Alteration around granite porphyry intrusives in the Tochibora and Maruyama deposits, Kamioka mine

Abstract

The Kamioka mining area, one of the largest zinc and lead producing area in Japan, includes two large mines; the Tochibora and Mozumi mines. The main Zn-Pb deposits are of skarn type, replacing crystalline limestone embedded in gneiss and migmatites of Hida metamorphic rocks. Besides the skarn type deposits, some minor mineralizations of other types and associated alterations are also present in the Kamioka mining area. This paper describes the alteration of post-skarnization of the Tochibora mine and discusses its geological implications. Dominant alteration mineralogy is: (1) hastingsite (potassian hastingsite), sodic plagioclase (An8-28), potassium feldspar, sericite and fluorite; (2) actinolite and fluorite; and (3) biotite, hastingsite±, sodic plagioclase (An20-30), potassium feldspar and fluorite. Migmatites and skarns are altered into above mineralogy, and product assemblage of the alteration is dependent on the original rock; hastingsite from migmatite (pyroxene monzonitic migmatite called "Isnishi migmatite") and actinolite from hedenbergite skarn. Occurrence of different amphiboles seems to be controlled by the aluminum content of the original rock. Such alteration is observed extensively around molybdenite-bearing granite porphyry intrusives near the Tochibora mine. Molybdenite-bearing veinlet exclusively occurs in and around the altered rock. These observations suggest that this alteration might correspond to potassic alteration zone commonly observed in and around porphyry molybdenum deposits.