Mineral Textures and Their Bearing on Formation of the Kuroko Orebodies
C. Stewart Eldridge, Paul B. Barton, Jr., Hiroshi Ohmoto, 1983. "Mineral Textures and Their Bearing on Formation of the Kuroko Orebodies", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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The widely accepted depositional model for Kuroko-type deposits postulates precipitation of most ore minerals on or above the sea floor from plumes of hydrothermal fluids and simple superposition of ore minerals to accumulate stratified massive ore piles; syndepositional slumping is commonly recognized. Our observations of ore textures using doubly polished thin sections suggest that most of the sphalerite, pyrite, galena, tetrahedrite, barite, and quartz in the deposits once formed in the open ocean water but were recrystallized when insulated in the middle and lower portions of the accumulating ore pile. Furthermore, most of the chalcopyrite in the lower part of the massive ores formed by replacing the other sulfides.
The depositional history for Kuroko-type deposits is divisible into five stages, all of which may have taken place nearly contemporaneously but at different sites within the accumulating sulfide deposits: (1) precipitation of primitive or facies 1 (black ore) minerals which are fine grained (<50 µm in size) and often colloform; sphalerite, galena, pyrite, tetrahedrite, barite with minor chalcopyrite, and quartz at the sites of mixing of hot hydrothermal fluids with cold seawater (i.e., on or near the sea floor); (2) resolution of facies 1 minerals by hotter hydrothermal fluids, resulting in the formation of coarser grained facies 2 minerals in the lower parts and the reprecipitation of facies 1 minerals in the outer parts of growing orebodies; (3) introduction of hotter and Cu-rich hydrothermal solutions into the ore pile, resulting in the replacement of facies 2 sulfides by facies 3 chalcopyrite (forming yellow ore) in the lower parts of orebodies and the migration of facies 2—and facies 1—zones outward; (4) introduction of hotter (and undersaturated with chalcopyrite) fluids into the ore pile, resulting in the dissolution of chalcopyrite and formation of pyrite ores in the lowermost part of the ore pile and migration of chalcopyrite, facies 2, and facies 1 ores upward and outward of the ore pile; and (5) precipitation of tetsusekiei (chert-hematite) ores on top of the massive ores.
Some deposits experience only the stages 1, 2, and 5, thus leading to the variation in the metal ratios among deposits. In most deposits, mechanical disruption of the ores takes place throughout all stages of ore formation, resulting in ores with spectacular clastic textures.
After sediments and volcanic rocks have covered the deposit, the waning stages of the thermal system heat and alter the hanging-wall rocks. During the subsequent heating stages, there occurs a local partial reequilibration of the sulfides so that some of the heterogeneous assemblages (such as mixtures of high and low sulfur copper mineral assemblages) are able to react, the only unreacted remnants being those encapsulated in pyrite or quartz.
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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.