Characteristics And Genesis Of The Guihuachong Copper Deposit In Anhui Province

The Guihuachong copper deposit is located in the transition zone between Tongling upfold belt and Fanchang depression basin. This deposit is a porphyry-skarn composite copper deposit newly discovered in the Tongling ore deposit concentrated region of Anhui Province, and appears characterized by occurrence of porphyry type orebodies. It is unique in major orebodies type and tectonic background. A detailed study has been carried out on the deposit, especially its porphyry type orebodies, with a contrast research on some other porphyry-skarn composite copper deposits of the region characterized by occurrence of skarn type orebodies. A genetic model for the formation of the Guihuachong porphyry-skarn composite copper deposit has been set up on the basis of ascertaining characteristics of the porphyry intrusion and deposit and discriminating alteration and mineralization zones and metallogenic periods.The petrological, mineralogical, isotopic chronological and petro-geochemical data prove that granodiorite porphyry in the Guihuachong copper deposit is a type of granite with mixing material source of mantle with crust, and belongs to a metaluminous high-K calc-alkaline series. Its age is defined to be(138.3±1.4) Ma by using LA-ICP-MS zircon U-Pb dating,suggesting that the rock was formed in the Early Cretaceous extending setting. A super-shallow magma chamber could exist during magma evolution process, and crystallization of magma occurred under a high oxygen fugacity condition after its rapid emplacement.The field geological investigation and microscopic observation indicate the occurrence of major porphyry- and secondary skarn-types of orebodies in the deposit, with ores of porphyry- and skarn-types. Based on their formation ways, the porphyry ores are divided into metasomatic, filling, and cryptoexplosive types. The metasomatic type of ores is characterized by metasomatic texture and disseminated structure, the filling type of ores by veined, net-veined, and massive structures, and the cryptoexplosive type of ores by brecciated structure. The alteration and mineralization zoning of the porphyry-type orebodies in the Guihuachong copper deposit is significant, from inner to outer, occurring potassic alteration zone, phyllite zone, and propylitization zone. Orebodies mainly exist in the phyllite zone. The alteration and mineralization process can be divided into four stages as follows: potassic alteration, phyllic alteration, quartz-polymetallic sulfides, and carbonation stages. The quartz-polymetallic sulfides stage is the most important mineralization stage. The amount of moving components in different alteration zones is calculated by using the method proposed by Grant(1986), showing larger variation of moving amount of major elements and little changing of moving amount of rare earth elements. The standard distribution pattern of REE in rock mass is consistent with the rock of alteration zone,and indicates that the rock mass is closely related to mineralization.The obtained geological and geochemical data support the formation of the skarn type copper deposit through contact replacement after intrusion of magma into carbonate strata and the generation of the porphyry type copper deposit through postmagmatic hydrothermal replacement, filling, and cryptoexplosion during the formation process of the Guihuachong composite copper deposit. The rapid cooling of magma after its rapid emplacement up to super-shallow level restrained the formation of skarn type copper deposit, but promoted the formation of porphyry type copper deposit. This may be the key reason for predomination by a porphyry type deposit in the Guihuachong porphyry-skarn composite deposit, but by a skarn type deposit in other porphyry-skarn composite deposits in Tongling.