滇东北地区层控Pb-Zn-(F-Ba)矿床的热液喀斯特成因

Hydrothermal Karst Genesis of Stratabound Pb Zn (F Ba)Deposits in Northeast Yunnan

滇东北地区上震旦统灯影组－下寒武统梅树村组中以白云岩为主岩的层控ＰｂＺｎ（ＦＢａ）矿床一直被认为是沉积或沉积改造矿床。但是，野外观察表明，它们是典型的、由成矿流体充填开放空间形成的后生矿床。被认为是同生成矿作用证据的条带状构造或“层状”构造却是成矿流体充填席状喀斯特洞穴的产物，以切割层理为特征。矿体具有典型喀斯特洞穴的形状。一些矿体呈球形———热液喀斯特洞穴所特有的形态。闪锌矿流体包裹体的均一温度为９０－１４０℃，共生萤石的ＲＥＥ地球化学特征和矿石的铅同位素数据表明，成矿金属来自上地壳而非玄武岩。成矿流体中水的δＤ和δ１８Ｏ值表明，成矿流体的水主要来自大气降水。ＰｂＺｎ（ＦＢａ）矿床有清楚的垂直分带：下部矿体形态不规则，有喀斯特洞穴的形态，Ｐｂ＞Ｚｎ，上部矿体呈层状、似层状（席状喀斯特洞穴形态），Ｚｎ＞Ｐｂ。

Stratabound Pb Zn (F Ba) deposits hosted by carbonate rock (dolomitite) of Zb dn 2-∈ 1 in northeast Yunnan have been considered to be sedimentary or sedimentary reworked deposits, But, our field observation indicates that they are typical epigenetic deposits with filling of ore fluid in open space. Banded or bedded structure to be regardered as an evidence for syngenetic ore formation turned out to be a product of filling of ore fluid in sheet karst cavities characterized by dissecting the bedding. The orebodies exhibit forms of typical karst cavities. Some orebodies occur in a spherical form which is regarded as characteristic of the hydrothermal karst cavities. Karst collapse breccia occurs in the margin of the orebodies. The footwall of the orebodies is uneven. Inverse triangular solution caves filled with barite, fluorite, galena and sphalerite occur in the footwall. The homogenization temperatures of the fluid inclusions in sphalerite from these deposits range from 90℃ to 140℃, while those in associated fluorite and barite from 70℃ to 120℃. The salinities of the inclusion fluid are estimated to be in the range of 10.24-23.99 wt NaCl %. The ore fluid had a density of 1.04-1.10g/cm 3. Sphalerite contains a large amount of organic or CO 2 inclusions. REE geochemistry of associated fluorite and lead isotope data of the ores suggest that the ore forming metals were derived from upper crust other than the basalt. The δD and δ 18 O values of inclusion fluid in sphalerite and fluorite suggest that water of the ore fluid was derived mainly from meteoric water. Strata from Sinian to Jurassic are conformable or pseudoconformable. The top horizon of Pb Zn (F Ba) deposits is Carboniferous and Permian. The basalt only contains a small amount of Pb Zn (F Ba) ore veins. There are no Pb Zn (F Ba) mineralization in tremendous carbonate rocks of the Triassic. The Pb Zn (F Ba) deposits are characterized by a clear vertical zonation: the lower orebodies occur as irregular shapes of karst cavities and are characterized by predominance of lead over zinc; the upper orebodies exhibit stratiform or stratoid forms of sheet karst cavities and are characterized by predominance of zinc over lead. From the above mentioned geologic geochemical characteristics such a metallogenic model is proposed. In the Late Permian, as a result of dissolution of salts and organic material, permeating downwards along the fractures and fissures meteoric water changed into a brine. Extracting the trace metals from the wall rocks on the way, this brine changed into an ore brine. In depth, under the influence of geothermal gradiant and basalt magmatism the ore brine changed further into a hot ore brine. Tectonism and magmatism drove it to rise along the fractures. The hot ore brine dolomitized and karstificated the carbonate rock of Zb dn 2,Simultaneously, when it run into sulfur which had been produced by reduction of sulfate by bacteria or abiotic reduction, galena and sphalerite (with barite and fluorite) were deposited out in karst cavities, forming irregular orebodies. When the hot ore brine flowed into carbonate rock of ∈ 1 under the pseudoconformity, because of shielding effect of the shale, it flowed laterally along the bedding. As a consequence sheet karst caves and stratiform or stratoid orebodies were formed.