甘肃厂坝铅锌矿床喷气沉积成因研究

A STUDY OF THE EXHALATIVE ORIGIN OF THE CHANGBALEAD-ZINC DEPOSIT,GANSU PROVINCE

厂坝铅锌矿床矿体为层状，具有韵律性沉积层序，矿体内发育喷气沉积岩，并且在矿体下盘发育特征的网脉状矿化蚀变带，在矿体外围有富阳起石蚀变岩。这些地质特征充分说明矿床由喷气沉积作用形成。矿床的微量元素、稀土元素及硫、碳－氧和铅等稳定同位素特征表明矿床的硫和碳来源于古海水，成矿金属来源于矿体下伏地层柱。据此，提出了厂坝超大型矿床的形成机制。

Changba Ph--Zn deposit, located in Cheng County, Gansu Province,is the only superlargeore deposit in the Devonian polymetallic mineralization belt in the Qinling Ranges. The stratiftitmorebodies show Characteristics of rhythmic sedimentation, varying upward from massive, banding to laminated structure.In the orebodies,there are such sedimentary exhalative rocks asquartz albitite and baritic rock interbedded with banded metallic sulfide ore.At the footwall ofthe orebody was found a typical network mineralized alteration zone which is composed of lenticular quartz albitite and sulfide networks. Around the orebodies, especially in the rocks underlyingthe orebodies, there are also actinolite--dominated altered rocks which are more extensive than thenetwork zone. These characteristics fully demonstrate that the ore deposit was foamed throughsedimentary exhalation. In addition, the S and C-O isotopic composition shows that the S and Cof the ore came from the sea sulfate and carbonate respectively.And the REE distributive patternand Ph isotope composition indicate that the metal elements of the ore were derived from the strata column underlying the orebodies.Based on these geological and geochemical characteristics, the following metallogenic mechanism is suggested. The heat anomaly in Changba area caused the sea water to seek downwardand formed a convection cell.During its convection, the heated sea water leached the metal elements out of the strata it passed through to form ore--bearing fluid.When the fluid exhalated tothe sea floor, it deposited sulfides and felsic minerals to form ore bed and sedimentary exhalativerocks respectively.At the same time i the fluid replaced the rocks around the outlet to form thenetwork mineralized alteration zone and the actinolite--dominated altered rock. Because this pramcess occurred periodically within a relatively short time and there were no geologic process to ruinthe formed orebodies,Changba became a superlarge exhalative ore deposit.