Studies on the chromites and their occurrences in the Tari district, Tottori Prefecture (2 nd report)

Abstract

Granite, granite porphyry and quartz porphyry were originated from same magma in the Tari district. It seems probably that ultrabasic rocks were intruded in Palaeozoic formation in the region. The ultra-basic rocks consist largely of serpentinized dunite and harzburgite. Serpentinite associated with disseminated chromite is generally pale yellow or pale green and wall rock serpentinite is dark green or black. The variation in forsterite content for the ultra-basic rocks tested by the writer is Fo_92-97 mo1ecu1ar percent. Iron oxide content in ultra-basic rock is rather low and magnesia content predominates over iron oxide content. Enstatite has sometimes altered to bastite and olivine to mesh serpentine. Serpentinized olivine is ophitically held in enstatite or bastite altered from enstatite. It seems that serpentinization come from within the ultra-basic rock (so-called autometamorphism) and serpentine is not crystallize directly from the residual liquid. Dusty magnetite is separated in serpentine derived from forsterite, but a part of iron still remains in the serpentine. Serpentinized ultra-basic rocks have a higher Fe₂O₃/FeO ratio than unserpentinized ultra-basic rocks as shown in Table of the chemical analyses.
Gabbros occur within the serpentinites and chromite bodies as cognate xenolith derived from the same magma that generated ultra-basic rockss and chromites. Uralite was produced around the periphery by the alteration of clinopyroxene. The optical properties and chemical compositions of the gabbros are described.