Geochemical characteristics of magnetite in Longqiao skarn iron deposit in the Middle-Lower Yangtze Metallogenic Belt, Eastern China
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The Longqiao Fe deposit is the only large stratabound Fe skarn in Eastern China. This extensive ore body is suitable for investigating the spatial variations in trace elements in magnetite from the intrusion to the wall rocks and tracing the evolution of the mineralizing fluid in a skarn deposit. Magnetite samples were collected every 200 m from the ore-forming gabbro-diorite to the edge of the ore body, including the diopside-magnetite and epidote-chlorite-magnetite stages. Our results show that magnetite proximal to the gabbro-diorite displays lower Mg, Al, and Si and higher contents of Ti and V than those found further from the gabbro-diorite. The hydrothermal fluids in both diopside-magnetite and epidote-chlorite-magnetite stages were influenced by reaction with marine sedimentary rocks. Magnetites from different parts of the ore body display a wide range of trace element contents and fall into the porphyry and skarn fields on a (Mn + Al) vs. (Ti + V) discrimination diagram. Based on our results and published data, we propose that the early-stage magnetite from skarn and iron oxide-apatite (IOA) Fe deposits is compositionally similar, and fluid compositional variations are largely dependent on differences in the composition of the wall rock and the intensity of fluid-rock interactions. This results in a large variation in (Ti + V) in magnetite compositions from skarn deposits in the (Mn + Al) vs. (Ti + V) discrimination diagram, which is different from IOA deposits. This study shows that magnetite trace element evolution trends can provide a reliable indication to recognize skarn and IOA deposit types.
KeywordsEastern China Longqiao Fe deposit LA-ICP-MS Magnetite Skarn
We thank Ou Bangguo and the Longqiao Mining Company, Limited, for their valuable support during fieldwork and Leonid Danyushevsky of the Centre for Ore Deposit and Exploration Science (CODES), University of Tasmania, for his help in carrying out the analyses. We are deeply indebted to two journal reviewers and associate editor Rolf L. Romer for their valuable suggestions to improve the manuscript and Prof. Georges Beaudoin for his useful comments.
This work was financially supported by the National Key Research and Development Plan (grant no. 2016YFC0600206), the National Natural Science Foundation of China (grant nos. 41672081, 41320104003, and 41172084), and the Public Welfare Project of Anhui Province (grant nos. 2015-K-2 and K-3).
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