Geochemical Characteristics And Chronology Of Skarn Minerals From The Jinchanghe Iron-copper-lead-zinc Polymetallic Deposit In The Baoshan Block,Western Yunnan

The Jinchanghe iron-copper-lead-zinc polymetallic deposit is located in the northern part of the Baoshan block in western Yunnan.It is a well-known skarn-type deposit in the southern part of the Sanjiang Tethys domain.In this paper,we present a detailed analysis of the skarn minerals in the deposit based on previous research results.By using the electron probe test（EPMA）and laser ablation micro-analysis test technology（LA-ICP-MS）,we were able to obtain the major,trace,and rare earth element data of skarn minerals（tourmaline and garnet）in the deposit.In addition,the LA-SF-ICP-MS U-Pb dating method was used to determine the direct mineralization age of the Jinchanghe deposit,indicating that it was formed in the Late Cambrian.This deposit may have been closely related to the arc magmatic rocks formed by the subduction of the Proto-Tethys Ocean.These specific conclusions are as follows:（1）The Jinchanghe deposit in the Baoshan block,western Yunnan,contains tourmaline in three different occurrences.Tur I consists of small particles,primarily in porphyritic（heteromorphic）;Tur II particles are of medium size,euhedral to semi-euhedral,and arranged in needle-column arrangements;the Tur III aggregates are mainly fan-shaped,radial,and bundle-shaped,containing large particle sizes and a high degree of euhedrality.According to the results,the alkaline tourmaline produced by the Jinchanghe deposit belongs to the schorl-dravite solid solution series,and its substitution mechanism is predominantly Fe Mg_-1,^x?Al(Na R²⁺)_-1and Fe³⁺Al_-1.The tourmaline in the Jinchanghe deposit primarily exhibits positive Eu anomalies and negative Ce anomalies,which indicates that oxygen fugacity was relatively high during its formation.Furthermore,the similar distribution patterns of rare earth elements in tourmalines in different occurrences suggest they evolved from the same hydrothermal fluid.During later evolution,the total amount of REE in residual fluids decreases,HREE does not change significantly,and LREE gradually enriches.In addition,tourmaline in the Jinchanghe deposit was mostly formed by fluids interacting with metamorphic volcanic rocks and metamorphic sedimentary rocks.（2）In the Jinchanghe deposit,garnets are composed of the grossular-andradite solid solution series,which can be divided into three generations:early（Grt I）,middle（Grt II）,and late（Grt III）.Grt I(And_55.69-64.47Gro_30.28-41.21Ura+Pyr+Spe+Alm_2.35-5.49),characterized by irregular shape and weak optical properties,primarily comprises Fe-rich grossular.Grt II(And_53.69-94.68Gro_2.78-45.07Ura+Pyr+Spe+Alm_0.59-2.12)is characterized by andradite with crystalline zones,of which Grt IIa is a homogeneous body,while Grt IIb shows oscillatory zones with abnormal interference colors.Meanwhile,Grt III(And_70.51-90.33Gro_6.34-27.20Ura+Pyr+Spe+Alm_1.03-3.32)is characterized by fine-veined andradite aggregates.Grt I,Grt IIa,and the rims of Grt IIb demonstrate HREE depletion,LREE enrichment,and positive Eu anomalies,indicating crystallization from weakly acidic magmatic-hydrothermal fluids.However,the Grt IIb core is HREE-depleted and LREE-enriched,showing a negative Eu anomaly,suggesting that it may have crystallized from a relatively neutral,Cl-rich fluid.There is a similar pattern of REE partitioning in Grt III and Grt II.However,Grt III is severely LREE-depleted with a positive Eu anomaly,indicating the presence of a weakly acidic environment.Accordingly,with the change in temperature and oxygen fugacity,the p H of the Jinchanghe deposit might have transitioned from early weak acidity to neutrality to late weak acidity.（3）In this study,LA-SF-ICP-MS U-Pb dating was also applied to date the garnets（Grt II,Grt III）of the Jinchanghe deposit,and reliable age data was obtained of520-496Ma,representing the skarn deposit.The metallogenic age of the Jinchanghe deposit indicates that the mineralization occurred in the Late Cambrian.Combined with the output characteristics of the samples,it is inferred that the formation of the deposit is linked to the Early Paleozoic magmatic activity widespread on the Baoshan micro-continent,as a result of the subduction of the Proto-Tethys Ocean.Based on the dating results of garnet（Grt I,Grt II）in the Douya deposit,it is estimated that the deposit was formed about 468-461Ma,indicating that the mineralization of the Douya deposit occurred during the Middle Ordovician.Furthermore,the metallogenic age of this deposit supports the hypothesis that the Jinchanghe deposit and adjacent deposits in the Baoshan block were formed in an older period.