查岗诺尔铁矿床赋存在大哈拉军山组中-上部的安山质火山碎屑岩和火山熔岩内,主矿体FeⅠ周围发育大量以石榴子石为代表的高温热液蚀变,精确厘定高温热液蚀变的时间是确定成矿作用是与火山作用有关还是由岩体侵入作用所导致的关键.石榴子石与磁铁矿体的关系最为密切,其形成时间稍早或同时于磁铁矿.石榴子石Sm-Nd同位素测试结果表明,147Sm/144Nd值为0.2792～0.5481,143Nd/144Nd值为0.512950～0.513501,7个样品拟合的线性关系良好,获得的Sm-Nd等时线年龄为(316.8±6.7) Ma,指示了高温热液蚀变的时间,表明主要磁铁矿体的形成时代为早石炭世晚期,成矿作用及高温热液蚀变可能不是矿区内二叠纪岩体侵入携带的岩浆热液与大理岩发生矽卡岩化所导致,而可能是大哈拉军山组火山岩喷发后的岩浆期后热液与下伏大理岩发生接触交代反应引起的.%The large-size Chagangnuoer iron deposit is hosted in andesite and andesitic volcaniclastic rocks in the middle-upper part of Carboniferous Dahalajunshan Formation, with widespread high hydrothermal alteration (represented by garnet) distributed extensively around the uppermost ore body Fel. The precise formation age of high hydrothermal alteration is the key to judging whether iron metallogeny was related to volcanism or intrusive activity. Garnet, which had a closely genetic relationship with magnetite, was formed simultaneously with or slightly earlier than magnetite. Garnet Sm-Nd isotope analyses reveal that 147Sm/144Nd values vary between 0.2792 and 0.5481 while 143Nd/144Nd ratios vary between 0.512950 and 0.513501. Data of seven isotope sam- pies form a good linear fitting relationship, yielding the Sm-Nd isochron age of (316.8 ± 6.7) Ma, which represents the formation epoch of high-temperature hydrothermal alteration. The result indicates that magnetite in-tergrown with garnet was formed at the late stage of Early Carboniferous period. Therefore, the iron metalloge-ny and high hydrothermal alteration might have resulted from the metasomatism between the post-magmatic hydrothermal solution derived from eruption of Dahalajunshan Formation volcanic rocks and the underlying marble, rather than the skarnization caused by the reaction between magmatic hydrothermal solution from Permian intrusion and marble in the ore district.
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