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毕机沟钒钛磁铁矿位于扬子板块北缘汉南杂岩的西北部,是基性-超基性杂岩体分异演化的产物。矿区范围出露的岩石主要包括橄长岩、辉长岩、辉长闪长岩和闪长岩,具有明显岩相分带和韵律旋回特征。目前,已发现的多个钒钛磁铁矿体均位于基性岩-超基性杂岩体的辉长岩相带内。本次研究对毕机沟矿区的含矿辉长岩和穿切含矿辉长岩的花岗岩脉进行了原位微区LA-MC-ICPMS锆石U-Pb定年,获得两者的侵位时间分别为(783±4)Ma(MSWD=0.09)和(759±4)Ma(MSWD=1.50),结果表明毕机沟钒钛磁铁矿的成矿时代大约为783 Ma,但不晚于759 Ma。结合前人区域成果资料,毕机沟含钒钛磁铁矿的基性-超基性杂岩体与该区新元古代大规模具岛弧性质的基性-超基性杂岩体的形成时代相近,我们认为毕机沟钒-钛磁铁矿可能形成于活动型大陆边缘环境。
The Bijigou vanadium-titanium magnetite is located in the northwestern part of the Hannan complex in the northern margin of the Yangtze plate and is the product of the differentiation of basic-ultramafic complexes. The exposed rocks in the mining area mainly include gneiss, gabbro, gabbro and diorite with distinct lithofacies zoning and rhythmic cycles. At present, a number of vanadium-titanium magnetite ore bodies have been found in the gabbro belt of basic-ultramafic complexes. In this study, LA-MC-ICPMS zircon U-Pb dating of both granodiorite and peridotite-bearing gabbro in the Bijigou mining area was carried out, and the emplacement time of both (783 ± 4) Ma (MSWD = 0.09) and (759 ± 4) Ma (MSWD = 1.50), respectively. The results show that the mineralization age of the vanadic-titanated magnetite in Bijigou is about 783 Ma but not later than 759 Ma. Based on the data of previous achievements, the basic-ultramafic complex of vanadium-bearing titanomagnetite and the formation of large-scale Neoproterozoic basic-ultramafic complex in this area In the similar era, we think that the vanadium-titanium magnetite in the Bijigou Formation may be formed on the active continental margin.