トルコのクロム鉱床 : アルプス型クロム鉱床へのアプローチ
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概要
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Turkey is well known for its chromite resources. Especially, the Guleman mine, southeastern Anatolia, is the most abundant in chromite reserve of this country. The mode of occurrence of the chromite deposits of this mine is briefly described based on the authors observation. Then the ultrabasic rocks and the associated chromite ores from the Andizlik-Zimparalik area, southwestern Anatolia, being studied by MTA Institute of Turkey are introduced. Comparison between chromite deposits of Turkey and those of Hokkaido, Japan is made and it is clarified that these are quite common to each other from the view point of ore genesis.<BR>Harzburgite or serpentinite, host rock of the chromite deposits in Turkey, generally occurs along the thrust formed during late Tertiary period. Chromite deposits enclosed in the harzburgite mass are classified into three kinds of ore : i.e. the disseminated ore partially including the stratiform ore, nodular ore and compact dense ore. The disseminated chromite ore is considered as a variety of harzburgite characterized by the presence of scattering fine-grained chromite, whereas, the mode of occurrence of the compact dense ore is different from that of the disseminated one. The dense ore shows podiform controlled by the joint directions of the host rock accompanying by the preceding disseminated ore.<BR>Chemical analysis for the individual chromite from the Andizlik-Zimparalik area was carried out by ENGIN (1972) and it was clarified that the chromite from the dense ore is rather rich in chromium compared with the chromite from the disseminated ore. Furthermore, he pointed out that the gangue minerals associated with the dense chromite ore are generally serpentine minerals or chlorite though forsterite is main gangue mineral of the disseminated ore.<BR>The above-mentioned features are one of the characteristics of the Alpine type chromite deposits, and these facts have been well known not only in Turkey but in Japan also. Following is a model interpreting the origin of the Alpine type chromite deposits.<BR>Harzburgite enclosing some chromite deposits is believed to be derived from the upper mantle material. Partial melting of upper mantle material rose through the crust in mush-state under the influence of the stress field of the Alpine orogeny. During this process, stratiform chromite occurs as the result of cumulus and most of chromite and olivine crystals may have been crushed or ground.<BR>Subsequently, gabbro, diorite, diabase, albitite or trondhjemite dykes derived from basaltic magma intrude. They are conformable with the joint directions and are particularly numerous in the marginal areas of the harzburgite.<BR>Hydrothermal solutions derived from the preceding dyke rocks in the deuteric stage may have formed the alteration of harzburgite and yielded the serpentinite. In addition, the hydrothermal solutions ascending along the fractures represented by the joint system may have changed the disseminated chromite ore to the compact dense ore occasionally including the nodular chromite. Hydrothermal minerals such as kämmererte or uvarovite are generally observed in the newly formed compact ore.<BR>Serpentinization resulted in average expansion of about 20% of the original volume. This expansion may have provided some of the impetus for renewed tectonic uplift to the present loci.<BR>During the uplift process of the serpentinite, above-mentioned chromite ore bodies may have changed their original shape and the Alpine type chromite ore bodies showing various irregular shape are formed.
- 社団法人 東京地学協会の論文