The Petrogenesis Of The Skarn And The Magmatic Origin In The Yangla Copper Deposit Area Along Jinshajiang Belt,Western Yunnan Province

The Yangla deposit is tectonically located in the western margin of the Jinshajiang suture zone. It is a well-known skarn deposit in Sanjiang metallogenic belt and is typical in Jinshajiang belt. This study was carried out based on fully understanding previous achievement and through field investigation, microscope observation, and analyses. The aim of the research is to explore the petrogenesis of skarn and the control of magmatism in the ore-forming process. We make a simple conclusion about the geochemical characteristics of the Cu-Mo-Au fertile magmatic rocks around the Yangtze block. The rocks are from Sanjiang, Qinhang and Qinling orogenic belt.The deposit is a typical calcic skarn deposit that is spatially and temporally associated with the granodiorite and quartz monazite porphyry rather than the later monzonitic granite. The ore-forming process can be divided into skarn stage, retroprograde altered stage, quartz sulfide stage and carbonate-sulfide stage. The skarn minerals include garnet and pyroxene, and are mainly produced from infiltration metasomatism. The electron microscope analysis reveals that the garnet is mainly composed of andradite and minor grossular, the pyroxene consists of diopside and hedenbergite. The characteristic match the typical skarn copper deposit around the world, and the skarnization provides the necessary flowing passage for later hydrothermal fluid.The intrusive units are composed of syn-mineralized dioritic enclaves, granodiorite, quartz porphyry(238-230 Ma), and post-mineralized granite stocks and monzogranite dikes(~223 Ma), bracketing a time span of ca. 15 Myr. The uniform Hf isotope characteristics during a ~15 Myr period suggest a primary control of the isotopic signature by a stable, long-lived, hot reservoir in the deep lithosphere. The occurrence of mafic enclaves and the identical intermediate initial εHf values(-5.9 to 1.7) of granodiorite, granite, and monzogranite, suggest that the felsic magmatic system was produced by remelting of Neoproterozoic lower crustal rocks that mixed with minor amounts of mantle-derived melts. The incorporation of the crustal component into the residual magma from which quartz porphyry formed, reduced the f O2 of the mineralized and barren quartz porphyry and settled much of dense sulfides out at depth.