SODIC-CALCIC ALTERATION OF GRANITES ASSOCIATED WITH MO MINERALIZATION AT THE KOMAKI DISTRICT, SOUTHWEST JAPAN

Hydrothermally altered granitic masses composed mostly of calcic plagioclase and quartz (so-called metasomatic anorthosite) are exposed at the Komaki district, Southwest Japan (Takagi and Sudo, 1994 Resource Geol., 44, 409-418).  The masses occur as two lenticular- and dome-shaped intrusions (50 to 200 m in diameter) in a Late Cretaceous to Paleogene hbl-bt granite batholith.  The metasomatic anorthosite contains no K-feldspar and biotite, and the modal ratio of plagioclase and quartz is about 2:1.  Plagioclase shows the compositions of oligoclase to andesine.  Titanite, actinolite, and muscovite occur as accessory phases.  Muscovite-rich zones are often found in peripheral parts of the masses.  Upper parts of the masses are mostly altered to kaolin-group minerals (mainly halloysite) and the host granite batholith are significantly weathered to grus several tens meter depth.

  The hydrothermal alteration to form the metasomatic anorthosite is very similar to the sodic-calcic alteration found from some porphyry copper deposits (Carten, 1986 Econ. Geol., 51, 1495-1519).  The formation temperature of the metasomatic anothosite constrained by the mineral assemblage ranges from 385 to 644 degree C at 50 to 200 MPa.

 A leucocratic two-mica granite with a quartz-vein type Mo deposit occurs at 2 km south of the metasomatic anorthosite masses.  The two-mica granite and the metasomatic anorthosite form a Rb-Sr whole-rock isochron yielding 65.7}1.7 Ma and 0.70624 of Sri.  Whole-rock delta ¹⁸O values of the metasomatic anorthosite (+4.5 to 5.1) are lower than those of the two-mica granite (+7.4 to 8.1); this suggests meteoric water contributions to the hydrothermal system.  The data above indicate that the metasomatic anorthosite masses are cupolas of the two-mica granite, and was formed by the sodic-calcic alteration associated with the Mo mineralization.  The hydrothermal alteration and subsequent supergene processes also brought about the kaolin mineralization and deep weathering of the host granite batholith.