1991 Volume 41 Issue 228 Pages 203-218
Hydrothermal alteration in the Kamikita Kuroko mineralization area have been examined by using samples from 16 drill holes and the field covering about 100km2 in area. Four types of alteration (I-IV) are distinguished spatially, temporally, and mineralogically in this area. Alteration I may be related to the intrusion of quartz diorite bodies at 13 Ma. This alteration is composed of five mineral zones which are zonally arranged from the center to the periphery as follows: the biotite-actinolite (I5) → the chlorite-epidote (I4) → the corrensite-laumontite (I3) → the smectite-zeolites (I2) → the smectite (I1) zones. Alteration II may be related to the Kuroko-type mineralization at 10 to 12 Ma. It comprises the K-feldspar, the illite-chlorite, and the mixed layer mineral zones which are arranged in turn from the stratigraphically lower to the upper parts. Alteration III was probably formed at almost same time or slightly later than the alteration II. The mineral assemblages of the laumontite and wairakite zones in the alteration III are similar to those in the higher-grade zones of alteration I, but the former is distinguished from the latter by the association of pumpellyite, prehnite, hedenbergite, or garnet. Alteration IV comprises the pyrophyllite-diaspore, the mixed layer mineral, and the alunite zones. The formation of the former two zones was at 6 Ma and the alunitization followed at 3 to 4 Ma.
Epidote, chlorite, or white mica is a common product in the higher-grade zones of all the types of alteration. These minerals were formed at approximately 200-300°C as inferred from comparison with the formation temperatures of the minerals in low-grade metamorphic environments and active geothermal fields. The compositional variability in these minerals, in addition to the difference in the mineral assemblages of each alteration, was caused by difference of geochemical conditions such as fugacities of CO2 and O2 and pH of solution.
