BASIC CONFIGURATION OF THE GENGMA BRUSH STRUCTURE AND ITS SIGNIFICANCE FOR MINERAL PROSPECTING

The study and determination of the Gengma brush structural type contained in the western Yunnan meridional structure are of great geological significance for studying the crustal movements, direction and mode of the stress action,magmatism and metamorphism in various times and formation and distribution of ore deposits in western Yunnan. The Gengma brush structural type has been defined through many years of mineral prospecting and by applying the geomechanic view-point and method. The Gengma brush structure consists of a series of compresso-shear faults and axial surfaces of folds.They are distributed in a vast area east of Mengding-Yunxian, west of Yunxian-Daxing-Muga and north of Ximeng （including the Mandon-Yishi area of northeast Burma）.The generation of the Gengma brush structure has a close genetic connection with the generation and development of the NE-trendingMengding-Yunxian fault.Therefore,the stress tnansformation of the Mengding-Yunxian fault is bound to bring about changes of the brush structure. As the Mengding-Yunxian fault has mainly undergone the meridional compression （which last the longest and acted the strongest）,the area where the Gengma brush structure lies is in the main in the compresso-shear stress field and in the state of counterclockwise shearing. The Gengma brush structure, especially its middle and southern sectors, has an important controlling effect on polymetallic ore deposits in Gengma, Changyuan and Ximeng. So mineral prospecting work in this area should be strengthened in the future, which will lead to the discovery of more and richer tungsten, tin, gold and silver deposits for the country.