Characteristics of the Late Devonian Organic Reef in Sanlixia,Zhenan,Shaanxi Province and its Control over the Occurrence of Pb一Zn Deposits

The reef front, reef core, reef flat and reef back could be distinguished in the Late Devonian organic reef in Sanlixia, which is 180 meters thick and has a width of 4000 meters. The N一S trend chain of reefs, including the reef in Sanlixia and other other south reefs,comprises the N一S barrier reef complex in Zhenan一Xunyang region. The fossils in the reef include stromatoporoids, corals, chaetets, alga, brachiopods, crinoids,gas- teropods, bryozooids, ostracods, lamellibranchiates tentaculitids etc.,totally amounting eleven kinds. The former four kinds are major reef一building organisms, and make up about 90 0 o of tolal organisms in the reef. Brachiopods and crinoids are major adherent organism. The organic reef developed in the margin of a horst type platform during the development of a continental margin rift一faulted depression basin, and the development of the reef could be divided into four stages; bank一reef stage, growth, prosperous stage and declining stage. Organisms during bank一reef stage were mainly brachiopods, crinoids。“alga, which formed organic bank. Organisms during the growth stage were mainly branched coral, tabular stromatoporoids and alga,which formed bafflestone and bindstone. All kinds of organisms could be found in different facies in the prosperous stage, for example, lamp stromatoporoids, corals,and chaetets formed framestone in the reef core, branch stromatoporoids and corals formed the baffle- stone in reef core and reef flat, tabular and crustified stromatoporoids and alga formed bindstone in reef flat. And the adherent organisms distrbuted in all facies. Branch coral were the major reef一building organism in the declining stage, which formed the bafflestone. The reef came to an end by the increasing of the water depth and mud. The synsedimentary rift一faulting resulted in the uplift of submarine topography, thus contributing to the growth of the reef-building organisms, and the relief of submarine topography and variation of sedimentary facies was enhanced by the rapid development of organic reef , which resulted in the lagoon graben basin suitable for containing and deposition of hot ore一bearing brine.