High-Grade Magnetite Mineralization at 1.86 Ga in Neoarchean Banded Iron Formations, Gongchangling, China: In Situ U-Pb Geochronology of Metamorphic-Hydrothermal Zircon and Monazite

摘要

Although less common than hematite ores, high-grade magnetite ores represent a distinct type of iron mineralization hosted by banded iron formations (BIFs). The Gongchangling iron deposit hosted in similar to 2.55 Ga BIFs in the North China craton represents one of the most economically important iron deposits in China. Located in mining area II, it is a high-grade (>50 wt % Fe) magnetite deposit and one of the largest of its type in the world. However, the lack of reliable age constraints on iron mineralization has hindered the testing of competing genetic models for the formation of the Gongchangling deposit. In situ U-Pb geochronology of monazite and zircon intergrown with garnet from the proximal alteration zone of a high-grade iron orebody yielded an age of 1.86 Ga, which represents the timing of formation of high-grade magnetite mineralization. This age is coeval with a tectonic extension event recorded in the northeastern North China craton. Our results preclude the previously suggested genetic link between high-grade magnetite mineralization and similar to 2.50 Ga regional metamorphism. Growth of authigenic monazite and zircon is likely related to the breakdown of detrital zircon, which has undergone metarnictization. In combination with previously published data, we propose that the development of zoned alteration associated with the deposit, which is characterized by the garnet-amphibole-magnetite assemblage in the proximal zone changing to a chlorite-quartz-dominated assemblage in the distal zone, can be attributed to a gradual decrease in temperature from >550 degrees to similar to 250 degrees C and to alteration minerals forming from leaching of the BIFs instead of by replacement of the wall rocks. Magnetite mineralization was controlled by the well-developed faults that cut the BIFs and provided conduits for silica-undersaturated alkaline meteoric fluids. Fluid flow likely took place in an extensional tectonic regime, similar to that invoked elsewhere for hematite mineralization but at greater depths.