Genesis and Characteristics of HydrothermalSedimentary Facies Forsiderite - Silver - Polymetallic Deposits in Yindongzi and Daxigou, Shanxi, China

Key words: 
• Phydrothermal sedimentary facies /
• subfacies geotheraml environment /
• rock and ore formation of hydrothermal sedimentary subbasin /
• tectomic environment /
•  /

Abstract: Siderite-silver-polymetallic deposits in Yindongzi and Daxigou,Shaanxi, Province, China, occur at the Middle Devonian hydrothermal sedimentary facies in the subbasin on the sea bed. There is a major economic orebody in Daxigou which is 2000 m long, from 10 m to more than 100 m thick, and there is the main economic orebody of silve-polymetals in Yingdongzi which is 1900 m long, from 1.0 m to 20.0 m thick. According to patterns of hydrothermal sedimentation,based on petrology,mineralogy, petrochemistry, geochemistry for the hydrothermal sedimentary facies,the hydrothermal sedimentary facies and their rocks can be classified into five subfacies, respectively,hydrothermal synsedimentary replacement-ebullitional subfacies,hydrothermal synsedimentary subfacies,hydrothermal synsedimentary-replacement subfacies,hot-brine vadose-replacement subfacies,and hydrothermal-mixing synsedimentary sub facies.First,the hydrothermal synsedimentary replacement-ebullitional subfacies consist of brecciated albite carbonate rock and ore-bearing brecciated albite siliceous rock.These brecciated rocks may have formed by the synsedimentary replacement and ebuilition of hydrothermal fluids along pouring passways (synfaults)in the subbasin on the sea bed,for hydrothermal fluids could explode and ebuilite along their passways covered by pre-existing hydrothermal sediments under the pressure of hydrothermal fluid overpassing total pressure of their overlying waterbody in the subbasin and their overlying sediments.Secondly,hydrothermal synsedimentary subfacies is made up of albite rock,sideritite, baritite. These rocks essentially stratiform with their elongation parallel to the bedding and with subfacies change and thinning-out. For examples,Bariteolites were formed by the Ba-richi sulfate hot-brine in a way of rapid chemical precipitation in the subbasin on the sea bed while sideriteolites were produced by Fe-rich carbonate hydrothermal fluid in the same way at the same subbasin.These rock layers are characterized by bedded,laminated,layer-massive structures which change regularly in the subbasin.Thirdly,the hydrothermal synsedimentary replacement subfacies include siliceous ferrodolomitite,ferrodolomite,dolomitic limeston and, siderite ferrodolomile.These hydrothermal rocks occur in the footwall rocks of the orebody or in the pinning-out layer of hydrothermal rocks.They might have been produced by the synsedimentation of the hydrothermal.At the same time, synmetasomatism between the hydrothermal fluids and soft sediments on the bed or the slope of the subbasin could take place because replacement structures are easily recognized in the field and lab study.Results of research on minerology,structures and textures of the rock and petrochemistry have suggested that SiO 2,Al 2O 3 and FeO from the hydrothermal in the subbasin has been extensively replaced in the underlying carbonate sediments on the sea bed. Fourthly,the hot-brine vadose-replacement subfacies is composed of scapolite biotite hornstone and scapolite slate. These rocks can be formed by K, Na and Cl-rich hot-brines during their diagenetic processes in the extensively geothermal environment but it is difficult for them to be formed by the processes related to magmatic activities during superimposed stage with the exception of the hornstone enclosing magmatic intrusions. Finally,the subfacies of mixing of different chemical components and physical-chemical states of the hydrothermal fluids,one of the most important subfacies,include sulfide rocks and albite chert with ore formation. These rock layers or ore layers are characterized by bedded,laminated,layer-massive, graded bedding and graded rhythmite.and chemical bedding, The most important characteristic exists in these rock layers or ore layers.The chemica lbedding is as follows: laminated albite layer→albite-sulfide layer→albite-carbonate mineral-sulfide layer→albite-barite laminated layer→lamineated barite layer→barite-albite-chert layer→ferrdolomite lamnated layer.