The Zoned Skarn Developed in Diorite Porphyry In the Shinyama Area, Kamaishi Mine, Japan

Abstract

Several zoned skarns are developed in the Shinyama Number 3 orebody of the Kamaishi mine at the contact between Paleozoic limestone and diorite porphyry intrusives of Cretaceous age. The textural relations and mineralogy of each skarn are strongly influenced by the primary host rock in which the skarn formed. Hedenbergite skarn is developed in areas in which the primary host rock was limestone. In this skarn the pyroxenes exhibit a distinctive granoblastic texture and twinning of individual grains is rare. A sequential arrangement of metasomatic alteration zones enveloping garnet veins is observed in the zoned skarn which formed in the diorite porphyry.
The succession of skarns observed with increasing distance from the garnet veins is: (1) garnet veins and/or garnet skarn, (2) ferrosalite skarn, (3) epidote pyroxene skarn, and (4) epidote amphibole skarn. The primary igneous texture of the diorite porphyry is generally quite well preserved throughout the entire endoskarn. There appears to be no relation, however, between the degree of preservation of the primary igneous texture in the successive skarns and the distance of the skarns from the garnet veins. The garnet veins are considered to have formed in the channelways through which the skarn-forming fluids flowed.
Petrographic examination of the mineralogy and textural relations of the different zoned skarns reveals that the first mineral to break down in the diorite porphyry is plagioclase. It is altered to epidote, clinopyroxene, garnet and calcite. Plagioclase phenocrysts appear to have been more reactive to the metasomatic fluid than plagioclase occurring in the groundmass. In the case of clinopyroxene the reverse relation is observed: clinopyroxene in the groundmass is readily altered to amphibole, whereas clinopyroxene phenocrysts appear resistant to mineralogical change in the zoned skarns.
The chemical compositions of clinopyroxene and epidote from the different skarns and from the fresh diorite porphyry were determined using electron microprobe. Clinopyroxenes display a gradual increase in their iron and manganese contents away from the diorite porphyry towards the ferrosalite skarn. Within the epidote pyroxene skarn a slight increase in the iron content and decrease in the aluminum content of epidote is observed from the epidote amphibole skarn towards the epidote pyroxene skarn.