Characteristics Of Apatite And It’s Geological Significance Of Hutouya Fe-Cu-Pb-Zn Polymetallic Deposit,Qimantag,Qinghai Province

Qimantag metallogenic belt is located in the western segment of East Kunlun orogenic belt,which is the junction of paleo-Asia and Tethys tectonic domain.It mainly experienced the breakup and closure of caledonian proto-Tethys ocean and the continental margin collisionorogeny of Indosinian Paleo-Tethys Ocean.The area had formed copper,iron,lead-zinc,tungsten-tin and other metal minerals mainly of skarn type,porphyry type and hydrothermal vein type through multiple periods of tectonic-magmatic activities.It has great prospecting potential and is one of the ten new strategic resource replacement bases in China.The Hutouya Fe-Cu-Pb-Zn polymetallic deposit is located in the middle of Qimantag metallogenic belt.Due to the complex strata,strong tectonic movements,multiple periods of magmatic activities,various mineralization types and variable metallogenic element combinations,there are still great differences in the understanding of the genesis of the deposit.In this study,apatites were selected from different magmatic rocks and mineralization types of the deposit,and technical means including SEM cathodoluminescence(CL),electron backscattering(BSE),electron probe(EPMA)and laser inductively coupled plasma mass spectrometry(LA-ICP-MS)were applied,to study the petrographic,chronological and geochemical characteristics of apatites.Combined with the analysis of geological characteristics of the deposit,the following conclusions and understanding were drawn:(1)The U-Pb dating results of apatite show that the hydrothermal vein type Cu-Pb-Zn ore was formed in 440～432 Ma,and the skarn type Fe ore was formed in 229～228 Ma.Therefore,it can be considered that there are Caledonian and Indosinian mineralization in Hutouya deposit.(2)The REE geochemical characteristics of the apatites indicate that the content of rare earth elements in hydrothermal vein type ore is 44～402ppm,and the distribution curve of rare earth elements shows a right descending type with weak Eu negative anomaly;The rare earth element content of skarn type Fe ore is between 3013～30434 ppm,with a right decreasing or slowly flat rare earth distribution mode,medium strong negative Eu anomaly.The REE geochemical characteristics of the two minerals are obviously different.(3)Combined with the geological characteristics of the deposit and the geochemical characteristics of apatite trace elements,it is believed that the Caledonian mineralization is related to the formation of Cl-rich,waterrich and high-oxygen fugacity magmatic fluid formed by metasomatism with the overlying mantle wedge during the subduction of the prototethyan oceanic crust in the early Paleozoic.The Indosinian mineralization is related to the partial melting of the lower crust after the late Paleozoic to early Mesozoic subduction of the Paleotethys Ocean,which resulted in the formation of Cl-poor,water-poor and low oxygen fugacity magmatic fluid.