关键词: 非Algoma类型, 铁迁移, 黄铁矿风化, 热力学分析, 陆壳来源物质

Abstract: Many accomplishments are made in the mechanistic study of iron source material migration for the formation of large-scale iron ore deposits during the Precambrian period. However, a few factors have yet to be fully resolved. In particular, still in debate are multiple theories on the genesis of cast iron deposits (non-Algoma type) of continental source. We have investigated the continental migration of iron source materials for the Ryongyon iron ore deposits in the Korean Peninsula as an example. From our geochemical study and the work of previous researchers, we showed that the iron ore was not Algoma type but formed from migration and sedimentation of weathered continental source materials under strong acidic conditions. As we know, during the Precambrian time, oxygen was depleted in the atmosphere and water with no ozone layers formed therefore no plants grew on land. Then, how did such a strong acidic environment occur? To clarify, we performed thermodynamic calculations on prevailing materials (at the time) that could convert water into acid in the environment. The results revealed that, when sulphide (such as pyrite) weathering occurred, it developed an iron source capable of medium degree weathering and transport. This result is consistent with the fact that the pH value was less than 3.5 for waters from the sulphide-rich stratum where iron content was much higher than in non-sulphide containing strata. Thus we conclude that sulphide weathering played  a non-negligible role in the formation of continental source iron deposits.

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