Podiform chromitites with dunitic envelopes, especially undeformed ones, usually cut mantle peridotite as dikes or pipes. Large-scale chromitites are usually set within the moderately refractory peridotite; i.e., the harzburgite with chromian spinel of intermediate Cr# (=Cr/[Cr+Al] atomic ratio), 0.4 to 0.6. Podiform chromitites are usually absent or very small in volume, in both fertile lherzolite and highly refractory harzburgite. This relationship indicates that the wall-rock chemistry controls to some extent the formation of chromitites, and is concordant with a petrogenetic model whereby podiform chromitite is formed by reaction between upwelling melt and mantle peridotite protolith and related melt mixing. Both the Cr# and the total amount of (Al+Cr) in the involved system, especially of orthopyroxene of the protolith, control the size and chemistry of the podiform chromitite, a reaction product. For fertile lherzolite wall, the Cr# of the system is too low to concentrate spinel because the mixed melt is relatively low in the degree of spinel-oversaturation. For highly refractory harzburgite wall, the amount of (Al+Cr) of the system is too low to precipitate large amounts of spinel. The moderately refractory harzburgite is satisfactory for the two criteria, and can be a host for large-scale podiform chromitites. Upper mantle beneath arc, especially arc proper and back-arc basin, and fast-spreading oceanic ridges is suitable for the podiform chromitite formation because the harzburgite with chromian spinel of intermediate Cr# is available there. Both melt/peridotite interaction and magma mixing can occur to precipitate large amount of chromian spinel at the Moho transition zone where upwelling melt is pooled within the harzburgite mantle.

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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.

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This paper reports the application and potential of detrital chromian spinel chemistry in exploration of chromite deposit. The areas investigated are in Hokkaido; the Mukawa, Sarugawa and Nukabira serpentinite masses in the southern part of the Kamuikotan tectonic belt, and the Takadomari serpentinte mass in the northern part of the belt. The ultramafic rocks from the masses usually contain chromian spinels, and detrital chromian spinels are found in nearby stream sediments. The detrital chromian spinel chemistry potentially shows the petrological characteristics of the ultramafic masses upstream, especially the presence or absence of chromitite. Around the Mukawa, Sarugawa and Nukabira masses, some of detrital chromian spinels have high Mg^#'s off a general Mg^#-Cr^# trend made up by peridotite-derived spinels, possibly indicating a presence of compact chromitite ores near the surface. On the other hand, several chromite placer deposits, which had been mined around the Takadomari serpentinite mass, for example at the Mitsui Horokanai mine, do not have high Mg^# characteristics. This means their source is not chromitites but chromite-bearing peridotites.
It is useful to examine the Mg^#-Cr^# relation of detrital chromian spinel grains to check the potential of massive chromitite ore distribution. If there are appreciable amounts of high-Mg^# chromian spinel grains which plot off the general Mg^#-Cr^# trend of the peridotite spinels, massive chromitite ores are possibly present in the mass upstream.

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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.

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Chromite is not only resource for chromium, chromia or other chrome-bearing chemicals but also raw materi-als for refractories. A few percent of chromite is con-sumed by refractory industry, and others for metal industry and some for chemical industry (OKITA, 1981). The chromite is classified based on their chemical com-positions and consumed in these industries as shown in Table 1. In this paper the usage and the properties of refractory chromite is reviewed. Chromite is a mineral having spinel structure and FeCr2O4 composition containing some MgO and A12O3. A rock composed of chromite is chromitite. Chromite and chromitite are generally called chrome ore in indus-tries. The word ‘chromite’ will be used in this paper to represent chrome ore i.e. chromite and chromitite. In addition, the word ‘magnesia’ usually means periclase in this paper.

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Trace concentration of gold in the currrent thermal water from the Hishikari gold deposit has been determined by selective preconcentration using anion exchange, followed by inductively coupled plasma quadrupole mass spectrometry (ICP-MS). The concentration of gold in the Hishikari thermal water after filtration is extremely low, 0.6 ± 0.4 ppt. The low gold concentration suggests that the thermal water is at present not transporting gold and is not interacting with the gold deposits in the waning geothermal system of the Hishikari gold deposits.

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