Study On Genesis Of Volcano Rocks-subvolcanic Rocks,and Its Relationship With Gold Mineralization In Mangzhuangshan-Yufeng Area,Gansu Province

The Mazhuangshan-Yufeng area is located at the northern of the Beishan orogenic belt,the Heiyingshan-Hanshan arc,and the southern wing of the Xingxingxia-Mingshui complex anticline.The north and south sides of the area are respectively controlled by the Xingxingxia strike slip fault and the Baishan-Lujing fault.At the same time,the area is located at the important north belt metallogenic belt of the Beishan.The identified gold deposits in the Mazhuangshan-Yufeng area include the Mazhuangshan gold deposit?large?,Nanpoziquan ore occurrence and the Xiuwenghala ore occurrence.The study of the Mazhuangshan-Yufeng volcanic rocks can contribute to reconstruct the tectonic background in the northern belt of Beishan in the late Paleozoic,and provide the geological evidence for the exploration of the genesis of the gold deposits and the prediction of the metallogenic regularities.The stratums of the Mazhuangshan-Yufeng area are mainly intermediate-acid volcaniclastic rocks of the second lower and upper rock group of the Lower Carboniferous Baishan Formation.The rock assemblage consists mainly of quartz porphyry,angenite,andesite and intermediate acid volcanic tuff of the same composition,of which the quartz porphyry of the subvolcanic facies is the metallogenic source rock of the metallogenic source rock in this area.The geochemical characteristics of the rocks indicate that the volcanic rocks in this area are homologous,and these volcanic rocks belong to the sub-alkaline rocks of calc-alkaline series,which are mainly the high and medium potassiumcalc-alkaline series and minor shoshonite series.The volcanic rocks are rich in silicon,potassium,alkali,depleted titanium,and peraluminous.There are a good correlation between SiO₂ and the other oxides.Andesite is high Magnesium-iron series.The volcanic rocks of this area have a high rare earth gross,and a right-hand curve distribution model with LREE enrichment,obvious Eu negative anomaly,especially in Yufeng region.Significant fractionation of trace elements,all samples show trace elements with enriched LILE,Rb,Ba,K,and Pb,Th,Hf,Nd,Zr,Sm,depleted HFSE,P,Ti,Nb,and Sr,relatively high Zr/Nb,La/Nb,Ba/Nb,Rb/Nb,Th/Nb,Th/La,Ba/La,Nb/Ta is close to the continental crust.The characteristics of rock assemblages,electron probe and geochemistry shows that the volcanic rocks in Mazhuangshan-Yufeng area are mainly the mixed crust-mantle material,but the hornblende andesite of the Nanpoziquan area from the mantle.The volcanic rocks in this area are mainly formed in the continental marginal arc environment under subduction.Zircon U-Pb age of the quartz porphyry in the Mazhuangshan area is 318.9±2.2Ma?1?,MSWD=0.068?,zircon U-Pb age of the Hornblende andesite in the Yufeng area is 290.2±4.0Ma?2?,MSWD=0.36?.At the same time,the older zircon appears in this area,indicating that the volcanic rocks in the Mazhuangshan-Yufeng area were formed in the Late Carboniferous,and there may be older strata in this area.According to the rock assemblage characteristics,geochemical characteristics,volcanic genesis,forming age and previous studies of volcanic rock from the Mazhuangshan-Yufeng area,these volcanic rocks are the products of the north-dipping subduction between the Heiyingshan-Hanshan arc and the Gongpoquan arc-accretionary system in the Late Carboniferous.The tectonic background is continental margin arc.The gold deposits in this area are the products of tectonic-magmatic activities in the same period.The physical and chemical characteristics of the Mazhuangshan-Yufeng area show that volcanic rocks are closely related to gold mineralization,and subvolcanic rocks are the parent rocks of gold.Subvolcanic rocks not only provide a source for mineralization,but also induce hydrothermal environment,surrounding rock alteration and other favorable geochemical environments for mineralization.