Mineralogical Studies of Ores
High-temperature hydrothermal fluids exit from the sea floor on the East Pacific Rise at 21°N. Mixing of the hydrothermal fluids with seawater leads to the precipitation of sulfide and sulfate minerals. These precipitates may become dispersed in the water column or may aggregate to form chimney like structures and basal mounds.
Our study of the distribution and textures of minerals in chimney samples collected during 1979 suggests a relatively simple model of chimney growth. Such growth starts with the rapid formation of a highly permeable wall composed of anhydrite and minor sulfides. Vertical growth rates of 30 cm/day for new chimneys were observed in 1.981 after preexisting chimneys were razed. Mixing of seawater and hydrothermal fluid through this wall causes the precipitation of additional sulfides and sulfates, which further reduces the permeability of the chimney wall. The temperature of the hydrothermal fluid inside chimneys gradually increases, and the fluid composition approaches the composition of fluid in the chimney roots. The mineralogy of chimney interiors changes gradually in response to changes in fluid temperature and composition. As chimney exteriors become increasingly isolated from the interior hydrothermal fluid, their temperature drops and new mineral assemblages are produced by reaction of ambient seawater and the mineral assemblages of the chimney walls. Weathering assemblages similar to supergene sulfide assemblages can develop during reactions of this type.
The process of basal mound formation is less well understood than the process of chimney formation. Stockwork fracturing is characteristic of the footwall of many ophiolite-hosted massive sulfide deposits, which suggests that basal mound growth commences with intense localized fracturing of the sea floor. Fractures produced during such events provide conduits for rising hydrothermal fluids, and sulfide and sulfate mineral precipitates accumulate on the sea floor around the fractures and may form a basal mound. Chimney structures develop around larger openings, while smaller openings clog with precipitates to form a low-permeability crust on basal mounds; hydrothermal circulation beneath such a crust was discovered in 1981. The upper part of the mounds is a transition zone between low-temperature ambient seawater and the high-temperature fluid within the mound interior; this transition zone is probably zinc rich.
The geologic preservation of sulfide-sulfate deposits on the ocean floor requires burial rates that exceed the rate of oxidation and dissolution.
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.