Study On The Gold Polymetallic Ore-forming System Of The Alkali-rich Porphyry Porphyry In The Heqing Area Of ??western Yunnan

The Beiya gold-polymetallic deposit is located at the western margin of Yangtze plate,also the largest one in the Jinshajiang-Honghe alkaline-rich porphyry blet.In this work,we specificly studied the mainly mineralization developed in the Beiya deposit and the petrological,istopic and chronological geochemistry of the Beiya alkaline-rich porphyry,concluded the new point of "porphyry metallogenic system"in the Beiya mining area.Main findings in this study are as follows:1.With the center of magma activity(the alkaline-rich porphyry),there are mainly three types of minetalization surrounded,named:(1)lenticular,stockwork porphyry Cu(Mo)-Au mineralization developed in the cracks in the porphyry,(2)non-shaped skarn Au-Cu-Fe ore bodies occur alone the contact part between the porphyry and the Triassic carbonate wall rocks,and(3)layer-like,veinlet epithemal Pb-Zn-Ag(Au)poly-metallic mineralization develops widely in the distal carbonate statas,which consist a zoning patterned porphyry metallognic system.2.We firstly obtained the 32.8±1.9 Ma(MSWD=0.21)ore-forming age of the Pb-Zn-Ag(Au)poly-metallic mineralization by using sphalerite Rb-Sr isotopic dating,demonstrating the distal Pb-Zn-Ag(Au)poly-metallic mineralization developed in the stratas is the evolution product of the hydrothermal derived from the Eocene alkaline-rich magma.3.Systematically analysis on the fluid inclusions in Beiya show the same "fluid inclusion group" pattern in both the ores of the three main mineralization and quartz phynocryst in alkaline-rich porphyry,indicating the ore forming fluids mainly originated from the magma hydrothermal.Migration of ore forming fluids in Beiya is primarily driven by the gradient differences in temperature,desity and pressure of fluids from the central alkaline-rich porphyry to the distal wall rocks.Sole fluid immiscibility,mixing of diverse-sourced fluids and water-rock interaction are the principal mechanisms to ore deposition in different mineraliration stages.Sulfur isotope compositions of sulfides in the three main mineralization are consist to those of the alkaline-rich porphyry as deep sourced magmatic sulfur,suggesting the sulfur in the Beiya polymetallic deposit are dominately originated from the alkaline-rich porphyry.Lead isotope compositons of ores are the same as that of the alkaline-rich porphyries but different to that of the wall rocks,implying a similar magma source as well.(87Sr/86Sr)0 values of sphalerites in the distal epithermal polymetallic mineralization are also identical to those of the alkaline-rich porphyries,comprehensively indicating the ore forming materials are originated from the Beiya alkaline-rich porphyry.4.Geochemical characteristics of the Beiya alkaline-rich porphyries show that they belong to shoshonitic rocks with a right inclined REE pattern and sligt Eu negative anormaly.They are also enriched in large ion lithophile element(LILE)and depleted in hight field strength element(HfSE),implying the related magma has experienced a subduction-related metasomatism.The berried Dashadi monzonitic granite porphyry in this study is formed at 35.4±0.5Ma,simultaneously with the ages of the othe porphyries in Beiya which are the products of the late Eocene magmatic peak in the Jinshajiang-Honghe alkaline-rich porphyry belt on the western margin of the Yangtze plate.The Beiya alkaline-rich porphyries have a thickend lower crust adakitic feature and the same Sr-Nd-Pb isotopic composition to the amphibolite xenolith hosted by the Eocene potassic felsic intrusion in western Yunnan,indicating that the magma are derived mainly from the lower crust.Besides,the magma also has a EMII mantle source feature.Combined with previous study,we conclude that the Beiya alkaline-rich magma with EMII mantle source feature are primarily derived from partial melt of the India-Euroasian collision thickend lower crust caused by the activities of a basaltic magma which produced from the partial melt of the thickend metasomatized lithospheric mantle,trigged by the upwelling of asthenosphere in late Ecocene at Beiya.Decompression tectonic made by the sliping of the lithosphere-penetrating Jinshajiang-Honghe fault belt,which is the product of the oblique collide of blocks on the western margin of the Yangtze plate during the transfer period of the main-cillision to late-collision stages of the Qianghai-Tibet plateau guided the upwelling of the alkaline-rich magma.The magma finally placed in the secondaty fault system(such as the Maanshan fault at Beiya)of the Jinshanjiang-Honghe fault in upper crust.5.Based on the structure investigation in the mining area,we found that east-west oriented compression and subsequently north-south oriented compression processes had activated at Beiya.Research on the typical detailed ore geology of the three main mineralization,combined with studies of the factors and sub-processes of metallogeny system comprehensively show that a zoning patterned three types of mineralization was made at different structures from evolutions of ore forming fluid and material derived from alkaline-rich magma,which was related to the movement of the large scaled Jianshajiang-Honghe strike-slip fault system at the end of Econece(36Ma),the resulting three types of ore deposits in Beiya jointly constitute a gold-polymetallic metallogenic system with the center of the alkaline-rich magma activities.