Abstract
The Ningwu ore district is the best-defined iron oxide–apatite (IOA) district in China and hosts >30 IOA deposits. At Ningwu, four volcanic cycles occurred over a short period of ∼ 10 Ma and yielded comparable lithogeochemical characteristics. However, only the sub-volcanic intrusive rocks from the Dawangshan Formation are associated with IOA-type mineralization. In this study, the mineralization related and unrelated formations have been distinguished based on zircon Hf-O isotope and trace element analyses to reveal the magma evolution and fertility of IOA deposits. Our results suggest that the Mesozoic volcanic rocks at Ningwu have three components of sources: enriched mantle, Archean crystalline basement, and Neoproterozoic crustal materials. The main sources for the magmas of the Longwangshan and Dawangshan formations include the Archean crust and enriched mantle, whereas those for the Gushan and Niangniangshan formations include Neoproterozoic crust and enriched mantle. According to the magmatic sources, the four volcanic formations in the Ningwu ore district can be divided into two groups: (1) the Longwangshan and Dawangshan formations and (2) the younger Gushan and Niangniangshan formations. The magmatic oxygen fugacities (fO2) for the Longwangshan and Dawangshan formations are ΔFMQ +0.6 to +2.2 and ΔFMQ +0.8 to +3.0, respectively. In comparison, the Gushan Formation has much lower magmatic fO2 values of ΔFMQ +0.2 to +0.6, whereas the Niangniangshan Formation yielded magmatic fO2 values peaking at ΔFMQ +0.6 to +2.4 and +3.2 to +4.2. Our new data, together with previous results, suggest that moderate magmatic fO2 (ΔFMQ +0.8 to +3), F-richness, high water contents, high proportions of enriched mantle materials, and a higher degree of magma evolution may affect the fertility of the magma for IOA deposit formation.
Published Version
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