Textures and trace element geochemistry of magnetite in the Beiya gold deposit, SW China

The Beiya gold deposit (323t of gold @ 2.47 g/t) is located in the centre of the Jinshajiang-Ailaoshan potassium alkaline porphyry Cu-Au metallogenic belt in SW China, which is the largest skarn gold deposit in China. As the dominant ore mineral at Beiya, magnetite has been selected as a petrogenetic indicator to understand the ore-forming process. In this study, magnetites have been divided into the euhedral-subhedral (M-E), anhedral (M-A), lamellar (M-L), and replaced (M-R) types based on their textures, which have been further determined by in situ laser ablation inductively coupled plasma mass spectrometry analyses. In detail, plots in discriminant diagrams constrain the Beiya magnetite a hydrothermal origin. Trace elements in magnetite vary with respect to different textures. Titanium concentrations are similar in all magnetite types, suggesting that all magnetites were precipitated in a relatively narrow temperature range. The decreasing V content combined with haematite after magnetite (martitization) suggests that the oxygen fugacity increased from M-E and M-A and then decreased to M-L and M-R. All magnetite types show similar content variations and similar (Mg + Mn)/Al ratios, indicating a decreasing degree of fluid-rock interactions. The rim-core texture indicates that the M-R type magnetite has experienced the dissolution-reprecipitation process. Mushketovite marks the later ore-forming environment and the iron-rich hydrothermal fluids transformed from high temperature and oxidized to low temperature and reduced features. Magnetite textures, combined with its trace element variations, could be used as a potential tool for inferring the ore-forming fluid evolution.