Abstract:

Gongchangling iron deposit is one of the most typical BIFtype iron deposits in AnshanBenxi area, and it is the biggest producer of highgrade iron ore. Judging from the field relationship the ore is inseparately related to altered rocks.The altered rocks are characteristic of zonal distribution. In the order from highgrade iron ore outsides there are: cummingtonite rock→garnet rock→chlorite rock→weakly altered amphibolite and→amphibolite. In the weakly altered rocks, the original appearance of the primary rock is still preserved, the alteration of minerals are incomplete, the relicts of primary minerals can still be found. However， in the intensively altered rocks, the primary minerals are rarely found. The comparison on geochemistry of the altered rocks, amphibolite and magnetite quartzite indicates that the trace element characteristics of weakly altered rock, garnet rock and chlorite rock are similar to those of the amphibolites, and the trace element characteristics of cummingtonite rock are much similar with those of the lowgrade iron ores. Combined with the evidences from the microscopic characteristics, contact relationship and characteristics of major elements, we hold that the altered rocks were formed by hydrothermal interaction with amphibolites expect for cummingtonite rock. Cummingtonite rock is more likely to be formed by the hydrothermal interaction with lowgrade iron ores. According to the varying regulation of (Fe+Mg)/Si, SiO2 and Fe2OT3 showed by various altered rocks and amphibolites, there is migration of Mg, Fe and Si occurred during the hydrothermal metasomatism. The results of plottings on tectonic discrimination diagrams indicate that the protolith of these rocks are backarc basin basalts.

Key words: Gongchangling, altered rocks, high grade iron ore, geochemistry, migration, back arc basin