Geochemistry of magnetite and its implication to genesis and exploration of the Duolong porphyry district

Duolong district, as one of the largest porphyry Cu-Au clusters in China, has magnetite wide spreading in porphyry intrusions and several porphyry deposits. This study investigates geochemistry of different types of magnetite in this district, with the aim of their genetic connections to Cu and Au mineralization in this district, meanwhile to refine the exploration tool for porphyry Cu deposits worldwide. Magmatic magnetite in the Duolong district is characterized by high Ti, V, Mg, Mn and Zn contents, low Ni/Cr ratios (usually > 0.2 ppm). In contrast, hydrothermal magnetite characterized by higher Cu, Pb, Co and Ni/Cr ratios (>1). Ni/Cr ratios and Ti and V contents indicate that magmatic magnetite crystallizes at a higher-temperature of ∼620-800 °C and near-FMQ redox conditions compare to that of hydrothermal magnetite formed at temperatures of ∼400-620 °C under more oxidizing conditions approaching the HM buffer. Hydrothermal magnetite shows a dispersed elemental distribution, like large variation of Ti, Vand Sn suggesting complex formation hydrothermal environments. Early low-grade Cu stage magnetite (Mag 2–1), which is mainly associated with quartz and pyrite, has higher Ti and V contents, compared to later main Cu mineralization stage magnetite (Mag 2–2), which is associated with chalcopyrite, bornite, quartz, and pyrite, suggesting a cooling process. This is coupled with sharply increasing Cu (±Mo, La, and locally Co) from Mag 2–1 to Mag 2–2, indicating precipitation of these metals may partly go into magnetite. Hydrothermal magnetite in the three deposits show difference that Ti, Al, Mn, Zn, and Ni are higher in the Duobuza and Naruo deposits, whereas Pb are higher in the Bolong deposit. These are consistent with different hydrothermal alteration features in these deposits that Duobuza and Naruo have wider area of higher-temperature potassic alteration, whereas Bolong has larger area of lower-temperature sericitic alteration. It suggests these elements are controlled by temperature, or alternatively, by a combination of temperature, pH, and hydrothermal fluid composition. Based on these findings, a four-quadrant magnetite discrimination scheme (Priority I-IV) is established to effectively distinguishes high-grade stage hydrothermal magnetite. Results suggest that low Ti + V contents and high Mo and Cu contents is a signature of high-grade mineralization center, thus can be used to guide exploration prospecting.