Some Ore Textures Involving Sphalerite from the Furutobe Mine, Akita Prefecture, Japan

Abstract

Observations of doubly-polished, uncovered thin sections of Kuroko from Furutobe reveal a paragenesis of great complexity. Sphalerite crystals up to 2 mm across show finely detailed growth banding, and the crystals occur both as broken fragments in "moved" ore and as later fillings between clasts. The "moved" ore exhibits a variety of sphalerite types (including coarse crystals as well as fine-grained aggregates) as clasts showing that coarse crystals did exist prior to the synsedimentary slumping. Growth of large crystals seems incompatible with rapid deposition on the sea floor, therefore an alternative model is suggested. The initial deposits were derived from submarine hot springs and consisted of very fine-grained sulfides with silica and barite. With con-tinued supply of hot fluid the lower part of the sulfide deposit covering the vent became heated, and solution and reprecipitation occurred below an upper, less permeable "blanket." Deposits formed on slopes slumped repeatedly yielding graded beds with coarse sphalerite and other sulfides incorporated in a melange of finer ore and gangue fragments.
Much of the sphalerite exhibits a fine "dusting" of chalcopyrite that appears to have developed subsequent to the growth of the host sphalerite, probably by a process of replacement, not of exsolution.