河北中关铁矿闪长岩体磷灰石和角闪石地球化学特征及对成岩成矿的记录OA

Skarn-type iron deposits are an important component of iron ore resources in China.To understand their formation mechanism,it is necessary to investigate the source,genesis,and physicochemical evolution of the ore-forming magma.This study presents BSE images and corresponding major and trace element compositions of hornblende and apatite from the diorite of the Zhongguan iron deposit.These data are used to identify the magma source,magma mixing,and crustal contamination processes,as well as the resultant changes in magmatic physicochemical conditions.Finally,the requisite magmatic conditions for the formation of the Zhongguan skarn iron deposit are proposed.The results show that the ore-forming magma evolved within a dual magma chamber system.In a deep chamber,hornblende phenocrysts crystallized at an average temperature of 943 ℃,pressure of 315 MPa,oxygen fugacity of ΔNNO+0.34,and water content of 5.97％.In contrast,the hornblende matrix crystallized in a shallow magma chamber at a lower average temperature(773 ℃)and pressure(89 MPa),with higher oxygen fugacity(ΔNNO+1.5)and Cl content(0.15％),and a lower water content(average 4.87％).Variations in the Al2O3 content of hornblende,along with the initial 87Sr/86Sr values of apatite(ranging from 0.7053 to 0.7076),suggest the ore-forming magma was mainly mantle-derived but was subsequently mixed with a minor amount of crustal materials.BSE imaging of apatite reveals obvious core-rim structures,with cores generally appearing brighter than the rims.The cores are enriched in SiO2,MnO,Ga,and REEs,whereas the rims are enriched in SO3,Na2O,and Cl.Furthermore,the rims exhibit higher δEu values than the cores.The compositional differences between phenocrysts and matrix of hornblende,in conjunction with the core-to-rim variations in apatite,indicate that the ore-forming magma underwent contamination by gypsum-bearing strata,assimilation of crustal material,and fluid exsolution during its ascent and emplacement.These processes led to a significant increase in the oxygen fugacity of magma,which promoted the enrichment of volatiles and iron.In summary,the assimilation of gypsum-bearing country rock and/or mixing with crust-derived magma,coupled with hypabyssal emplacement,are necessary conditions for the formation of the high-grade Zhongguan skarn iron deposit.