The Furutobe mine, known as one of typical kuroko type deposits, started its production in 1963 after 1.8 million tons of ore reserve was confirmed by more than 100 surface diamond drilling holes.
For the first decade of operation, exploration effort was rewarded by the discovery of additional orebodies in the vicinity of original main orebodies. But when it comes to the second decade, exploration was already limited to small area which was left by a number of drill holes, grid interval of which was only 43 meters and the potentiality looked less promising. However, this detailed exploration both by diamond drilling and tunneling based on new geological concept turned out to be successful by discovering unexpected and small but high grade orebodies.
The followings are the principal geological concepts which will be referred case by case in this report.
1. Reconstruction of fault displacement of post ore nature.
2. Detailed examination of the occurrence of dyke rocks such as dacite and dolerite which intruded the orebody.
3. Examination of detailed succession of ore horizon and tracing of this horizon.
4. Re-examination of white dacite of the footwall.

View full abstract

Amphibole asbestos, crocidolite (riebeckite) and amosite (grunerite) can be abundant in Precambrian iron-formations. The phase relations involving these amphiboles are evaluated in the system Na-Fe-Mg-Si-O-H on the basis of reported mineral assemblages from various rock types and available thermodynamic data. The phase relations may be divided into two types: 1) low-to medium-temperature (150°C-450°C) and 2) hightemperature at about 500 (±50)°C. The lower temperature stability relations are most applicable to the occurrence of amphibole asbestos. These relations involve three invariant points defined by assemblages riebeckite + grunerite (minnesotaite)+quartz+magnetite, riebeckite + quartz + magnetite + hematite, and riebeckite + acmite + quartz + hematite. With decreasing XRKFEof riebeckite the higher temperature assemblages approach those of low-to medium-temperature. The phase relations suggest that crocidolite may be replaced by amosite when there is no effective barrier against the possible escape of alkali-bearing solutions from the rock system. Similar replacement would also be promoted by increasing temperature. Some textural observations of the two asbestos minerals seem to support such replacement by amosite.

View full abstract

The purpose of this paper is to describe geological modes of occurrence of silica and silicate minerals in the Shakanai kuroko deposits and thereby to provide some additional constraints in studies of the kuroko mineralization. Some of our observations and conclusions are as follows:
(1) Silica and silicate minerals in the deposits are mostly composed of quartz and a variety of clay minerals. Their total contents in the economically valuable portions of the deposits are estimated to be 15 to 25 percents in weight.
(2) They are in part derived evidently from some altered rocks in the deposits originating from mudstone, dacitic pyroclastics, dacitic hyaloclastites and other exotic breccias. Contribution of these altered rocks is, however, comparatively insignificant, being estimated to be less than 10 percents in weight.
(3) The rest of quartz are derived from "cherty rocks" as well as from "siliceous ores". On the other hand, a majority of clay minerals are derived from certain peculiar "clayey rocks", which are interspersed among ore-lumps in the deposits. The "clayey rocks" are rather homogeneous in structure and show no distinct textures indicative of the pre-existence of mudstone, lavas and pyroclastic rocks. At least some of the silica and silicate minerals constituting the "cherty rocks", "siliceous ores" and "clayey rocks" are suggested to be closely related in origin to the kuroko mineralization.
(4) All the rocks in the foregoing are emplaced definitely within the kuroko deposits. This fact strongly suggests that the different kinds of rocks were all supplied and deposited within the period of kuroko mineralization. In other words, the duration of kuroko mineralization is suggested to have been long enough for the emplacement of all the rocks in the deposits.

View full abstract

Recent success in the exploration and development of the epithermal gold deposits in the western United States, stimulated the study of transportation and deposition of gold and other metals at shallow depths, combined with the study on the metal contents of geothermal waters. The Nansatsu-type disseminated gold deposits occur in "mushroom"-shaped highly silicified bodies which accompany hydrothermally altered minerals by acid solutions such as alunite and kaolinite, and seem to correspond to the so-called "hot spring type" gold deposits. Until recently fluid inclusion data of these deposits were almost none, because of the fine size of fluid inclusions, but a few data on fluid inclusions were obtained from the Kasuga and Iwato deposits of the Nansatsu-type, and the physicochemical condition of fluids was considered based on these data. Temperature of fluids generally distributes in a range between 120°and 275°C and salinity is low ranging from about 0.5 to 2.0 wt% NaCl equivalent. The temperature range of the Kasuga deposit is apparently higher than that of the Iwato. This difference is probably owed to the difference of the size of orebody and the location of samples. In most of the samples, the coexistence of gaseous and liquid inclusions is observed, suggesting the remarkable boiling phenomena. Taking into account of this fact, the pressure of fluids is estimated to be 10-40 bars.

View full abstract

The distribution of minor elements (Co, Ni, Mn, Cr, Ag, In, Bi, Cd, Sb) in pyrrhotites from the Simokawa Mine, copper deposits in Hokkaido, has been investigated to conduct geochemical exploration using sulfide minerals. 49 pyrrhotites were separated from various representative parts of the deposits and host rocks, and were analyzed by atomic absorption spectroscopy. It is observed that Ni, Cr and Mn are comparatively enriched in pyrrhotites from the host rocks and Co in pyrrhotites from the ores. These effects suggest that geochemi-cal dispersion patterns of Co, Ni, Cr and Mn in pyrrhotites clearly reflected a restricted mineralization. Fairly high anomalous concentrations of Ag, In, Bi, Cd and Sb were not found in the minerals.

View full abstract

Calcareous nannofossils were identified from some drilling cores of the southern Shakanai mining area. The result is as follows.
1. The M3 mudstone is correlated to CN4 of Coccolith biostratigraphic zone by BUKRY, and the lower part of M2 mudstone to CN5a and CN5b. But, nannofossils were not found from the M1 mudstone and the upper part of M2 mudstone.
2. Stratigraphic horizon of kuroko deposits can reasonably be correlated to CN5a (13.9 Ma-12.3 Ma), and may possibly extend to a part of CN5b and CN4.
3. Globorotalia-Hopkinsina-Prunopyle Zonule, which is foraminiferal and a radiolarian fossil zone, established in the Shakanai mining area, is thus subdivided into CN4, CN5a and CN5b.

View full abstract