Genesis Of Skarn In Xiekeng Copper And Gold Deposits, Xunhua County, Qinghai Province

Xiekeng Cu-Au deposit, which is located in the east of Qinghai-Tibet Plateau and to the west end of the western Qinling orogenic zone, is a small-scale skarn-type polymetal ore deposit, which is mainly composed of Cu-Au ore. The skarn of the deposit has its special characteristics and carrys reliable information on mineralization by further reseach.Acording to studying on the Skarn and ore of Xiekeng Cu-Au deposit, it is found that the skarn of the deposit is main diopside-garnet skarn. Garnet is main andradite and grossular, which shows anomalous interference colour and zoned texture; Diopside is mainly composed of salite; Ore-mineral association is chalcopyrite, pyrite, niccolite, magnetite and gold.Basing on studying the petrological characteristics and geochemical characteristics of constant element, trace element and rare earth element on skarn, the original rock of the skarn are traced and the geochemical characteristics of trace elemen and rare earth element are summarized:According to thin sections and analysis of constant element of skarn, the rock outcrops in the deposit is not intermediate-acid intrusive rocks, such as diorite, granite-porpphry, but intermediate-basic intrusive rock, such as gabbro, pyroxene-diorite and anorthosite-porpphry.Through tracing the original rock of the skarn, which mainly comprises marlitic sediment and carbonate, secondly is an igneous, basic volcanic rock and partial marlitic rocks.The characteristics of trace element: During the course of metamorphosation of protolith to skarn, it shows strong inheritance to the substance of protolith. At the same time, in the process of metamorphosation of different protolith to skarn, except some elements, most elements show the similar evolution characteristics: Among strong incompatible elements, Ba and Rb strongly deplete, LREE show enrichment to some extent; Compatible elements all deplete; Nd and Sm changed inevidently in the whole process.The characteristics of rare earth element: In the process of protolith to skarn, HREE show high concordance that they all deplete and the degree of fractionation is high; Intermediate REE show relatively high concordance, which all deplete, but the degree of fractionation is high and the difference exist in the differentiation of Eu. The anomaly of Eu changed from negative anomaly to positive anomaly in marlite skarn and diorite granite-porpphry skarn; The anomaly of Eu of diorite skarn is inevident; A comparatively high discrimination exists in the process of the differentiation of LREE: La, Ce and Pr of diorite skarn enrichded and La, Ce and Pr of diorite granite-porpphry skarn depleted, but Nd of the two tpyes of skarn inevidently changed; La of marlite skarn enriched, Pr and Nd depleted, Ce inevidently changed. Though the differentiation existed in the REE pattern graph between the skarn and the original rocks, they all keep the same shape with the REE pattern graph of the original rocks(Especially the basical same shape of the REE pattern graph of diorite skarn and diorite) and it shows the homology between skarn and the original rocks.Integrating the laws and characteristics of the differentiation in the trace element spider graph and the REE pattern graph, it is concluded that there are two genetic types: one genetic type is contact-metasomatic metamorphism that hydrothermal is involved, the other is contact-thermal metamorphism that hydrothermal is not involved and skarn is formed by the contact-thermal metamorphism from diorite.