4.65% - 19.05% NaCleq, C, H and O isotope is −20.3‰ - −3.7‰, −64.7‰ - −53.6‰, 1.49‰ - 1.99‰, reflects this period hydrothermal reformation mixed with atmospheric water.

Chen, J.Y., Fan, H.H., Wang, S.Y. and Gu, D.Z. (2017) C-H- O Stable Isotope, Elements and Fluid Geochemistry of Uraniferous Leucogranites in Gaudeanmus Area, Southern Central Zone, Damara Orogen, Namibia. Journal of Geo- science and Environment Protection, 5, 1- 18. https://doi.org/10.4236/gep.2017.52001