GEOLOGY AND GEOCHEMISTRY OF THE CHANGBA AND DENGJIASHAN SEDEX LEAD-ZINC DEPOSITS IN THE QINLING OROGENIC BELT, CHINA

The Qinling Devonian polymetallic belt in China contains about 30 Pb-Zn deposits with total Pb+Zn reserves of about 20 Mt. These mineral deposits can be subdivided into two types that are represented by the world class Changba and the Dengjiashan deposit. Both deposit types are characterized by stratiform and lenticular orebodies with bedded and laminated structures. The major minerals are sphalerite, pyrite, galena, quartz, calcite, ankerite and barite. Sulfur isotope values for sulphides and barite are characterized by enrichment of ³⁴S, which is believed to be the result of intensive bacteriogenic reduction of the Devonian marine sulfate. Lead isotope ratios form an array indicating mixing between the Upper Crust and Mantle reservoirs. The Changba deposit is hosted in Mid-Devonian Eifelian clastic rocks and limestone, and has a feeder zone underlying the largest bedded orebody, which exhibits three sulfide facies zoned from bottom to top: banded sphalerite intercalated with quartz albitite, interbedded massive pyrite and sphalerite, and banded sphalerite intercalated with banded baritite. The REE pattern of sulfide layers and associated quartz albitite are similar to that of the host biotite quartz schists. The Dengjiashan deposit is hosted in chert conformable with underlying bioclastic micrite and overlying phyllites of the Mid-Devonian Givetian sedimentary sequence. Sedimentary facies indicate that the mineralization took place when the Dengjiashan sub-basin was subsiding from an open platform to a shallow marine setting under an extensional tectonic environment. The REE patterns of sulfide layers and host cherts suggest that the mineralizing fluids were dominated by oxydized seawater that became enriched in Eu2+ by leaching feldspar in the basement. It is concluded that the deposits formed as exhalative accumulations in Mid-Devonian sedimentary basins. The Changba deposit formed on a continental slope, whereas the Dengjiashan deposit formed in an organic bank environment.