This study summarizes geologic and geochemical features of the “unconformity-related type” (Athabasca and Alligator Rivers districts), “Oklo-Mounana type” (Franceville district) and “sandstone-type” (Grants district) uranium deposits, in order to compare mechanisms of ore-formation and the role of organic matters in uranium mineralizations.
The unconformity-related type and Oklo-Mounana type uranium deposits occur mostly in Early Proterozoic sedimentary rocks, and are controlled frequently by basal unconformity of the overlying unmetamorphosed sediments and/or steeply dipping faults. Principal mineralization in these types of deposits is considered to have been caused by reactions of uraniferous oxidizing solution with hydrocarbon-bearing reducing fluid derived from carbonaceous materials (graphite and bitumen). The uranium deposits in the Grants district are located in felsic sandstone members within the Upper Jurassic Morrison Formation, and uranium enrichment can be regarded as a result of interactions of diagenetic uraniferous solution with amorphous organic matter (humic substance). These phenomena clearly indicate that all of the three types of deposits are "epigenetic", although these deposits passed through distinct thermal histories in the diagenetic-hydrothermal metallogenic processes. In the processes of uranium fixation, such carbonaceous and humic materials are inferred to have played an important role as actual reductants.

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The Cheongyang W-Mo mine, about 120km south of Seoul, is composed of fissure-filling quartz veins within Cretaceous granite porphyry (70.5±1.6 Ma) and Precambrian banded biotite gneiss. The spatial distribution of mineralized veins exclusively within or near the granite porphyry and related acidic dykes indicates that W-Mo mineralization was associated with intrusion of the granite porphyry. The major ore minerals are wolframite, scheelite and molybdenite. Ore mineralization can be divided into three distinct stages: greisen, vein (early W-Mo, and late base-metal sulfides), and vug (carbonates).
Fluid inclusion data indicate that the hydrothermal system at Cheongyang evolved from initial high temperatures (near 400°C) to later lower temperatures (near 150°C) from fluids with salinities between 0 and 6 wt. % eq. NaCl. Deposition of W-Mo minerals occurred at temperatures between 300° and 400°C mainly as a result of pH increase due to vapor loss by fluid boiling. Later deposition of base-metal sulfides and carbonates was the result of increasing influx of cooler meteoric waters. Fluid inclusion evidence of boiling during the early W-Mo mineralization indicates pressures 100 to 260 bars. Fluid inclusion data for the Cheongyang W-Mo deposits are similar with those of W-Mo deposits in the Hwanggangri district, and are quite different with those of deposits in the Pusan-Yangsan district. This difference in temperature and compositions of hydrothermal fluids for Korean vein-type W-Mo deposits may reflect different pressure-depth conditions of W-Mo mineralization associated with late Cretaceous granitic activity in Korea.
Sulfur isotope compositions of sulfide minerals are consistent with an igneous source of sulfur with a δ³⁴S_Σs value near 4 per mil. There is a systematic decrease in calculated δ¹⁸O_water values with increasing paragenetic time in the Cheongyang hydrothermal system, from values of ≈8 per mil for greisen formation and W-Mo mineralization, to ≈3 per mil for base-metal sulfide mineralization, to ≈-2 per mil for carbonate mineralization in vugs. There is a small increase in δ¹⁸O_water values from greisen formation (6.3-6.8‰) to W-Mo mineralization (7.0-7.8‰), likely indicating boiling of ore fluids during W-Mo mineralization. The trend of overall decrease in δ¹⁸O_water values with time is interpreted to indicate progressive meteoric water inundation of an early magmatic W-Mo hydrothermal system.

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The Oligocene volcanism of the Okushiri Island, situated on the northeastern border of the Japan Sea, may have taken place as the volcanic front of the Eurasian continental margin prior to the formation of the Japan Sea basin. The Oligocene volcanic rocks from the Okushiri Island consist of High-Mg andesite (HMA), High-Ti basalt to andesite (HTV), and Low-Ti andesite (LTV). The rocks of HMA have high contents of MgO, Cr and Ni, and have low Sr and high Nd isotopic ratios. The patterns of primordial mantle-normalized incompatible elements indicate that the HMA rocks are enriched in Rb, Ba, K and Sr and depleted in Nb, and show patterns which resemble those of calc-alkaline rocks from back arc side of the NE Japan arc. The HTV rocks, comparable with the high alumina basalt series, are characterized by high concentrations of both LIL and HFS elements, high Sr isotopic ratio. The primordial mantle-normalized geochemical patterns and the Sr and Nd isotopic ratios of the HTV are similar to those of an enriched island arc tholeiite in the NE Japan arc. By reference to melting experiments at high pressure, the HTV magmas were inferred to have been segregated at 50-60km depth. The LTV rocks are locally exposed, in which contamination phenomena are apparent from the petrographical and geochemical point of views. In comparison with the NE Japan arc magma, we discussed the chemical heterogeneity of the wedge mantle, the characteristics of the Oligocene frontal volcanism at the Eurasian continental margin, and the temporal variation of the Sr isotopic ratios of the Cenozoic volcanic rocks from SW Hokkaido, Japan.

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