J. Min. Petr. Econ. Geol. 85, 522-530, 1990 Vanadium minerals in siliceous sedimentary rocks from Unuma, Gifu Prefecture, Japan, with special reference to volborthite and roscoelite SATOSHI MATSUBARA, YASUJI SAITO and AKIRA KATO Department of Geology, National Science Museum, Shinjuku, Tokyo 169, Japan Mesozoic sedimentary rocks belonging to the southern zone of Mino Terrane crop out along the norther side of the Kiso River, Unuma, Kagamihara City, Gifu Prefecture. They consist of chert, mudstone and sandstone. In the mudstone are found volborthite and roscoelite in associa tion with cuprite, chalcocite, brochantite, palygorskite and montroseite. Among them, the first and the last one were firstly found in Japan. They are concentrated in and around a bitumen bearing horizon, which is the source of these copper and vanaidum minerals. The chemical analysis of volborthite gives CuO 50. 37, V2O5 38. 33, H2O (calc.) 11.38, sum 100.08%, corresponding to Cu3.00 [(OH)2 | V2,00 O7.00]•E2 H2O. That of roscoelite does SiO2 46.42, Al2O3 11.38, V3O5 26.42, FeO 1.50, MgO 1.32, CaO 0.41, K2O 7.82, H2O (calc.) 4.18, sum 99.45%, yielding the empirical formula (K0.72 Ca0.03) ƒ°0.75 (V3+1.52 Al0.29 Mg0.14 Fe2+0.09) ƒ°2.03 [(OH)2 | (Si3.33 Al0.07) ƒ°4.00 O10], characterized by the low content of total interlayer cations. Copper and vanadium were originally contained in the bitumen, which liberated them perfect ly during the diagenetic process to form such copper and vanadium minerals of lower valencies as chalcocite, roscoelite and montroseite, then cuprite was formed. The formation of volborthite owed the origin of copper and vanadium to chalcocite and probably to montroseite, respectively. rock of bedded manganese ore deposit of the Introduction Kurisu mine, Aichi Prefecture situated nearby There are some examples of concentration the present locality contains minor vanadium of vanadium in older sedimentary rocks in (Hamachi and Sakamaki, 1961). In chert-dom Japan. The most notable ones are represented inant sedimentary rocks exposed near the mine by such vanadium minerals in metamorphosed is found a thin bitumen layer in mudstone. bedded manganese ore deposits or manganese The bulk analysis of the material including rich metamorphic rocks as haradaite bitumen and brochantite gave vanadium up to (Watanabe et al., 1982), suzukiite (Matsubara et 0.64 wt.% V2O5 (Watanabe, pers. comm.). Geo al., 1982), nagashimalite (Matsubara and Kato, logical and mineralogical surveys of this time 1980), roscoelite (Matsubara, 1985), ardennite on the area near the mine led to the find of (Enami, 1986; Matsubara and Kato, 1987), gold volborthite, roscoelite and a copper and va manite (Momoi, 1964) and its manganese ana nadium-free bitumen. Volborthite and the logue (Hirowatari and Fukuoka, 1988). associated roscoelite were subjected to the Besides them, spessartine, jacobsite and titanite chemical and X-ray powder studies. Also, in contact metamorphosed bedded manganese such secondary copper minerals as chalcocite, ore deposits contain minor vanadium (Ma cuprite, brochantite and malachite are found in tsubara and Kato, 1986a, 1986b). Also, a wall fractures of the sedimentary rocks together (Manuscript received, Augast 20, 1990; accepted for publication, September 10, 1990) Vanadium minerals in siliceous sedimentary rocks 523 with some fissure minerals like chlorite and The thickest parts of the bitumen reach a half palygorskite. centimeter and the intermittent pieces are The present work is for the description of traceable more than a few meters within the volborthite as the first occurrence in Japan and grey part of the mudstone surrounded by red of the associated minerals represented by ros one, suggesting that the presence of bitumen coelite, and for the consideration of the geneses invited the reduction of the hematite imparting of vanadium minerals. the red colour. We express our sincere gratitude to the Volborthite is found along fractures devel late Dr. Takeo Watanabe, for his information oped in and near the bitumen-bearing horizons on the occurrence of bitumen at the locality. in the mudstone as thin bright yellow to green Also, we are greatly indebted to Dr. Saburo ish yellow filmy coatings, botryoidal colonies, Chii, who helped our field works. greyish yellow green rosettes, grey green tables, yellow stains in palygorskite fibers, and Occurrence a brilliant yellow lining material of thin quartz The locality is situated about 1.5km east veinlets. Coatings are all thin and rather of Unuma Station, Takayama Line of JR Tokai shredded and nearly monominerallic, except for Company (Fig. 1), where Mesozoic cherts, mud microscopical stains of goethite. Botryoidal stones, and sandstone and nodules of rhodo colonies are composed of powdery material or chrosite are exposed. The geology including very minute rosette aggregates of tablets. the locality point is described by Kondo and They are over- and underlain by powder or film Adachi (1975). The cherts are composed of of brochantite. Grey green tablets of volborth white, grey and red ones and rather well bedded ite are devoid of yellowish colour unless scrat with a few centimeter thickness. The mud ched. The most typical brilliant yellow colour stones include red and grey ones, and the latter of volborthie is seen as stains in palygorskite comprises bitumen accompanied by copper and fibers and lining material of quartz veinlets. In vanadium minerals here described. Rhodo Fig. 2, one of the representative forms of vol chrosite nodules are grey white spheroid borthite observed under the scanning electron involved within grey chert. The maximum microscope is demonstrated. dimension reaches 30cm. A part of grey mudstone involves thin black bitumen with brilliant fracture surfaces. Fig. 2. A representative form of volborthite occur Fig. 1. Index map of the locality point (cross ring as a crust near bitumen. Bar indi marked). cates 10ƒÊm. 524 Satoshi Matsubara, Yasuji Saito and Akira Kato Roscoelite is concentrated in bitumens with parallel fractures as if it were a subparallel aggregate of prismatic mineral, where the col onies of volborthite are also developed. Under the microscope films of volborthite are composed of aggregates of very fine grains as well as the stains in palygorskite fibers. Some tablets have linear outlines in part, where the grain size reaches the order of 0.n milli meter. It is greenish yellow in colour without any notable pleochroism. Roscoelite is found as microscopical clots Fig. 3. Montroseite (needle-like) enclosed within roscoelite aggregates (dark gray). Bright in fractured bitumen finely disseminated by spots or their aggregate is chalcocite. The quartz, chalcocite and minor illite. The fea darkest area (left under) is bitumen. Bars tures of fractures are very variable, such as indicate 10ƒÊm. cell-like, birds-eye-like and parallel textures even in one thin section. Fractures are inter found, the maximum dimension being a few sticed by quartz with a twin-like linear bound millimeter across. It is commonly surrounded aries, which are perpendicular to walls, or its by brochantite films. Also, very minute moss curved derivatives. The clots of roscoelite like cuprite is found beneath films of volborth reaching a millimeter across are developed in ite. On account of its stained surface, it is fracture crossings or in minor fractures and noticed with difficulty. finely disseminated by opaque mineral re Most of light green films and colonies are presented by chalcocite. And, neither quartz brochantite, but if it is found in remote places nor bitumen is disfigured around the roscoelite from the bitumen-bearing horizon, it consists of clots, except for the later intersection by the malachite or the mixture with brochantite, aggregate of quartz and illite. though the colour of mudstone is still grey and Also the colours of the present roscoelite in minor volborthite is accompanied therewith. thin section is markedly different from those of The secondary minerals stated below are the known roscoelite. That is, the present all from such grey mudstone parts. Goethite material has light brown colour and the pleo occurs as brown films and fracture-filling veins, chorism is very weak with axial colours rang and even in fractures of bitumen itself. ing from very pale to light brown, whereas the Brownish yellow films developed along the pleochroic colours of known roscoelite are fissures parallel to the bedding are composed of from olive-green to greenish brown (Fleischer jarosite or natrojarosite after the X-ray pow et al., 1984). der study, the latter being less frequent and In association with above roscoelite and accompanied by palygorskite. A white to quartz, very minute needles forming a point colourless coating on weathering surfaces of radiating texture are found (Fig. 3). As stated grey mudstone is identified as gypsum. The later, the chemical composition corresponds to occurrence of this coating is more frequent in that of montroseite, in which a part of V3+ is grey mudstone rather remote to bitumen, where substituted by Fe3+. In the bitumen-bearing the veinlets of calcite are common. Therefore, horizon, minute grains of chalcocite are visibly a part of calcium forming gypsum came from Vanadium minerals in siliceous sedimentary rocks 525 calcite of this occurrence. The microscopic observation of mudstone Chemical analyses of vanadium, copper and devoid of bitumen informs that the essential manganese minerals constituents are illite and quartz, the latter Chemical analyses of vanadium, copper forming radiolarian remains. Besides them, and manganese minerals were made using minor albitic plagioclase is detected after the Links Systems energy-dispersive X-ray X-ray powder study. spectrometer. The results given Table 1 are The rhodochrosite nodules consist of for volborthite, roscoelite, montroseite, chal rhodochrosite, quartz and apatite after the cocite and rhodochrosite. Volborthite chemical study. The apatite is amorphous to analysed were made on crystal and films, but X-ray. The last two phases are interstitially both of them are essentially identical and corre present among microspherulitic bodies of spond to the ideal figures.