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Tetsusekiei, translated literally from Japanese as iron quartz, occurs primarily in the immediate hanging wall of the massive stratiform Kuroko ore and, in places, well within the overlying tuff, There are two forms: bedded, which is predominant, and networks cementing hanging-wall breccia fragments. Bedded tetsusekiei is discontinuous and usually covers an area about twice that of the ores without exceeding a thickness of 20 to 30 cm. It appears to have formed late in the development of the stratiform ores and consists of two contrasting constituents: a predominantly aluminosilicate (chlorite + sericite) tuffaceous or, simply, clastic constituent, and a predominantly chert, hematite, and pyrite or, simply, chemical constituent. Textures, mineralogy, and compositions of the tetsusekiei and related tuffs are consistent with the formation of tetsusekiei by the mixing in various proportions of the chemical and clastic constituents. The major source of iron, sulfur, and silica in tetsusekiei was hydrothermal.

During intensification of the hydrothermal system at the time of sulfide deposition, tetsusekiei formed at low temperatures (<150°C) around and at some distance from zones of main fluid discharge whereas, during declines in hydrothermal activity, it formed in close spatial association with the ore. High temperatures (~300°G) recorded by fluid inclusions in cavity-filling quartz, chalcopyrite disease in sphalerite, and sulfidation of hematite and chlorite, indicate that tetsusekiei was chemically and texturally modified after deposition. Several stages of tetsusekiei formation are envisaged to have occurred during the lifetime of the ore-forming system: Tetsusekiei can provide exploration guides at several different scales: (1) its recognition in the field identifies a possible ore-forming horizon; (2) the alteration index, R = forumla, increases from about 50 to 90 percent at Fukazawa over a distance of about 3 km with a concomitant decrease in oxygen isotope values approaching the ore; and (3) trace elements of exhalative input are present at elevated concentrations. The areal distributions of trace elements with respect to the Fukazawa orebodies are complicated by multiple input sources from scattered hot springs along linear structures and do not provide a simple panacea for exploration.

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