The skarn-type iron deposit is an important guarantee of iron ore resources in China. It is necessary to study the source, genesis and evolution of physical-chemical conditions of ore-forming magma for understanding formation mechanism of skarn-type iron deposits. In this paper, the BSE images and corresponding major and trace element compositions of the hornblende and apatite in diorite from the Zhongguan iron deposit are conducted. The source of magma, magma mixing and crustal contamination process, as well as the changes of magmatic physical and chemical conditions caused by these processes are identified. Finally, the necessary magmatic conditions for the formation of the Zhongguan skarn iron deposits are proposed. The results show that the ore-forming magma experienced a double magma chamber system. In the deep magma chamber, the hornblende phenocrysts crystallized in the environment of temperature (average 943 °C), pressure (average 315MPa), oxygen fugacity (average ΔNNO + 0.34) and water content (average 5.97 %). In contrast, the hornblende matrix crystallized in the environment of temperature (average 773 °C), pressure (average 89MPa), higher oxygen fugacity (average ΔNNO + 1.5) and Cl content (average 0.15 %), and lower water content (average 4.87 %) in the shallow magma chamber. The changes of Al2O3 content of hornblende, as well as the 87Sr/86Sr（i）values (from 0.7053‰ to 0.7076‰) of apatite indicate that the ore-forming magma is mainly mantle-derived and mixed with a small amount of crust-derived materials. The obvious core-rim structures are shown in the BSE images of apatite, and the core is generally brighter than that of the rim. The cores are mainly enriched in SiO2, MnO, Ga, REEs et al. In contrast, the rims are more enriched in SO3, Na2O and Cl. Besides, the δEu values of the rims are higher than those of the cores. The chemical composition differences between phenocrysts and matrix of hornblende, as well as the core-to-rim compositional variations of apatite indicate that the ore-forming magma has undergone contamination of gypsum rock strata, crustal material mixing and fluid exsolution during the upward emplacement process, resulting in a significant increase in oxygen fugacity of magma, and promoting the enrichment of volatiles and Fe in magma. In summary, gypsum rock contamination or crust-derived magma mixing and hypabyssal emplacement are necessary conditions for the formation of the Zhongguan skarn high-grade iron deposits.