Significance of magma/peridotite reaction for size of chromitite: example for Wakamatsu chromite mine of the Tari-Misaka ultramafic complex, southwestern Japan

摘要

Wakamatsu chromite mine has a largest chromitite-pod called "nanago-ore body (7th ore body)" which is 40 X 210 X 25m (2.1 X 10~5m~3) in size in Japan. The other chromitite-pods which are intermediate size (4.5 X 10~4 approx 1.2 X 10~3m~3) are also distributed in Wakamatsu mine. We have clarified that the relationships between chromian spinel in chemistry and size of chromitite for exploration for podiform chromitite. Major results of this work are as follows.(1) Nanago ore body is larger than the other chromitite bodies by one order of magnitude.(2) Chromitite bodies are divided into two groups by Cr# of chromian spinel from chromitite that are high-Cr# group (Cr#:0.52-0.57, nanago and 10th ore bodies) and low-Cr# group (Cr#:0.41-0.48, 51st, 52nd, 53rd, 54th, 55th, and 56th ore bodies).(3) Cr# of chromian spinel in dunite envelops varies from 0.45 to 0.62 in "nanago and 10th ore bodies group" and 0.48 to 0.57 in "the other chromitite ore bodies group."(4) Chromian spinels from dunite envelops in both Nanago and the other chrmitite bodies are plotted in relatively high-Cr# and low-V_2O_3 field on Cr#-V_2O_3 diagram of Matsumoto and Arai (1997).The results of above characteristics clearly show that spinel precipitation and concentration are more intense at nanago ore body than at the other chromitite bodies of the Wakamatsu mine. And that to explore the podiform chromitite, Cr#-V_2O_3 relation is useful petrological exploration tool not only for relatively large chromitite but also for intermediate one.