TITHOFACIES AND PALEOGEOGRAPHY ENVIRONMENT OF STRATA-BOUND Pb Zn ORE DEPOSITS IN FENGXIAN-TAIBAI COUNTIES, SHAANXIPROVINCE

The ore field richin Pb zn ore deposits in the Fengxian-Taibai counties is located in the middle of the Qinling Mts.. The orebearing bed mainly exists in the transitional sedimentary beds between the Gudaoling Group and the xinghongpu Group of the Middle Devonian Series. It can be divided into two ore-bearing beds. The upper bed is the predominant one, forming the huge and the medium-sized ore deposits in this area. This paper briefly deals with the distribution of the sea and the land, the sea floor reliefs, the hydrodynamic conditions and the physico-chemical conditions during the sedimentary period of the main ore-bearing bed of the ore field in the Fengxian-Taibai counties. By the texture analysis section, it describes the evolutionary process of facies from the platform marginal shoal facies to the platform marginal bioherm facies-the localized platform facies-the platform marginal shoal facies-the platform marginal bioherm facies of the Middle Devonian (Lower Givetian) since the ore field is divided into two along the South-north line of Hetaoba, we have found a marked difference of the ore-forming characteristics between the east and the west of the ore field. The geographical distribution of some huge and medium-sized ore deposits clearly shows that the restricted environment forming the barrier island of the bioherms is favorable for the ore-formation. The field work has proved that the distribution of ore deposits is controlled by the barrier island of the bioherms in the neighboring region of the Weiziping. The ore-bearing bed was formed at the end of the two reef-building periods during the Middle Devonian (Lower Givetian), and the formation was closely related to the bioherms. Thus, the advantageous foundation was provided for the ore beds of later concentration under all kinds reconstruction process. In a word, the ore field in the Fengxian-Taibai Counties belongs to a kind of the strata-bound ore deposits, and some promising areas have been indicated in further prospectings.