Clusters of Kuroko deposits of Miocene age in the Hokuroku district of northern Honshu lie along one of a conjugate set of lines with NNW and NE strike and most of the larger deposits lie at or close to the intersections of these lines. A wide variety of structural features of northern Honshu have NNW and NE strikes crosscutting the predominant N-S structural trends of Miocene and younger age and are thought to be of basement (pre-Tertiary) origin. It is proposed that fractures in the basement of the Hokuroku district and particularily their intersections were responsible for the distribution of the Kuroko deposits. Presumably, such sites facilitated the diapiric rise of dacite domes and provided a suitable plumbing system for metal-bearing hydrothermal fluids. Several intersections of the NNW and NE lines do not have known Kuroko deposits and may be good exploration targets.

View full abstract

Kuroko deposits occurring in mudstone at the Matsuki mine, called "A" ore deposits, are mainly composed of very high grade yellow ores. Seven unit orebodies have been found. Sulphide orebodies are divided into the coarse porous and the fine banded orebodies. Both occur within the black clay zone that has originated from the mudstone. The deposits have a regular lateral zoning, that is, the sulphide ore zone, the gangue minerals zone and the black clay zone from its center to periphery. Each zone reveals also the following lateral variations: i) Yellow ore → black ore for the sulphide ore zone and ii) Barite lens → carbonate lens → siliceous lens + nodule for the gangue minerals zone. Fossils of arenaceous foraminifera are found in the interior of orebodies as well as in the hanging-wall and foot-wall of the deposits. The existence of benthonic foraminifera in the mineralized zones and the mode of occurrence of orebodies suggest strongly that "A" ore deposits were formed in the seawater-bearing unconsolidated mudstone, just after sedimentation of the lower half of M2 mudstone. Relatively reduced environment resulted from the weak preceding volcanism may be the main cause to have formed this peculiar deposit as compared with common Kuroko deposits.

View full abstract

The Fukazawa Zn-Pb-Cu-Ag deposit is a typical Kuroko deposit in the Hokuroku district. The felsic volcanic rocks around the deposit have suffered hydrothermal alteration. The footwall of the stratiform massive ores is characterized by the Mg-chlorite-sericite assemblage. FeMg-chlorite, sericite and feldspar are present in the outer zone. The regional diagenetic alteration is also recognized in upper part of the volcanic pile.
The chemical variations involved in alteration of the footwall volcanic rocks of felsic composition are the depletion of Na₂O and CaO and the increase of MgO, S and As. These features are recognized in the case of the Iwami deposit of the Kuroko type. CI in weakly altered volcanic rocks are 300-500 ppm in the Iwami area and 20-40 ppm in the Fukazawa area. The low Cl content of volcanic rocks in the Fukazawa area might be due to leachine by thermal waters related to base metal mineralization of younger age.
Trends of chemical variation of altered rocks can be represented by AL₂O ₃-MgO-(CaO+Na₂O+K₂O) mole proportional diagram. Alteration associated with Kuroko mineralization reveals the extensive Mg enrichment in the felsic volcanic rocks. The Mg enrichment is found in the case of Archean massive sulfide deposit in Cananda. The Mg enrichment associated with Na depletion can be considered as a unique characteristic of hydrothermal alteration involved sea water.

View full abstract

Quartz is the only silica mineral in black, yellow and siliceous ores of the Uwamuki No.4 (Kuroko) deposit, Kosaka mine. Intimate textural relationship between quartz and sulfide minerals in Kuroko ores suggests that these minerals were precipitated simultaneously. Filling temperatures, T, of fluid inclusions in quartz from the siliceous ore (MARUTANI, 1977) can be fitted by;
T (°C)=289.7+0.2159y
where y represents vertical distance in meters from estimated sea bottom at the time of siliceous ore mineralization. The steep thermal gradient within siliceous orebody may be a consequence of mixing between ascending hydrothermal solution and down-circulating cool sea water. Net contamination of sea water is computed to be less than 25%.
Contents of quartz and barite in six black ores are measured. The ratios quartz/sulfides and barite/sulfides show excellent parallel relation. Mixing of cool and oxygenic sea water with discharged hydrothermal solution causes both decrease in temperature and introduction of sulfate ion. Consequently, quartz and barite precipitate simultaneously with sulfide minerals in black ore. Common occurrence of bipyramidal quartz in black ore indicates that the quartz grew up in open space. Besides, the grain size of quartz increases towards the margin of the black orebody. It will be ascribable to gradational slow cooling of the discharged hydrothermal solution on sea floor.

View full abstract

Filling temperatures and salinities of fluid inclusions in quartz from the bedded and stockwork siliceous orebodies of Uwamuki No.4 Orebody and No.7 Stockwork Orebody of Uchinotai Western Deposit of the Kosaka mine were studied. Filling temperatures in quartz were in a range of 260°-310°C for the bedded orebodies, and 280°-320°C for the stockwork orebodies. The temperature range of sphalerite was 200°-240°C for the bedded siliceous orebodies and 235°-280°C for the stockwork orebodies, and apparently lower than those of associated quartz. Thermal gradients in the stockwork orebodies were fairly gentle and most of temperatures concentrated around 300°C, but the data obtained from drilling cores indicate a tendency of increase in temperature at deeper levels. Salinities range from 2.5 wt. % to 5.5 wt. % and slightly increase at lower levels. From the data on fluid inclusions, it is inferred that the depth of sea floor was deeper than 1, 000 m at the Kuroko mineralization.

View full abstract

Lateral zoning of Miocene ore deposits of the Green Tuff belt is discussed in two mining areas, with reference to newly obtained sulfur isotopic evidence. The ore deposits show a distinct lateral zoning around the Miocene tonalite-granodiorite stocks in the Hokuroku district, having xenothermal-type copper vein or normal copper vein deposits in the center and the kuroko type at the margin. Rock sulfur of the granitoids has an average δ34S of about +6 permil and the vein and kuroko sulfur is somewhat depleted in 34S. Rhyolites associated with the kuroko mineralization have similar δ34S values to the granitoids. There is possibility that the Miocene plutonic and volcanic rocks as well as the ore-forming sulfur have been derived from a common deep-seated source material.

View full abstract