According to the lithology and geochemistry characters of the iron horizons ofthe Longqiao iron deposit, stratigraphic correlation with Middle TriassicZhouchongcunian gypsum-bearing rock series in the Nanjing area, coupled withthe distribution of the ore horizons and regional tectonics and a tectonic analysisof volcanic basin, it has been shown that iron horizons would be originallyiron-rich sedimentary (siderite, ankerite, magnetite, hematite) source beds, in theMiddle Triassic Ma'anshan'an (Zhouchoncunan) stage deposited in the lagoonal-sabkh sedimentary environmet. The source beds might have played an importantrole as embryo beds which have caused the preference of thermometamorphismand hydrothermal superimposition on such ?particular beds during the magmaticintrusion at the Yanshanian.