Occurrence of the Iron–rich Melt in the Heijianshan Iron Deposit, Eastern Tianshan, NW China: Insights into the Origin of Volcanic Rock–hosted Iron Deposits

Abstract

Long‐standing controversy persists over the presence and role of iron–rich melts in the formation of volcanic rock‐hosted iron deposits. Conjugate iron–rich and silica–rich melt inclusions observed in thin‐sections are considered as direct evidence for the presence of iron‐rich melt, yet unequivocal outcrop‐scale evidence of iron‐rich melts are still lacking in volcanic rock‐hosted iron deposits. Submarine volcanic rock‐hosted iron deposits, which are mainly distributed in the western and eastern Tianshan Mountains in Xinjiang, are important resources of iron ores in China, but it remains unclear whether iron‐rich melts have played a role in the mineralization of such iron ores. In this study, we observed abundant iron‐rich agglomerates in the brecciated andesite lava of the Heijianshan submarine volcanic rock–hosted iron deposit, Eastern Tianshan, China. The iron‐rich agglomerates occur as irregular and angular masses filling fractures of the host brecciated andesite lava. They show concentric potassic alteration with silicification or epidotization rims, indicative of their formation after the wall rocks. The iron‐rich agglomerates have porphyritic and hyalopilitic textures, and locally display chilled margins in the contact zone with the host rocks. These features cannot be explained by hydrothermal replacement of wall rocks (brecciated andesite lava) which is free of vesicle and amygdale, rather they indicate direct crystallization of the iron‐rich agglomerates from iron‐rich melts. We propose that the iron‐rich agglomerates were formed by open‐space filling of volatile‐rich iron‐rich melt in fractures of the brecciated andesite lava. The iron‐rich agglomerates are compositionally similar to the wall‐rock brecciated andesite lava, but have much larger variation. Based on mineral assemblages, the iron‐rich agglomerates are subdivided into five types, i.e., albite‐magnetite type, albite‐K‐feldspar‐magnetite type, K‐feldspar–magnetite type, epidote‐magnetite type and quartz‐magnetite type, representing that products formed at different stages during the evolution of a magmatic‐hydrothermal system. The albite‐magnetite type represents the earliest crystallization product from a residual iron‐rich melt; the albite‐K‐feldspar‐magnetite and K‐feldspar‐magnetite types show features of magmatic‐hydrothermal transition, whereas the epidote‐magnetite and quartz‐magnetite types represent products of hydrothermal alteration. The occurrence of iron‐rich agglomerates provides macroscopic evidence for the presence of iron‐rich melts in the mineralization of the Heijianshan iron deposit. It also indicates that iron mineralization of submarine volcanic rock‐hosted iron deposits is genetically related to hydrothermal fluids derived from iron‐rich melts.

Chronolgy and Geochemistry of the Sijiaying Iron Deposit in Eastern Hebei Province, North China Craton: Implications for the Genesis of High-Grade Iron Ores

“…Accordingly, this study identified the primitive sedimentary type and hydrothermally altered type of high-grade iron ore from the ore texture, structure, and geochemical characteristics. Although the detailed alteration process and mechanism are not clear, we preliminarily suggest that the genesis of high-grade ores is probably related to the hydrothermal process of migmatization, which may lead to the desilication of adjacent BIFs [14].…”

“…In addition, most large iron ore deposits, such as Gongchangling, Nanfen, Xingshan and Sijiaying [8], contain a certain number of high-grade iron ore resources. High-grade iron ore mainly includes two types (primitive sedimentary and hydrothermally altered); however, the hydrothermal origins are still controversial, with origins mainly in metamorphic hydrothermal or migmatized hydrothermal environments [9][10][11][12][13][14]. Many geologists constrain the metallogenic age of BIF volcanic interbeds based on their zircon ages.…”