2. Key Laboratory of Mineral Ｒesources Evaluation in Northeast Asia，Ministry of Natural Ｒesources，Jilin University，

Changchun 130061，China;

3. College of Earth Sciences，Institute of Disaster Prevention，Sanhe 065201，Hebei，China

with typical banded structures． To determine the forming age，tectonic setting and genetic type of the Tadong iron deposit，the whole-rock geochemistry analysis and zircon U--Pb dating for the ore-hosting biotite k-feldspar

gneiss of the Lalagou Formation were conducted． The results of microscopic petrography and whole rock major

and trace elements indicate that the protolith of the biotite k-feldspar gneiss is rhyolitic-dacitic volcanic tuff which was formed under an oceanic arc setting related to plate subduction． Zircon CL images and U--Pb dating results of  the biotite k-feldspar gneiss show that the weighted mean age of nineteen magmatic zircons is ( 725. 6 ± 7. 4) Ma，which represents the age of the protolith and indirectly indicates that the sedimentary age of the ore-hosting strata in the Tadong iron deposit is the Middle Neoproterozoic． By comparing characteristics of the Tadong iron deposit and

others in adjacent areas，the authors conclude that the formation time and the ore type of the Tadong iron deposit are different from those of other typical BIFs，which suggests that the genetic type of the Tadong iron deposit should

be volcanic-sedimentary metamorphic type．