Zapla iron ore bodies in Jujuy state, northern Argentina are located within Paleozoic Silurian marine sedimentary rocks and can be categorized into ironstone deposit. Iron ores contain oolitic hematite as main iron mineral as well as siderite and chamosite. Hematite replaced biotite and/or muscovite along their cleavage or grain boundary, which indicates hematite is precipitated by chemical reaction. Silurian basins in northern Argentina has high potential resources for ironstone deposit but economic aspects of ore body can be controlled by magnitude of lateral vertical extensions and local grade variation of iron beds.