A tentative discussion on the genetic type and ore-forming process of main late Paleozoic magnetite deposits in West Tianshan Mountains, Xinjiang

Absract: The late Paleozoic magnetite ore belt is an important iron metallogenic belt in West Tianshan Mountains, Xinjiang. For a long time, there has been controversy concerning the metallogenic setting and genetic type. Based on previous researches in combination with lots of field investigations and indoor studies conducted by the authors in the past three years, this paper deals tentatively with the geological characteristics, distribution patterns and metallogenic types of iron deposits in West Tianshan Mountains, Xinjiang. The Sr-Nd-Pb isotopic compositions of ore minerals show that the metallogenic setting was the island arc under the subduction-collision of ocean and land. The metallogenic age of iron ores was younger than that of volcanic rocks but older than that of intermediate-acid intrusive rocks in the same ore district. The iron material might have come from the volcanic rocks, where their partition models were very similar in trace elements and REE. These iron deposits all belong to marine volcanic type, which can be further divided into three subtypies: volcanic effusion type, volcanic-subvolcanic hydrothermal type, superimposition type of volcanic effusion and volcanic hydrothermal solution. The geological features of different subtype deposits are not different. The iron ores were relatied to the metasomatism of fluid along the zone of subduction. Early iron ores were probably derived from the fractional crystallization of Fe-rich magma after replacement, and the late ores might have resulted from the metasomatism and precipitation of volcanic hydrothermal solution along faults and fractures.