The Uchinotai-east deposit occur in the same stratigraphic horizon as the other major kuroko occurrences, namely the uppermost part of the Miocene Nishikurosawa formation. Based on variations in mineral assemblages and textural features, ores in the northern sector of the deposit are subdivided into seven zones. In stratigraphically ascending order, these are denoted networked siliceous ore, siliceous ore, powdery yellow ore, yellow ore, black-yellow ore, black ore, and the ferruginous-chert bed.
Of these, the siliceous ore can be regarded as an explosion product of the networked siliceous ore set in dacitic pyroclastic rocks and brecciated dacite. The rest of the ore horizons in this northern sector are very similar to those of other kuroko deposits previously described, although there are some differences in classifications. The mode of occurrence of deposits in the southern sector, however, is different from the usual kuroko model, being richer in lead, zinc, and barium.
Furthermore, the foot-wall rocks are pyroclastic flow deposits and different from those to the north.
The filling and freezing temperatures were studied for fluid inclusions in ore minerals. With the exception of some conflicting results (both intra-and inter-grains) from some minerals near the top of the deposit, even the highest indicated temperatures of ore formation did not substantially exceed 300°C. The salinities of fluids in inclusions appear to be lower than 10 wt.% equivalent NaCl solution.
Analyses of minor elements indicate that the sphalerite in this ore deposit has a high content of cadmium and manganese, and its FeS content is lower than 2 mol.%. The variation in As/Sb ratios for bulk ore samples from the northern sector is distinct from that of the south, while samples from the central sector of the deposit show an intermediate character. It also seems that the Co/Ni ratio in pyrite abruptly decreases in the sulfide beds above the powdery yellow ore zone.
On the basis of the experimental data, the great variation of filling temperatures for fluid inclusions (both intra-and inter-grains of ore minerals) presents the possibility of gas bubbles having been trapped with the liquid at the time of formation. This means that the ore minerals may have formed under a condition of degassing or bubbling. According to the chemical data, it is possible that the hydrothermal solution could have been bubbling or boiling even at great oceanic depths, depending upon the content of dissolved gases.

View full abstract

Many pyrometasomatic type and vein type copper-iron-lead-zinc-gold ore deposits are enclosed mainly in Permian limestone and slate of the Kamaishi mining district, Northeastern Japan. Those deposits are considered to have formed during acidic igneous activity of Cretaceous age. In present study it was revealed that nickel and/or cobalt-bearing minerals invariably occur as minor to trace components in the deposits.
Ultrabasic to acidic intrusives are distributed in the mining district. Nickel and cobalt contents of those rocks were measured by Atomic Absorption Spectroscopy Method to deduce geochemical behaviour of the both metals during magmatic differentiation. Based on the analytical data, nickel and cobalt, especially nickel, in silicate magma are considered to be difficult to remain in the residual solution after formation of acidic igneous rocks, probably because nickel (and cobalt) are easily incorporated into mafic silicate minerals during its crystal-lization. This suggests that nickel (and cobalt) in the deposits were not derived from acidic igneous rocks of the mining district, but from other sources.
The fact that the mining district is located in the Hayachine tectonic belt, into which ultrabasic rocks and metagabbro with high nickel-cobalt contents intruded, leads to a following hypothesis as to the origin of the both metals in the deposits: The ore solution from acidic igneous rocks ascended along shear zone of the tectonic belt and leached out nickel (and cobalt) from the ultrabasic rocks and metagabbro.

View full abstract

The Oshirabetsu gabbroic complex of Paleogene age, occurring near the southern tip of Hokkaido, represents one of the igneous centers in the Hidaka metamorphic belt. The gabbroic rocks are highly variable in lithology but mainly consist of olivine gabbro, troctolite, coarse-grained gabbro, norite and diorite. All rock-types are massive in appearance and neither layered nor flow structure is observable, while microscopic investigation indicates that the olivine gabbro and part of the troctolite are cumulus phases. Major and trace elements study of the representative rock-types (TAKAHASHI, 1983) returns smooth variation trends which indicate that the rocks are the products of a series of magmatic differentiation with the, characteristics of calc-alkali rock series. Minor amounts of sulfides, pyrrhotite, chalcopyrite and pentlandite, are observed throughout the complex, and ore grade concentration of nickel-bearing pyrrhotite with graphite occurs in olivine gabbro and more commonly in norite masses.
Sulfur isotopic measurement has been made for 19 rock samples (12 from the gabbroic complex and 7 from the country rocks) and 6 sulfide ores. All the sulfurs examined give negative δ³⁴S(CDT) values. A systematic decrease of δ³⁴S in parallel with magmatic differentiation is observed in the complex rocks : -1.6 to -1.7 (trocto-lite), -3.3 (gabbro), -4.7 to -6.9 (norite) and -6.0 to -8.7‰ (diorite). Ore sulfides have δ³⁴S or -4.4 to -6.7‰, being almost identical with the values for norite sulfur. Similar values observed for olivine gabbro (-5.0 to -6.2‰) are considered to have been caused by the contamination with ore sulfur. Sulfur in the Hidaka sedi-mentary and metamorphic rocks into which the Oshirabetsu gabbroic rocks are intrusive, shows the typical sedimentary δ³⁴S value, ranging from -5.3 to -12.3‰.
The isotopic results obtained may be best explained with a model that assumes magmatic assimilation of country rock sulfur. The systematic δ³⁴S variation in the gabbroic complex strongly suggests that the assimilation has taken place in the magma chamber more or less continuously during the course of magmatic differentiation. Assuming the isotopic values of the initial magmatic and the average country rock sulfur to be 0 ± 1 and -9‰, respectively, isotopic mass balance calculation indicates that at least half or more of the sulfur in the norite has been sedimentary rock origin. A similar conclusion may be drawn for the ore sulfur. Preliminary δ¹³C data of the associated graphite ore also conform to this view.
Examination of available δ³⁴S data for "magmatic" sulfide ores from the world reveals that very few deposits consist of the genuine magmatic sulfur. The role of country rock sulfur in the formation of this group of deposits is probably more essential than has been suggested and the assimilation of sedimentary sulfur by magma is quite a common phenomenon, or may be a necessity, in any remarkable mineralization of "magmatic" sulfides.

View full abstract

Over 1900 rock samples from drill core of Hokuroku district were chemically analysed and subjected to Principal Component Analysis and Cluster Analysis hoping to delineate the interrelationship among the major and minor elements in relation to the alteration and mineralization associated with the kuroko deposits.
The principal component analysis on all of the analytical data (N=1917) showed that factor 1 governed the major oxide chemistry of the rock, i.e., the contribution of Fe, Ti, Mg and Ca were significant while Si showed very significant negative contribution. Basic rocks thus showed higher factor scores while acidic varieties showed lower scores.
In factor 2 on the other hand, Cu, Pb, Zn and Ba had large contribution while Na and Al showed large negative contribution. This combination and behaviour of the elements were thought to be of prime interest since this factor might have a possibility to suggest the existence of mineralization with associated alteration.
The fact that Na does not show high contribution in factor lwhile it does in factor 2 is another very interesting finding. This is more apparent in the case of the subset of igneous rocks (N=1047).
Younger dacitic intrusives are thought to be the least affected by alteration associating mineralization. They were subjected to a principal component analysis and were found that the contribution of Na in factor 1 also negligible. Here factor 2 was thought to be controlling the diagenetic zeolitic alterations.
The cluster analysis showed several interesting clusters of which further investigation should be carried on in association with the data on geology, X-ray diffraction and others.

View full abstract

New method to calculate the statistical frequency distribution of geochemical data with microcmoputer was developed by using the CHEBYCHEV or SPLINE function approximating the cumulative frequency distribution and its first derivative without any assumption on the type of statistical distribution of data.
The method was applied to several sets of data on the geochemical exploration of some gold-silver mineralization zones in Japan. Computation of frequency distribution was ascertained to be very useful for statistical treatment of geochemical data, not only in defining the geochemical anomalies, but also in estimating the type of statistical distribution of data.

View full abstract

In the systems of chloroauric acid aqueous solution-silica gel, -calcium carbonate and-activated carbon, the concentration changes of chloroaurate complex ion in the sample solutions with time are examined and determined quantitatively during 61 to 71 days after the sample preparations in terms of inductively coupled plasma emission spectrometer.
As the results, kiesel-gel (230 mesh under) as an additive has large reduction capacity for chloroaurate complex ion in comparison with wako-gel (100 and 200 mesh) but calcium carbonate has not large reduction capacity. On the other hand, activated carbon (granule) has the largest reduction capacity such that chloroaurate complex ion is reduced perfectly to gold metal in a few days.

View full abstract

Using a Plasma torch furnace, melting experiments of sulfides such as pyrrhotite, molybdenite, tungstenite and some Cu-Fe-S compounds were carried out. The melting products of these compounds, under extreme high temperatures, show some unusual mineralogical characteristics. Typical examples are (1) the values of d(102) of hexagonal pyrrhotite with a few wt.% of Cu are larger than that of stoichiometric FeS (2.0932A) and 10 months annealing of the products produced an assemblage of mackinawite+troilite, and (2) bornite+pyrrhotite assemblage is always produced by melting the compounds whose composition is between chalcopyrite and cubanite.
Usefulness of the Plasma melting method is stressed in spite of the unavoidable sulfur loss during the melting, uncertainty of the temperature determination and heterogeneity of the melting products.

View full abstract