THE DIAGENESIS OF CARBONATE ROCKS IN THE MIDDLE-UPPER DEVONIAN SERIES AND ITS RELATION TO THE FORMATION OF Pb-Zn ORE DEPOSIT IN HEQING DISTRICT, CENTRAL CHINA

On the basis of geology, petrology and geochemistry, the diagenesis of carbonate rocks in the Middle-Upper Devonian series in Heqing district and its relation to the formation of Pb-Zn ore deposit are studied by means of field investigation and various techniques in laboratory in this pater. In the studied area the outcroped strata are mainly three formation: the Middle Devonian Qiziqiao Formation (D_2q), the Upper Dvonian Shetianqiao Formation (D_3s) and Xikuangshan Formation (D_3x). According to the differences in lithologic characters, Qiziqiao Formation can be subdivided into three members, Shetianqiao Formation and Xikuangshan Formation both into two members. All of them are mainly composed of limestone with few of terrigenous sedimentary rocks. Sedimentary characteristics show that the Middle-Upper Devonian series may be classified into five microfacies (1) intertidal mud flat (including tidal-flat in clear water and tidal-flat in muddy water), (2) grain beach, (3) bioreef, (4) restricted bulf, (5) subtidal tempestite. The field investigation, observation under microscope and distribution characteristics of carbon and oxygen stable isotopes and the trace elements show that the Middle- Upper Devonian carbonate rocks have four principal diagentic manners i. e. : compaction, cementation, neomorphism and dolomization. Compacton is one of the most common diagentic manners, causing decrease of porosity and compact of lime. Neomorphism make micrite enlarge into microsparry and pseudosparry, and considered as the second one. The Middle- Upper Devonian carbonate rocks in studied area experienced four diagenetic environments. They are: (1) Vadose zone of freshwater, (2) Phreatic zone of freshwater, (3) Seawater and (4) Burial- underface dorage zone, among which the influnce of burial-underface dorage zone diagenetic environment is the largest to carbonate rocks in the area. The properties of carbon-oxygen stable isotopes and trace elements suggest that the diagenetic environments of bioreef, grain beach, tidal-flat in clear and subtidal tempestite are open relatively and the diagenetic fluid are active and fresh water flow in. But the diagenetic fluid of tidal-flat in muddy water and restricted gulf are stagnant relatively. As the result of the the open diagenetic environment over ore-bearing strata and the stagnant of diagenetic fluid beneath it, burial-underface dorage diagenesis made mineral elements to be enriched into industrial deposit.