In-situ Trace Element Study Of Altered Minerals In The Yangla Copper Deposit In Northwestern Yunna

The Yangla Cu-polymetallic deposit in northwest Yunnan,as a typical representative of the contact account genesis of the"Three Rivers"metallogenic belt in southwest Yunnan,has preserved the information of hydrothermal evolution intact.At present,the Yangla copper deposit is facing resource depletion,and there is an urgent need to improve the theory of ore finding and innovate exploration techniques to achieve a technical breakthrough in exploration to alleviate the resource crisis.On the whole,it appears that the Yangla deposit is characterised by a series of alterations from the intrusive body to the wall rocks,with distinctive alteration zones in which characteristic alteration minerals such as garnet,epidote and chlorite are developed,among which silicification associated with granodiorite is most closely related to mineralization.However,the lack of distribution pattern of alteration、mineral fingerprint records and indications for mineralisation exploration,affects the comprehensive understanding of the"fluid evolution-alteration-mining"of the deposit directly and hinders further mineralisation exploration.In this paper,the geological characteristics,alteration mineralisation and mineralisation processes of the Yangla Cu deposit are investigated using detailed geological mapping,systematic microstructure and LA-ICP-MS trace element analysis.In this paper,the geological characteristics,alteration mineralisation and mineralisation processes of the Yangla copper deposit are investigated using detailed geological mapping,systematic microstructure and LA-ICP-MS trace element analysis.In this paper,we analyse the alteration mapping of the Line 41 section in the section of 3250and find that garnet skarn and pyroxene skarn are mainly formed close to the rock（near source）and actinolite-pyroxene skarn is mainly formed away from the rock（distant source）.The main skarn assemblages developed at the Yangla copper deposit are garnet and pyroxene,which are subject to different types of retrograde alteration,forming retrograde alteration minerals such as actinolite and chlorite.The garnets of the Yangla Cu deposit can be divided into Grt Ⅰ and Grt Ⅱ.Grt Ⅰ is belongs to the grossular-andradite garnet solid solution(And_57.87-77.08Gro_21.21-42.45)and is enriched in trace elements such as HFSE,Y,Sc,Ti,V,Co and Ni;Grt Ⅱ is more enriched in Fe,of the calcium-iron garnet series The Grt Ⅱ is more Fe-rich and belongs to the andradite garnet series(And_93.63-98.95Gro_0.31-5.84),with metallic mineral assemblages such as pyrite and chalcopyrite.Based on the chemical compositions of garnet,it is suggested that the Grt Ⅰ may have formed under mildly acidic,oxidized and low W/R conditions,whereas the Grt Ⅱ may have formed under acidic,oxidation,and high W/R conditions.The pyroxenes of the Yangla Cu deposit are mainly diopside and hedenbergite.Diopside(Hed _4.07%-30.07%)mainly coexistes with the Grt Ⅰ and enriched in Sc,Zr and Hf,while hedenbergite(Hed_{69.10%-94.98%})coexistes with the Grt Ⅱ and enriched in Y.The amphibole group in the area is mainly iron-actinolite and tremolite,which belong to the actinolite-tremolite series.The chlorite in Yangla copper deposit can be divided into early and later types.The early type of chlorite（Chl-Ⅰ）coexists with the andradite,chalcopyrite and other sulfides,and the later type of chlorite（Chl-Ⅱ）coexists often with a large amount of calcite.Both types of chlorite belong to Mg-rich chlorite with trioctahedral structure,indicating that they were formed in a relatively reduced environment.Its tschermark substitution mechanism is the main substitution mechanism of two types of chlorite major elements;The calculation results of geological thermometer of chlorite range from 140℃to 281℃,with an average of 224℃.The temperature of the two types of chlorite gradually decreases from Chl-Ⅰto Chl-Ⅱ,indicating that the mineralization in this period belongs to the range of medium and low temperature hydrothermal alteration;In the early mineralization stage（Chl-Ⅰ）→late mineralization stage（Chl-Ⅱ）,the concentrations of trace elements（Sc,Ti,Ga,V）in chlorite decrease,which may be related to the gradual decrease of hydrothermal fluid temperature,indicating that the occurrence of high concentrations of Ti,Sc,Ga,V and other elements may indicate that they are closely related to mineralization,thus their occurrence can be used as an effective mineralization indicator.Therefore,the study of the composition of trace elements in chlorite is of certain significance for the exploration of the deposit.