MINING GEOLOGY, 28, 293•`300, 1978 Some Ore Textures Involving Sphalerite from the Furutobe Mine, Akita Prefecture, Japan* Paul B. BARTON,Jr.** 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. revealed in my small collection , are startling Introduction and, even more surprising, apparently have not In recent years the Miocene Kuroko deposits been described in the voluminous and other- of Japan have become recognized almost wise comprehensive Japanese literature. universally as synsedimentary, hydrothermal Perhaps the reason for this lack is that most, deposits formed essentially at the seawater- if not all, previous workers relied entirely on sediment interface (TATSUMIand WATANABE, standard thin sections, polished sections, and 1971; ISHIHARA,1974; LAMBERTand SATO, hand specimens, whereas I used uncovered, 1974). They appear to be prototypes for doubly-polished thin sections. Sphalerite has an many stratabound massive sulfide ores found index of refraction of about 2.4, but bonding in older rocks throughout the world. Many media for thin sections usually have indices careful studies of the Kuroko deposits have of refraction in the 1.5 to 1.6 range. There- revealed significant features regarding these fore, unless both the upper and lower surfaces fascinating ores, but not all questions are yet are polished, so much light is scattered from satisfactorily answered. Previous studies specifi- the rough sphalerite surfaces that very im- cally of the Furutobe mine include: TAGUCHI portant features go undetected. In our labora- and LU (1966). TANAKA and LU (1969), tory we use a single microscope set up for and TANAKAet al. (1974). simultaneous reflected and transmitted light, In 1970 I had the opportunity, through and control the illumination by means of the kindness of the Mitsubishi Metal Corpora- separate, foot-operated switches. The color tion, to visit briefly and collect a few specimens plates show typical kuroko from Furutobe as from the Furutobe mine. Some of the textures paired reflected and transmitted light photo- graphs. The wealth of information available * Received April 7, 1978. to viewers of doubly polished sections is ** U .S. Geological Survey, National Center, Reston, apparent from the figures. I will discuss here Virginia, U.S.A. 22092 Keywords: Sphalerite, Ore textures, Kuroko deposits, only some of the more spectacular textures Diagenesis, Chalcopyritization revealed in these samples. 293 28(4), 1978 Some Ore Textures Involving Sphalerite from the Furutobe Mine 295 discontinuous record that would be far more Description of Textures tedious to reconstruct than my collection of The observations described below were specimens warrants. Another potential diffi- made principally on sphalerite. Other minerals culty is the probability that any single stratum may also possess informative textures, but of sphalerite may undergo facies changes as only sphalerite has the broad range of chemical it is traced through the ore zone. and optical properties that make a study The different varieties of sphalerite can such as this one possible. occur in close juxtaposition in such a way The crystal size of sphalerite can be large. that they must have originated separately Plates I-1 and II-1 show crystals as much as and been brought together physically by the a millimeter across, and other crystals as slumping process after crystallization (note much as 2 millimeters across are present in especially Plate I-2). A very significant fact my samples. Considerably larger crystals have is that coarse, well-crystallized sphalerite been reported (see ISHIHARA, 1974). occurs both in clastic fragments (Plate I-4) A wide variety of sphalerite is present, as is and as open-space fillings between fragments. shown in the illustrations. In particular, the The coarse sphalerite in the clastic fragments transmitted-light photographs reveal strikingly presumably formed in some earlier open contrasting characteristics, even for materials space. that appear superficially similar in reflected Some of the sphalerite clasts appear to be "primitive" (Plates I light. -1 and I-2) because Many previous studies (see ISHIHARA, 1974) they show little growth morphology, appear have clearly recognized extensive intradeposi- to be made up of many small crystals, and tional slumping that creates the clastic contain numerous ragged inclusions of galena, "moved" ore and also develops the widespread pyrite, chalcopyrite, and tetrahedrite. These graded beds of ore and gangue, with or without may be the closest thing to the initial, wallrock components. When viewed at even sphalerite-bearing sediment preserved in my lower magnification than illustrated, e.g., samples. 10 •~ or less, the different types of sphalerite Although no new measurements have been serve to identify different clasts of sulfide ore. made in the course of this study, it is likely Many of the clasts are also recognizable by that many of the fluid inclusion studies reported distinctive mineral associations with or with- in the literature for the black ore (for an out sphalerite. A complex, fragmented excellent summary see TOKUNAGAand HONMA, "stratigraphy" of intricately banded sphalerite 1974) were based on the coarse, recrystallized is displayed in the Furutobe samples, but the sphalerite and that the coarse sphalerite in clastic nature of the samples produces a the clasts was not distinguished from the Plate I. Photomicrographs of sphalerite from the Furutobe mine. Each microscope field is shown in transmitted (A) and reflected light (B). Sphalerite is medium grey in reflected light and is colorless to yellow to brown to red in transmitted light. In transmitted light chalcopyrite is yellow, pyrite is pale yellow, galena is white, and tetrahedrite is an intermediate grey between galena and sphalerite. Barite and quartz are dark grey in reflected light and colorless in transmitted light. Plate I-1 : Two stages of sphalerite; on the left is an example of the 'primitive' type (width 2 mm). Plate I-2 : Two additional stages of sphalerite; on the left is an example of the 'primitive' type. The crystal on the right is intensely altered along its edges by the 'chalcopyrite disease' (width 0.5 mm). Plate 1-3 : Multiple generations of sphalerite. The grain in the upper-right-center shows complex development of the 'chalcopyrite disease' that may be related to regrowth of a fractured crystal prior to the addition of chalcopyrite (width 2 mm). Plate I-4 : A fractured fragment of growth-banded sphalerite is overgrown by sphalerite that is so full of inclusions of other sulfides that it is opaque. The red banding is typical of that associated with fine tetrahedrite (width 0.5 mm). 28(4), 1978 Some Ore Textures Involving Sphalerite from the Furutobe Mine 297 coarse sphalerite filling vugs between clasts. under attack. It appears to be a reaction of It would be surprising if any of the "primitive" copper in solution with FeS in sphalerite and sphalerite had been studied for inclusions, as may be associated in time with the replacement such sphalerite is usually too fine grained and of pyrite by chalcopyrite, but documentation too opaque for good optical examination (see of this relation requires more work. Some of Plates I-1 and 1-2). the sphalerite-chalcopyrite features are alll Much of the sphalerite suffers from the too easily confused with exsolution textures, "chalcopyrite disease" , a feature common but I have found no chalcopyrite exsolved from to many hydrothermal deposits (P. M. BETAKE sphalerite at Furutobe. and P. B. BARTON, Jr., unpublished research). Another significant feature is the presence This malady has several symptoms, the most of hydrothermal leaching and regrowth of common of which is the development of a fine sphalerite (Plates II-1, II-2). This phenom- "dusting" of •… 1 micrometer-sized chalcopyrite enon is common in conventional hydrothermal inclusions in solid sphalerite. It begins along ores, but was a surprise in the black ore part grain boundaries and cracks and spreads into of a Kuroko deposit because its mode of the adjacent crystals (Plates I-2, II-1, II-2). deposition (TATSUMIand WATANABE,1971; Eventually, it may spread sufficiently to make SATO,1972) suggests rapid, irreversible precipi- the entire grain appear