Age of mineralization

The age of Kuroko mineralization can be estimated from (1) K-Ar ages of igneous rocks associated with Kuroko deposits and (2) foraminiferal assemblages in mudstone directly overlying Kuroko deposits. 

Considering uncertainty of foraminiferal and K-Ar ages it seems reasonable that the Kuroko deposits in Hokuroku district formed in 14-12 Ma (more likely 13.6- 

However, several small Kuroko deposits (e.g., Yunosawa in Hokuroku district, Kuroko deposits in Hokkaido) occur in the formation younger than middle Miocene age (16-14 Ma), suggesting younger ages (12-13 Ma). 

The Shakanai No. 1 deposit in the Shakanai mine is a good example of the B sub-type (Kajiwara, 1970a). The ore of the B sub-type deposits consists of predominantly of galena and sphalerite with lesser amounts of chalcopyrite. Ore deposits of this sub-type are usually not directly associated with dacite lava dome. However, it is known that domeshaped dacite occurs below some of this sub-type deposit (Kajiwara, 1970a Tanimura et al., 1974). Total ore quantity of a single unit deposit is generally small, about one million tons. 

C sub-type deposits are often called typical Kuroko deposits. Sato (1970) and Horikoshi (1976) published the schematic sections of Kuroko deposits referring to the general geology of this sub-type. The major Kuroko deposits belong to this sub-type. The largest Kuroko deposit is the Doyashiki deposit in the Hanaoka mine which belongs to C sub-type. The total ore quantity may be more than 10 million tons. The second largest deposit of Kuroko deposits is the Motoyama deposit of this sub-type. About 7 million tons of 

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K-Ar ages data on adularia and sericite in the veins and altered host rocks indicate that ages of mineralization vary widely, ranging from 1 Ma to 68 Ma and from 1 Ma to 24 Ma for the Se-type and Te-type, respectively (Tables 1.17 and 1.18). 

Age of mineralization Late Cretaceous-Quaternary Miocene-Present... 

Ages of mineralization in the Hidaka and Kitakami regions may be Cretaceous, considering the ages of associated granitic rocks. 

Kubota, 1991). For example, in the south Kyushu (Nansatsu district), the southwest Hokkaido and the north Hokkaido epithermal Au deposits are hosted by andesitic rocks (calc-alkaline rocks) and ages of mineralization are close to those of andesitic volcanic activity (Watanabe, 1990). 

Miyashita (1995) proposed that the epithermal mineralizations in Hokusatsu district (Kyushu) are related to strata volcanoes in the volcanic depression and not to caldera formation, hy compiling the data on the ages of mineralization and volcanic activities, gravity and geology. He considered that the Hokusatsu volcanic depression zone is an extension of the Okinawa Trough. This volcanic depression started from 5-6 Ma (Kamata and Watanabe, 1985), where bimodal volcanism is found. 

Figure 11.18 Apparent K-Ar ages of minerals from Idaho Springs Formation (Front Range, Colorado, 1350-1400 Ma) in zone subjected to contact metamorphism by intrusion of a quartz monzonite (Eldora stock, 55 Ma). Reprinted from S. R. Hart, Journal of Geology, (1964), 72, 493-525, copyright 1964 by The University of Chicago, with permission of The University of Chicago Press.

Geochronological measurements (isochrone methodology) are based on the radioactive decay of the parent nuclide to the daughter nuclide using the fundamental Equation (8.8) for calculating the ages of minerals. 

Aldrich, L. T., G. L. Davis, G. R. Tilton and G. W. Wetherill Radioactive ages of minerals from the Brown Derby Mine and the Quartz Creek granite near Gunnison, Colorado. J. geophys. Res. 61, 215 (1956). 

Note.—Since this paper was written, one by Professor Barrell14 has appeared, in which the use of the uranium-lead ratio for determining the age of minerals is defended. There are also two papers by Holmes and Lawson,15 and another by Holmes,16 in which the same position is taken. There is evidently room for further discussion of the subject, but as yet I see no good reason to change my own views. 

Variations of the isotopic ratio are found because of radioactive decay of 4°K, which enables the ages of minerals to be determined. The variations also occur due to small fractionations that come about only because of the differing weight of the Ar isotopes, as in the Earth s atmosphere. And variations are found in meteorites owing to differing nucleosynthesis components in the materials from which small meteoritic samples were assembled. 

U-series Constraints on Crystallization Ages of Mineral Populations... 

Moore M. A. and England P. C. (2001) On the inference of denudation rates from cooling ages of minerals. Earth Planet. Sci. Lett. 185, 265-284. 

The typical effects of the earliest stages of "diagenesis" (involving transformations of organic matter, "aging" of mineral components and formation of new equilibria between solid and dissolved species) have been demonstrated by Salomons (1980) with respect to the behaviour of trace metals at the sediment/seawater interface (Table 3-5). 

The Ar-Ar technique is able to achieve higher levels of internal precision than K-Ar dating since it does not depend upon separate absolute measurements but instead requires only the ratios of Ar isotopes and can achieve precision of better than 0.25%. However, external precision and accuracy are affected by the uncertainty in the age of mineral standards, as we will see in the following section. In order to achieve optimum precision in the mass spectrometric measurements, the neutron flux (which affects the magnitude of the J value) must be carefully selected. The flux must be sufficient for precise... 

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