Geochemical Characteristics Of The Neutral-acidic Rocks In The Jinping Block And Its Relation To Mineralization

The Jinping block is located in southern segment of the Jinshajiang-Red River alkali-rich intrusive belt.For the outcrop in the area of middle-acid rocks,mainly include syenogranite,syenogranite porphyry,grain-refining syenogranite and quartz syenite porphyry.Mineral compositions are mainly potassium feldspar,plagioclase,amphibole,biotite,quartz.Accessory minerals consist of apatite,sphene,zircon,monazite,xenotime.The ore bodies and dykes occur together or cut each other.The studies on petrogeochemistry characteristics of the middle-acid rocks show that the SiO2 contents vary from 65.63%to 72.02%,the Na2O+K2O contents vary from 6.38%?10.38%and the A/CNK values focus on 1.15?2.69.In the SiO2 and K2O discrimination diagram,most of the samples fall in the shoshonite series.The SiO2-AR discrimination diagram shows,most of the samples fall in the alkaline series.In the Na2O-K2O diagram,samples fall into high potassic-potassic zone.The middle-acid rocks are characterized by alkaline and sub-alkaline,peraluminous and enrichment in potassium,and belong to shoshonitic rocks.The trace element and REE analysis indicate that the total REE values of middle-acid rocks in the Jinping block are 60.61?433.89?g/g,the ratio of LREE/HREE is 9.1?26.65.These rocks are obviously enriched in LILEs,and depleted in HFSEs.They also have high ?REE and LREE contents and fractionation between LREEs and HREEs.The 8Eu values range from 0.8 to 0.8,and weakly negative Eu anomaly.These rocks are enriched in large ion lithophile elements,and relatively depleted in high field-strength elements.All kinds of the middle-acid rocks may originate from the same parent magma.Zircon U-Pb age is 33.66?34.49 Ma,Lead isotope compositions 208Pb/204Pb values are 38.985?39.022,207Pb/204Pb values are 15.600?15.612,20bPb/2O4Pb values are 18.706?18.828 respectively,distributing relatively stable,showing the characteristics of organic belt Pb sources;The average value of(87Sr/86Sr)i is 0.70733,the 143Nd/144Nd values range from 0.512247 to 0.512268;The ?Nd(0)values rang from-4.02 to-7.0,which are characterized by high(87Sr/86Sr)i values and low ?Nd(t)values.The Jinping block middle-acid rocks formed in late collisional setting at?34.5Ma which were directly derived from partial melting of the mantle Combining former carbon,hydrogen,oxygen,sulfur,plumbum isotopes data,we conclude that the ore-forming fluid was related to magmatic-derived fluids and sourced from magmatic water and metamor phicwater,and the metallogenic materials were mainly from mantle as well as strata.The resources of mineralizing matter include mantle degassing and formation.Integrated the studies on geological and geochemical characteristics of the middle-acid rocks,the mineralization process analysis is as follows:On the influence of the ascent along faults of large scale alkali-richmagma of Cenozoic.in the process of magmatic gas-liquid transport mixed shallow fluid and extracted ore-forming elements from the formation and relative geological bodies,when the ore-forming fluid transport to the proper structural migrated and finally precipitated them under some physical-chemical condition.