Abstract: Located in the middle of Eastern Tianshan Mountains, the large-size Yamansu iron deposit is hosted in Andesitic volcanic clastic rock or andesitic tuff of the Lower Carboniferous Yamansu Formation, with a lenticular marble beneath the main ore body. The ore bodies occur as lamellar stratoids and lenses, controlled by EW-striking faults and circular faults. Wall rock alteration is strong, including garnet skarn and complex skarn. Electeon microprobe analyses show that the end member of garnet is andradite(And) with an average content of 63.59%, grossularite(Gro) with an average content of 34.11%, almandite(Alm) and spessartine(Sps) with an average content of 4.44%. Components of garnet and pyroxene are And45.68~100Gro0.67~57.95(A1m+Sps)11~29.03, which indicates that characteristics of this skarn minerals are quite similar to those in calcic skarn from the major large iron deposits, suggesting that they probably resulted from skarnization; In the Ca+Al+Mn versus Ti+V discriminant diagram showing spot analyses of magnetite and hematite, almost all data from the Yamansu ore deposit fall into the region of the skarn type iron deposit. In addition, in the ternary plot of Ti02-Al203-Mg0 of magnetite, many data from the Yamansu ore deposit tend to be seated in the sedimentary metamorphogenic and contact meatasomatic region while less data drop into magmatic mafic-ultramafic region. These two diagrams may suggest that the formation mechanism of magnetite from this ore deposit may be similar to that of magnetite from skarn iron deposits. Combining the geological characteristics with the study of mineralogy, the authors hold that most samples through a process of hydrothermal metasomatism, it indicates that iron formation was related to magmatic hydrothermal metasomatism.