Abstract
Exploration for podiform chromitite has been difficult, because its occurrence is usually very irregular in peridotites and mechanism of spinel concentration has not been fully understood. We characterize chromian spinel in chemistry and morphology and propose a method of petrological exploration for podiform chromitite as a case study at some ultramafic complexes from the Sangun zone of Southwest Japan.
Significance geological and petrological features of podiform chromitite are as follows.
(1) Chromitite is always enclosed by dunite.
(2) Hydrous mineral inclusions (Na-phlogopite and pargasite) in chromian spinel is only found in relatively small chromitite.
(3) DR# [=degree of roundness of chromian spinel; (area/(round-length)2)/0.0796(≅value of circle)] is usually low (<0.4) in harzburgite and high (mostly 0.7-0.9) in dunite. However, the DR# is relatively high in harzburgite and low in dunite near relatively large chromitite.
(4) TiO2 content and Fe3+#(=Fe3+/(Cr+Al+Fe3+)) of chromian spinel is higher in dunite and lower in harzburgite, ex-cept in those near the relatively large Chromitite.
(5) Chromian spinels in both dunite and harzburgite near the large chromitite are relatively high-Cr# and low-V2O3.
The genesis of podiform chromitite with such characteristics as mentioned above is only accounted by the mantle-melt interaction and related magma mixing processes in the magma conduit within the upper mantle. The extraordinary morphological and chemical characteristics of spinels only found near chromitite pods especially indicate the presence of a "transitional lithology" between dunite and harzburgite as reaction zone of mantlemelt interaction. To explore the podiform chromitite, it is of vital importance to clarify the presence and distribution of the transitional lithology between dunite and harzburgite which has chromian spinel with extraordinary morphological and chemical characteristics.
