In the Chugoku district, Southwest Japan, wolframites occur mainly in veins in and around the Late Cretaceous to Early Tertiary granitic rocks. In the present paper, the mineralogical characteristics and regional variation in Fe/Mn ratios of the wolframite series from 27 localities in the district, together with those from some other regions are discussed in terms of the lattice parameters and mineralogical association.
The relationship obtained between chemical composition and the values of a₀, b₀, c₀ and β agree mostly with the previous results. Among them, it is worthwhile to note that with the exception of Mn-rich member, the B value almost remains constant, conflicting with the previous studies. In calculation, however, B can also change linearly with variation of the composition accompanying with the linear change of the values, d₀₁₁-d₁₁₀ and/or quartz (102)-wolframite (200).
The unit cell volume of the series expands with the increasing substitution of iron with manganese with or without other cations. The lengthening of a₀, b₀, and c₀, especially of the former two is responsible for the expansion. The compositional distribution of the natural wolframite series, characterized by the relatively rare occurrence of Mn-rich member, may be indicative of the degree of difficulty in the Fe/Mn substitution and the possibility of the presence of immiscibility gap in the series.
As was pointed out by Sasaki (1959), the natural wolframites deviate slightly from the relationships between composition and unit cell parameters obtained with synthetic compounds. This is best explained by the influence of some cations such as Nb and Y other than Fe and Mn in the crystal structure of the mineral, supporting the conclusions reached by the previous workers. In this connection, the presence of 1 per cent order of Nb in Mnrich wolframites from the Sekigane mine may give an evidence in support of this.
Regional variation of Fe/Mn ratios in the wolframite series from the Chugoku district is examined in relation to the granite province with which the wolframite deposits are intimately associated. There seems to exist a tendency, though not so remarkable, that wolframites in the granitic rocks of the Sanin-Shirakawa Zone, characterized by relatively higher oxidation state, are rich in manganese, while wolframites in those of the Sanyo-Naegi Zone, characterized by relatively lower oxidation state, are rich in iron. All the samples with composition close to huebnerite end member came from the manganese deposits or Paleozoic formation nearby or rich in manganese deposits. Sasaki (1960) also reached the same conclusion.
Thus, it is concluded that composition of the wolframite series may vary according to the bulk chemical composition of surrounding rocks through which the ore fluids migrated and to the oxidation state of the ore fluids and perhaps of related granitic magma. Other possible factors to affect on the Fe/Mn ratios are physicochemical ones such as temperature and acidity of the ore fluids.

View full abstract

Nickel-bearing pyroaurite-sjögrenite was found in a brucite serpentinite mass in the Sambagawa metamorphic terrane at Yoshikawa, Aichi Prefecture, Japan, showing various modes of occurrence, such as vein-, nodule-, crusty-, platy- and earthy-types. Optical properties, X-ray diffraction, DTA-TG and IR patterns vary with each other depending on the modes of occurrence.
Electron probe microanalysis gave variable contents of nickel for four portions of two flaky crystals in the sample of crusty-type.
Clear correspondences among optical properties, basal spacings and DTA patterns were observed, showing that the mineral with larger basal spacings possesses smaller refractive index ω as well as lower peak temperature in endotherm at low temperature region. Splitting of the basal reflections suggestively results from mixing of two species of pyroaurite-sjögrenite, corresponding to the optical observation.

View full abstract

K-Ar age determinations were carried out on twelve samples of various acidic rocks (six volcanic rocks, two pyroclastics and four granitic rocks) which were obtained from the western part of Aizu district. The district studied is one of the important acidic petrographic provinces in the Green tuff region of Northeast Japan, and is widely covered by the acidic volcanic rocks and pyroclastics of Neogene period.
The ages of six volcanic rocks range from 8 to 23m. y., and they are generally correlated to the stratigraphic units of the Neogene in Northeast Japan. Dating results on four granitic rocks from the Tagokura granitic body showed the age range of 39 to 65m. y., corresponding to the Late Cretaceous to Eocene. A sample of dacitic welded tuff from the Miyako River area gave an age of 44m. y. It is pointed out that the welded tuff may be correlated to the Late Cretaceous to Paleogene acidic igneous rocks such as Nohi rhyolites, Asahi rhyolites, Tagawa acidic rocks and others, on the basis of the age and lithofacies of the rock. However, further geological and geochronological data are necessary to settle the problem.

View full abstract