The Mamut porphyry copper deposit of upper Miocene age is located on the northern end of the island of Borneo. Copper mineralization is associated with an adamellite porphyry intrusion, and is localized in the intrusion as well as in the wall rocks which are composed of serpentinite and clastic sedimentary rocks. This paper deals with the serpentinite which has been altered hydrothermally by the mineralization.
The next four types of alteration for serpentinite are seen in the Mamut ore deposit: (1) weakly altered type, (2) chlorite type, (3) talc type and (4) tremolite type. The alteration process is considered to have proceeded in the following succession: (1) serpentinization of olivine and pyroxene, (2) formation of talc and chlorite from serpentine minerals, and (3) formation of tremolite from talc and chlorite. The alteration and mineralization process caused by the steps (1) and (2), which are considered to have been related to the intrusion of adamellite porphyry, is characterized by pervasive silicification and weak dissemination of chalcopyrite. Many fractures have been developed along the margin of the adamellite porphyry apophysis as the result of its cooling and shrinking. The process of the step (3) may be correlated with the introduction of mineralizing fluid, which has a high salinity and a high copper concentration, into the zone of well fractured adamellite porphyry and contiguous wall rocks. This alteration and mineralization process related to the step (3) is characterized by passive silicification and has completed the building up of the Mamut copper ore deposit.

View full abstract

The Iwami kuroko mining district is in the Miocene Green Tuff region of southwest Japan. Around the kuroko-type deposits, the rocks are altered by hydrothermal solutions related to the mineralization. X-ray diffraction analyses of rock samples collected from 15 drill holes in the district reveal that the altered rocks consisting mainly of sericite, chlorite and quartz occur around the kuroko-type deposits. The chlorite is of Fe-Mg variety and Ia polytype is common while IIb polytype is detected from the rocks adjacent to the Iwami deposit. Interstratified chlorite-saponite is rarely found.
Diagenetic alteration of the Green Tuff formation in this district is characterized by the presence of zeolite and montmorillonite. The basal spacing data of montmorillonites indicate that the exchangeable ion ratio Na/Ca increases with depth and toward the ore zone, which is probably a marginal effect of the kuroko-type mineralization.
Filling temperature data ranging from 290° to 160°C are obtained for vein calcite and quartz from one of the drill holes (SI-31), which is in the area strongly influenced by the kuroko-type mineralization. The temperatures from 184° to 74°C or lower are measured for the samples from another drill hole (SI-30), the altered rocks in which consist mainly of zeolite, montmorillonite and quartz or low cristobalite.
Salinities for the inclusions having the filling temperatures higher than 160°C in SI-30 and SI-31 are 0.5-0.7 wt% NaCl equivalent. On the other hand, small inclusions with filling tempratures lower than 90°C in SI-30 seem to be composed of nearly the pure water indicating an extensive dilution of the hydrothermal fluid with ground water.

View full abstract

The Maruyama orebody of the Tsumo mine belongs to the skarn type copper-zinc-tungsten deposit. Skarns are mainly classified into the following three types judging from their modes of occurrence and the mineral assemblages; banded skarn, massive skarn (vesuvianite-garnet-clinopyroxene skarn, wollastonite-garnetclinopyroxene skarn, and garnet-clinopyroxene skarn) and vein skarn (hedenbergite skarn). A copper-zinc mineralization took place in wollastonite-garnet-clinopyroxene skarn and garnet-clinopyroxene skarn, while a tungsten (scheelite) one is recognized not only in hedenbergite vein skarn, but also in the part of wollastonite-garnetclinopyroxene skarn.
Most of fluid inclusions in garnet, clinopyroxene and wollastonite are homogenized into liquid phase at about 300°-350°C. When the temperature correction is made considering salinity of the inclusion fluid and the formationlpressure on the basis of the sphalerite geobarometer, it is concluded that the formation temperature of the skarn minerals was about 400°-350°C.
Skarn minerals contain fluid inclusions, some of which are evidently primary in origin. Many of fluid inclusions contains solid phases, such as halite, sylvite, carbonate and metallic minerals. The variable volume ratio of the solid phases to the whole vacuole may suggest that the skarn minerals of the deposit were formed from saline solution of various concentrations. On the contrary, fluid inclusions in scheelite do not contain any solid phase. This implies that the mineral precipitated from a solution whose salinity was lower than 26 wt.% NaCl. Small grains of chalcopyrite are included in a sphalerite crystal. The fact that they array along a paticular plane may suggest that some of them is not the exsolution product, but minerals trapped during the growth of host mineral.
In general, fluid inclusions in skarn minerals have no liquid CO₂ phase at room temperatures. This turns out to be true for those in the Tsumo deposits. This fact agrees well with the conclusion derived from the solubility data of CO₂ for NaC1 solution and the calculated data for the stability field of the skarn minerals. However, the presence of gaseous inclusion, though the abundance is few, may suggest that two phase locally appeared during the skarn mineralization.

View full abstract