"Brown ore" from the Fukasawa kuroko deposits, northeast Japan: Its characteristics and formation process

Brown-colored sulfide ore (brown on) occurs in the easternmost part of the Tsunokakezawa No. 1 orebody of the Fukasawa kuroko-type deposits, northern Honshu, Japan. As this type of ores also occur in the marginal or uppermost part of several other kuroko deposits in Japan, the formation of brown ore appears to be repeated in the process of kuroko formation. The brown ore i s characterized by its higher Ag concentration (up to around 2000 g/t) than ordinary black ore (Zn-Pb ore) of volcanogenic massive sulfide deposits. T he brown ore from the Fukasawa deposits can be divided into following three ore types based on its texture and mineral composition: pyritic brown ore, principal brown ore and "diseased" brown ore. Primary precipitation textur es such as framboidal- and colloform-textures and compositional zoning with in sulfide grains are significant in the brown ores. This seems to be due t o lack of overprinting high temperature mineralization resulting in preserv ation of primary features. The Ag-Au mineralization is widely observed with in the brown ores. Silver and gold are especial ly concentrated in the bari te veinlets in the principal brown ore, which are supposed to be fillings o f conduit of hydrothermal solution precipitated in the latest stage of hydr othermal activity. This mineralization seems to occur at waning stage of br own ore formation by ore solution at a lower temperature (around 250 degree s C) than that of main part of brown ore (around 270 degrees C). Relatively low fluid temperature and contribution of oxic ambient seawater may be res ponsible for the development of the Ag-Au mineralization in the brown ore. The occurrence of framboidal-rich pyritic brown ore having negative delta(3 4)S values (less than -10 parts per thousand) and filamentous texture of sp halerite, seeming remnant of bacteria, indicate the presence of intensive m icrobial activity in the hydrothermal area for brown ore formation. Formation environment of each ore type of the brown ore is supposed to be a s follows: Pyritic brown ore is likely to have formed on the sea-floor arou nd redox boundary at temperature (around 240 degrees C) lower than ordinary black ore. Principal brown ore seems to have been formed beneath the shell of the pyritic brown ore at temperature around 270 degrees C. Footwall of the brown ore is disseminated tuff breccia corresponding to feeder zone of hydrothermal fluid. Overprinting chalcopyrite mineralization is not observe d in the brown ore except in limited part of "diseased" ore, which occurs j ust above the disseminated tuff breccia. Based on the features distinct from the ordinary black ore, the brown ore c an be regarded as a product in the marginal part of submarine hydrothermal system, where temperature and flow rate of hydrothermal solution was relati vely low and microbial activity was intensive. The brown ore seems to well preserve its primary features after its deposition and might show the initi al feature of some part of the ordinary stratiform black ore.