Metallogenic Geological Conditions And Prospecting Direction Of Lan Nitang Porphyry Copper Deposit In Shangri-La,Yunnan

The Lannitang porphyry copper deposit in Shangri-La,Yunnan province is located in the southern part of the Yidun island arc within the southwest Sanjiang metallogenic belt,which belongs to the Tethyan metallogenic domain of three main metallogenic domains of the porphyry copper deposit.In the study area,the strongly and widely mineralization and hydrothermal alteration constitute the unique features,various deposit types and abundant mineral resources are exposed.This study through field investigation,in combination with geochemical features of rocks and ores,the issues like hydrothermal alteration zoning,diagenetic age and setting,element geochemical features and metallogenic setting of the Lannitang copper deposit are discussed.Through field investigation and microscopic observation,this study defines that the hydrothermal alterations of Lannitang porphyry show some distinctions with those in the common porphyry deposits worldwide,the hydrothermal alterations include muddy zone(Ka),silicified phyllic zone(SiSe),prophylitic zone(ChEp)and rare potassium phenomenon.The above features are consistent with the zonation patterns of monzonite,mineralization in the silicified zone and phyllic zone is strong and in the prophylitic zone and muddy zone is weak.The exposed strata in the deposit mainly includes Qugasi group and Tumugou group,the second segment(T3t2)of the Tumugou group is the host strata of ore.Hydrothermal alteration and metamorphism are strongly occurred in accompany with mineralization in the contact zone between strata and porphyry.The high-grade mineralization is mainly occurred in the quartz monzonite porphyry,mineralization is better in the shallower emplacement.The regional mineralization is obviously controlled by structure,e.g.series of NNW trending complex fold affected by regional tectonic with fold related secondary cross extension joint and longitudinal extension joint.The secondary fault is usually the ore-hosting structure.The Triassic strata and late Indosinian intermediate to acid intrusive pluton are mainly exposed in the deposit.Based on element geochemical results,this study obtains crystallization ages of 204.9 ± 1.5Ma and 208.4 ± 1.8Ma for the quartz monzonite porphyry,the ages are comparable with those porphyries in Pulang,Chundu and Xuejiping.The rock samples are intermediate to acid rocks and they are plotted in the calc-alkaline field of the sub-alkaline series.The rocks show Sr*<1 which are related to the crystallization differentiation of plagioclase.Their Nb contents display negative anomaly,suggesting the ore-forming magma is I-type magma.The rocks are relatively enriched in LREE and depleted in HREE without Eu anomaly,indicating the crystallization of feldspar is not obvious.In combination with depletion of trace elements(Sr,Nb,Ta)and tectonic discrimination diagrams,the intrusive rocks in the Lannitang deposit are proposed to have formed in a volcanic arc setting related to plate subduction.Studies have shown that the Lannitang area have favorable geological condition of mineralization,there constitutes a complete "source,transportation,storage,cover"metallogenic system.During the mineralization,the intermediate to acid magmatic rocks provide the main ore-forming material and fluid.The Zange fault formed before the mineralization is the main rock-and ore-transmitting structure,the secondary fault F1 is the host structure of rocks.The joints and cracks in the intermediate to acid intrusion,structures in the contact zone of wall rock,and cracks in the wall rock are the host structures of ores.The wall rocks are performed as geological barriers during the mineralization,the result is consistent with the fact that the mineralization in the Lannitang deposit is enriched in the quartz monzonite porphyry.