Clay vein and its implication for uranium exploration activity in the northern part of the Alligator Rivers Uranium Field, northern Australia

摘要

Clay veins have been found by uranium exploration drilling around the Black Rock uranium prospect in the northern part of the Alligator Rivers Uranium Field (ARUF), northern Australia. The mineralogical and chemical features are described to clarify relations with uranium mineralization, because it is not accompanied by uranium mineralization. X-ray diffraction and chemical analysis for major elements indicate that the clay vein consists mainly of chlorite (clinochlore to ferroan clinochlore) and lesser mica clay mineral (t-1M dominant). The clay vein is compared with the clay alteration zone around the uranium deposits in ARUF in terms of mode of occurrence, mineral and chemical compositions. Mineral composition of the clay vein is only in accordance with that of the inner alteration halo of the clay alteration zone. It is, however, different from mineral composition of the outer alteration halo in terms of lack of Fe chlorite in the clay vein. Chemical composition of the clay vein is similar to that of the clay alteration zone, except for lack in the vein of high iron content which is observed in some samples of the alteration zone. As a whole, the feature of the clay vein corresponds to the inner alteration zone around the uranium deposit in ARUF. The mode of occurrence of the clay vein is very different from that of the clay alteration zone. Mode of occurrence, and mineral and chemical compositions of the clay vein resemble a chlorite vein in the Lower to Middle Proterozoic sand-stone above the Jabiluka deposit, one of major uranium deposit in the ARUF. Because of the similarity between the clay and the chlorite veins, the clay vein is regarded as marginal facies of an alteration zone. The fluid that formed the clay vein is estimated to have been oxidized, because of the existence of hematite and ubiquitous Mg chlorite. This nature is in accordance with the mineralizing fluid that formed the inner alteration zone in the Nabarlek deposit. In conclusion, the vein-forming fluid was possibly related to the uranium mineralization, judging from similarity of mineral and chemical compositions between the clay vein and the inner clay alteration zone, and of nature of vein- and ore-forming fluids. In the uranium deposits in ARUF, ferrous iron in Fe chlorite and mafic mineral such as amphibole have acted as an important reductant against oxidized ore-forming fluid. No uranium mineralization in the clay vein is estimated to have resulted due to the lack of Fe chlorite. Therefore, alteration accompanying Fe chlorite needs to be confirmed to find a uranium mineralization. In the Jabiluka deposit, a general spatial correlation between the intensity of chlorite in the sandstone and the close proximity to ore below has been suggested. Because the mode of occurrence, mineral and chemical compositions of the clay vein resemble those of the chlorite vein, deeper parts of the distributed area of the clay vein are most prospective as an exploration target. Conclusively, confirmation of following points should be important in any future exploration to reach uranium ore; more frequent occurrence of the clay vein, a strongly altered zone below the clay vein and an alteration zone accompanying Fe chlorite.