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The variation of Na 2 O in the acid lava underlying Kuroko deposits has been examined as a convenient exploration guide. First, the precise spatial distribution of the low Na 2 O anomaly around certain Kuroko deposits was established using a large number of samples collected systematically from the footwall acid lava. Then the applicability of using the anomaly for delimiting promising areas around the Kosaka and the Fukazawa deposits was examined. The following results have been obtained. 1. The most economical threshold can be set at 0.36 percent Na 2 O in the Kosaka area and 0.32 percent Na 2 O in the Fukazawa area. The small difference in the Na 2 O value of the thresholds is compatible with previous results. 2. The areal distribution of the low Na 2 O anomaly, however, is somewhat different in extent and shape from that presented previously. The difference in distribution is probably due to a different interpretation of high Na 2 O masses near the footwall of orebodies; namely, they are. regarded as either unaltered remnants or as intrusions. 3. The general areal extent of the anomaly in this study is two or three times as large as the presently known extent of ore and has a high correlation with small offsets of ore distribution. The variation of Na 2 O in the footwall is particularly useful for detailed exploration, but the extent may be too small for regional explorations.

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