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Hydrothermal alteration halos associated with the formation of volcanogenic massive sulfide deposits were examined in terms of their size, chemical composition, mineralogy, and oxygen isotopic composition. Selected examples are: (i) Uwamuki deposits, Kosaka mine, northeast Japan (Kuroko); (ii) Seneca prospect, southwest British Columbia, Canada (Kuroko type); (iii) South Bay mine, northwest Ontario, Canada (Archean volcanogenic massive sulfide); and (iv) Corbet mine, northwest Quebec, Canada (Archean volcanogenic massive sulfide).

Substantial mineralogical zoning is observed within thin Miocene footwall volcanic rocks of rhyolitic composition beneath the Uwamuki deposits. From core to margin the sequence is quartz + sericite; sericite + chlorite + quartz; remnant albite + sericite + chlorite + quartz; and kaolinite + quartz + sericite ± chlorite ± albite zones. Whole-rock δ 18 O (SMOW) values decrease gradually toward the center from 8.7 to 10.4 per mil in the kaolinite zone to 6.7 to 8.6 per mil in the quartz + sericite zone. Estimated temperatures of formation of these zonesare 210° to 250°C and 240° to 310°C, respectively. The. mineralogical zones have formed by lateral migration of hydrothermal solution which had ascended along the maiu discharge conduit. Impermeable Permo-Triassic chert and phyllite basement which unconformably underlies the volcanic rocks is virtually free from hydrothermal alteration (δ 18 O (SMOW) = 8.2-19.0‰).

The mineral assemblage of the footwall tuff breccia at the Seneca prospect is quartz + chlorite ± albite ± sericite ± apatite ± epidote ± calcite ± K-feldspar ± maguetite ± sphene ± rutile. There is no apparent mineral zoning except for a laumontite-bearing zoue more than 1,200 m away from the orebody. On the other hand, whole-rock oxygen isotope composition and the ratio Fe/(Fe + Mg) in chlorite change systematically from δ 18 O (SMOW) = 6.9 per mil and Fe/(Fe + Mg) = 0.3 at the center of the mineralization to 11.6 per mil aud 0.6 at the margin. There is only very weak hydrothermal alteration withiu an impermeable lava flow unit which underlies the tuff breccia.

Identical alteration zoning in terms of whole-rock δ 18 O values and chlorite compositions were observed in the flow top breccia of the footwall andesite lava of the Corbet deposit where δ 18 O decreases inward by about 7 per mil and the Fe/(Fe + Mg) ratio of chlorite decreases by 0.3 over a distance of about 1,500 m. In contrast, no oxygen isotope variation is observed in the rhyolite lava dome beneath the massive sulfide orebody of the South Bay deposit. However, both mineral assemblages and mineral compositions show remarkable lateral zoning for more than 1,000 m away from this deposit in response to outward increases in pH of the solution and in Fe/(Fe + Mg) ratios of ferromagnesian minerals during alteration reactions. Replacement of primary(?) ilmenite by rutile aud sphene uear the orebody was observed at both of these Archean volcanogenic massive Sulfide deposits.

Many lines of evidence suggest the existence of contemporaneous high-level felsic plutons below many Kuroko and Archean deposits. These plutons are regarded as being indispensable as the source of metals and of heat to drive the geothermal systems. The overall similarities in the chemical, mineralogical, and isotopic characteristics of the studied footwall-rock alteration suggest that the genesis of volcanogenic massive sulfide deposits has uot changed appreciably for more than 2,700 m.y.

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