Skarn Mineralogy Characteristics And Metallogenic Mechanism Of The Yangla Deposit In Northwestern Yunnan,China

The Yangla copper deposit is located near the middle part of Sanjiang Tethyan orogenic belt.It is a famous copper deposit in the Sanjiang Metalogenic Domain,and catains siganficant amounts of Cu,Pb and Zn.This study aims to resolve the problems relating with skarn mineral assemblages and paragenesis,ore-forming processes and to build a metallogenetic model in the Yangla deposit,which combined with modern analytical technology such as Back Scatter Electron?BSE?,Electron Probe Microanalysis?EPMA?,LA-ICP-MS and stable isotope geochemistry.According to this study,the Yangla deposit is a typical skarn deposit.The ore bodies are layered occurred in the contact zone between acid rocks and carbonate rocks or nearby,but the deposit is not a product of carbonate rocks dirsct contact with magmatic rocks.From the magmatic rocks to marble are,in order,the endoskams is rare,garnet skams?Grt>Pyx?to pyroxene skams?Pyx>Grt?.Mineral assemblages and paragenesis imply the mineralization occurred three stages:prograde stage?e.g.,garnet and pyroxene?,retrograde stage?e.g.,grunerite-actinolite,epidote and chlorite?,quartz and Fe-Cu-Pb-Zn sulfides stage,calcite and Fe-Cu-Pb-Zn sulfides stage.The mineral assemblages and paragenesis of Yangla deposit is similar to typical skarn-type deposit.The electron microscope analysis shows that the garnet is mainly composed of andradite,the pyroxene consists of hedenbergite.Three garnet types have been identified in this deposit:Al-rich garnet(And_74-99,GrtI)has asymmetric girdle.Fe-rich garnet(And_71-99,GrtII)has symmetric girdle.Fe-rich garnet(And_93-98,Grt?)is homogeneous body without girdle.The REE distribution patterns are characterized by LREE enrichment and positive Eu anomalies,which indicate the lg?O₂ in the early skarn stage is high.The mineral assemblages and paragenesis of garnet and pyroxene confine the early skarn was formed between 430⁶00?,with lg?O₂ from-26^-19.Sulfur isotopic study shows that the sulfur of the sulfides is from the magma.Pb,H,O isotopic study shows the ore-forming fluids are mainly magmatic hydrothermal.The ore-forming fluids were origined from magma mixing with precipitated water in the later period,while the mineralization elements were mainly from magma.The ore-forming fluids evolved from high temperature and high salinity conditions to low temperature and low salinity conditions,and experience dextraction of magma,phase separation of supercritical fluid,decompression and boiling,the filling or metasomatism between magmatic hydrotherm-volatile and carbonate wall rocks.In the processes of water-rock interaction,magmatic hydrothermal and meteoric water was boiled and mixed.Boiling and mixing could be a significant mechanism of the formation of mineralization.