Abstract
Peridotite and chromitite in Wakamatsu mine, a large chromite mine, at the Tari-Misaka ultramafic complex are described in order to understand the genesis of podiform chromitite. Rocks in and around the Wakamatsu mine are thermally metamorphosed by granite and the main silicate mineralogy and zonal structure of relic spinel are controlled by the thermal metamorphic temperatures dependent on the distance from the granite. The primary lithologies, however, can be determined by relic textures. Dunite is relatively predominant at Wakamatsu mine area, and chromitite occurs as pods within the dunite. Primary peridotites are variable in textural and chemical characteristics of chromian spinel dependent on the distance from the chromitite pod. Harzburgite far from chromitite, typically exposed at Misaka, has low-Ti (≤0.2 wt% of TiO2) and highly anhedral spinel. Harzburgite close to the boundary with dunite has less anhedral and more Ti-rich (up to 0.5 wt% of TiO2) spinel. Dunite also has relatively Ti-rich (up to 0.5 wt% of TiO2) spinel, which tends to be more abundant and euhedral towards chromitite. Chromitite has rather constant mineralogy irrespective of spinel modal amount, with up to 0.7 (mostly 0.2 to 0.4) wt% of TiO2. Cr#(=Cr/(Cr+Al) atomic ratio) of spinel in all lithologies is confined in a narrow range, from 0.4 to 0.6. These characteristics possibly indicate that a melt/harzburgite interaction controlled the genesis of the podiform chromitite and enclosing peridotites. The low-Ti harzburgite was passed by a relatively Ti-rich melt to leave dunite-chromitite and harzburgite envelope with relatively high-Ti mineralogy.
