Genesis of Early Cretaceous porphyrite-type iron deposits and related sub-volcanic rocks in the Ningwu Volcanic Basin, Middle-Lower Yangtze Metallogenic Belt, Southeast China

ABSTRACT

The Ningwu Volcanic Basin (NVB), located in the Lower Yangtze Metallogenic Belt, is characterized by the widespread occurrence of porphyritic iron-ore deposits. These deposits are clustered in three areas within the basin: the northern, central, and southern districts. Our study shows that the differences are existed in geochemistry of the rocks and the mineralization of the deposits among these three districts. Though results of Sr–Nd isotopes indicate that the parental magma of the sub-volcanic rocks in NVB were derived from EMI and EMII, however, the sub-volcanic rocks from the northern and southern districts have higher ε_Nd(t) values (t = 130 Ma) and lower initial ⁸⁷Sr/⁸⁶Sr ratios (t = 130 Ma) than those from middle district. The rocks from northern and southern districts are similar to EMI in geochemistry; in contrast, rocks from the middle district are close to EMII. Meanwhile, the deposits from different districts show some differences in mineralization. The magnetites from the northern ore deposits are rich in Cu, Mo, Zn, and Mg and poor in Ca, Al, and Ti, which probably indicates that the magnetites from the northern ore deposits closer to hydrothermal genesis than to magmatic genesis, but the magnetites from the centre are closer to both magmatic and hydrothermal geneses. Based on the fact that a part of the sub-volcanic rocks is closer to EMI and another part to EMII in geochemistry, we suggested that the incorporation of the lower and upper crustal materials into the source regions of the sub-volcanic rocks has taken place by different manner and processes. Integrated with analysis of the tectonic evolution of NVB, we come to the conclusion that the sub-volcanic rocks have recorded the geochemical imprints of the subduction of Yangtze block towards North China block during Indosinian Period and the delamination of the lower crust of Yangtze block during Yanshanian Period.

KEYWORDS:

• Iron-ore deposit
• sub-volcanic rocks
• Sr–Nd isotopes
• magnetite
• Ningwu Volcanic Basin (NVB)
• Middle-Lower Yangtze Metallogenic Belt

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Acknowledgements

This study is supported by the DREAM project of MOST China (2016YFC0600404), National Science and Technology Support Program of China (Project 2011BAB04D01), and the National Natural Science Foundation of China (No. 41372087). We thank Mrs Brätz Helene for his help in EPMA analyses in Key Lab of the GeoZentrum Nordbayern, University of Erlangen-Nuremberg, in the process of LA-ICPMS.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study is supported by the DREAM project of MOST China (2016YFC0600404), National Science and Technology Support Program of China (Project 2011BAB04D01), and the National Natural Science Foundation of China (No. 41372087).