ANALYSIS OF THE CONDITIONS FOR ELECTRUM AND POLYMETALLIC DEPOSIT FORMATION IN THE WUDANG MOUNTAINS AREA,HUBEI PROVINCE,AND PRELIMINARY RECOGNITION ON THE DIRECTION IN MINERAL PROSPECTING

The Wudang Mountains area is a part of the Wudang-Huiyang uplift in the Southern Qinling Indosinian miogeosynclinal foldbelt of the Qinling geosynclinal system. The exposed strata include the Proterozoic Wudang Group, Sinian, Cambrian, Lower Ordovician and Tertiary. In recent years, a large-sized electrum deposit and a medium-sized polymetallic deposit have been demonstrated preliminarily and a number of polymetallic occurrences that deserve further work have been found. Our recent research results show that the conditions of formation and law of distribution of the electrum and polymetallic deposits in the area are controlled by the following factors. 1. The distribution of the deposits are controlled by definite stratigraphic positisons of the Proterozoic Wudang Group. According to the lithologic associations, structural superimposition relations and part of isotopic age data, the Wudang Group may be divided into five formations; from below upwards they are: the Loutai Granulitite Formation, the Yintonggou Metavolcanic Formation, Cangping Quartzite Formation, Shuangtai Phosphatic Schist Formation and Longwangmiao Metavolcanic Formation. Electrum and polymetallic ores are controlled by the Yintonggou Metavolcanic Formation and the orebodies are localized in the upper part of this formation. 2. Ore deposition is controlled by definite lithologies. The Yintonggou Metavolcanic Formation is divided into two units according to the lithologic association: the upper unit is dominated by parametamorphic rocks, and the lower unit by metavolcanic rocks. According to the textures and chemical composition of the rocks the lower unit may fall into the K-rich acid volcanic rocks and Narich acid volcanic rocks. Electrum and polymetallic rocks and their contacts. The controlling effects of these lithologies are mainly related to their metal content. As shown by statistical analysis of the background contents of the ore-forming elements in varius lithologies in the area shows that the contents of silver and gold as well lead, zinc, molybdenum and copper in the K-rich acid volcanic rocks are higher than in other lithologies and that the lead and zinc contents in the Na-rich acid volcanic rocks are higher than in other lithologies too. 3. Distribution of mineral deposits, mode of occurrence of orebodies and association of elements are controlled by Yintonggou-aged paleovolcanic structures. From studies of the sulfur and oxygen isotopes in the deposits and the cobalt-nickel ratio in pyrite, it is considered that such type of deposit in the area is of volcanic origin. According to the position where the deposits are localized and reconstruction of volcanic structures, this type of deposit occurs at the top of the Yintonggou-aged paleovolcanic domal structure and in its vicinity. This controlling effect is also manifested in （1） control over the mode of occurrence of orebodies and intensity and zoning of wallrock alteration, and （2） control over the zoning of the element association. In other words, for the deposits located at the top of the paleovolcanic dormal structure, the wall-rock alteration is strong, the orebodies mainly occur as veins and the element association is dominated by silver and gold; for the deposits far from the paleovolcanic domal structure, the wall-rock alteration is weak, the orebodies mainly occur as layers, the element association consists mainly of lead, zinc and copper and the orebodies exhibit strong structural deformation. 4. Tectonism and metamorphism have palyed a certain role in the modification and enrichment of the deposits. Isotopic age determination on the ore-bearing wall-rocks and orebodies shows the time of ore formation to be greater than 1,307m. y. After their formation,the orebodies were subjected to polydeformation and the ore veins exhibit the features of polyformation. According to the mutual relations of the ore veins and their deformation features, three phases may be distinguished: Phase Ⅰ——ore formation phase; Phase Ⅱ——deformation-modification and metamorphism-en- richment phase;Phase Ⅲ——destruction-reconstitution phase. Analysis of the isotopic age data in the area shows:Phase Ⅰ＞1,307m.y.; Phase Ⅱ 308—422m.y.,belonging to late Caledonian time; Phase Ⅲ＜300 m.y.-＞200m.y.,belonging to late Variscan time. The above-mentioned analysis shows that the leading ore-controlling factor is the Yintonggou-aged paleovolcanic domal structures,These paleovolcanic domal structures often occur as brachyanticlines. Through studies it is revealed that these brachyanticlines often occur in the compounding loci of E-W-trending anticlinal structures and N-S-trending anticlinal structures. This understanding points out the direction in further mineral prospecting in the area.