In-situ compositional analyses of garnets from the Baoshan Cu-Pb-Zn polymetallic deposit in the southern Hunan and their geological significances
The Baoshan Cu-Pb-Zn polymetallic deposit is located in the central-western part of the southern Hunan W-Sn polymetallic ore concentration area.It is tectonically located in the collisional contact zone between the Yangtze block and Cathaysian block.There are mainly two types of garnets developed in the deposit.The type-Ⅰ garnets(Grt-Ⅰ)are gray-grayish green ones with obviously rhythmic rings.The type-Ⅱ garnets(Grt-Ⅱ)are brownish red or light red ones without ring.They are relatively broken and occurred in form of vein.In this study,laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)was used to analyze compositions of two types of garnets.Our results show that the Grt-Ⅰ is mainly rich in Fe,belonging to andradite(And91-93Gro5-8Spe+Pyr+Alma.71-0.79).Compared with the Grt-Ⅰ,the Grt-Ⅱ has relatively low Fe content and relatively high Al content.It is mainly characterized by the increase of grossularite composition.It belongs to the andradite-grossularite solid solution series(And52-66Gro32-47Spe+Pyr+Almi.10-1.37).The total REE contents of garnets are relatively low(42.60-201.50 ppm,excluding Y).The Grt-Ⅰ garnets have relatively high values of ΣREE,LREE/HREE,and δEu comparing to the Grt-Ⅱ garnets.The Grt-Ⅰ garnets have strong Eu positive anomalies with steeply right-declined REE distribution patterns,while the Grt-Ⅱ garnets have medium Eu negative anomaly,with slightly left-declined REE distribution patterns.The two types of garnets are both depleted in the LILE including Rb,Ba,and Sr,but enriched in the HFSE including Nb,Zr and Hf.The enrichment degrees of the Grt-Ⅰ garnets are generally lower than those of the Grt-Ⅱ garnets The two types of garnets have different end member components,Eu anomalies and other characteristics,indicating that the Grt-Ⅱ garnets whic are rich in Fe were formed in an oxidized(high oxygen fugacity)and alkaline environment,while the Grt-Ⅱ garnets were formed in a weakly oxidized-weakly reduced and weakly alkaline environment.This shows that the oxygen fugacities of the formation environments of garnets from the Grt-Ⅰ to the Grt-Ⅱwere decreased gradually,and the formation environments of garnets were changed from the oxidation one for the Grt-Ⅰ to weakly oxidized-weakly reduced one for the Grt-Ⅱ.The contents of ore-forming metal elements including Cu,Zn,Mo and Pb are relatively low in two types of garnets,while Sn contents of garnets are up to tens or even hundreds of times of the average Sn content of the crust(2.00 ppm),indicating that the garnet skams in depth could be favorable for the enrichment of Sn and could have Sn metallogenic potential.
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