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Oxygen isotope analyses of 151 samples of Miocene volcanic and sedimentary rocks from a 20 × 15-km area of the Hokuroku district, Japan, show a consistent relationship between alteration assemblages and δ 18 O values of whole rocks. Excluding quartz-rich sandstones, which are resistant to hydrothermal alteration, the δ 18 O values of all types of igneous rocks (dacite, andesite, basalt) and sedimentary rocks (tuff and mudstone) lie within the ranges of 16.9 ± 2.7 (1 σ value) per mil in the zeolite zone, 11.1 ± 2.5 per mil in the montmorillonite zone, and 6.7 ± 1.3 per mil in the sericite-chlorite zone. Essentially, all the Miocene rocks in the Hokuroku district are altered to zeolite or higher grade zones.

Concentric zoning patterns of the whole-rock δ 18 O vaines and of alteration minerals are particularly well developed in the footwall volcanics around the Fukazawa Kuroko deposit. The δ 18 O values of the whole rocks gradually increase from less than 8 per mil in the sericite-chlorite zone that extends about 0.5 km outside of the ore zone, to 8 to 14 per mil in the montmorillonite zone, a 1- to 3-km-wide zone occurring outside the sericite-chlorite zone, to mostly greater than 14 per mil in the peripheral zeolite zone.

In the hanging-wall rocks above the Fukazawa deposit, δ 18 O and mineralogical zonations extend at least 400 m above the ore horizon (i.e., to the present-day surface), but these zones gradually contract upward. Such features together with the radiometric ages of the igneous rocks suggest that the Fukazawa mine was the site of discharge of high-temperature (T > 200 º C) hydrothermal fluids for a period of over 3 million years, although" the hydrothermal activity may have been episodic and decreased in intensity with time.

Comparison of the spatial variation of δ 18 O values of the footwall rocks in the Fukazawa area with that of Na, K, Mg, Ca, Sr, Cu, Pb, Zn, and S contents and of the magnetic susceptibility shows that the areal extent of the δ 18 O anomaly is mnch larger than that of other geochemical anomalies, and that the variability of δ 18 O values within a site is much less than that of the elemental compositions. This suggests that the analysis of whole-rock δ 18 O values is a useful method of exploration for volcanogenic massive sulfide deposits (and also for vein deposits occurring in submarine volcanic sequences) during the reconnaissance stage whereas other geochemical methods are more suitable in the developments, tage. In fact, 18 of 21 drill sites where low δ 18 O (<8‰) rocks were discovered in this study are located within 1 km of known Kuroko-type or vein-type deposits. Analyses of whole-rock δ 18 O values are also useful in the exploration for these types of deposits in metamorphosed and deformed terrains, because the oxygen isotope zoning is less likely to be destroyed than the alteration mineral zoning during regional metamorphism.

The δ 18 O values of the altered volcanic rocks around the Knroko deposits and their correlation with alteration assemblages can be interpreted as a result of interaction between the rocks and seawater at different temperatures (25°-200°C for the zeolite zone, 150°-300°C for the mont-morillonite zone, and 200 o -400°C for the sericite-chlorite zone) under water-dominated (i.e., water/rock > 1) conditions. This interpretation also explains most of the hydrogen isotope data of the whole-rock samples (δD = —34 to — 80‰) from the Hokuroku district.

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