Studies on Alteration Halos and Hydrology
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.
Figures & Tables
This paper consists of three parts. The first is an overview of the geologic history of the Green Tuff region where all Kuroko deposits occur. The second part presents a description of the stratigraphy and an interpretation of the structural and igneous history of the Hokuroku district, the most important Kuroko mining district. The third part is an analysis of the role of submarine calderas in Kuroko genesis.
The sequence and causes of the major geologic events that have occurred in Japan and its vicinity since the Cretaceous are interpreted as follows: (1) an active but shallow-dipping north-northwestward subduction of the Pacific plate under the Asian continent during a period from approximately 130 to 65 m.y. ago resulted in ilmenite series magmatism in the outer zone of Japan, then still a part of mainland Asia; (2) about 65 to 40 m.y. ago, the direction of the subducted Pacific plate changed to westward and the angle of subduction steepened, initiating back-arc spreading in the Japan basin province and migration of Japan away from the Asian mainland until about 30 m.y. ago; (3) during the period 65 to 30 m.y. ago, the basaltic crust created in the Japan basin province was subducted eastward under the Yamato Ridge province, resulting in calc-alkaline and magnetite series igneous activity in the inner zone of Japan; (4) about 25 m.y. ago, the first sea (proto-Japan Sea) was formed in the Japan basin province as a result of the eustatic rise of the sea following cessation of spreading there about 30 m.y. ago; (5) back-arc spreading was active in the Yamato basin province during the period between 25 and 5 m.y. ago, cansing bimodal volcanism and subsidence in the flanking Inner Honshu and Yamato Ridge provinces [the Hokuroku basin (i.e., a Kuroko-bearing basin), Niigata oil field basin, and Akita oil field basin were all fault-bounded, deep (>2,500 m) marine basins created by rapid subsidence of crustal blocks within a few million years around 17 m.y. ago, although Kuroko mineralization and the accumulation of organic matter were not synchronous]; and (6) the dip of the subducted Pacific plate returned to a shallow angle about 5 m.y. ago, causing the cessation of back-arc spreading and the initiation of subsidence of the Yamato basin province and uplift of the flanking Inner Japan and Yamato Ridge provinces. The Green Tuff activity is, therefore, synonymous with the tectonic and igneous activity that accompanied the formation of the Japan Sea and the Japanese islands during the period from ~65 m.y. ago to the present.